RELATION BETWEEN MARTENSITIC-TRANSFORMATION TEMPERATURE AND DEGREE OFLONG-RANGE ORDER IN THE BETA-PHASE OF AG-ZN-AL ALLOY

The relation between the martensitic transformation temperature and lo ng 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 dow n to room temperature by quenching from a temperature a little above t he critical temperature (about 340 K). By aging at a temperature aroun d or below room temperature, ordering of atomic arrangements proceeds to result in the C11(b) structure. It was observed that the resistivit y of specimens lowers in accordance with this ordering. It was also fo und that the martensitic transformation temperature, at which the crys tallographic structure changes to the SR-type structure, lowers marked ly with ordering. This indicates that ordering affects the relative st ability between the matrix phase and the martensitic phase. A good lin ear relation was obtained between the decrease in transformation tempe rature (Delta T) and the change in resistivity. By making use of the m aximum value of resistivity change as a reference, the resistivity cha nge was related to the degree of long range order (phi). Then, the rel ation Delta T = -1100 phi(2) [K], was obtained. A themodynamical consi deration, based on the Bragg-Williams-Gorsky approximation, has been g iven for the effect of ordering on the transformation. The theoretical results agree fairly well with the experimental relation.