According to the generally accepted opinion, metals are the most durable and stable materials. However, there are alloys that can recover after deformation without applying an external load. They are also characterized by other unique physical and mechanical properties that distinguish them among structural materials.
The essence of the phenomenon
The shape memory effect of alloys is that a previously deformed metal spontaneously recovers as a result of heating or simply after unloading. These unusual properties were noticed by scientists back in the 50s. XX century Even then, this phenomenon was associated with martensitic transformations in the crystal lattice, in which there is an ordered movement of atoms.
Martensite in materials with a shape memory effect is thermoelastic. This structure consists of crystals in the form of thin plates, which are stretched in the outer layers, and in the inner ones are compressed. The “carriers” of deformation are interphase, twin, and intergranular boundaries. After heating the deformed alloy, internal stresses appear, trying to return the metal to its original form.
The nature of spontaneous recovery depends on the mechanism of the previous exposure and the temperature conditions under which it occurred. Of greatest interest is the repeated cyclicity, which can amount to several million strains.
Metals and alloys with a shape memory effect also have another unique property - the nonlinear dependence of the physical and mechanical characteristics of the material on temperature.
Varieties
The above process can occur in several forms:
- superplasticity (superelasticity), in which the crystalline structure of the metal withstands deformations significantly exceeding the yield strength in the ordinary state;
- a single and reversible shape memory (in the latter case, the effect is repeatedly reproduced during thermal cycling);
- ductility of direct and reverse transformation (accumulation of deformation during cooling and heating, respectively, when passing through a martensitic transformation);
- reverse memory: when heated, first one deformation is restored, and then, with a further increase in temperature, another;
- oriented transformation (accumulation of deformations after elimination of the load);
- pseudoelasticity - restoration of inelastic deformations from elastic values in the range of 1-30%.
The return to the initial state in metals with the shape memory effect can occur so intensively that it cannot be suppressed by an effort close to the ultimate strength.
Materials
Among alloys with such properties, titanium-nickel (49– 57% Ni and 38–50% Ti) are most common. They have good performance characteristics:
- high strength and resistance to destruction by corrosion;
- significant coefficient of shape recovery;
- the high value of internal stress when returning to its initial state (up to 800 MPa);
- good compatibility with biological structures;
- effective vibration absorption.
In addition to titanium nickelide (or nitinol), other alloys are also used:
- two-component - Ag-Cd, Au-Cd, Cu-Sn, Cu-Zn, In-Ni, Ni-Al, Fe-Pt, Mn-Cu;
- three-component - Cu-Al-Ni, CuZn-Si, CuZn-Al, TiNi-Fe, TiNi-Cu, TiNi-Nb, TiNi-Au, TiNi-Pd, TiNi-Pt, Fe-Mn-Si and others.
Alloying additives can strongly shift the temperature of martensitic transformations, affecting the properties of reduction.
Industrial use
The use of the shape memory effect allows solving many technical problems:
- creation of tight tube assemblies is similar to the flaring method (flange joints, self-tightening clips and couplings);
- manufacturing of clamping tools, grips, pushers;
- design of "super springs" and batteries of mechanical energy, stepper motors;
- the creation of compounds from dissimilar materials (metal-non-metal) or in hard-to-reach places when the use of welding or soldering becomes impossible;
- manufacture of reusable power elements;
- case sealing of microcircuits, sockets for their connection;
- production of temperature regulators and sensors in various devices (fire alarms, fuses, valves of heat engines and others).
There are great prospects for the creation of such devices for the space industry (self-expanding antennas and solar panels, telescopic devices, tools for installation work in outer space, drives of rotary mechanisms - rudders, shutters, hatches, manipulators). Their advantage is the absence of pulsed loads, which introduce disturbances in the spatial position in space.
The use of alloys with the effect of shape memory in medicine
In medical materials science, metals with these properties are used for the manufacture of technological devices such as:
- stepper motors for stretching bones, straightening the spine;
- filters for blood substitutes;
- devices for fixing fractures;
- orthopedic devices;
- clamps for veins and arteries;
- pump parts for an artificial heart or kidney;
- stents and endoprostheses for implantation in blood vessels;
- orthodontic arches for correction of the dentition.
Disadvantages and Prospects
Despite the wide possibilities, alloys with a shape memory effect have drawbacks that limit their widespread adoption:
- expensive components of chemical composition;
- sophisticated manufacturing technology, the need to use vacuum equipment (to avoid the inclusion of nitrogen and oxygen impurities);
- phase instability;
- low machinability of metals by cutting;
- difficulties in accurately modeling the behavior of structures and manufacturing alloys with specified characteristics;
- aging, fatigue and degradation of alloys.
A promising direction in the development of this field of technology is the creation of coatings from metals with a shape memory effect, as well as the manufacture of such alloys based on iron. Composite structures will allow combining the properties of two or more materials in one technical solution.