TRANSFORMATION RELAXATION AND AGING IN A CUZNAL SHAPE-MEMORY ALLOY STUDIED BY MODULATED DIFFERENTIAL SCANNING CALORIMETRY

The reverse martensitic transformation and aging processes in a polycr ystalline Cu-23.52 at. pet Zn-9.65 at, pet Al shape-memory alloy have been studied using the recently developed modulated differential scann ing calorimetry (MDSC) technique, and some new findings are obtained. By separating the nonreversing heat flow from the reversing heat flow, MDSC can better characterize the thermodynamic, kinetic, and hysteret ic features of thermoelastic martensitic transformations. Two kinds of exothermal relaxation peaks have been identified and separated from t he endothermal reverse martensitic transformation: one is associated w ith the movement of twin interfaces or martensite-parent interfaces, a nd another is due to the atomic reordering in the parent phase via a v acancy mechanism. The martensite aging processes have been examined, a nd two stages of the aging process been distinguished: the first stage of aging is characterized by the stabilization of martensite, as mani fested in the increase in the reversing enthalpy of the reverse marten sitic transformation and in the transformation temperatures, and the s econd stage is, in fact, the decomposition of the martensite on prolon ged aging, accompanied by a decrease in the transformation enthalpy. T he results suggest that the mechanisms of the relaxation in the marten site and in the parent phase may be quite different.