Relation between martensitic transformation temperature and degree of long range order in the β phase of Ag-Zn-Al alloy

The relation between the martensitic transformation temperature and long range order in the beta phase of an Ag-Zn-Al alloy was investigated mainly by means of electrical resistivity measurements. In this alloy, the disordered state (of the bcc structure) can be brought about down to room temperature by quenching from a temperature a little above the critical temperature (about 340 K). By aging at a temperature around or below room temperature, ordering of atomic arrangements proceeds to result in the C11(b) structure. It was observed that the resistivity of specimens lowers in accordance with this ordering. It was also found that the martensitic transformation temperature, at which the crystallographic structure changes to the SR-type structure, lowers markedly with ordering. This indicates that ordering affects the relative stability between the matrix phase and the martensitic phase. A good linear relation was obtained between the decrease in transformation temperature (Delta T) and the change in resistivity. By making use of the maximum value of resistivity change as a reference, the resistivity change was related to the degree of long range order (phi). Then, the relation Delta T = -1100 phi(2) [K], was obtained. A themodynamical consideration, based on the Bragg-Williams-Gorsky approximation, has been given for the effect of ordering on the transformation. The theoretical results agree fairly well with the experimental relation.