Entropy Change Caused by Martensitic Transformations of Ferromagnetic Shape Memory Alloys

摘要

In this paper, our most recent findings on the influence of magnetic order on the main transformational caloric and elastic properties of shape memory alloys (SMAs) are reviewed. It is argued that ferromagnetic order has a strong influence on the temperature interval of martensitic transformation (MT), the characteristics of stress-induced MT, and the shear elastic modulus of SMA. The problem of separation of the magnetic contributions to the entropy change ???? S and heat Q exchanged in the course of martensitic transformation (MT) of SMA is considered in general terms, and theoretical formulas enabling the solution of the problem are presented. As an example, the ???? S and Q values, which were experimentally determined for Ni-Mn-Ga and Ni-Fe-Ga alloys with different Curie temperatures T C and MT temperatures T M , are theoretically analyzed. It is shown that for Ni-Mn-Ga martensites with T M < T C , the ratio of elastic and magnetic contributions to the entropy change may be greater or smaller than unity, depending on the temperature difference T C ?￠???? T M .