![]() on the so called temperature memory effect (TME), which is characterized by a delay of the thermal reverse MT, which takes place at a higher temperature, after some previous uncompleted heating cycles covering only partially the transformation. On the other hand, several works were published in the last years 9–14 9. Then, the martensite acquires an improved stability and the stress-induced reverse transformation only happens for very low values during the withdrawal of the stress. which are partially attributed to the release of the elastic energy, stored at the growing front of martensites, on the surface of micro and nano pillars. On the one hand, some size effects were recently reported on the superelastic behavior of SMA at a small scale, 6–8 6. Indeed, we will shortly describe two effects, apparently associated with this phenomenon, which were reported in the literature. If such evolution occurs, it would constitute an important issue for the reliability of SMA behavior, because the thermal shape memory effect and the recovery of the superelastic effect could be compromised due to the stabilization of the martensite. The important point is that the elastic energy stored during the forward MT on cooling constitutes the driving force for the reverse MT on heating, and in the case that such stored elastic energy would be released, the transformation will be accordingly delayed to a higher temperature range. In the absence of any plastic deformation or dissipative processes, the creation of the martensite interfaces and the stored elastic energy are responsible for the hysteresis associated with the MT, as well as for the broadening of the transformation temperature range. In spite of that, these self-accommodating groups of martensite are not free of local stresses, which are elastically stored in the lattice of both phases and finally, at the end of the MT, in the martensite lattice. However, in SMA the MT is thermoelastic, which means that the stresses are accommodated through the formation of multiple martensite domains (or variants) promoting local reversible elastic strains, to minimize the increase of the local stresses and prevent plastic deformation. In classical non-thermoelastic MT, these stresses produce intensive plastic deformation around the interfaces and the MT becomes irreversible like in the case of many steels. Although the main strain associated with the MT is a shearing, it could involve shuffling, distortions, and expansions of the parent phase lattice and consequently important stresses appear in between the two lattices of austenite and martensite. Wayman, Engineering Aspects of Shape Memory Alloys ( Books on Demand, 1990). Nishiyama, Martensitic Transformation ( Elsevier, 2012). Wayman, Shape Memory Materials ( Cambridge University Press, 1999). Shape memory alloys (SMAs) are functional materials characterized by their specific properties of shape memory and superelastic effects, which are based on a reversible first order diffusionless structural phase transition, called martensitic transformation (MT), taking place between the high temperature phase, austenite, and the low temperature phase, martensite, via an atomic lattice shearing responsible for the change of shape. The neutron experiments have allowed a complete description of the strains during martensitic transformation, and the obtained conclusions can be extrapolated to other SMA systems. In addition, the thermal expansion coefficients of both martensite and austenite phases were measured. The observed effects and the measured strain relaxations are in agreement with the predictions of the model proposed to explain this behavior in previous calorimetric studies. The evolution of the stresses is measured through the strain relaxation, which is accessible by neutron diffraction. These changes are associated with the relaxation of the mechanical stresses elastically stored around the martensitic variants, due to the different self-accommodating conditions after uncompleted transformations. Two different effects are observed, the d-spacing position shift and the narrowing of various diffraction peaks, along uncompleted transformation cycles during the thermal reverse martensitic transformation. A careful study of the influence of partial cycling on the neutron diffraction spectra in the martensitic phase is presented. This work is focused on the analysis of the strain evolution along the temperature memory effect appearing in these alloys after partial thermal transformations. In situ neutron diffraction is used to study the strain relaxation on a single crystal and other powdered Cu-Al-Ni shape memory alloys (SMAs) around martensitic transformation temperatures. ![]()
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