On the asymmetry of the forward and reverse martensitic transformations in shape memory alloys

TL;DR

This paper investigates the asymmetry in phase transformation behaviors of shape memory alloys during cooling and heating, analyzing calorimetric and acoustic emission data to understand underlying relaxation mechanisms and their dependence on external magnetic fields.

Contribution

It introduces a classification of asymmetry in acoustic emissions during martensitic transformations based on empirical correlations and relaxation energy differences.

Findings

Asymmetry in acoustic emissions can be positive or negative.

Relaxation of elastic strain energy differs between cooling and heating.

Magnetic emission noises show similar asymmetry influenced by magnetic fields.

Abstract

Differential Scanning Calorimetric, DSC, runs taken during martensitic phase transformations in shape memory alloys, often look differently during cooling and heating. Similar asymmetry is observed e.g. for the numbers of hits or the critical exponents of energy and amplitude distributions (\epsilon and \alpha, respectively) in acoustic emission measurements. It is illustrated that, in accordance with empirical correlations, the above asymmetry of acoustic noises can be classified into two groups: the relative changes of the exponents during cooling and heating (\gamma \epsilon=(\epsilon h-\epsilon c)/\epsilon c as well as \gamma \alpha=(\alpha h-\alpha c)/\alpha c)) are either positive or negative. For positive \gamma values the number of hits and the total energy of acoustic emission are larger for cooling, and the situation is just the reverse for negative asymmetry. Our interpretation is based on the different ways of relaxation of the elastic strain energy during cooling as well as heating. It is illustrated that if the relaxed fraction of the total elastic strain energy (which would be stored without relaxations) during cooling is larger than the corresponding relaxed fraction during heating, then the asymmetry is positive. Magnetic emission noises, accompanied with martensitic phase transformations in ferromagnetic alloys, show similar asymmetry than those observed for thermal (DSC) and acoustic noises and depends on the constant external magnetic field too.