Tensile stress-strain behavior of Ni 57.5 Mn 22.5 Ga 20.0 single crystal exhibiting a hightemperature 2M-martensitic phase stable up to 360ºC has been studied in the course of thermal and mechanical cycling. The ultra-large reversible strains, about 9%, caused by the shape memory and superelasticity effects, have been observed up to 400 ºC being the instrumental temperature limit. Abnormally large two-way shape memory effect with 9% of strain magnitude has been found. The cycling procedure and the variation of thermal/mechanical routs of the training of samples revealed the destabilization (rejuvenation) of martensite. This physical effect is opposite to the well-known phenomenon of martensite stabilization. A destabilization effect is explained phenomenologically in terms of internal stressing of the alloy sample by the crystal defects.Keywords: Ni-Mn-Ga single crystal, high-temperature shape memory and superelasticity, two-way shape memory effect, destabilization of martensite.2
Abstract. The Landau theory has been developed for the description of martensite aging. The characteristic features of the theory are: i) the multicomponent non-scalar character of the order parameter describing the slow reconfiguration of lattice defects after martensitic transformation (MT); ii) the complete agreement with Symmetry-Conforming Short-Range-Order principle formulated by X. Ren and K. Otsuka; iii) the applicability to the different MT-s and various defects related to aging phenomena. The physical values interpreted as the components of internal stress, which stabilizes certain variant of martensitic phase, have been composed of the components of slow non-scalar order parameter. An applicability of the developed theory to the description of influence of aging on the MT temperature and yield stress was demonstrated.
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