The influence of long range order on the martensitic transformation in Cu–Zn

Abstract

Abstract The transformation between the α (fcc) and β (bcc) equilibrium phases at elevated temperatures is compared with that between the long range ordered B2 phase and the martensite at low temperatures, taking the Cu–Zn system as a prototype. The low temperature vibrational entropy difference, as measured from the martensitic transformation, is extrapolated to elevated temperatures at which disordered α and β are stable. This makes it possible to compare the corresponding enthalpy differences and deduce pair interchange energies, which depend only on pair distance but not on the crystal structure. This result is consistent with the observation that in many alloys based on the noble metals the enthalpy difference between the equilibrium structures is controlled mainly by the electron concentration e / a although the much larger mixing enthalpy of each of these structures depends strongly on the specific system. It also permits to evaluate the short range order (SRO) contribution at elevated temperatures using the cluster variation method. It is shown that the Gibbs free energies due to SRO are almost the same in the α and β phases, although they are non-negligible.