We have performed experimental and theoretical investigation of the anomalous form of the compression diagrams and shape memory restoration curves in Ni_49Fe_18Ga_27Co_6 alloy crystals deformed by uniaxial compression along the [011]_ A crystallographic direction ( A -austenite) in the temperature range of 200–350 K. It is found that in the investigated temperature range, all compression diagrams contain anomalous segments of smooth and sharp decrease in deforming stresses. It is shown that the segments of a smooth decrease in stress are associated with peculiarities in martensite reaction L1_2 → 14M, while segments of a sharp drop are due to instability of martensite reactions 14M → L1_0 and L1_2 → L1_0. A possible source of reaction instability is associated with interfacial stresses at the interfaces between the martensite and austenite phases (lamellas) due to different elastic moduli of contacting phases. The magnitude of these stresses is significant in the case of 14M → L1_0 and L1_2 → L1_0 transformations, which induces a sharp drop of the deforming stress, while the restoration of the shape memory effect is of a burst nature. It is established that the contribution of interfacial stresses to the free energy of martensite transformation is smaller than the dissipative (entropy) contribution to this energy; however, interfacial stresses higher than a certain threshold strongly affect transformation kinetics and, hence, determine the strongly anomalous shape of pseudoelastic deformation curves and burst restoration of the shape memory effect.
Abstract. The paper is a presentation of two new phenomena which have been recently revealed and investigated by authors in well-known shape memory alloys such as Cu-Al-Ni and Ni-Fe-Ga. The first is a strong decrease of the martensitic transfornation heat evolved (absorbed) in single crystals of Cu-13.5 wt. % Al - 4.0 wt. % Ni alloy in the course of their in-situ calorimetric study under constrained shape memory deformation conditions. The second is a burst character of emerg-ing shape memory strain in the course of heating single crystals of Ni49Fe27Ga18CoSubscript text6 ferromagnetic alloy. The crystals were preliminary compressed along [110] crystallographic axis for the purpose of giving them a shape memory deformation. As a result of burst martensitic transition, the monocrystalline samples standing on a hard base and loaded additional weight of 400 g threw up this weight on 66 mm height .
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