Phase Stability of Martensitic Structures

Abstract
In the first part of the paper the relative stabilities of the different martensitic phases, mainly in Cu-Zn-Al, are compared. It is shown that the differences in enthalpy of formation between the fcc type 3R or 6R and the 9R, 18R or 2H structures are above all due to a lattice distortion which reduces the energy per stacking fault, considering 9R, 18R and 2H as a basic face centered lattice into which faults are introduced on each third or second plane. In the second part the factors are discussed which are important in the relative stabilities not only between the matrix and martensite phases, but also for the equilibrium phases in the noble metal alloys. These are : i) the vibrational entropy difference ΔS, which is a function mainly of electron concentration e/a. It depends little, if at all, on the special alloy system, on the degree of long range order, or on temperature ; ii) the stability of the average periodic lattice which is a smooth function of e/a, without any indication of nesting effects at intermediate e/a ; iii) the pair interchange energies which depend strongly on the alloy system. They are a function of the pair distance but otherwise are structure independent. They determine most of the enthalpy of formation and simultaneously account for the long range order contribution.