ENTROPY CHANGE BETWEEN THE BETA-PHASE AND THE MARTENSITE IN CU-BASED SHAPE-MEMORY ALLOYS

A study of the entropy change DELTAS between the beta phase and the ma rtensite in Cu-based shape-memory alloys is presented. The critical re solved shear stress as a function of the temperature [tau(T)] was dete rmined by compression experiments. This allowed the calculation of DEL TAS by means of a Clausius-Clapeyron type relationship. The results ob tained were compared with those resulting from tension tests and calor imetric determinations taken from the literature. A good agreement was found between the three methods for the wide range of compositions av ailable. Moreover, a correlation seems to exist between the experiment ally determined DELTAS and the corresponding electron concentration e/ a. The entropy change can be expressed in terms of the specific heats, using the models of Debye and Einstein. However, by relating the char acteristic temperatures (Debye or Einstein) to the elastic constants ( C' and C44) of the beta phase at the transformation temperature, it is found that the dependence of DELTAS on e/a cannot be explained. It is shown that a relation between the phonon frequency at the Brillouin z one and C' with composition must be considered.