Recent development of shape memory alloys is reviewed, emphasis being placed on theTi–Ni, Cu–based and ferrous alloys which are considered as practical materials for applications among many shape memory alloys. Crystal structures of the parent and martensitic phases are described, and the crystallography of the martensitic transformations is also briefly explained. The origins of the shape memory effect and the shape memory mechanisms are discussed on the basis of the crystal structure and the crystallography of the martensitic transformations. Since an applied stress also induces the martensitic transformations, successive stages of the martensitic transformations are reviewed briefly in Cu–based and Ti–Ni alloys, which show martensite-to-martensite transformations upon loading. Then, characterization of the shape memory alloys are reviewed in detail; i.e., phase diagrams, transformation temperatures, transformation process, stress-induced transformation, aging effects, cycling effects, fracture, fatigue, grain refinement, and so on.

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Grain refinement effect in Ti-doped Cu–Al–Ni shape memory alloys has been studied using microscopic and microanalytical methods. Titanium is found to decrease as-cast grain size and reduce grain coarsening rate during betatization/homogenization heating but does not affect much on large grain refinement during hot working. A eutectic phase separation takes place during casting and Ti-rich eutectic equilibrates on subsequent heating to produce homogeneously distributed μm-order X_L particles at more than 0.38 mass% Ti additions, whereas two-orders-of-magnitude smaller X_S phase tends to precipitate in the β phase which has less than 0.05 mass% solid solubility of Ti.
The grain refinement upon casting is considered to originate from the constitutional supercooling effect and the reduced diffusion rate by presence of Ti. However, the ultimate refinement effect depends significantly on the pinning effect of X_S which suppresses the growth of β grains.

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Fine grained Cu–Al–Ni–Ti alloys have been subjected to post quench ageing tretments in the β-phase. The resulting changes in transformation characteristics, particularly As, and the physical and mechanical properties of martensite have been studied. The As shifts are discussed in terms of quenched-in-short range disorder and the subsequent recovery on ageing of first and second neighbour order. The role of order and defects, particularly vacancies, is also discussed in relation to the observed changes in the physical and mechanical properties of martensite. It is also shown that optimising the ageing treatment can lead to the desired properties.

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The effect of heat treatments on martensitic transformations in monocrystalline Cu–14.1Al–4.2Ni (mass%) (alloy 1) and Cu–14.7Al–4.2Ni (mass%) (alloy 2) shape memory alloys has been investigated by means of differential scanning calorimetry measurements, optical microscopy and tensile tests of martensitic specimens. As a result, it was found in alloy 1 that not only transformation temperatures shifted but also thermally formed martensite changed from β₁' to γ₁' with heat treatment. Meanwhile, in alloy 2, the γ₁' martensite was thermally formed regardless of heat treatment. To clarify the reason for the change of thermally formed martensite, equilibrium temperatures, To, for the β₁_??_γ₁' and β₁_??_β₁' transformations and their variation within heat treatment were estimated by utilizing previous data on stress-induced martensitic transformations and associated pseudoelasticity of the alloys (Trans. Jpn. Inst. Met., 28 (1987), 264). Consequently, by taking into consideration the variation of To temperatures and, in addition, supercooling neccessary for the nucleation of the γ₁' and β₁' martensites, the origin was successfully elucidated in terms of a simple scheme that the martensite phase with the highest M_S temperature changed from the β₁' to the γ₁' with heat treatment. Applicability of this scheme was discussed on similar phenomena occurred in other alloy systems in which martensite phases changed with chemical composition.

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Morphological characteristics of the orthorhombic martensite in a Ti–Ni–Cu shape memory alloy have been investigated and the following have been found:
(1) In the cubic (B2) to orthorhombic transformation of the Ti–Ni–Cu alloy, no lattice invariant shear is needed. This is due to the fact that one of the principal distortion of transformation is nearly zero.
(2) The experimentally obtained habit planes are {334} of the parent lattice and agree very well with the prediction by the phenomenological theory of martensite.
(3) Self-accommodation in the orthorhombic martensite is achieved by combination of three variants around each one of the ‹111› axes of the parent latice.

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The stepwise character of the thermoelastic martensitic transformation in NiTi alloys is investigated in terms of the enthalpy changes connected with the transfomation. Differential scanning calorimetry has ben adopted to evidence the discontinuous heat flow developed during both the direct and reverse martensitic transformation. The results are examined at the light of a simplified fractal model advanced to describe the self-similarity of martensitic microstructures.

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Measurements of the effect of heat treatment following cold work on the yield and shape memory behaviour in a NiTi alloy are reported. Tensile behaviour is found to depend on test temperature and initial structure as well as annealing temperature. Minimum values of stress required for stress induced martensite, martensite reorientation and parent phase yield were found to be associated with the as-recrystallized structure. Measurements of reversion and permanent strains were found to depend on applied stress.

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The martensitic transformation and the R-phase transition were observed successively upon cooling and heating both in solution-treated and age-treated Ti–Ni specimens, and they wre correlated with the electrical resistance vs. temperature curves for both cases. Specific self-accommodating morphologies were observed both for the R-phase and martensite variants in an age-treated specimen, while no specific self-accommodating morphology was observed for martensite variants in a solution-treated specimen, the reason for the difference being discussed. The interaction between the R-phase and martensite variants was also observed to affect the morphology of the R-phase variants upon heating.

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The crystal structures, internal structure of the martensitic phases and the effect of chromium addition on the martensitic transformations on the TiPd alloys were investigated by elecrical resistivity measurements and electron microscope observation. It was found that the chromium addition is very effective in reducing the Ms temperature from about 800 K for the stoichiometric TiPd alloy to liquid nitrogen temperature with increasing chromium content, in both the alloys Ti₅₀Pd_50-x Cr_x(x=2 to 10 at%) and Ti₅₄Pd_46-x Cr_x(x=2 to 6 at%). In the chromium-added alloys, the 9R martensite phase and a peculiar intermediate incommensurate phase appear besides the twinned B19(2H) martensite, like in the case of the TiPd–Fe alloys. The internal structure of the 9R martensite was found to be {114}_9R twins which corresponds to {128}_18R type twin of the 18R martensite phase of the Cu–Al alloys. The features of the incommensurate phase, i.e., large incommensurability of superlattice reflections, its dependence on the alloy composition, configuration of variants are quite the same as those in the TiPd–Fe alloys.

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Behavior of martensitic transformation and shape memory effect in an Fe–31%Ni–10%Co–3% alloy were studied. The specimen which was ausaged at 873 K for 3.6 ks produces a thin plate martensite (Ms=193 K) with a large thermal hysteresis (i.e., As – Ms=150 K, Af – Ms=315 K). Despite the large thermal hysteresis, the forward and reverse transformations of thin plate martensite occur by a reversible movement of interface in the same manner as thermoelastic martensite. The ausaged specimen which is fully austenitic at room temperature shows a complete shape memory effect when the specimen is heated up above Af after the deformation at temperatures near or below Ms. The ausaged specimen hardly exhibits the shape memory effect when deformed at room temperature. However, when the ausaged specimen is once subzero cooled to liquid nitrogen to produce a large amount of martensite (about 70%), the almost complete shape memory effect occurs even by the deformation at room temperature. Furthermore, the subzero cooled specimen shows the partial shape memory effect when the specimen is cooled below Ms after the deformation at temperature around As. The deformation mode of specimen at various temperatures was investigated by microstructural observations and the origin of the shape memory effect in the present alloy was discussed.

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