Many of today’s shape memory systems are based on the Ni-Ti alloy used near to its equiatomic composition. However, regular Ni-Ti-based systems operate with a substantial hysteresis which can for some applications be a hinderance. In the present lecture we will present results on different ternary system Ni_50-xTi₅₀X_x (X = Pd, Pt, Au) in which different amounts of ternary compound substitution on Ni positions lead to special ratios between the austenite and martensite lattice parameters. The applied alloying elements and conditions for the low hysteresis were obtained based on the non-linear crystallographic theory of Ball and James. As a result of the compatibility between austenite and martensite, the hysteresis of the transformation becomes very narrow and the amount of microtwinning, necessary to yield an invariant plane strain, decreases drastically. Also, when changing the amount of alloying element, the type of stable microtwinning changes from Type I & II into compound. Detwinning and rearrangement of the martensite plates lead to a radically different microstructure which can account for the decrease of hysteresis. Still, practical samples appear to be rather inhomogeneous containing large precipitates disturbing local compatibility conditions.

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