On the thermodynamic analysis of martensite stabilization treatments

Abstract

It is shown that the broadenings of the forward and reverse martensitic transformations are proportional to the second derivatives of the elastic energy by the transformed martensite volume fraction. The shifts of the transitions, beside the effect of chemical contribution to the equilibrium transformation temperature, To, can depend on the change of the elastic and dissipative energies as well. The effect of mechanical stabilization (by making the surfaces roughened) in Ni49Fe18Ga27Co6 single crystal, where the transformation too place by single interface motion, can be well treated in thermoelastic approximation. In Ni50Mn28.5Ga21.5 single crystal (where the usual multivariant martensite structure developed) not only the dissipative energy, but the slope of the second derivative of the elastic energy for cooling has also been altered after mechanical stabilization and there can exist an elastic energy contribution to the width of the hysteresis too. Symmetry conforming short range ordering under uniaxial stress (SIM-aging) in Ni53Mn25Ga22 and Ni51Fe18Ga27Co4 single crystals caused an asymmetric change in the second derivatives of the elastic energy: the sharpening of the corresponding branches of the hysteresis was stronger for heating than cooling. The thermoelastic balance condition did not fulfill for the SIM-aged samples and the elastic energy contributions to the shift of the transformation temperatures could even compensate the increase of To (caused by the stabilization by symmetry conforming short range ordering). It is also illustrated that the dissipated energy per one cycle could decrease even if the area of the hysteresis has been increased.