One of the basic perceptions by people of the phenomena of the outside 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 beings, have a special property that allows them to exhibit 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 its initial state, the material has a certain structure. In the figure it is indicated by regular squares.

When deformed (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. What is unusual is that in shape memory materials the martensite is thermoelastic.

When heated, the thermoelasticity of martensite plates begins to appear, that is, internal stresses arise in them, which tend to return the structure to its original state, that is, 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 undergoes self-deformation in the opposite direction and restores its original structure, and with it its shape.

Characteristics of the shape memory effect

Shape memory effect characterized by two quantities.

A brand of alloy with a strictly maintained chemical composition. (See further “Shape Memory Materials”)
Temperatures martensitic transformations.

In the process of manifestation shape memory effect participate martensitic transformations two types - direct and reverse. Accordingly, each of them manifests itself in its own temperature range: MN and MK - the beginning and the end direct martensitic transformation during deformation, AN and AK - the beginning and end during heating.

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

This implies the need for strict maintenance of the chemical composition of the alloy for an unambiguous functional manifestation shape memory effect. Which brings metallurgical production into the sphere of high technology.

Shape memory effect several million cycles appear.

Preliminary heat treatments can be strengthened shape memory effect.

Reversible possible shape memory effects, when a material “remembers” one shape at one temperature, and another at another temperature.

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), at which temperatures reverse martensitic transformation 200 - 400˚C.

Superelasticity

Another phenomenon closely related to shape memory effect is superelasticity.

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

Superelastic behavior is an order of magnitude higher than elastic behavior.

Superelasticity 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

The leader among materials with shape memory in terms of application and knowledge it 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 is a stable body-centered cubic lattice of the CsCl type and undergoes thermoelastic behavior upon deformation. martensitic transformation with the formation of a phase of low symmetry.

Another name for this alloy, adopted abroad, is nitinol comes from the abbreviation NiTiNOL, where NOL is the abbreviation for the US Naval Ordnance Laboratory, where the material was developed in 1962.

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

Titanium nickelide has:

Excellent corrosion resistance.
High strength.
Good shape memory characteristics. High shape recovery coefficient and high restoring force. Deformation up to 8% can be completely restored. The recovery stress can reach 800 MPa.
Good compatibility with living organisms.
High damping capacity of the material.

Flaws:

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 difficult, especially cutting. (High strength reverse side).
High price. At the end of the 20th century, it was worth slightly 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 further “Use of shape memory materials”).

Other alloys

At the end of the 20th century shape memory effect was found in more than 20 alloys. Except titanium nickelide Effect shape memory detected 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 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.
- Flaws:
  - Worse in terms of shape memory characteristics.
  - Worse mechanical and corrosion 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 Osaka University, Japan. Temperatures martensitic transformation in the range from 100 to 200˚C.
Fe-Mn-Si. 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 that undergo martensitic transformations, including such pure metals as titanium, zirconium and cobalt.

Production of titanium nickelide

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

Removing surface defects - roughing with an emery wheel.

To more completely equalize the chemical composition throughout the volume of the ingot, homogenization is carried out at a temperature of 950 - 1000˚C in an inert atmosphere.

Application of materials with shape memory effect

Titanium nickelide connecting sleeves

A bushing first developed and introduced by Raychem Corporation, USA, for connecting hydraulic system pipes of military aircraft. There are more than 300 thousand such connections in the fighter, but there have never been any reports of their failures.

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 internal protrusions are flared with a plunger.

A cold bushing becomes smooth from the inside.

Using special pliers, the sleeve is removed from the cryostat and placed on the ends of the pipes being connected.

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

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

In essence, 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 joining method is good for the final connection when assembling a structure, when welding becomes difficult to access due to the interweaving of components and pipelines.

These bushings are used in aviation, space and automotive applications.

This method is also used to join and repair submarine cable pipes.

In medicine

Gloves used in the rehabilitation process and designed to reactivate active muscle groups with functional insufficiency. Can be used in the 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 cutter, after which they take the form of filters having a given location.
Clamps for pinching weak veins.
Artificial muscles that are powered by electric current.
Fastening pins designed for fixing prostheses to bones.
Artificial extension device for so-called growing prostheses in children.
Replacement of femoral head cartilage. The replacement material becomes self-clamping under the influence of the spherical shape (femoral head).
Rods for spinal correction in scoliosis.
Temporary clamping fixing elements for artificial lens implantation.
Glasses frame. In the lower part, where the glass is secured with wire. Plastic lenses do not slip out when cooled. The frame does not stretch when wiping the lenses and prolonged use. Effect used superelasticity.
Orthopedic implants.
Wire for correcting dentition.

Heat alarm

Fire alarm.
Fire dampers.
Alarm devices for bathtubs.
Mains fuse (protection of electrical circuits).
Device for automatic opening and closing of windows in greenhouses.
Thermal recovery boiler tanks.
Ashtray with automatic ash removal.
Electronic contactor.
System for preventing the exhaust of gases containing fuel vapor (in cars).
A device for removing heat from a radiator.
Device for turning on fog lights.
Temperature regulator in the incubator.
Container for washing with warm water.
Control valves for cooling and heating devices, heat machines.

Other Applications

Focus Boro, Japan, uses titanium nickelide in drive devices for recorders. The input signal from the recorder is converted into an electric current, which heats a titanium nickelide wire. By lengthening and contracting the wire, the pen of the recorder is set in motion. Since 1972, several million such units have been manufactured (data for the end of the 20th century). Since the drive mechanism is very simple, breakdowns are extremely rare.
Electronic kitchen stove of convection type. A titanium nickelide sensor is used to switch ventilation between microwave heating and circulating hot air heating.
Sensitive valve for room air conditioner. Adjusts the wind direction in the air conditioner vent for cooling and heating purposes.
Coffee maker. Determination of boiling point, as well as for turning valves and switches on and off.
Electro-magnetic food processor. Heating is produced by eddy currents that arise at the bottom of the pan under the influence of magnetic force fields. To avoid getting burned, a signal appears that is driven by an element in the form of a titanium nickelide coil.
Electronic storage dryer. Drives the flaps during regeneration of the dehydrating agent.
In early 1985, shape-memory alloys, used to make bra frames, began to successfully conquer the market. The metal frame at the bottom of the cups consists of titanium nickelide wire. The property of superelasticity is used here. At the same time, there is no feeling of the presence of wire, the impression is of softness and flexibility. When deformed (when washed), it easily restores its shape. Sales - 1 million units per year. This is one of the first practical applications of materials with shape memory.
Manufacturing of various clamping tools.
Sealing of microcircuit housings.
The high efficiency of converting work into heat during martensitic transformations (in titanium nickelide) suggests the use of such materials not only as highly damping ones, but also as the working fluid of refrigerators and heat pumps.
Property superelasticity used to create highly efficient springs and mechanical energy accumulators.

Literature

V. A. Likhachev et al. “Shape memory effect”, Leningrad, 1987
A. S. Tikhonov et al. “Application of the shape memory effect in modern mechanical engineering,” M., 1981.
V. N. Khachin “Shape Memory”, M., 1984

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

The ability to recover deformation 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 properties, fatigue resistance, corrosion resistance and unusual properties such as thermomechanical memory, reactive stress, based on the thermoelastic martensitic transformation. A feature of alloys with SME is the pronounced dependence of most properties on the structure. The values of 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 equiatomic composition (equal number of atoms), usually called titanium nickelide or nitinol. Less commonly used are cheaper copper-based alloys Cu-AI-Ni and Cu-Al-Zn.

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

Alloys with SME are often classified as so-called smart materials, which make it possible to create fundamentally new designs and technologies in various branches of mechanical engineering, aerospace and rocket technology, instrument making, energy, medicine, etc. Let us consider some applications of alloys with SME.

The exploration of near and far space is associated with the creation of orbital stations and large-scale space construction. It is necessary to construct such bulky objects as solar panels and space antennas. In Fig. Figure 1.1 shows a diagram of a spacecraft with self-deploying elements. The antennas consist of a Ti-Ni alloy sheet and rod that are coiled and placed in a recess in the artificial satellite. After the satellite is launched and put into orbit, the antenna is heated using a special heater or the heat of solar radiation, as a result of which it goes into outer space.

To accommodate various technical facilities, residential and production modules, it is necessary to build large platforms in open space conditions. Delivery of bulky units into outer 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, are not

Rice. 1.1.

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

Rice. 1.2. Connecting tubular parts (/) using a coupling (2) made of metal with shape memory:O - before assembly;b - after heating

suitable for space conditions. Special requirements are placed on ensuring exceptionally high safety.

Taking these features into account, our country has created a unique technology for connecting elements in outer space using a coupling made of TN-1 alloy. This technology was successfully used to assemble a truss structure made of aluminum alloys with a total length of 14.5 m and a cross-section in the form of a square 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). Using a mandrel, the coupling was deformed at low temperature so that its inner diameter was larger than the outer diameter of the elements being connected. After heating above the reverse martensitic transformation temperature, the inner diameter of the coupling was restored to the diameter that the coupling 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 Kvant astrophysical module of the Mir orbital complex was carried out in 1991 in just four spacewalks and took a total of about a day.

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

Couplings for thermomechanical connection of pipes are used in many designs (Fig. 1.3). They are used to connect the hydraulic systems of the F-14 fighter jet, and no accidents associated with oil leaks have been reported. The advantage of couplings made from 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 do not deteriorate. Couplings like this

Rice. 1.3. Connecting pipes using shape memory effect:

A - insertion of pipes after expansion of the coupling; b- heating

type are used for pipelines of nuclear submarines and surface ships, for repairing pipelines for pumping oil from the bottom of the sea, 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.

Rivets and bolts are usually used to permanently connect parts. However, if it is not possible to carry out any actions on the opposite side of the parts being fastened (for example, in a sealed hollow structure), performing fastening operations becomes difficult.

Stoppers made of an alloy with a shape memory effect allow in these cases fastening using spatial shape restoration. The 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, A). Before the fastening operation, the stopper is immersed in dry ice or liquid air and cooled sufficiently, 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, V), 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 use opens up wide possibilities

Rice. 1.4. The operating principle of a shape memory stopper makes it possible to create new effective treatment methods. Alloys used in medicine must have not only high mechanical characteristics. They must not be subject to corrosion in a biological environment, must be biologically compatible 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 an implanted organ made of metal is active relative to a biological structure, then degeneration (mutation) of biological cells of the peripheral structure occurs, an inflammatory rush of blood, impaired circulation, and then necrosis of the biological structure. If the implanted organ is inert, then a fibrous structure appears around it, caused by 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 biocompatibility at the level or even higher than commonly used corrosion-resistant steels and cobalt-chrome 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 caused by habit or a painful condition, lead to severe deformation when walking. This not only causes severe pain, but also has a harmful effect on the internal organs. In orthopedic surgery, spinal correction is usually performed using a Charinton rod, made of corrosion-resistant steel. The disadvantage of this method is that the initial corrective force decreases over time. 20 minutes after installation, the corrective force decreases by 20%, and after 10-15 days - up to 30% of the original. Additional adjustment of the force requires repeated painful operations and does not always achieve the goal. If an alloy with SME is used for the Kharinton rod, then the rod can be installed once, and there is no need for repeated surgery. If, after surgery, the Charinton rod is heated to a temperature slightly higher than body temperature, then the necessary corrective force can be created. Alloys based on Ti-Ni with additions of Cu, Fe and Mo are effective for this purpose; after restoration of their shape, they exhibit high elasticity in the temperature range

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

Plate for bone connection. Methods of medical care in case of bone fractures are to use plates made of corrosion-resistant steel or Co-Cr alloys to fix the fracture zone in a state where a compressive force acts on the bone.

If an alloy with a shape memory effect is used for the connecting plate, then it becomes possible to firmly fix the fracture zone by externally heating the plate to a temperature slightly higher than body temperature after surgery, and there is no need to perform longitudinal compression of the bone during surgery.

Intraosseous pins. Such pins are used in providing medical care for fractures of the tibia. Moreover, pins, mainly made of stainless steel, are inserted into the bone marrow, thereby fixing the bone. When using 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 diameter of the hole to create a large degree of deformation. In this regard, there is a risk of damaging the tissue in the area into which the pin is inserted.

The surgery is simplified when using Ti-Ni-based shape memory alloys for the studs. Pre-cooled pins restore their original shape at body temperature, which increases the degree of fixation.

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

The devices are used to effectively treat bone fractures through both continuous and discrete skeletal traction.

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

The disadvantage of correction wire is low elastic elongation and, as a consequence, plastic deformation. When making wire from 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 a continuous increase in electricity consumption. The limited reserves of fossil fuels, overcoming the energy crisis and the acceptable cost of electricity production have necessitated the use of nuclear energy and the large-scale construction of nuclear power plants (NPPs) in all developed countries of the world. Nuclear energy is the energy of the future.

According to 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, causing it to rotate. The turbine shaft is connected to the generator shaft, which produces electrical energy when rotated. The difference between nuclear power plants and thermal power plants is the method of heating water to boiling. If a thermal power plant burns coal or fuel oil to heat water, a nuclear power plant uses the thermal energy of a controlled chain reaction of uranium fission for this purpose.

Light water reactors (LWRs) are currently used in most countries 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 the most common.

In Fig. Figure 1.5 shows a diagram of a nuclear power plant equipped with a light water reactor (with water under pressure). Reactor vessel 9 contains the core 10 and the first circuit. Water circulates in the primary circuit, which is a coolant and slows down

Rice. 1.5. Scheme transfers warmth between PWR station elements:

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 - litel container. Water removes heat from the core to the heat exchange zone (steam generator 7), where the heat is transferred to the second circuit in which steam is generated. Energy conversion occurs 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. In this way, any radioactive fission products that may escape from the fuel into the primary water are isolated from the environment.

In the primary circuit, the 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 significantly 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 passing through the reactor vessel cover. Steam leaving the turbine 3, condenses in a water-cooled condenser 6, in which the remaining thermal energy is discharged. Some cooling systems use cooling towers.

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

Nuclear energy places increased demands on the structural materials used, their production technology and performance monitoring. When exposed to irradiation, structural materials undergo structural transformations that have a negative impact primarily on mechanical properties and corrosion resistance. Of all types of radiation (neutrons, A- and p-particles, y-radiation), neutron irradiation has the strongest effect.

Radiation-resistant materials These are materials that maintain stability of structure and properties under conditions of neutron irradiation (Table 1.11).

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

The most dangerous consequence of radiation is radiation swelling. In Fig. 1.6 presents the characteristics of radiation swelling of a number of grades of steels and alloys. Swelling can be suppressed by structurally forced recombinant Table 1.11

Effect 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 release
		Increasing yield strength
	Polystyrene	Decrease in tensile strength
	Ceramic materials	Reduced thermal conductivity, density, crystallinity
	Plastics	Not suitable for use as a construction material
	Carbon	Significant reduction in ductility, doubling of yield strength, increased transition from ductile to brittle fracture
	Corrosion-resistant steels	Threefold increase in yield strength
	Aluminum	Reduced ductility without complete embrittlement

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

Radiation resistance of reactor materials can be achieved if a set of conditions are met. These include

Rice. 1.6.

V- volume; DR - change in volume

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

Each metal and alloy has its own crystal lattice, architecture and dimensions.
which are strictly specified. For many metals, with changes in temperature and pressure, the lattice does not
remains the same and a moment comes when its restructuring occurs. Such a change
type of crystal lattice - polymorphic transformation - can be carried out by two
ways:
1) at high temperatures due to diffusion with high atomic mobility;
2) at low temperature due to the collective, coordinated movement of atoms, which
leads to a change in the shape of the volume 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.
When cooled, the alloy passes into the martensitic phase, in which the lattice cells become
with beveled parallelepipeds. When heated, the austenite phase is restored, and with it
The original shape of the alloy product with shape “memory” is also restored.
Martensitic transformation is one of the fundamental methods of crystal restructuring
lattice in the absence of diffusion, characteristic of 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 regime.
A change in shape is the main feature of the martensitic transformation, which is associated with the effect
the effect of “memory” of alloys, a necessary condition, 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 martensitic transition. The transition is slowed down due to the appearance of an interface
old and new phases and increasing free energy. Growing crystals of the martensitic phase
deform the surrounding volume, which resists this. Elastic energy appears
preventing further crystal growth. When this energy exceeds the elastic limit
guests, intense deformation of the material occurs in the vicinity of the phase boundary and
crystal growth stops. In steels the process occurs almost instantly (individual
martensite crystals grow to final sizes).
Reverse transition of martensite to austenite (high-temperature phase, diffusion-free
shear rearrangement of the lattice is difficult), occurs at high temperatures, when in open-hearth
austenite crystals grow on a sieve without transitioning to their original form (the atoms do not fall into their
previous places).
In alloys with “memory”, upon cooling, martensite crystals grow slowly, at
upon heating disappear gradually, which ensures dynamic equilibrium of the interface
between them and the initial phase. The boundary between the phases behaves similarly if the cooling
Replace heating and heating by applying and removing the load, respectively - thermoelastic
equilibrium of phases in a solid.
Thermoelastic martensitic transformation is accompanied by a reversible change in shape
austenite crystals, which mainly provides the “memory” of metals.
56 Intelligent polymer materials (IPM)
A direct consequence of the thermoelastic martensitic transformation is the reversible
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
The “memory” of the form has little practical significance. Collective movement
atoms in a certain direction, accompanied by spontaneous (martensitic)
neu) deformation of the material (lattice rearrangement), in which the proximity and interatomic
the bonds of atoms are not broken (the possibility remains to return to their previous positions,
to the original form), takes place only under certain conditions. "Memory" of an individual
crystal is not yet a memory of the entire volume of the alloy, which usually has a polycrystalline
personal structure.
Individual crystallites (grains) differ in the orientation of their crystal lattices.
The shift of atoms during martensitic transformation occurs in the lattice along certain planes.
bones and directions. Due to the different orientation of the grains, the shears in each grain occur
in different directions and, despite significant deformation of individual crystals,
the sample as a whole does not experience a noticeable change in shape. This happens when
if the crystals are oriented in the same direction. The control force, which, when mar-
Tensite transformation organizes the preferential organization of crystals, is
external load.
During the martensitic transformation, atoms move in the direction of the external
load (the sample as a whole experiences deformation). The process continues until
the entire material will not deform in the direction of the force without breaking 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 material.
The “memory” effect is based on thermoelastic phase equilibrium and control action
loads. Special thermomechanical processing of alloys creates micro-
stresses, the action of which during martensitic transitions is similar to the action of external
loads. When cooled, the alloy spontaneously takes one shape, when heated
returns to the original one (the plate curls into a ring when cooled, when heated -
turns around or vice versa).
Materials with shape memory can exhibit superplasticity (significant de-
formations, when the martensitic transformation is caused by the application of an external load, and
not by cooling, which is used to create spring shock absorbers and 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 restructuring of the crystal lattice is accompanied by the release
or heat absorption, if an external load causes martensitic transformation, then
mechanical energy turns into thermal energy; with memory effects, a process is also observed
converting heat into work).
Change in shape (with periodic temperature changes) of metals with memory
accompanied by the manifestation of powerful interatomic forces. Expansion pressure of materials
this type reaches 7 t/cm2. Depending on the type of material, products of different 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 according to a special regime, in 2–3
times cheaper and 1.5 times lighter than titanium nickelide), an alloy based on titanium with 28–34% manganese and
5–7% silicon, terfenol (magnetostrictive alloy, dampens vibrations at low frequencies
vibrations).
Smart polymer materials (IPM) 57
Manganese-based alloys have a temperature range of maximum thermal sensitivity
softness at 20–40 °C and restore the desired shape in the temperature range from
–100 to 180 °C
Alloys of the Cu-Zn- system were obtained by powder metallurgy (Fukuda Metal Co.).
Al with shape memory effect by sintering (700 MPa, 900 °C, 0.1 wt.% aluminum fluoride
powders of Cu-Zn (70:30), Cu-Al (50:50) and copper alloys (grain size 20–100 µm). Alloy
restores its shape after stretching by 10%.
When cooled, the alloy passes into the martensitic phase, in which, thanks to the changed
depending on the geometric parameters of the crystal lattice cells, it becomes plastic and when
mechanical impact, a product made from an alloy with “memory” (nitinol, etc.) can be given
virtually any configuration that will be maintained until the temperature
will exceed the critical value at which the martensitic 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 is not fully restored.
Nitinol alloys have been developed that simultaneously “remember” the shape of products,
corresponding to high and low temperatures. Memory effect in nitinol alloys
clearly defined, and the temperature range can be precisely adjusted in the range from non-
how many degrees to tens of degrees, introducing modifying elements into the alloys, however
cyclicity margin, 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 microns of alloys
with titanium and nickel nanoparticles, change the length by 12–13% over 5 million iterations and
used in artificial muscles. Nano Muscle Actuator, Nano
Muscle, USA, Johnson Electric, KHP, 2003) develops a thousand times more power than
human muscles and 4000 times faster than an electric motor at actuation speed
0.1 seconds with a smooth transition from one state to another at a given speed (mic
roprocessor control).
Materials with magnetomechanical memory have been developed (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
qualitative change in properties, conductor-semiconductor, paramagnetic-ferrous transitions
romagnet), which is promising for creating MI actuators for radio engineering purposes
to reduce radar signature.

Moscow State University

them. M.V. Lomonosova

Faculty of Materials Science

Topic: “Materials with shape memory.”

V-year student of FNM

Kareeva I.E.

Moscow 2000

Introduction……………………………………………………2

Mechanism for implementing the shape memory effect………...3

Areas of application………………………………………………………..7

Preparation of alloys with shape memory…………………….9

Degradation……………………………………………………………..10

Conclusion……………………………………………………………..11

References………………………………………………………..12

Introduction.

Shape memory materials (MSM) were discovered in the late 60s of this century. Within 10 years (late 70s - early 80s), many reports appeared in scientific journals describing various possibilities for their use. Currently, functional properties are defined for MPF: one- and two-way memory effect, pseudo- or superelasticity, high damping ability.

MPFs have already found wide application in medicine as long-term functioning materials implanted into the body. They exhibit high elastic properties, are able to change their shape with temperature changes and do not collapse under alternating load conditions. The complex nature of martensitic-type phase transformations occurring in alloys based on titanium nickelide is clearly manifested in porous structures. Phase transitions in such alloys are characterized by wide hysteresis and a long temperature range in which the material exhibits shape memory and superelasticity effects. In addition to alloys based on Ni-Ti, martensitic transformations exist, for example, in systems such as Pt-Ti, Pt-Ga, Pt-Al.

Depending on the martensitic transformation temperature and mechanical properties, shape memory alloys have a wide range of applications.

The mechanism for implementing the shape memory effect.

Martensite.

Martensite is a structure of crystalline solids that arises as a result of a shear, diffusion-free polymorphic transformation upon cooling. Named after the German metallurgist Martens (1850 - 1914). As a result of lattice deformation during this transformation, a relief appears on the metal surface; internal stresses arise in the volume, and plastic deformation occurs, which limits the growth of the crystal. The growth rate reaches 10 3 m/s and does not depend on temperature, so the rate of martensite formation usually limits the nucleation of crystals. The counteraction of internal stresses shifts the nucleation of crystals well below the point of thermodynamic equilibrium of the phases and can stop transformations at a constant temperature; therefore, the amount of martensite formed usually increases with increasing supercooling. Since the elastic energy must be minimal, martensite crystals take the form of plates. Internal stresses are also relieved by plastic deformation, so the crystal contains many dislocations (up to 10 12 cm -2), or is broken into twins with a thickness of 100 - 1000 Å. Intragrain boundaries and dislocations strengthen martensite. Martensite is a typical product of low-temperature polymorphic transformations in pure metals (Fe, Co, Ti, Zr, Li and others), in solid solutions based on them, in intermetallic compounds (CuZn, Cu 3 Al, NiTi, V 3 Si, AuCd).

Martensitic transformations.

Ni-Ti intermetallic compounds with a composition close to eutectic are characterized by a transition from the cubic (austenitic phase) to the monoclinic (martensitic) phase at room temperature. Such transformations usually occur in alloys at high stresses, but as a result of the memory effect or superelasticity, transformations can also occur at low stresses. Austenitic Ni-Ti alloys exhibit superelastic behavior under mechanical loads and tension (8%) caused by martensitic transformation. Upon unloading, martensite becomes unstable and turns into austenite, with compensation of all macroscopic stresses.

Martensitic transformation is a polymorphic transformation in which a change in the relative arrangement of the atoms that make up the crystal occurs through their ordered movement, and the relative displacements of neighboring atoms are small compared to the interatomic distance. The restructuring of the crystal lattice in microregions usually comes down to the deformation of its cell, and the final phase of the martensitic transformation is a uniformly deformed initial phase. The magnitude of the deformation is small (~1-10%) and, accordingly, the energy barrier that prevents the uniform transition of the initial phase to the final phase is small, compared to the binding energy in the crystal. A necessary condition for the martensitic transformation, which develops through the formation and growth of regions of a more stable phase in a metastable one, is the preservation of ordered contact between the phases. The ordered structure of interphase boundaries with a small barrier for a uniform phase transition ensures their low energy and high mobility. As a consequence, the excess energy required for the nucleation of crystals of a new phase (martensitic crystals) is small and, with some deviation from phase equilibrium, becomes comparable to the energy of defects present in the initial phase. Therefore, the nucleation of martensitic crystals occurs at a higher rate and may not require thermal fluctuations. A significant role during the martensitic transformation is played by internal stresses arising due to the elastic adaptation of crystal lattices mating along the phase boundaries. Elastic stress fields lead to a displacement of the equilibrium point of the interacting phases relative to the position of true thermodynamic equilibrium for isolated, undistorted phases; Accordingly, the temperature at which the martensitic transformation begins can differ significantly from the true equilibrium temperature. The desire to minimize elastic stress energy determines the morphology, internal structure and relative position of martensite crystals. The new phase is formed in the form of thin plates, oriented in a certain way relative to the crystallographic axes. Plates, as a rule, are not single crystals, but are packages of plane-parallel domains—regions of a new phase that differ in the orientation of the crystal lattice (twins). Interference of voltage fields from different domains leads to their partial destruction. A further reduction in elastic fields is achieved by the formation of ensembles of regularly arranged plates. That is, as a result of the martensitic transformation, a polycrystalline phase is formed with a peculiar hierarchical order (assemblies - plates - domains) in the arrangement of structural components. An increase in internal stresses during the martensitic transformation under certain conditions leads to the establishment of a two-phase thermoelastic equilibrium, which reversibly shifts when external conditions change: under the influence of mechanical loads or when temperature changes, the sizes of individual crystals and their number change. Martensitic transformations are found in many crystalline materials: pure metals, numerous alloys, ionic, covalent and molecular crystals.

There are great prospects for reversible shape changes during martensitic transformation (the creation of superelastic alloys that restore their original shape when heated after plastic deformation - the memory effect), as well as the connection between martensitic transformation and the appearance of superconducting properties in some metals. Martensitic transformations form the basis of numerous structural transformations, due to which, with the help of thermal and mechanical treatment, a directed change in the properties of crystalline materials is carried out.

Features of porous titanium nickelide alloys.

The presence of a wide temperature range of martensitic transformation in porous titanium nickelide compared to cast titanium is reflected in the temperature curves of electrical resistance. It has been shown that the martensitic transition is incomplete in porous alloys and occurs over a wider temperature range than in cast alloys. Thus, an important feature of porous titanium nickelide compared to a non-porous (cast) alloy of the same composition is the wide temperature range of phase transformations. It is approximately 250 0 C, i.e. significantly exceeds the range (30-40 0 C) of transformations of the cast alloy. The increase in the temperature range of phase transformations is due to the structure of porous titanium nickelide. The size factor is also significant, since the martensitic transformation in thin bridges and massive regions manifests itself differently. The action of these factors leads to the fact that phase transformations in porous materials based on titanium nickelide begin in different regions at different temperatures, extending the hysteresis along the temperature axis, correspondingly expanding the temperature ranges of transformations and the intervals of manifestation of shape memory effects and superelasticity in porous alloys based on nickelide titanium.

Fig. 1 Temperature dependences of the reversible memory effect and the yield strength in porous (1) and cast (2) alloys based on titanium nickelide.

Figure 1 shows the shape memory effect in porous and cast alloys. In a porous alloy, the shape memory effect manifests itself in a wider temperature range than in a cast alloy, and the residual plastic deformation in a porous material is more significant (in Fig. 1) than in a cast one. In cast titanium nickelide, almost complete (up to 100%) restoration of shape occurs after deformation by 6 - 8% and subsequent heating above the MT temperature range (Fig. 1). As the degree of deformation of cast titanium nickelide increases, dislocation defects are formed, which, unlike martensitic transformations, are irreversible. The stage of reversible deformation according to the martensitic mechanism is replaced by the stage of irreversible plastic deformation. Even at low loads, areas arise in which the magnitude of elastic deformation exceeds the limit. In contrast, in porous alloys, even with minimal deformations, the degree of shape restoration does not exceed 85%. The degree of shape restoration depends on porosity, pore size distribution, and the level of martensitic shear stress, i.e. associated with the peculiarities of deformation of porous bodies. Analysis of the deformation dependences of titanium nickelide with different porosities shows that the yield strength of the alloy decreases with increasing porosity.

Areas of use.

Non-medical use.

The first shape memory alloy was used in the F-14 aircraft in 1971, it was Ni-Ti-Fe. The use of Ni-Ti-Nb alloy has been a great advance, but also Fe-Mn-Si alloys have received a lot of attention, despite their lower recovery voltage.

There are potential applications for nitinol in the production of consumer goods. For example, an interesting invention: a device - an ashtray holder, which lowers a burning cigarette into the ashtray, preventing it from falling, say, onto the tablecloth.

The reliability of shape memory devices depends on their service life. Important external parameters for controlling system operating cycles are time and temperature. Important internal parameters that determine the physical and mechanical properties are: alloy system, alloy composition, transformation type, and lattice defects. These parameters control the thermomechanical history of the alloy. As a consequence, the maximum memory effect will be limited depending on the number of cycles required.

Space payloads such as solar panels or satellite antennas currently use mainly pyrotechnic deployment methods, which create many problems. The use of shape memory materials will eliminate all these problems and will also provide the opportunity to repeatedly test the performance of the system on the ground.

Recent research on Ni-Ti alloys has shown that super elastic behavior results in improved wear resistance. Pseudoelastic behavior reduces the area of elastic contact during sliding. Reducing the area of elastic contact between two sliding parts increases the wear resistance of the material. A special type of wear is cavitation erosion, which creates specific problems in hydraulic machines, ship propellers, and water turbines. Comparative studies of various materials have shown that Ni-Ti alloys have higher resistance to cavitation erosion than conventional alloys. In the martensitic state, the Ni-Ti alloy has very good resistance to cavitation erosion. But manufacturing working parts subject to corrosion entirely from Ni-Ti alloy is too expensive, so the best way is to use a Ni-Ti alloy combined with steel.

Medical use.

In medicine, a new class of composite materials “bioceramics–titanium nickelide” is used. In such composites, one component (titanium nickelide) has superelasticity and shape memory, while the other retains the properties of bioceramics.

The ceramic component can be porcelain, which is widely used in orthopedic dentistry and is a fragile material. The high fragility of porcelain is due to the fact that contact stresses arise at the boundaries of various phases and grains, significantly exceeding the level of average applied stresses. Relaxation of contact stresses in a ceramic material is possible if energy dissipation occurs in the zone of these stresses due to a phase transformation in titanium nickelide. A change in temperature or the application of a load causes a martensitic transformation in titanium nickelide, which leads to effective stress relaxation in the matrix when the composite material is loaded, allowing the solid component to bear the applied load. It is known that the elastic restoration of the volume of porous compacts made of superelastic titanium nickelide powder is associated with the rupture of interparticle contacts and is determined by the strength of the briquette, which depends on the porosity and magnitude of the contact adhesion forces. Weakening these forces by adding other components to the titanium nickelide powder, such as finely dispersed tungsten or silicon carbide, significantly increases the elastic effect, since strong titanium-nickel contacts of the same name are replaced by opposite ones. Since the magnitude of the elastic effect decreases with decreasing titanium nickelide content in the compact, the concentration dependence of elastic volume recovery is usually extreme. In the porcelain–titanium nickelide composite material, the components interact weakly and after sintering, the contacts between the ceramic and metal components are weakened. When loaded, they rupture first and elastic volume recovery increases. As a result, the deformation is reversible and the composite exhibits properties similar to superelasticity. The biocompatibility of the composite material “dental porcelain–titanium nickelide” was studied histologically, assessing the tissue response in rats to implantation of composite material and porcelain samples under the skin of the anterior abdominal wall. The nature of tissue reactions, their prevalence and features of cellular changes in both cases turned out to be unambiguous. Thus, bioceramics–titanium nickelide composite materials are biocompatible.

Preparation of alloys with shape memory.

Shape memory alloys are produced by fusing individual components. The melt is quickly cooled and high-temperature treatment is carried out.

A whole class of composite materials “bioceramics - titanium nickelide” for medicine has been proposed. In such materials, one component (titanium nickelide) has shape memory and superelasticity, while the other retains the properties of bioceramics. The most commonly used ceramic component is porcelain, which is widely used in orthopedic dentistry and is a fragile material. To make such samples, powders of titanium nickelide and porcelain mass are used, which, after mixing and drying, are sintered in a vacuum.

Degradation

Martensitic transformation in NiTi-based alloys is an athermal process, the rate of which is entirely determined by the rate of temperature change near the thermodynamic equilibrium of the phases. Therefore, all specific mechanical effects in NiTi that accompany the martensitic transformation, such as shape memory and transformation plasticity, can be realized in very short times under appropriate heating and cooling conditions. In high-speed devices, to accelerate the exchange of heat with a heat agent (liquid or gaseous), thin-gauge tape, wire and pipes with micron linear dimensions in cross-section are used. In this case, the state of the free surface of the alloy becomes of great importance. Since even small variations in composition lead to changes in temperature kinetics and completeness of transformation, segregation of elements and oxidation of the surface significantly change the special properties of the material. This circumstance acquires particular importance due to the need for preliminary thermal or thermomechanical treatment of the material.

Studies have shown the tendency of titanium nickelide on the free surface under thermal influences. In an atmosphere containing oxygen, the alloy oxidizes to form an oxide layer containing mainly TiO 2 oxide. It can be assumed that since titanium is chemically very active, in an oxygen-free environment titanium atoms will form compounds with any non-inert gas, for example, in a nitrogen atmosphere - nitrides. The formation of oxides along grain boundaries and on the surface can only be avoided by heat treating samples in a vacuum or in an inert environment.

Conclusion

The memory effect or shape memory is the ability of a product, when heated, to restore its original shape, changed due to plastic deformation. The most well-known memory alloy is nitinol.

Shape restoration is caused by martensitic transformation or reversible twinning in the structure of the metal material.

In the case of the memory effect, which occurs through the mechanism of martensitic transformation, when the alloy is heated, stresses arise in the pre-deformed steel lattice. Restoration of the previous shape is carried out only in the case of coherence between the deformed crystal lattice of the material and the martensitic phase formed during heating. In coherent crystal lattices at the phase interface, the number of cells of the main and resulting phases of the alloy is the same (only the directions of the atomic planes of the crystal lattices are slightly different). In partially coherent lattices, the regularity of the alternation of atomic planes is disrupted, and a so-called edge dislocation appears at the phase boundary. In incoherent crystal lattices, the directions of atomic planes are very different. Martensite crystal growth occurs only up to incoherent interphase boundaries.

The martensitic phase in steel is formed if the free energy of the system A≠ 0. If the energy of elastic deformation of the steel lattice is equal to the energy of formation of the martensite phase in it, then A = 0 and the growth of martensite crystals ends. This equilibrium depends on temperature and is called thermoelastic.

Shape restoration according to the second mechanism is associated with the formation of twins in the crystal lattice of metallic materials under mechanical load and their disappearance upon heating. When a steel sample in the martensitic state is deformed, retwinning or reorientation of martensite crystals occurs. This causes a change in the shape of the sample. When heated, the structure and orientation of the crystals of the initial phase are restored, which leads to the restoration of the shape of the product. Exceeding the critical level of deformation leads to the formation of irreversible twins, the disappearance of which is possible only during recrystallization.

Complete restoration of shape is observed for alloys with thermoelastic martensite: Cu - Al - (Fe, Ni, Co, Mn), Ni - Al, Ti - Ni, Ti - Au, Ti - Pd, Ti - Pt, Au - Cd, Ag - Cd, Cu - Zn - Al.

Nitinol is one of these alloys. Ti - Ni . The temperature range of the memory effect in nitinol is 550-600 0 C. The main properties of nitinol:

Elastic modulus E=66.7...72.6 MPa;

Tensile strengthσ =735...970 MPa;

Relative elongation l=2…27%;

Specific electrical resistance ρ=65…76 μOhm× cm;

Melting temperature Tmelt=1250…1310 0 C;

Density d = 6440 kg/m 3.

Memory alloys are used for tubular permanent connections that eliminate the need for welding and soldering, in washers for electrical contact connections that provide constant pressure and, accordingly, contact resistance, self-expanding antennas of spacecraft, etc.