Shape-restoring metal. Shape memory metals. The mechanism of the shape memory effect. Application of shape memory alloys in medicine

One of the basic perceptions by people of the phenomena of the external world is the durability and reliability of metal products and structures that stably retain their functional form for a long time, unless, of course, they are subjected to supercritical influences.

However, contrary to common sense, there are a number of materials, metal alloys, which, when heated, after preliminary deformation, demonstrate the phenomenon of returning to their original shape. That is, these metals, not being living things, have a special property that allows them to display a kind of memory.

Phenomenon

To understand the shape memory effect, it is enough to see its manifestation once. What's happening?

Demonstration of the shape memory effect
There is a metal wire. This wire is bent.
We begin to heat the wire. When heated, the wire straightens, restoring its original shape.

The essence of the phenomenon

Why is this happening?

The essence of the phenomenon

In the initial state, the material has a certain structure. In the figure, it is indicated by regular squares.

During deformation (in this case, bending), the outer layers of the material are stretched, and the inner ones are compressed (the middle ones remain unchanged). These elongated structures are martensite plates. Which is not unusual for metal alloys. Unusually, martensite is thermoelastic in shape memory materials.

When heated, the thermoelasticity of martensite plates begins to manifest, that is, internal stresses arise in them, which tend to return the structure to its original state, i.e., to compress the elongated plates and stretch the flattened ones.

Since the outer elongated plates are compressed, and the inner flattened ones are stretched, the material as a whole carries out autodeformation in the opposite direction and restores its original structure, and with it the shape.

Shape memory characteristics

Shape memory effect characterized by two values.

  • An alloy grade with a strictly consistent chemical composition. (See below "Shape Memory Materials")
  • Temperatures martensitic transformations.

In the process of manifestation shape memory effect are involved martensitic transformations two types - direct and reverse. Accordingly, each of them manifests itself in its own temperature range: MN and MK - beginning and end direct martensitic transformation when deformed, AH and AK - the beginning and end when heated.

Temperatures martensitic transformations are a function of both the grade of the alloy (alloy system) and its chemical composition. Small changes in the chemical composition of the alloy (intentional or as a result of marriage) lead to a shift in these temperatures.

Hence, it is necessary to strictly maintain the chemical composition of the alloy for an unambiguous functional manifestation. shape memory effect... Which translates metallurgical production into high technologies.

Shape memory effect several million cycles appear.

The preliminary heat treatments can be enhanced shape memory effect.

Reversible shape memory effects, when the material at one temperature "remembers" one form, and at another temperature another.

The higher the temperature reverse martensitic transformation, the less pronounced shape memory effect... For example, weak shape memory effect observed in alloys of the Fe-Ni system (5 - 20% Ni), in which temperatures reverse martensitic transformation 200 - 400˚C.

Hyperelasticity

Another phenomenon closely related to shape memory effect is an hyperelasticity.

Hyperelasticity - the property of a material subjected to loading to a stress significantly exceeding the yield point to completely restore its original shape after removing the load.

Superelastic behavior is an order of magnitude higher than elastic.

Hyperelasticity observed in the temperature range between the beginning of the direct martensitic transformation and the end of the reverse one.

Shape memory materials

Titanium nickelide

Leader among materials with shape memory for application and study is titanium nickelide .

Titanium nickelide is an intermetallic compound of equiatomic composition with 55 wt% Ni. Melting point 1240 - 1310˚C, density 6.45 g / cm3. The initial structure of titanium nickelide, a stable body-centered cubic lattice of the CsCl type, undergoes a thermoelastic martensitic transformation with the formation of a phase of low symmetry.

Another name for this alloy, adopted abroad, is nitinol comes from the abbreviation NiTiNOLwhere NOL is the abbreviated name for the United States Naval Artillery Laboratory, where this material was developed in 1962.

Item from titanium nickelide can perform the functions of both a sensor and an actuator.

Titanium nickelide possesses:

  • Excellent corrosion resistance.
  • High durability.
  • Good shape memory characteristics. High coefficient of shape recovery and high restoring power. Deformation up to 8% can be completely restored. In this case, the recovery stress can reach 800 MPa.
  • Good compatibility with living organisms.
  • High damping capacity of the material.

Disadvantages:

  • Due to the presence of titanium, the alloy easily attaches nitrogen and oxygen. To prevent reactions with these elements during production, vacuum equipment must be used.
  • Processing in the manufacture of parts is complicated, especially by cutting. (Reverse side of high strength).
  • High price. At the end of the 20th century, it cost a little less than silver.

At the current level of industrial production, products from titanium nickelide (along with alloys of the Cu-Zn-Al system) have found wide practical application and market sales. (See “Applying Shape Memory Materials” below).

Other alloys

At the end of the 20th century shape memory effect has been found in more than 20 alloys. Besides titanium nickelide Effect shape memory found in systems:

  • Au-Cd. Developed in 1951 at the University of Illinois, USA. One of the pioneers of shape memory materials.
  • Cu-Zn-Al. Along with titanium nickelide has a practical application. Temperatures of martensitic transformations in the range from -170 to 100˚C.
    • Advantages (compared to titanium nickelide):
      • Can be smelted in normal atmosphere.
      • Easy to cut.
      • The price is five times cheaper.
    • Disadvantages:
      • Worse in terms of form memorization.
      • Poor mechanical and corrosive properties.
      • During heat treatment, grain coarsening easily occurs, which leads to a decrease in mechanical properties.
      • Problems of grain stabilization in powder metallurgy.
  • Cu-Al-Ni. Developed at the University of Osaka, Japan. Temperatures martensitic transformation in the range from 100 to 200˚C.
  • Fe-Mn-Si. The alloys of this system are the cheapest.
  • Fe-Ni
  • Cu-Al
  • Cu-Mn
  • Co-Ni
  • Ni-Al

Some researchers believe that shape memory effect is fundamentally possible for any materials undergoing martensitic transformations, including those of such pure metals as titanium, zirconium and cobalt.

Titanium nickelide production

Melting takes place in a vacuum-skull furnace or in an electric arc furnace with a consumable electrode in a protective atmosphere (helium or argon). The charge in both cases is titanium iodide or a titanium sponge pressed into briquettes, and nickel grade N-0 or N-1.

To obtain a uniform chemical composition over the cross-section and height of the ingot, double or triple remelting is recommended.

The optimal mode for cooling ingots in order to prevent cracking is cooling with a furnace (no more than 10˚ per second).

Removing surface defects - grinding with an emery wheel.

For a more complete leveling of the chemical composition over the volume of the ingot, homogenization is carried out at a temperature of 950 - 1000˚C in an inert atmosphere.

Application of shape memory materials

Titanium nickelide connecting sleeves

The bushing, first developed and implemented by Reichem Corporation, USA, for connecting pipes of the hydraulic system of military aircraft. The fighter has more than 300 thousand of such connections, but there have never been any reports of their breakdowns.

The use of such bushings is as follows:

Application of connecting sleeves
The bushing is in its original state at a temperature of 20˚C.
The sleeve is placed in a cryostat, where, at a temperature of -196˚C, the inner projections are expanded by a plunger.
The cold sleeve becomes smooth from the inside.

Using special pliers, the sleeve is removed from the cryostat and put on the ends of the pipes to be connected.

Room temperature is the heating temperature for a given alloy composition. Then everything happens “automatically”. The inner protrusions “remember” their original shape, straighten and cut into the outer surface of the pipes being connected.

The result is a strong vacuum-tight connection that can withstand pressures up to 800 atm.

In fact, this type of connection replaces welding. And it prevents such disadvantages of the weld as the inevitable softening of the metal and the accumulation of defects in the transition zone between the metal and the weld.

In addition, this connection method is good for the final connection in the assembly of a structure, when welding due to interlacing of nodes and pipelines becomes difficult to access.

These bushings are used in aeronautical, aerospace and automotive applications.

This method is also used to connect and repair pipes of submarine cables.

In medicine

  • Gloves used in the rehabilitation process and designed to reactivate groups of active muscles with functional impairment. Can be used in intercarpal, elbow, shoulder, ankle and knee joints.
  • Contraceptive coils, which, after insertion, acquire a functional form under the influence of body temperature.
  • Filters for introduction into the vessels of the circulatory system. They are introduced in the form of a straight wire using a catater, after which they take the form of filters with a given location.
  • Clamps for pinching weak veins.
  • Artificial muscles that are powered by electrical current.
  • Fastening pins for fixing prostheses to bones.
  • Artificial extension device for the so-called growing prostheses in children.
  • Replacement of the cartilage of the femoral head. The replacement material becomes self-clamping under the action of a spherical shape (femoral head).
  • Rods for the correction of the spine in scaliosis.
  • Temporary clamping elements for implantation of an artificial lens.
  • Spectacle frame. At the bottom, where the glass is secured with wire. Plastic lenses will not slip out when cooled. The frame does not stretch with lens wiping and long-term use. Effect used hyperelasticity.
  • Orthopedic implantators.
  • Wire for correcting the dentition.

Heat alarm

  • Fire alarm.
  • Fire dampers.
  • Bath alarm devices.
  • Mains fuse (protection of electrical circuits).
  • Device for automatic opening and closing of windows in greenhouses.
  • Heat recovery boiler tanks.
  • Ashtray with automatic ash shaking.
  • Electronic contactor.
  • System for preventing the exhaust of gases containing fuel vapors (in cars).
  • A device for removing heat from a radiator.
  • Device for turning on fog lights.
  • Incubator temperature regulator.
  • A container for washing with warm water.
  • Control valves for cooling and heating devices, heat engines.

Other applications

  • Focusu Boro, Japan uses titanium nickelide in recorder drives. The input signal of the recorder is converted into an electric current, which heats the titanium nickelide wire. By lengthening and shortening the wire, the pen of the recorder is set in motion. Since 1972, several million such units have been manufactured (data at the end of the 20th century). Since the drive mechanism is very simple, breakdowns are extremely rare.
  • Convection type electronic cooker. To switch ventilation during microwave heating and heating with circulating hot air, a titanium nickelide sensor is used.
  • Room air conditioner sensitive valve. Adjusts the direction of the wind in the air vent for the cooling and heating air conditioner.
  • Coffee maker. Determination of the boiling point, as well as for on / off valves and switches.
  • Electro-magnetic food processor. Heating is carried out by eddy currents arising at the bottom of the pan under the influence of magnetic force fields. In order not to get burned, a signal appears, which is activated by an element in the form of a coil of titanium nickelide.
  • Electronic storage dryer. It drives the flaps during the regeneration of the dehydrating agent.
  • In early 1985, the shape-memory alloys used to make bras were successful in conquering the market. The metal frame at the bottom of the cups is made of titanium nickelide wire. The property of superelasticity is used here. At the same time, there is no sense of the presence of wire, the impression of softness and flexibility. When deformed (during washing) it easily restores its shape. Sales - 1 million pieces per year. This is one of the first practical applications of materials with shape memory.
  • Manufacturing of various clamping tools.
  • Sealing of microcircuit cases.
  • The high efficiency of converting work into heat during martensitic transformations (in titanium nickelide) presupposes the use of such materials not only as high-damping materials, but also as a working medium for refrigerators and heat pumps.
  • Property hyperelasticity used to create high-efficiency springs and mechanical energy accumulators.

Literature

  • V. A. Likhachev et al. "Shape memory effect", L., 1987
  • AS Tikhonov et al. "Application of the shape memory effect in modern mechanical engineering", M., 1981
  • V. N. Khachin "Memory of form", M., 1984

For a long time, inelastic deformation was considered completely irreversible. In the early 1960s. a vast class of metallic materials was discovered in which the elementary act of inelastic deformation is carried out due to structural transformation. Such materials possess the reversibility of inelastic deformation. The phenomenon of spontaneous restoration of shape - shape memory effect (SME) - can be observed both under isothermal conditions and with temperature changes. During heat changes, such metallic materials can be reversibly deformed many times over.

The deformation recovery ability cannot be suppressed even under high force. The level of reactive stresses of some materials with SME can be 1,000 ... 1,300 MPa.

Metals with SME are among the most prominent representatives of materials with special properties. The increased interest in this metallurgical phenomenon is due to the unique combination of high conventional mechanical characteristics, fatigue resistance, corrosion resistance, and unusual properties such as thermomechanical memory, reactive stress based on thermoelastic martensitic transformation. A feature of alloys with SME is the pronounced dependence of most properties on the structure. The values \u200b\u200bof physical and mechanical characteristics change several times during the reversible austenite-martensite phase transition for different alloys, usually in the temperature range -150 ... + 150 ° C.

Of the large number of alloys with SME, the most promising for practical application are Ti-Ni alloys of equi-atomic composition (equal number of atoms), usually called titanium nickelide or nitinol. Less commonly used are cheaper copper-based alloys Cu-AI-Ni and Cu-A1-Zn.

The shape memory effect is that a specimen having a definite shape in the austenitic state at an elevated temperature is deformed at a lower martensitic transformation temperature. After overheating, accompanied by the reverse transformation, the original characteristic form is restored. The shape memory effect is manifested in alloys characterized by thermoelastic martensitic transformation, lattice coherence of the initial austenite and martensite phases, relatively small transformation hysteresis, and also small volume changes during transformations. In titanium nickelide, volumetric changes are about 0.34%, which is an order of magnitude less than in steels (about 4%).

SME alloys are often referred to as the so-called intelligent materials that make it possible to create fundamentally new designs and technologies in various branches of mechanical engineering, aerospace and rocket technology, instrumentation, energy, medicine, etc. Let us consider some objects of application of SME alloys.

The development of near and deep space is associated with the creation of orbital stations and large-scale space construction. The construction of such bulky objects as solar panels and space antennas is necessary. In fig. 1.1 shows a diagram of a spacecraft with self-deploying elements. The antennas consist of a Ti-Ni alloy sheet and rod, which are wound into a spiral and placed in a recess in an artificial satellite. After launching the satellite and putting it into orbit, the antenna is heated using a special heater or the heat of solar radiation, as a result of which it goes out into space.

To accommodate various technical objects, residential and industrial modules, it is necessary to build large platforms in open space. Delivery of bulky units into open space is technically possible only in parts with subsequent installation work. Methods of joining parts used in mass production, such as welding, soldering, gluing, riveting and others, non-

Fig. 1.1.

/ - antenna; 2 - solar battery; 3 - energy emitter; 4 - mechanical stabilizer

Fig. 1.2. Connecting tubular parts (/) with a shape memory metal coupling (2):about - before assembly;b - after heating

suitable in space conditions. Special requirements are imposed on ensuring extremely high safety.

Taking these features into account, our country has created a unique technology for connecting elements in open space using a coupling made of TN-1 alloy. This technology was successfully used in the assembly of a truss structure made of aluminum alloys with a total length of 14.5 m and a square cross-section with a side of 0.5 m.

The truss consisted of individual tubular parts / 28 mm in diameter, which were connected to each other using a coupling 2 made of metal with shape memory (fig. 1.2). The sleeve was deformed with a mandrel at a low temperature so that its inner diameter was larger than the outer diameter of the elements to be joined. After heating above the temperature of the reverse martensitic transformation, the inner diameter of the box was restored to the same diameter that the box had before expansion. In this case, significant compressive reactive forces were generated, the connected elements were plastically deformed, which ensured their strong connection. The assembly of the truss and its installation on the astrophysical module "Kvant" of the orbital complex "Mir" was carried out in 1991 in just four spacewalks and took about a day in total.

The same construction principles can be used for the installation of large-sized subsea structures at great depths.

Couplings for thermomechanical connection of pipes are used in many designs (Fig. 1.3). They are used to connect the pipelines of the hydraulic systems of the F-14 jet fighter, and there have been no accidents associated with oil leaks. The advantage of couplings made of shape memory alloys, in addition to their high reliability, is the absence of high-temperature heating (unlike welding). Therefore, the properties of materials near the joint are not degraded. Couplings of such

Fig. 1.3. Connecting pipes using shape memory effect:

and - introduction of pipes after expansion of the coupling; b - heating

types are used for pipelines of nuclear submarines and surface ships, for the repair of pipelines for pumping oil from the seabed, and for these purposes, large-diameter couplings are used - about 150 mm. In some cases, Cu-Zn-A1 alloy is also used for the manufacture of couplings.

For fixed connection of parts, rivets and bolts are usually used. However, if it is impossible to perform any action on the opposite side of the fastened parts (for example, in a sealed hollow structure), performing the fastening operations is difficult.

The shape memory alloy stops allow in these cases the fixing using spatial shape restoration. Stoppers are made of an alloy with a shape memory effect, and in the initial state the stopper has an open end (Fig. 1.4, and). Before carrying out the fastening operation, the stopper is immersed in dry ice or liquid air and sufficiently cooled, after which the ends of the stopper are straightened (Fig. 1.4, b). The stopper is inserted into a fixed hole for fastening (Fig. 1.4, in), when the temperature rises to room temperature, the shape is restored, the ends of the pin diverge (Fig. 1.4, d), and the fastening operation is completed.

The use of shape memory alloys in medicine is of particular interest. Their application opens up wide possibilities


Fig. 1.4. The principle of operation of the stopper with shape memory effect, the possibility of creating new effective methods of treatment. The alloys used in medicine must have more than just high mechanical characteristics. They must not corrode in a biological environment, must have biological compatibility with the tissues of the human body, ensure the absence of toxicity, carcinogenicity, and resist the formation of blood clots, maintaining these properties for a long time. If the implanted organ, made of metal, is active relative to the biological structure, then there is a degeneration (mutation) of biological cells of the peripheral structure, inflammatory blood flow, impaired circulation, then the death of the biological structure. If the implanted organ is inert, then a fibrous structure arises around it, due to collagen fibers formed from fibrous germ cells. The implanted organ is covered with a thin layer of this fibrous structure and can exist stably in biological organisms.

Special experiments carried out on animals have shown that alloys based on the Ti-Ni system have biological compatibility at the level or even higher than the commonly used corrosion-resistant steels and cobalt-chromium alloys and can be used as functional materials in biological organisms. The use of alloys with SME for treatment has shown their good compatibility with tissues and the absence of rejection reactions by the biological structures of the human body.

Spine correction.Various curvatures of the spine, both congenital and due to habit or a painful condition, lead to severe deformity when walking. This not only causes severe pain, but also has a harmful effect on the internal organs. In orthopedic surgery, the spine is usually corrected using a Harinton rod made of corrosion-resistant steel. The disadvantage of this method is the reduction in time of the initial corrective effort. In 20 minutes after installation, the corrective force decreases by 20%, and after 10-15 days - up to 30% of the original. Additional correction of effort requires repeated painful operations and does not always achieve the goal. If an SME alloy is used for the Harinton rod, then the rod can be installed 1 time, and there is no need for a second operation. If, after the operation, the Harinton rod is heated to a temperature slightly higher than the body temperature, then the necessary corrective force can be created. Effective for this purpose are alloys based on Ti-Ni with additions of Cu, Fe and Mo, which exhibit high elasticity in the temperature range after restoration of the shape

Corrective devices with such alloys create a constant magnitude stress on the spine during the entire treatment period, regardless of the displacement of the support points of the device.

Bone plate.Methods of medical care in the case of bone fractures consist in fixing the fracture zone in such a state when a compression force acts on the bone with the help of plates made of corrosion-resistant steel or Co-Cr alloys.

If a shape memory alloy is used for the connecting plate, it becomes possible to firmly fix the fracture zone by external heating of the plate to a temperature slightly higher than the body temperature after the operation, while there is no need to carry out longitudinal compression of the bone during the operation.

Intraosseous hairpins.Such pins are used in the provision of medical care for fractures of the tibia. Moreover, the pins, mainly made of stainless steel, are inserted up to the bone marrow, thereby fixing the bone. With this method, the bone is fixed due to the elastic properties of corrosion-resistant steel, so it is necessary to insert a pin with a larger diameter than the hole diameter to create a large degree of deformation. In this regard, there is a risk of damaging the tissue in the area where the hairpin is inserted.

The surgical procedure is simplified by using Ti-Ni based shape memory alloys for the studs. The pre-cooled stiletto heels restore their original shape at body temperature, which increases the degree of fixation.

Devices for skeletal traction.The property of the material is used to create significant stresses in a given temperature range when restoring the shape.

The devices are used for the effective treatment of bone fractures by both permanent and discrete skeletal traction.

Teeth correction wire.To correct the position of the teeth, for example, a malocclusion, a corrosion-resistant steel wire is used, which creates an elastic force.

The disadvantage of a correcting wire is a low elastic elongation and, as a result, plastic deformation. When making a wire from a Ti-Ni alloy, even with an elastic deformation of 10%, plastic deformation does not occur, and the optimal corrective force is maintained.

Technological progress is associated with the continuous growth of electricity consumption. The limited reserves of fossil fuels, the overcoming of the energy crisis and the acceptable cost of electricity production made it necessary to use atomic energy and large-scale construction of nuclear power plants (NPPs) in all developed countries of the world. Nuclear energy is the energy of the future.

By the principle of operation, nuclear power plants and thermal power plants (TPPs) differ little from each other. At nuclear power plants and thermal power plants, water is brought to a boil and the resulting steam is fed to the blades of a high-speed turbine, forcing it to rotate. The turbine shaft is connected to the generator shaft, which generates electrical energy when it rotates. The difference between nuclear power plants and thermal power plants is in the way water is heated to boiling. If coal or fuel oil is burned in a thermal power plant to heat water, then in a nuclear power plant for this purpose, the thermal energy of a controlled chain reaction of uranium fission is used.

Most countries now use light water reactors (LWRs) to generate electricity. Reactors of this type have two modifications: pressurized water reactors (PWR) and boiling water reactors (BWR), of which pressurized water reactors are more common.

In fig. 1.5 shows a diagram of a nuclear power plant equipped with a light water reactor (with water under pressure). The reactor vessel 9 contains the core 10 and the first circuit. In the first circuit, water circulates, which is a heat carrier and slows down


Fig. 1.5. Schemetransmission warmthbetween elements of the PWR station:

1 - concrete shell; 2 - shell made of corrosion-resistant steel; 3 - turbine; 4 - generator; 5 - cooling tower; 6 - capacitor; 7 - steam generator; 8 - circulation pump; 9 - reactor vessel; 10 - active zone; 11 - pressure compensator; 12 - container with a sprinkler. Water removes heat from the core to the heat exchange (steam generator 7), where the heat is transferred to the second loop, in which steam is generated. Energy conversion takes place in the generator 4, where steam is used to generate electricity. The primary circuit with all piping and components is enclosed in a specially designed container 12. Thus, any radioactive fission products that can escape from the fuel into the primary water are isolated from the environment.

In the first circuit, water is under a pressure of 15.5 MPa and at a maximum temperature of 315 ° C. These conditions prevent water from boiling, since the boiling point of water at a pressure of 15.5 MPa is much higher than 315 ° C.

In each reactor 16-25 cells (depending on the design) are left free for control rods. They are moved by a control rod through the reactor vessel lid. Steam leaving the turbine 3, condenses in a water-cooled condenser 6, in which the remaining heat energy is discharged. Some cooling systems use cooling towers.

The cost of plant equipment that generates and transmits energy (reactor vessel, heat exchangers, pumps, tanks, pipelines) is about 90% of the cost of the plant. The equipment must be properly designed and manufactured from economical but guaranteed reliable materials.

The nuclear power industry places high demands on the structural materials used, their production technology and performance monitoring. Structural transformations under the influence of irradiation of structural materials have a negative effect primarily on the mechanical properties and corrosion resistance. Of all types of radiation (neutrons, and- and p-particles, y-radiation) the strongest effect is exerted by neutron irradiation.

Radiation resistant materials are called materials that retain the stability of the structure and properties under neutron irradiation (Table 1.11).

The corrosion rate of aluminum-based alloys in an aqueous medium under irradiation conditions increases 2-3 times. Austenitic chromium-nickel steels in wet steam are susceptible to intergranular corrosion and stress corrosion cracking.

The most dangerous consequence of exposure is radiation swelling. In fig. 1.6 shows the characteristics of radiation swelling of a number of steel grades and alloys. Swelling can be suppressed by structural-forced recombin Table 1.11

Effects of neutron irradiation on various materials

Integral flux of fast neutrons, neutron / cm 2

Material

Exposure to radiation

Polytetrafluoroethylene, floor and methyl methacrylate and cellulose

Decreased elasticity

Organic

liquids

Gas emission

Increasing the yield point

Polystyrene

Decreased tensile strength

Ceramic

materials

Decrease in thermal conductivity, density, crystallinity

Plastics

Unsuitable for use as a construction material

Carbonaceous

Significant reduction in ductility, doubling of the yield stress, increased transition from ductile to brittle fracture

Corrosion resistant steels

Threefold increase in yield strength

Aluminum

Reduced ductility without complete embrittlement

metals due to continuous decomposition of the solid solution with a certain dilatation at the interface of the matrix with the resulting secondary phase. The strong fields of structural stresses arising during the decay favor the recombination of radiation defects and significantly reduce the swelling. Developed precipitation hardening is a way to suppress radiation swelling.

The radiation resistance of reactor materials can be achieved when a set of conditions are met. These include


Fig. 1.6.

V - volume; DR - volume change

optimal chemical composition and structure of materials, conditions of their operation: levels of operating temperature, neutron flux and properties of a corrosive medium.

The effect consists in the ability of an unloaded material under the influence of external stress and temperature changes to accumulate deformation (10–15%), which is reversible either upon heating or in the process of removing the external stress (superelasticity). Deformation can accumulate under active loading, as well as when the temperature of the alloy under the influence of uniaxial or shear stress changes. A typical operating cycle for such a material is shown in Figure 1. Deformation at stage b – c (Figure 1) is accumulated due to the reorientation of martensite crystals (the effect of martensitic inelasticity) and remains after the removal of loads. The shape memory effect manifests itself at the stage c – d (Figure 1), where the material independently restores its shape and can develop significant efforts.

Figure 1 - Scheme of rod deformation with the shape memory effect (a – d) and the dependence of the volume fraction of martensite q on temperature T (e).

Shape memory alloys, in addition to titanium nickelide, include AuCd, Cu – Al – Zn, AgCd, etc. The shape memory effect is based on martensitic transformations, for which a weak dependence of the temperatures of the beginning and end of the transformation on the rate of temperature change, most often reversible the nature of the transformation, a noticeable mismatch (hysteresis) of the temperatures of the forward and reverse reactions, and other signs. The high-temperature modification is usually called austenite, and the low-temperature modification is called martensite (Figure 1). The temperatures of martensitic transformations strongly depend on the chemical composition of the alloys, their thermal and mechanical treatment. For example, the characteristic temperatures of titanium nickelide are in the range of 30–80 ° С, rarely going beyond this range, but alloying with iron reduces them by about 150–200 ° С, that is, to –170… –70 ° С.

The kinetics of martensitic transformations has a pronounced hysteresis (Figure 1e). If the material is cooled from the austenitic state, then initially no phase transformations occur. However, starting from a certain characteristic temperature, which is usually denoted by M s, the first crystals of martensite appear, therefore, the proportion of the martensite phase in the bulk of the material also increases. With further cooling, their size and number increase until the crystals fill the entire volume at a temperature M f. Such a transformation is called direct and, in the presence of an external load, is accompanied by the appearance of large deformation (the effect of transformation plasticity). During subsequent heating, starting from the temperature A s, martensite begins to transform into austenite. In this case, the accumulated deformation begins to slowly disappear until the temperature rises above A f and the shape is restored.

Such alloys are used as biomedical implants: stents, orthodontic wires, filters, fixators, braces for osteosynthesis, plates, etc. ...

When using alloys with SME in medicine, it is necessary that they ensure not only the reliability of the performance of mechanical functions, but also chemical reliability (resistance to deterioration of properties in a biological environment, resistance to decomposition, dissolution, corrosion), biological reliability (biological compatibility, absence of toxicity, carcinogenicity, resistance to the formation of blood clots and antigens). Simple metal elements have a strong toxic effect, but in combination with other elements, the effect of mutual weakening of toxicity is found. However, more important than the formation of ions is the solubility of passivating films formed on the surface of metals. For example, chromium-nickel alloys, cobalt-chromium alloys, pure Ti, Ti – 6Al – 4V [% (at.)] Alloy used as biological materials contain elements that have a strong toxic effect in the form of simple elements, but passivating films formed in contact with biological organisms are fairly stable.

Each metal and alloy has its own crystal lattice, architecture and sizes of co-
second are strictly specified. For many metals with a change in temperature and pressure, the grid does not
remains the same and the moment comes when its restructuring takes place. Such a change
type of a crystal lattice - polymorphic transformation - can be carried out by two
ways:
1) at high temperatures due to diffusion at high mobility of atoms;
2) at low temperatures due to the collective, coordinated movement of atoms, which
leads to a change in the shape of the bulk of the alloy (diffusion-free shear thermoelastic mar-
tensitic transformation with the formation of a new crystal lattice - martensite).
At high temperatures in the austenitic state, the alloy has a cubic lattice.
Upon cooling, the alloy passes into the martensite phase, in which the lattice cells become
with beveled parallelepipeds. When heated, the austenite phase is restored, and with it
the original shape of a product made of an alloy with shape memory is also restored.
Martensitic transformation is one of the fundamental methods of crystal restructuring.
lattice in the absence of diffusion, typical for steels, pure metals, non-ferrous
alloys, semiconductors, polymers.
Memory effect - restoration of the original shape and size of crystals after
their changes during deformation as a result of thermoelastic martensitic transformation
during heat treatment according to a certain mode.
Shape change is the main feature of the martensitic transformation, with which the effect is associated
the effect of the "memory" of alloys, a condition necessary, but not sufficient for the manifestation of "memory".
The free energy of martensite crystals is less than that of the initial phase, which stimulates
development of the martensitic transition. The transition is inhibited by the appearance of the interface
the old and new phases and the increase in free energy. Growing crystals of the martensite phase
deform the surrounding volume that resists it. Elastic energy arises,
preventing further crystal growth. When this energy exceeds the elastic limit
guests, an intense deformation of the material occurs in the vicinity of the interface and
the growth of crystals stops. In steels, the process takes place almost instantly (individual
crystals of martensite grow to a finite size).
Reverse transition of martensite to austenite (high-temperature phase, diffusionless
the shear rearrangement of the lattice is difficult), occurs at high temperatures, when in open-hearth
sieve, austenite crystals grow without a transition to their original form (atoms do not fall on their
former places).
In alloys with "memory", on cooling, martensite crystals grow slowly, at
heating disappear gradually, which ensures dynamic equilibrium of the interface
between them and the original phase. The boundary between the phases behaves similarly if the cooling
replacement and heating, respectively, by applying and removing the load - thermoelastic
phase equilibrium in a solid.
Thermoelastic martensitic transformation is accompanied by a reversible change in shape
crystals of austenite, which mainly provides the "memory" of metals.
56 Intelligent polymer materials (IPM)
A direct consequence of thermoelastic martensitic transformation is reversible
a change in the shape of a solid as a result of periodic cooling and heating (thermal
engine). Metals with "memory" (for example, nitinol), "remember" their original
shape when heated after preliminary deformation of the sample.
By the end of the 1960s. the field of physical research and technical
applications of the shape memory effect in alloys.
There are hundreds of alloys with martensitic transformation, but the number of alloys where the effect
"Memory" of the form is of practical importance, insignificantly. Collective movement
atoms in a certain direction, accompanied by spontaneous (martensite
no) material deformation (lattice rearrangement), in which the neighborhood and interatomic
the bonds of atoms are not broken (it remains possible to return to previous positions,
to the original form), passes only under certain conditions. "Memory" of the individual
crystal is not yet a memory of the entire volume of the alloy, which usually has a polycrystalline
facial structure.
Individual crystallites (grains) differ in the orientation of the crystal lattices.
The shift of atoms during the martensitic transformation occurs in the lattice along certain planes.
bones and directions. Due to the different grain orientation, the shifts in each grain pass through
in different directions and, despite significant deformation of individual crystals,
the sample as a whole does not experience a noticeable change in shape. It happens when
if the crystals are oriented in one direction. The governing force, which when mar-
Tensitic transformation organizes the predominant organization of crystals, is
external load.
During the martensitic transformation, atoms move in the direction of the action of the external
loads (the sample as a whole is undergoing deformation). The process develops until
all material is not deformed in the direction of the force without breaking the interatomic
bonds and violation of the proximity of atoms. When heated, they return to their original positions,
restoring the original shape of the entire volume of the material.
The "memory" effect is based on thermoelastic phase equilibrium and control action
load. Special thermomechanical treatment of alloys creates micro-
stresses whose actions during martensitic transitions are similar to the action of an external
load. When cooled, the alloy spontaneously takes one shape, when heated
returns to the original one (the plate folds into a ring when cooled, when heated -
unfolds or vice versa).
Shape memory materials can exhibit superplasticity (significant de-
formation, when the martensitic transformation is caused by the application of an external load, and
not cooling, which is used when creating spring shock absorbers, batteries
mechanical energy), have high cyclic strength (there is no accumulation
structural defects) and a high ability to dissipate mechanical energy (with open-hearth
sieve transformations, the rearrangement of the crystal lattice is accompanied by the precipitation
or heat absorption, if an external load causes a martensitic transformation, then
mechanical energy is converted into heat; with memory effects, there is also a process
converting heat into work).
The change in shape (with a periodic change in temperature) of metals with memory co-
is accompanied by the manifestation of powerful interatomic forces. Expansion pressure of materials
this type reaches 7 t / cm2. Depending on the type of material of the product of various sizes
and configurations bend, expand, twist (the shape can be programmed).
Shape memory metals include alloys nitinol, nitinol-55 (with iron), nickelide
titanium VTN-27, titanium alloys VT-16, VT23 (heat treatment in a special mode, in 2–3
times cheaper and 1.5 times lighter than titanium nickelide), titanium-based alloy with 28-34% manganese and
5-7% silicon, terfenol (magnetostrictive alloy, dampens vibrations at low-frequency
vibrations).
Intellectual polymer materials (IPM) 57
Manganese-based alloys have a temperature range of maximum thermal sensing
temperature at 20-40 ° C and restore the given shape in the temperature range from
–100 to 180 ° С
Alloys of the Cu-Zn- system were obtained by powder metallurgy (Fukuda Metal Co.)
Al with shape memory sintering (700 MPa, 900 ° C, 0.1 wt% aluminum fluoride
powders of alloys Cu-Zn (70:30), Cu-Al (50:50) and copper (grain size 20–100 microns). Alloy
restores its shape after stretching by 10%.
Upon cooling, the alloy passes into the martensitic phase, in which, due to the changed
the geometric parameters of the cells of the crystal lattice becomes plastic and at
mechanical effect of a product made of an alloy with "memory" (nitinol, etc.)
virtually any configuration that will persist as long as the temperature is not
exceeds the critical value, at which the martensite phase becomes energetically unfavorable,
the alloy passes into the austenitic phase with the restoration of the original shape of the product. However,
deformations should not exceed 7–8%, otherwise the shape will not be completely restored.
Nitinol alloys have been developed that simultaneously "remember" the shape of the products,
corresponding to high and low temperatures. Memory effect in nitinol alloys
clearly expressed, and the range of temperatures can be accurately adjusted in the range from non-
how many degrees to tens of degrees, introducing modifying elements into the alloys, however
cyclic reserve, the number of controlled deformations (iterations) does not exceed 2000,
after which the alloys lose their properties.
Conductive fibers formed from filaments with a diameter of 50 μm alloys
with titanium and nickel nanoparticles, change the length by 12-13% within 5 million iterations and
used in artificial muscles. Nanomuscle (Nano Muscle Actuator, firm Nano
Muscle, USA, Johnson Electric, KHP, 2003) develops a thousand times more power than
human muscles and 4000 times larger than an electric motor at an actuation speed
0.1 seconds with a smooth transition from one state to another at a given speed (mic-
proprocessor control).
Materials with magnetomechanical memory (magnetoelastic martensitic
the transition is stimulated by a magnetic field directly or in combination with temperature
and load) and electromechanical memory (martensitic transformation is accompanied by
a qualitative change in properties, transitions conductor-semiconductor, paramagnet-fer-
romagnetics), which is promising for the creation of actuators IM for radio engineering
to reduce radar signature.

Some alloys have a surprising property: remember their shape. Work on the study and application of such alloys is carried out in many countries. The spring was compressed, and then released, it immediately returned to its original state. The same will happen with a curved steel ruler, stretched by a piece of rubber ... In all these cases, the material returns to its original size and shape. It seems natural and does not surprise anyone. But this happens only within the limits of elastic deformation. If the elastic limit of the material is exceeded, plastic deformation occurs. Now, after removing the load, it will not take its original shape, for this it is necessary to deform the material in the opposite direction. These were the generally accepted, customary ideas.

Relatively recently, researchers have discovered alloys that, even after plastic deformation, were able to "remember" their original shape. Imagine that a piece of wire made of such an alloy is bent so that it takes the shape of the word "MEMORY". The wire can then be crumpled. But as soon as it is slightly heated, it will again independently "write" the word "MEMORY". Naturally, such experiences are surprising and are perceived rather as a focus.

The study of the phenomenal properties of metals showed that its mechanism is determined by very subtle processes occurring with the crystal lattice, in particular, the phenomenon that is called "thermoelastic phase equilibrium in a solid." At first it was predicted theoretically by G.V. Kurdyumov, a full member of the Academy of Sciences of the Ukrainian SSR, and then he and his collaborator L. G. Khandros established experimentally.

Even the popular presentation of the essence of the problems associated with the shape memory effect in alloys presupposes the presence of some obligatory volume of information from the field of metal science.

Martensitic transformation

Each metal and alloy has its own crystal lattice, the architecture and dimensions of which are strictly specified. But for many metals with a change in temperature and pressure, the lattice does not remain the same: there comes a moment when its rearrangement occurs. Such a change in the type of crystal lattice - polymorphic transformation - can be carried out in two ways.

For clarity, let's imagine a lattice in the form of a building made of children's cubes. How now from the same cubes (atoms) to construct a building of a different architecture (to "produce" a polymorphic transformation)? The answer is obvious: dismantle the old building and fold the new one. Of course, now each cube can be anywhere in the new building, surrounded by other neighbors. This is understandable, because during the restructuring, the path of any cube is individual - in no way connected with others. It is according to this scheme that the rearrangement of the lattice occurs, if the mobility of atoms - diffusion - is high enough to ensure their movement to new places. This is possible when the polymorphic transformation occurs at a high temperature.

And how will the lattice rearrangement occur in those cases when the temperature of the polymorphic transformation is low? From an energy standpoint, the lattice of the high-temperature modification must necessarily be rearranged, but the diffusion of atoms is practically absent, since the energy of their thermal vibrations is insufficient for separation from neighbors. So, there must be another, diffusion-free method?

Indeed, this method was discovered in the study of one of the most ancient processes of heat treatment of steel - hardening. As a result, it forms a phase with a new crystal lattice - martensite; Accordingly, the method of lattice rearrangement was called martensitic transformation.

Later it turned out that the martensitic transformation is generally one of the fundamental methods of rearrangement of the crystal lattice. It is characteristic not only of steels, but also of pure metals, non-ferrous alloys, semiconductors, and polymers whenever the lattice rearrangement is forced to occur in the absence of diffusion.

What are the features of the rearrangement of the lattice with such a diffusionless method of transformation? Let's go back to our dice model. Now the old building cannot be disassembled into cubes - there is no diffusion. One possibility remains: without tearing the cubes apart (without destroying interatomic bonds), move them in whole cooperatives, practically simultaneously from old positions to new ones. It is clear that such a collective, coordinated movement has the character of a shear (therefore, the martensitic transformation is sometimes called shear).

The cooperative shift of atoms inevitably leads to a change in the shape of the bulk of the alloy. Shape change is the main feature of martensitic transformation.

The memory effect of alloys is associated with it. But one should not think that any alloy that undergoes martensitic transformation has memory. As will become clear from what follows, a change in form during such a transformation is a necessary condition, but still insufficient for the manifestation of memory.

In the long history of the study of martensitic transformations, three key events can be distinguished that had a direct impact on the formation of a new, stupid direction dealing with the study and application of the shape memory effect in alloys.

First event... In 1949, the journal "Doklady Akademii Nauk SSSR" published an article by GV Kurdyumov and LG Khandros "On Thermoelastic Equilibrium in Martensitic Transformations". Its authors discovered a previously unknown feature of martensitic transformation in one of the copper alloys.

Here we will have to turn to the considered classical picture of the martensitic transformation. The free energy of the nascent martensite crystals is less than that of the initial phase. This is what stimulates the development of the martensitic transition. However, there are also obstacles. First of all, this is an increase in free energy due to the appearance of the interface between the old and new phases. In addition, growing crystals of the martensitic phase are forced to deform the surrounding matrix, which, of course, resists this. As a result, elastic energy arises, which prevents further crystal growth. The accumulation of elastic energy is like a spring that contracts as the crystal grows. When this energy exceeds the elastic limit, a kind of destruction of the spring occurs, which causes an intense deformation of the material in the vicinity of the phase interface. The growth of the crystal stops. This process can occur extremely quickly, like an explosion, and then individual crystals of martensite grow almost instantly to their final size. In steels, martensitic transformation occurs in this way.

The reverse transition of martensite to austenite (this is the name of the high-temperature phase of the steel from which it was formed) can no longer occur by the reverse “explosive” mechanism. The spring was broken, the boundaries between the phases were violated, and now the reverse diffusionless shear rearrangement of the lattice is difficult. A significant overheating of the alloy is required for austenite crystals to begin to nucleate and grow in the depths of martensite. Moreover, their original form, as a rule, is not restored (atoms do not fall into their previous places).

The peculiarity of the martensitic transformation, which was observed in the copper alloy, was that the martensite crystals grew slowly upon cooling, and gradually disappeared upon heating. If we continue the analogy with the spring, then we can say that in this case it has time to stop the growth of the crystal before it collapses. The martensite crystal turns out to be, as it were, spring-loaded, which ensures the dynamic equilibrium of the boundary between it and the initial phase: upon cooling, the boundary shifts in one direction, and upon heating, in the opposite direction.

The new phenomenon is called thermoelastic phase equilibrium in a solid.

The thermoelastic martensitic transformation is also accompanied by a change in shape, but in this case this change is (which is very important) reversible: the original shape of the austenite crystals is restored. And, as it became clear later, it is this transformation that basically provides the memory of metals.

Second event... In 1958, at the World Exhibition in Brussels, the attention of visitors was attracted by the device of American scientists T. Reed and D. Lieberman. Its main part was a thin (3 mm in diameter) long (100 mm) rod made of a gold-cadmium alloy (66% gold). At one end, it was rigidly fixed in the rack and was in a horizontal position. A weight (about 50 g) was suspended on the free end of the rod, under which the rod was bent. The rod's behavior was unusual. When heat was supplied from the heater to the rod, it straightened and lifted the load, but as soon as the fan cooled the rod, it was bent again, etc. This was a working model of a heat engine, which had a solid working body made of gold-cadmium alloy as a result of cooling and heating reversibly changed its shape, which was a direct consequence of thermoelastic martensitic transformation.

This clearly demonstrated the previously unknown shape memory property of metals.

Third event... In the early 60s, in an American laboratory, as a result of a search for a material that would be strong, relatively light, and at the same time could work in aggressive environments, a nickel-titanium alloy (1: 1) was created.

In the course of processing, this alloy unexpectedly showed a property the existence of which the researchers did not even suspect: a pre-deformed specimen recalled its original shape when heated.

The discovery in the "ordinary" alloy of a unique property (which was then given the name "memory effect") was perceived as a sensation.

The effect was so strong that it literally took your breath away from the prospects of its use. On the other hand, the randomness of the discovery made did not allow immediately giving a correct explanation of the nature of the effect, and this, naturally, held back its wide practical application.

The new material nitinol (formed from the words Nickel, Titan and NOL - the abbreviated name of the laboratory where it was obtained) and its remarkable property of memory have become the object of intensive study. But only after a few years it became clear that in this case, too, the memory of the alloy is a consequence of the martensitic transformation.

Under the influence of all three events, by the end of the sixties, a whole area of \u200b\u200bphysical research and technical applications of the shape memory effect in alloys was formed.

When every crystal is on its own

There are hundreds of martensitic transformation alloys. But not all of them are able to remember the form. And in general, only a few alloys are known where this effect is manifested so strongly that it can be of practical importance. What's the matter?

As already mentioned, during the martensitic transformation, there is a collective movement of atoms in a certain direction, accompanied by spontaneous (martensitic) deformation of the material. Since with this method of rearrangement of the lattice, the neighborhood and interatomic bonds of the overwhelming majority of atoms are not violated, they retain the ability to return to their previous places, and the material, respectively, to its original form.

But this is only an opportunity, and special conditions are needed for its implementation.

In the general case, the reverse rearrangement of the structure does not necessarily have to occur through the "backward" motion of atoms. There can be several directions that lead to the original lattice architecture. Everything is determined by the complexity of the crystal lattice of martensite: the more complex it is, the fewer these directions. When the martensite lattice is so complex that it does not provide a choice at all, then there is only one option for its rearrangement - the "backward" movement of atoms to their initial positions. Only in this case, the martensitic transformation provides the crystal with a memory of the original shape. It is precisely this transformation and memory that was observed in individual crystals in the above event No. 1.

But the memory of an individual crystal is not yet the memory of the entire alloy volume. And that's why.

The alloy, as a rule, has a polycrystalline structure, that is, it consists of many individual crystallites (grains), which differ from each other in the orientation of the crystal lattices - like children's cubes randomly poured into a box. Since the shift of atoms during the martensitic transformation occurs in the lattice along certain planes and in a certain direction, then due to the different orientation of the grains, the shifts in each grain will be carried out in very different directions. Therefore, after the martensitic transformation, despite significant deformation of individual crystals, the sample as a whole does not undergo a noticeable change in shape.

It is clear that a noticeable change in the shape of the entire sample will occur only if a certain order in the arrangement of the crystals is created. Ideally, make sure that they are all oriented in the same direction.

This is exactly what the researchers who demonstrated the manifestation of alloy memory in events 2 and 3 succeeded.

The second event (like the third) differs from the first in that the transformation in the alloy occurs with the participation of an external load.

It is the control force that organizes the preferential orientation of the crystals during the martensitic transformation.

How does this happen? At the moment of transition during cooling, when the atoms must leave their old places and take new ones, from all possible directions they will choose only those that coincide with the direction of the external force. This is natural, since otherwise the atoms would have to do additional work against the external load, which is clearly disadvantageous from an energy point of view. So, the process of martensitic transformation makes the atoms leave their positions and hit the road, and the external load sets the direction of movement.

As a result of such an organized movement of atoms, the sample as a whole undergoes deformation in the direction of the external force. Let us recall how in event No. 2 the rod was bent in the direction of the load during cooling. When heated, when the atoms are forced to return to their original positions, the original shape is restored, even against the action of an external force (load), since atoms simply do not have other directions of motion, except for the opposite.

It is interesting that an external load can control the movement of atoms not only during the martensitic transformation itself, but also after its completion, as it was in event No. 3. In this case, it can change the already existing situation with chaotically oriented crystals of martensite.

Under the action of the load, the number of crystals with martensitic deformation coinciding in direction with the applied force increases. The process continues until all the crystals line up and the sample as a whole is deformed in the direction of the force. Let us emphasize once again that this occurs without breaking interatomic bonds and disrupting the proximity of atoms. Therefore, when heated, they return to their original positions, restoring the original shape of the entire volume of the material.

In this case, the external load acts on the martensite crystals, like a magnet on iron filings, which line up in a magnetic field in a strictly defined order.

These are the mechanisms due to which the shape memory effect is realized, based on thermoelastic phase equilibrium and the control action of the load.

The effect described in event No. 3 is essentially a memory of the material for one, high-temperature shape. In event No. 2, the presence of an external force (load) made it possible to achieve memory for two geometric shapes: the alloy assumed a low-temperature shape when cooled, and a high-temperature shape when heated.

It turns out that it is possible to "teach" the alloy to memorize two forms without any permanent source of external force. The idea of \u200b\u200bsuch a method was proposed by Soviet scientists and recognized as an invention (copyright certificate No. 501113). Its essence consists in a special thermomechanical treatment of the alloy, which creates microstresses inside the material, the effect of which on atoms during martensitic transitions is similar to the effect of an external load. As a result, the alloy, when cooled, spontaneously takes one shape, when heated, it returns to its original shape, etc. For example, you can "teach" the plate to fold into a ring when cooled, and when heated, to unfold, or vice versa.

Another unusual property is often observed in materials with shape memory - superelasticity (rubber-like behavior). This effect manifests itself if the martensitic transformation is caused not by cooling, but by the application of an external load. Then the transformation and "putting things in order" in the arrangement of the crystals occur simultaneously. As a result, significant deformation of the alloy is observed, which disappears during unloading. Moreover, the value of reversible deformation is ten times higher than that of the best spring materials. The use of such alloys opens up new possibilities for creating highly efficient spring shock absorbers, mechanical energy accumulators, etc.

Another feature of memory alloys: high cyclic strength, that is, the ability to withstand large alternating loads without destruction. The use of such materials is especially effective in case of significant deformations. In this case, the "durability" of products from memory alloys can be thousands of times greater than products from traditional materials. Let us recall, for example, how quickly any wire collapses when it is subjected to bending-bending in one place. Memory alloys can in principle withstand any number of such cycles.

Cyclic resistance is provided by the same special mechanism of martensitic transformation, which is not accompanied by disruption of the proximity of atoms and the destruction of interatomic bonds, and therefore, there is no accumulation of structural defects, which ultimately lead to the formation of cracks and destruction of conventional alloys.

Finally, about one more property of memory alloys. It turned out that they have a high ability to dissipate mechanical energy. This is due to the fact that, during martensitic transformations, the rearrangement of the crystal lattice is accompanied by the release or absorption of heat. Therefore, if an external load causes martensitic transformation, then an intense transition of mechanical energy into heat occurs. By the way, with memory effects, the opposite process is observed: the conversion of heat into work.

Memory alloy professions

Among all the known shape memory materials, nitinol is the most promising for technology. It is it that is most often used in devices and devices for various purposes. This is facilitated not only by its excellent memory, but also by a whole range of other useful properties: high corrosion resistance, significant strength, manufacturability.

Today, the areas where the use of memory alloys is most promising have already been clearly identified. First of all, it is energy. With their help, attempts are made to create heat engines using low-temperature heat sources. In 1977, a film about such devices was shown at an international conference on martensitic transformations in Kiev. The scheme of the heat engine is extremely simple (recall that its prototype was the device described in event No. 2). Working elements made of nitinol and fitted around the circumference of the wheel, getting into cold water, are forcibly deformed, for example, flat plates bend in a semicircle. Then, in hot water, the plates are straightened and at the same time do the work. Part of it goes to deformation of the working elements that are at this time in cold water, and the other part goes to drive the wheel, which, in turn, rotates the electric generator.

So far, only models of such engines exist. But even they show a high efficiency of converting heat into work using memory alloys. At the same time, it should be emphasized once again that heat engines are used to operate heat, which is still difficult and expensive to turn into work by other means, and often even impossible. Such heat, as a rule, “disappears” today (solar energy, geothermal sources and thermal waste from power plants, etc.).

Naturally, materials with shape memory are also effective for the reverse process: "pumping" heat, that is, as a working fluid for refrigerators or heat pumps.

Another application of memory alloys is the sealing and joining of various parts. In particular, nitinol bushings are used to connect pipelines. A sleeve is made of the alloy, the inner diameter of which is slightly less than the outer diameter of the pipeline, it is cooled and distributed in diameter so that it can be freely put on the ends of the pipeline. Then the sleeve is heated, and it restores (remembers) its original size, tightly crimps the pipeline and thereby makes a tight connection. The high reliability of such a connection is evidenced, for example, by the following fact. More than 100,000 nitinol bushings have been installed on F-14 fighters (USA) - and not a single case of destruction of connections or breakdowns during operation.

With the help of nitinol, the housings of radio engineering devices are also sealed without the use of welding or soldering. Here, the flat cover is pre-deformed into a hemisphere and is freely installed in the device body. When heated, the cover returns to its original flat shape, while cutting into the grooves of the body, reliably isolating the device from the external environment.

Memory alloys are also used as working elements of various temperature-sensitive, signaling and executive devices and mechanisms.

Self-deploying devices, such as antennas made of nitinol, are of great interest for space technology. A large-sized product is rolled (deformed) and transported in such a compact form to its destination, where, after heating, it regains its shape.

Nitinol is also used in medicine. Abroad, for example, methods of treating scoliosis (spinal deformity) using a nitinol rod are being developed.

Original work is being carried out by the Siberian Institute of Physics and Technology in cooperation with the Chita and Tomsk Medical Institutes, the Kurgan Research Institute of Experimental and Clinical Orthopedics and Traumatology. A number of new surgical devices have been developed for joining and fusing bone fragments, prosthetics and filling of teeth. The possibilities of using nitinol to create new medical instruments are also being investigated.

These examples, of course, do not exhaust all areas of use of memory alloys. The track record of their professions is undoubtedly wider - and it is constantly growing.

It is not only possible to induce a martensitic transition in an alloy and, accordingly, to control a reversible change in shape, not only by means of heating and cooling or loading. This role can be played by an electric or magnetic field. Therefore, in principle, it is possible to create, for example, alloys with magnetoelastic martensitic transformation. In such materials, a magnetic field, either alone or in combination with temperature (or load), should stimulate the martensitic transition and thereby lead to a reversible change in shape, that is, to shape memory.

In general, alloys where a martensitic transition can be induced by a magnetic field are known. However, the martensite in them, as a rule, is not elastic and, therefore, without memory. And in alloys where thermoelastic transitions are observed, they are practically insensitive to changes in the magnetic field strength. But there is no doubt that materials with magnetomechanical memory must exist.

Let us dwell on one more interesting direction, which is associated with the study of alloys with memory.

A change in the type of a crystal lattice during a martensitic transformation, in addition to a reversible change in shape, must, of course, also cause changes in all other properties that are determined by the structure of the lattice. It is obvious that, along with unusual mechanical behavior, memory alloys should also differ in a special set of reversibly changing physical properties. To control them, it is enough to slightly change the temperature or apply a small external load. The situation is unique. In theory, this is exactly the case. And the practical task is to find alloys where the required properties will change significantly. There are already first successes in this direction. Thus, it was experimentally observed that when nitinol is loaded above a certain value, its electrical resistance abruptly increases by tens of percent.

V. Khachin, Candidate of Physical and Mathematical Sciences.

 

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