Observations concerning the β→α_m massive transformation in Cu-Zn alloys

Abstract

The occurrence and progress of the massive transformation in copper-rich Cu-Zn alloys, have been studied during quenching and heating by metallographic and electron-microscopic techniques. The transformation occurred predominantly at compositions within the single-phase α field in samples containing a diffusion gradient, and in two-phase samples containing both the α and β phases. α_m particles produced during quenching from the β-phase field have been found to grow massively following a rapid reheating into the α-phase field. Equilibrium α, or Widmanstätten α particles, have not been observed to continue by massive growth, nor did they act as preferred sites for nucleation of the massive phase. These observations are consistent with a growth model suggested recently by Karlyn, Cahn, and Cohen.³ However, in samples rapidly quenched from the β phase, the onset of the massive transformation occurred in the two-phase field some 10° to 20°C above the equilibrium a-phase boundary. This observation is not consistent with a model that requires the predominant growth of the massive phase to occur within the α single-phase field, and includes delay times that increase on approaching the α/(α + β) phase boundary. No low index orientation relationship was found to exist across the faceted interfaces between the massive and the retained β(β′) matrix. In addition, no regular interfacial dislocation arrays were evident at these boundaries. These observations add further support to the notion that the advancing interface is structurally comparable to a high-angle, incoherent boundary.