Interfacial structure and crystallographic studies of transformations in β and β' Cu-Zn alloys - II. Martensitic formation of α₁ plates in β'

Abstract

A TEM study was made of the structure of the broad faces and edges of α₁ plates formed in β' Cu-Zn alloys. Thickening kinetics of α₁ plates were also measured. α₁ plates were found to be fully coherent and also free of internal structure during the earliest observed stage of their growth. A single array of misfit dislocations bounding stacking faults, and also growth ledges, develop during the second growth stage and a second array of misfit dislocations appears during the third stage of growth. The inter-dislocation spacings in the two arrays are roughly 2 and 12 nm, respectively. Both interphase boundary structure and thickening kinetics are shown to be inconsistent with various shear mechanisms of growth but to fit comfortably into the framework provided by diffusion-controlled ledge-wise thickening. A ledge structure was also observed on α₁ plate edges, in agreement with deductions made by Simonen and Trivedi from their measurements of the lengthening kinetics of these plates.