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Vladimir PUSHIN, Dr. Sc. (Phys. & Math.), Institute of Metal Physics, Urals Branch, RAS
One of the central tasks of both basic and applied research in the field of solid state physics and the materials science has been and remains the development of metallic materials, devices and products thereof. In most cases (speaking of alloys of iron, aluminum, etc.) these are used as structural materials for the production of general and also special-purpose equipment.
Among the materials of this kind which have appeared on the scene in the latter third of the 20th century a special place is taken by what we call functional materials. These are intended for applications in electrical engineering and electronics, cryogenics and aerospace, and also in the medical and other fields. One typic case in point is a group of metal alloys with thermoelastic martensite * modifications and thermomechanical memory. Since the 1970s the area of practical applications of such materials has continued to grow.
Belonging to this group of materials are alloys developed on the basis of titanium nickelide. These are remarkable for high strength and elasticity, unique thermomechanical memory effects (form memory, one-time and reversible; superelasticity, damping, etc.). They are also distinguished for high reliability, what we call mechanothermal and thermocyclic longevity, weldability, corrosion resistance, biological compatibility and for relative simplicity of chemical composition and technological-metallurgical process and the simplicity of subsequent reprocessing. Despite their high cost, in comparison, for example, with alloys with the similar basic parameters on the basis of Cu-Zn-Al, they are irreplaceable in, let us say, medicine where they are used as functional material of a new generation.
Applied on a broad scale both here in Russia and in industrially advanced countries over the last two decades have been alloys with thermomechanical memory. Experts have been investigating their basic properties, have systematically been studying their structural and phase transformations, and also some unique temperature, deformation and stress parameters of their thermomechanical memory effects.
The phase and structural transformations occurring in solid bodies under various external impacts determine a whole range of their important characteristics, above all what are called structural-sensitivity properties (strength, high-temperature strength, deformation factors, breaking viscosity, etc.). For example, high strength of steels results from martensite transformations in them, and in aluminum alloys it is ensured at early stages of ageing (decay of oversaturated solid solutions). The heat resistance and thermal stability ofintermetallic compounds obtained on the basis of titanium, nickel and suchlike materials are ensured, in the main, by their orderly atomic structure which is retained up to some very high melting temperatures. This list of transformations-non-diffusive, martensite, diffusion-controlled decay with the separation of excessive phases or atomic ordering-practically exhausts the range of all possible phase and structural transitions which can take place in the solid state in metals, their alloys and compositions with both other metals and non-metals. This can be accompanied by simple transformations (polymorphism in pure metals) or combined reactions-at changes of temperature, pressure, stress and deformation in any order of succession or simultaneously at several stages of transition (for example, decay and atomic ordering, decay
* Martensite-main structural component of tempered steel. It accounts for the "memory effect" at heating and cooling of metals and alloys .-Ed.
and martensite transformation, atomic ordering and martensite transformation).
Subject to the martensite (shift) transformation are a number of pure chemical elements and many alloys, intermetallic and chemical compounds. It represents a special variety of phase transitions noted by lack of diffusion and orderliness of the shift recombination of the atomic-crystalline lattice of the high-temperature phase (also called austenite) into the low-temperature phase lattice (martensite) at which shifts of adjacent atoms relative to one another are minimal: they do not exceed the interatomic distances and do not alter the immediate environment. The physical cause of marten-site transformations is the appearance and development of an unstable crystal lattice and modifications of its symmetry resulting in free energy reduction.
An important role in understanding the nature and the basic regularities and conditions accompanying martensite transformations in different materials has been a physical concept advanced back in the late 1930s by Academician Georgi Kurdyumov: these transformations should be regarded as a phase transition in a single-component system accompanied by considerable inner stresses as a result of its own deformation in the transformed crystal
area. Let us now examine this problem in three aspects: thermodynamic, crystal-geometric and kinetic.
Out of these three, the thermodynamic one, is based on an analysis of the dependence of temperature or pressure on the free energies of the initial austenite and martensite phases. For steels and alloys with non-thermoelastic martensite transformations the values of the chemical motive force at the start of a direct transition (at cooling) and of the reverse force (at heating) are close.
In alloys with thermoelastic martensite transformations the value of temperature hysteresis is very small (an order of magnitude smaller as compared with steels) and much smaller (by one or two orders) is the motive force of the transformation. Substantially increased is the role of elastic energy accumulated in the direct transformation and cooperating in the reverse transformation undirectionally with the chemical one. This feature determines, in the final analysis, the appearance of the thermomechanical memory effects based on the mechanisms of micro-structural memory.
What we call the crystallogeometrical, and actually phenomenological, analysis of martensite transformations and numerous experimental studies of various materials have shown that volume changes can be visualized as a superpositioning of the pure deformation of the lattice and the deformation with an invariant lattice. The existing crystallogeometry theories explicity describe the crystallography of martensite transformation, explaining the available data on the invariant (or inter-phase) planes, different in different materials, and also other crystallo-graphic phase characteristics in alloys (orientation correlations of phases, the value and direction of macroscopic deformities, martensite substructure, etc.).
The most complicated aspect in the formulation of this theory is the analysis of the mechanisms and kinetics of origin and also of the growth of martensite crystals. Suppositions on the existence of some nucleating centers, whose crystal structure is identical with that of the observed martensite, have found no experimental proof. Therefore most researchers assumed that martensite originates heterogeneously at a high rate of crystal growth. Winning support in recent years has been a concept claiming that there is no need for a coincidence of the structures of the subcritical nucleating centers and the end martensite. And one can consider a range of metastable - intermediate and heterogeneous - nanostructural states of the lattice. As has been established, an important role in the development of pre-transition states and thermoelastic martensite transformations proper is a strong pre-martensite softening of the lattice.
The discovery of the temperature-elastic martensite transformation and the resulting effects of reversible form changes provided a foundation for the development of a new class of functional materials. Their potentialities and range of applications appear to be very broad and have not been yet identified to the full. As has been pointed out, titanium-based alloys, used on a broad scale in engineering and medicine, are clearly in the lead among the memory metallic compounds according to the known parameters. This is true of titanium nickelide above all.
Known today are these basic mechanisms of inelastic deformation of solid bodies: by slipping, twinning and martensite transformations. Martensite inelasticity is the third (after resilience and plasticity) basic type of deformation behavior of crystalline materials. In alloys with thermoelastic martensite transformations the most significant, in practical terms, features of their unique mechanical behavior are the following effects:
- form memory (FM), which means that the alloy regains, completely or in part, the earlier deformation assumed in the martensite state and, accordingly, regains the original form;
- superelasticity (superresilience, or rather pseudoresilience), when, loaded, an alloy undergoes considerable non-elastic deformation which disappears fully or in part when the load is removed;
- transformation plasticity (super-plasticity) - the property of an alloy to undergo considerable (up to 100 percent) pseudoelastic and plastic deformation under load in a broad temperature range;
- multiple reversible form memory;
- high inner friction and damping. As demonstrated by X-ray structural studies (at various stages of deformation and different temperatures), this depends on the direct and reverse transformations which possess, under the effect of the initiating external stresses, a texture-an oriented generation and growth of martensite crystals. And this ensures a microscopic deformation of the samples. As the load is removed, the martensite transformation is reversed and the accumulated deformation in the end returns along the lower curve of the hysteresis loop. In other words, the alloy after load removal persists fully (or in part) in the martensite state. The initial form and, consequently, the structure of the alloy are restored only at a definite temperature. Finally, martensite transformation can be intensified following a sufficiently deep progression of plastic deformation through the ordinary mechanisms of slipping and twinning at the initial austenite phase. In this case the superplasticity effect comes into play which, in its turn, is fully irreversible mechanically.
The cause of the multiple reversible form memory (MRFM), as proved by X- ray studies, is what we call the oriented martensite phase growth as a result of which macroscopic elastic deformation is accumulated. The degree of texturation or crystallographic orientation of the products of transformation and, consequently, the value of the reversible inelastic deformation increase, and the boundaries of the temperature hysteresis effect are reduced with the external load reaching a
critical value. Apart from that, the restoration of inelastic deformation at heating depends on a high degree of reversibility (thermal and crystallographic) of the thermoelastic martensite transition.
As different from form memory (FM), MRFM occurs many times over without any apparent changes in the cycle parameters. But the regularities of formation (value, sign, thermal stability) can vary depending on the mode of the cycle. The elementary carriers of internal stress, which provide for oriented generation of martensite crystals and their growth, are above all dislocations, their accumulations as well as sub-boundaries produced by deformation or thermocycling and forming some stable configurations, and also dispersed particles of the second phase. At the present time MRFM can be obtained by several methods which differ from one another by the kind of effects which generate oriented and highly reversible direct and reverse thermoelastic martensite transformation.
Of practical importance, apart from the deformation and temperature effects, are the load effects of generation and relaxation of stresses in thermoelastic martensite transformations. Finally, martensite transformations are accompanied by some exothermal (at direct transition) and endothermal (at reverse transition) effects. Accordingly, the growth of martensite crystals is accompanied by heat release, and their attenuation down to ultimate disappearance-by heat absorption and the cooling of alloys.
Now, what atomic-crystalline structure and microstructure do FM alloys possess at the initial high-temperature (austenite) and at marten-site states or stages?
As it turned out, most such alloys possess the atomic-orderly structure of austenite. Naturally enough, their microstructure can be different depending on the technology of synthesis-they can be mono-, poly- or nanocrystalline, and single or multiphase. Doping of FM alloys with different chemical elements provides for an oriented modification of the temperature, deformation and stress
parameters of the thermomechanical memory and also of other physical- mechanical properties.
The development and utilization of FM alloys in various branches of production is proceeding at a sufficiently rapid pace. The first patent on a temperature switch from an alloy on the basis of the system of Au-Ag-Cd was issued in the United States in 1960. The development of devices utilizing FM alloys was stepped up following the publication in 1965 of data on some record, and also unique in many respects, properties of titanium nickelide. A large number of patents have been issued already on FM alloys and products and devices developed on their basis.
So far, however, out of a large number of such materials only alloys on the basis of two systems of Ti-Ni and Cu-Zn-Al are applicable for practical uses. And whereas the former possess better properties, the application of the latter is prompted mainly by economic considerations. This being so, titanium nickelide alloys are used for the production of highly dependable devices with a long service life. In cases where no such rigid requirements exist, and the number of cycles is limited (such as security alarms which are triggered only in emergencies), one can use alloys of the Cu-Zn-Al type at lower cost. A concrete choice of alloys is dictated by a set of different requirements: design- related, functional, technological, aesthetic, economic, corrosion-preventive, biological and even clinical-biological ones (if the alloy is meant for medical uses).
The action of functional elements of alloys with thermomechanical memory can be unidirectional (FM) or bidirectional (MRFM), or what we call multiple.
In engineering applications alloys with memory are more frequently used as unidirectional elements. Thus
more than 100,000 joints of titanium nickelide with FM for unwelded hermetic sealings of hoses have been used in hydraulic systems of F-14 jets only in the 1970s-1980s. Apart from the high dependability, such joints are also remarkable for the absence of high-temperature heating and, if needed, can be easily dismounted at reduced temperatures. Sealings of this kind are used in aircrafts and also on submarines and surface ships, for repairs and assembly of piping for oil and gas, including pipelines on the bottom of the sea.
FM materials are used for the manufacture of plugs and catches for permanent connections (instead of bolts or rivets), when assembly operations on the reverse side of the assembled parts are difficult or simply impossible (say, in hollow sealed structures). All sorts of clamps and seals have been developed for the attachment of fixed parts and sections.
Parts of bidirectional action with form memory are used as connectors, temperature sensors, adjustment or activating devices. Their main advantages are small size and dependability.
Used successfully for several years now in Russia are fire alarm sensors with MRFM elements. Developed on the same basis have been spark plugs for cars, water pumps and air ventilators of the automobile industry. In recent years considerable attention has been given to the development of heat engines operating on the basis of low-heat sources of energy (hot water, steam). And there have been some well-known examples of using FM alloys in domestic appliances (air conditioners, dryers, thermostats, etc.).
High resistance to corrosion as well as biological compatibility, combined with a range of other unique properties of alloys based on FM titanium nickelide, provide for a really broad potential for their medical uses and applications.
The uses of metallic materials in this area are developing in several main directions. In some cases this is linked with prosthetic implants - elements and appliances which remain for a long time, or for life, in direct contact with biological tissues (bones, muscles, nerves or tendons). In other such cases- mainly in dentistry-the implants are in direct contact with biological surfaces (teeth, skin or mucous membrane). In all of such cases the quality requirements for implants are really high, and one should also take into account the possibility of their exposure to medicinal drugs, including utilization of container-implants when medicinal preparations are administered to the ailing organs directly. Another and no less important area is associated with the development of medical equipment and instruments for various applications. In such cases a direct contact with biological tissues is either of short duration or absent altogether.
Detailed and systematic studies of titanium-nickelide alloys have proved that they can perform some what we call long-term mechanical functions and that they also possess chemical (resistance to degradation in biological media, decomposition, dissolution and corrosion) and biological stability (biocompatibility, bioinertia, absence of toxicity or carcinogenic effects, and resistance to thrombosis and formation of antigens). In Russia FM alloys based on titanium nickelide have passed federal, clinical and technical tests and have been cleared for use in implants.
The main areas of medicine in which Ti-Ni FM alloys are used, or can be used, are the following: neuro-surgery, cardiovascular surgery, pulmonology, general surgery, gastroenterology, urology, proctology, gynecology, oncology, sexopathology, microsurgery, traumatology and field surgery, orthopedics and dentistry.
One can single out three classes of alloys used for medical applications depending on their purpose and the critical temperatures of direct and inverse martensite transformations and memory effects. In the late 1980s we developed for the first time surgical tools of titanium nickelide for endoscopic extraction of stones and alien bodies from hollow organs (ureters, bile ducts, etc.), their bougieurage (probing) and endoscopic electrosurgery. Such devices have been patented in Europe, the United States and Canada (and there are also a number of such patents in Russia) and have been awarded with Gold Medals at the World Show of Inventions in Brussels (1995). All were cleared for use by the Ministry of Public Health of the Russian Federation in 1993 and are now used on a large scale in this and many other countries.
Ti-Ni alloys with FM are also used in devices for osteosynthesis. The basic difference of new pins and locks from the traditional ones, intended only for rigid fixation of fractured bones, is this: they provide for a constant and uniform compression of fractures during the whole period of coalescence. We have also developed and have been successfully applying medical appliances for the healing of fractures and pseudo-arthritis of the neck of the femur-this grave and very common scourge, especially of old age (from 25 to 40 percent of all fractures).
It should also be stressed that implants (locks and fixatives, endoprostheses and suchlike devices) with FM for uses in traumatology make it possible to provide conditions for a comprehensive rehabilitation of the locomotor system of patients. They belong to a rapidly developing and promising area of what is called biological or low-invasive osteosynthesis.
FM materials are also actively used in maxillofacial surgery. An important factor in such cases is a very small size of the basically new portable and highly efficient medical devices and appliances for various applications.
According to our experience, the on-going cooperation of metals scientists, engineers and medical practitioners provides for a steady expansion and improvement of medical uses of FM materials discussed in this review.
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