ON RELATIONS BETWEEN THE TRANSFORMATION TEMPERATURES, STRESSES, PRESSURES AND MAGNETIC FIELDS IN SHAPE MEMORY ALLOYS

摘要

It is shown that in the austenite to martensite transformation of shape memory alloys the transformation strain, epsilon~(tr), and the change in magnetization, DELTA M, usually has definite field dependence while the change in entropy or volume can be considered constant. Under uniaxial loading the field dependence of epsilon~(tr) and DELTA M is related to the change of the variant/domain distribution with increasing field parameters. For the description of this, the volume fraction of the stress induced (single) variant martensite structure, eta, is used. Expressions for the elastic and dissipative energy contributions to the start and finish temperatures and stresses as well as for relationships between these terms obtained. The stress dependence of the elastic and dissipative energy contributions is correlated with the eta dependence of epsilon~(tr). It is also shown that the well known linear relation between the temperature and the critical stress (magnetic field) necessary to induce martensite formation is simply and plausibly obtained from our formalism. This form of the Clausius-Clapeyron relation is more linear as compared to the one giving the field dependence of the equilibrium transformation temperature, because in the latter the field (or eta) dependence of epsilon~(tr) or DELTA M directly appears, while in the former one e.g. only the saturation value of epsilon~(tr) has to be used which corresponds to a well defined single variant martensite structure.