On the stability of martensitic and equilibrium phases in the noble metal alloys.

The transformation between the long range ordered (LRO) parent phase and the martensite differs from that between the disordered fee alpha phase solid solution and the intermediate bcc beta equilibrium phases at elevated temperatures by the LRO contribution. It is shown that the LRO, which is described in terms of pair interchange energies, can be determined from the experiments. It thus has become possible to derive the enthalpy difference between the equilibrium phases, which hitherto could not be done with the required accuracy. From the entropy difference for the martensitic transformation, the entropy difference at elevated temperatures for the equilibrium phases has been deduced. From it and the temperature at which both phases have the same Gibbs free energy, the enthalpy difference has also been determined. Both methods and the results from ab initio calculations show an excellent agreement among each other. On the basis of the present results it can be concluded that the disordered bcc beta phase in Cu-Zn and in other noble metal alloys is less stable energetically than the fee alpha phase, but that its presence at elevated temperatures as an equilibrium phase is due only to the vibrational entropy from the large amplitude soft phonon modes. Their collapse on lowering the temperature leads to the formation of the martensitic phases, when diffusion controlled decompositions are suppressed.