Experimental and theoretical study of mechanical stabilization of martensite in Cu–Al–Ni single crystals

Abstract

A direct correlation between the hysteresis of the partial stress–strain ( σ– ɛ) cycles corresponding to the β → γ′ martensitic transformation of a 82.5%Cu–13.5%Al–4.0%Ni (wt.%) single crystal and the induced amount of mechanical stabilization (MS) determined by calorimetry has been established. The results show a good agreement with the coefficient d σ/d T typically obtained from stress-inducing the transformation at different temperatures. In addition, the experimental σ– ɛ curves have been quantitatively reproduced by a statistical model of superelasticity. In partial cycles, a unique probability distribution function (probabilities of martensite appearance/disappearance) can be used for a correct description of all the forward cycles while different distributions, which depend on the maximal strain values achieved during the loading branch, have to be considered for the reverse cycles. Conversely, no mechanical stabilization of martensite has been observed during the β → β′ transformation of 75.0%Cu–17.4%Zn–7.6%Al (wt.%) single crystal in tension mode and, in this case, the forward and reverse partial transformations have been successfully modelled using the same probability distribution function. Thus, the asymmetry in the forward and reverse probability distributions needed for the β ↔ γ′ transformation in Cu–Al–Ni explicitly manifests the immediate stabilization of the mechanically induced martensite.