Shape memory effect in TiNi single crystals with thermal-induced martensite transformation or strain glass transition

Abstract

In present work, we investigated the thermoelastic B2-B19' martensitic transformations in thermocycles under constant stress, σapp, (ε(Т) curves) and in loading/unloading cycles (σ(ε) curves) in quenched [001]-oriented single crystals of Ti-50.7 at%Ni and Ti-51.7 at%Ni alloys. Single crystals undergo either thermal-induced В2-В19' martensitic transformation (Ti-50.7 at%Ni) or strain glass transition (Ti-51.7 at%Ni), depending on their chemical composition. The transformation type defines the morphology of a martensite during cooling under stress. In Ti-50.7 at%Ni alloys during stress-assisted cooling the morphology is the mix of self-accommodation martensite and oriented martensite. In opposite, in Ti-51.7 at%Ni alloys the morphology consists of oriented martensite only. The following parameters in stress-assisted cooling/heating cycles are determined by transformation type and morphology of a martensite: the minimal stress, σmin, for oriented martensite formation; the coefficient dεfor/dσ that defines the strain dependence on stress, εfor(σapp), and the presence of additional stage in the dependence Msσ(σapp). Similar in loading/unloading cycles the transformation type defines the energy barrier that should be overcome in order to start the stress-induced martensitic transformation, and, so, the stress level, σcr(Ms0).