Irreversibility in two-stage martensitic transformations of Cu-Al-Ni and Cu-Zn-Al-Mn alloys

The thermoelastic martensitic transformation undergone in shape-memory alloys is considered to be thermodynamically reversible. However, the existence of irreversible heat dissipated is made evident in the special case of several Cu-based alloys which display a complex transformation from b.c.c. to a mixture of 18R and 2H martensites. The simultaneous growth of both the martensites is suggested as the main responsibility for the irreversible behaviour. In this sense, as a first approach, the entropy production associated to the irreversible heat is related to the degree of coexistence, and shows a maximum for a structure consisting of 50% 18R and 50% 2H.status: publishe