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

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 (ε and α, 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 (γε=(εh-εc)/εc as well as γα=(αh-αc)/αc)) are either positive or negative. For positive γ 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.