According to conventional wisdom, metals are the most durable and resistant materials. However, there are alloys that can recover their shape after deformation without applying an external load. They are also characterized by other unique physical and mechanical properties that distinguish them from structural materials.
Essence of the phenomenon
The shape memory effect of alloys is that a pre-deformed metal spontaneously recovers as a result of heating or simply after unloading. These unusual properties were noticed by scientists as early as the 1950s. 20th century Even then, this phenomenon was associated with martensitic transformations in the crystal lattice, during which there is an ordered movement of atoms.
Martensite in shape memory materials is thermoelastic. This structure consists of crystals in the form of thin plates, which are stretched in the outer layers, and compressed in the inner ones. The "carriers" of deformation are interphase, twin and intercrystallite boundaries. After heating the deformedalloy, internal stresses appear, trying to return the metal to its original shape.
The nature of spontaneous recovery depends on the mechanism of the previous exposure and the temperature conditions under which it proceeded. Of greatest interest is the multiple cyclicity, which can amount to several million deformations.
Metals and alloys with a shape memory effect have another unique property - a non-linear dependence of the physical and mechanical characteristics of the material on temperature.
Varieties
The above process can take several forms:
- superplasticity (superelasticity), in which the crystal structure of the metal can withstand deformations that significantly exceed the yield strength in the normal state;
- single and reversible shape memory (in the latter case, the effect is repeatedly reproduced during thermal cycling);
- forward and reverse transformation ductility (accumulation of strain during cooling and heating, respectively, when passing through a martensitic transformation);
- reversible memory: when heated, first one deformation is restored, and then, with a further increase in temperature, another;
- oriented transformation (accumulation of deformations after removal of the load);
- pseudoelasticity - recovery of inelastic deformations from elastic values in the range of 1-30%.
Return to the original state for metals with the effectshape memory can be so intense that it cannot be suppressed by a force close to the tensile strength.
Materials
Among the alloys with such properties, the most common are titanium-nickel (49–57% Ni and 38–50% Ti). They have good performance:
- high strength and corrosion resistance;
- significant recovery factor;
- large value of internal stress when returning to the 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 greatly shift the martensitic transformation temperature, affecting the reduction properties.
Industrial use
Application of the shape memory effect allows solving many technical problems:
- creation of tight pipe assemblies similar to the flaring method (flanged connections, self-tightening clips and couplings);
- manufacture of clamping tools, grippers, pushers;
- design"supersprings" and accumulators of mechanical energy, stepper motors;
- creating joints from dissimilar materials (metal-nonmetal) or in hard-to-reach places when welding or soldering becomes impossible;
- production of reusable power elements;
- case sealing of microcircuits, sockets for their connection;
- manufacture of temperature controllers and sensors in various devices (fire alarms, fuses, heat engine valves and others).
The creation of such devices for the space industry (self-deploying antennas and solar batteries, telescopic devices, tools for installation work in outer space, drives for rotary mechanisms - rudders, shutters, hatches, manipulators) has great prospects. Their advantage is the absence of impulse loads that disturb the spatial position in space.
Application of shape memory alloys in medicine
In medical materials science, metals with these properties are used to make technological devices such as:
- stepper motors for stretching bones, straightening the spine;
- filters for blood substitutes;
- devices for fixing fractures;
- orthopedic appliances;
- clamps for veins and arteries;
- pump parts for artificial heart or kidney;
- stents and endoprostheses for implantation in blood vessels;
- orthodontic wires for correcting the dentition.
Disadvantages and prospects
Despite their great potential, shape memory alloys have disadvantages that limit their widespread adoption:
- expensive chemistry components;
- complicated manufacturing technology, the need to use vacuum equipment (to avoid the inclusion of nitrogen and oxygen impurities);
- phase instability;
- low machinability of metals;
- difficulties in accurately modeling the behavior of structures and manufacturing alloys with desired characteristics;
- aging, fatigue and degradation of alloys.
A promising direction in the development of this area 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.
