On the stability of the martensitic phases in Cu–Zn–Al, and its relationship with the equilibrium phases

The relative stability of the martensitic phases in Cu–Zn–Al shape memory alloys was used to evaluate the high-temperature equilibrium between the face-centered cubic and disordered hexagonal phases in a set of binary Hume-Rothery compounds. The equilibrium was found to be around an electron concentration of 1.44 in agreement with the available experimental binary phase diagrams. In the calculation, the contribution arising from the distortion of the martensitic structure due to long-range order was considered. It was additionally rationalized that, although the 18R phase is a common martensitic structure, it is not observed in the disordered state.