The effect of thermal cycling on the thermoelastic martensitic transformation in a Cu-Zn-Al alloy

The effect of thermal cycling between the M_fA_f on the thermal hysteresis, temperature dependence, and the morphology of the martensitic transformation in a Cu-Zn-AI alloy was studied. The hysteresis behavior of the cyclic transformation, as characterized by following the change in electrical resistivity accompanying the transformation changed in two respects: First, the M_s and A_f temperature shifted to higher temperatures while the M_f and A_s temperatures remained constant. Second, the amount of martensite undergoing cyclic transformation decreased. These changes appeared to saturate at 300 cycles and could be partly recovered during room temperature aging. The morphology and mode of transformation also changed. During the first few cycles, transformation occurred in two simultaneous ways: (1) as martensite plates which are observed to grow and thicken in a continuous manner and (2) as plates which form in a “burst” process. After 60 cycles no “burst” type transformation occurred during cooling and plate growth and thickening was stepwise. Formerly Visiting Scholar, Department of Materials Engineering, Rensselaer Polytechnic Institute, Troy, NY