The effect of martensite ageing in shape memory alloys is a serious problem in their applications. Although the origin for the effect is still controversial, one feasible origin is believed to be the reordering of martensites with ageing. If that is the case, the free energy or the enthalpy of transformation (ΔH) is expected to change with ageing. To test such a possibility, DSC measurements were carried out for the reverse transformation of the ζ'2 (trigonal) martensite in an Au-49.5 at% Cd alloy single crystal, and the change in ΔH with ageing was found for the first time.
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