Stability of martensite in noble metal alloys

The stability of the martensite is controlled by the electron concentration and by the electronegativity, which is described in terms of pair interchange energies. Although the contribution of the conduction electrons determines the difference in stability between the disordered equilibrium face centred cubic and body centred cubic phases at elevated temperatures, they are only partly responsible for the enthalpy difference between the β phase matrix and the martensite. For a quantitative evaluation of the martensitic transformation it is therefore necessary to have also information on the differences in electronegativity, which control a large fraction of the enthalpy of mixing of the disordered phases, and contribute to the changes from long-range ordering. Long-range ordering takes place generally at lower temperatures in the β phase matrix, and is inherited in the martensite after the diffusionless transformation. The aim of this paper is to derive pair interchange energies that permit to describe quantitatively the martensitic transformation in noble metal alloys. As their prototypes the binary Cu–Zn, Cu–Al and ternary Cu–Zn–Al alloys are analysed in detail, because most information is available for them. It will be shown that for the quantitative description of the enthalpy of formation of the martensite and of the equilibrium phases, pair interchange energies are required which include important contributions also from more distant than first and second nearest neighbour pairs. However, the difference in enthalpy of formation between the martensite and the high temperature β phase can be well described by composition independent nearest and next nearest neighbour pair interchange energies alone.