Massive and martensitic transformations in the Ag−Al system

The Ag?Al system in the composition range of stability of the high temperature β phase exhibits two-phase field characteristics which make it an ideal system for examining massive phase transformations. Experiments have established that the β phase can be retained to room temperature following cooling rates of the order of 10⁵ deg per sec. Subsequently, the M_s temperatures have been determined for the composition range Ag-24.4 to 25.0 at. pct Al. Since the ζ phase can also be retained, it has been possible to examine the β→ζ _m , β→μ _m and ζ→μ _m 3composition-invariant, nonmartensitic phase transformation at a single composition. In addition, the two high-temperature phases, β and ζ, are associated with a congruent point at Ag-24.5 at. pct Al, the width of the two-phase field increasing at higher and lower aluminum concentrations. This has permitted an investigation of the morphology of the ζ _m -product for a range of compositions and cooling rates, the latter determining the extent of undercooling at which the reaction occurs. In particular, that morphology representative of a given degree of supercooling has been compared for several compositions and the interdependence of cooling rate and parent/product coherency (as revealed by the shape of the resulting ζ _m -phase) has been examined.