PHASE-STABILITY OF MARTENSITIC STRUCTURES

In the first part of the paper the relative stabilities of the differe nt martensitic phases, mainly in Cu-Zn-Al, are compared. It is shown t hat the differences in enthalpy of formation between the fee type 3R o r 6R and the 9R, 18R or 2H structures are above all due to a lattice d istortion which reduces the energy per stacking fault, considering 9R, 18R and 2H as a basic face centered lattice into which faults are int roduced 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 equilibriu m phases in the noble metal alloys. These are: i) the vibrational entr opy difference Delta S, which is a functi sn mainly of electron concen tration e/a. It depends little, if at all, on the special alloy system , on the deg-ree of long range order, or on temperature; ii) the stabi lity of the average periodic lattice which is a smooth function of e/a ,without any indication of nesting effects at intermediate e/a; iii) t he pair interchange energies which depend strongl:y on the alloy syste m. They are a function of the pair distance but otherwise are structur e independent. They determine most of the enthalpy of formation and si multaneously account for the long range order contribution.