The β to ζ massive transformation in Ag-Al alloys

Abstract

The progress of a single β ζ m reaction interface has been studied as a function of undercooling and cooling rate during the β to ζ m massive transformation in Ag-24.5 at.% Al and Ag-25.5 at.% Al alloys. Measurements of the interface velocity during growth of single crystals have demonstrated that the growth rate increases with an increasing degree of undercooling below the equilibrium transformation temperature and reaches a value of 1.2 cm/sec at an undercooling of 18°C. These results are discussed in terms of the manner of interface migration as related to the magnitude of the driving free energy. With small driving free energy, growth of the ζ m phase appears to agree with a lateral ledge growth model involving two-dimensional nucleation of ledges. At large values of driving free energy a continuous growth model accounts reasonably well for the observed behavior. The transformation kinetics of the b.c.c. β phase under isothermal conditions indicate that there is a delay time prior to the initiation of the β → ζ m reaction. The isothermal delay time decreases as the degree of undercooling increases and appears to be associated with the nucleation of the ζ m phase. It is suggested that nucleation is the rate controlling step in the present β → ζ m reaction and that this situation may prevail in all massive transformations that occur under both isothermal and continuous cooling conditions.