Two-phase (α/β)-brass bicrystals produced by a solid-solid diffusion method: II. Diffusion-induced twins in the α-phase

Abstract

A now type of annealing twin fault has been found to grow under certain conditions in the α-phase adjacent to the interphase boundary of two-phase (α/β)-brass bicrystals produced by a solid-solid diffusion method in which a phase transformation is present. It appears that the characteristics of the twin formation depend on the crystallographic orientations of α-brass crystals with reference to the inter-phase plane; i.e. the activated twinning {111} planes generally have the smallest angle to the isoconcentration planes in the diffusion zone. It has been observed, by an analysis of the concentration profile with an EPMA, that the growth and propagation of the twins during the diffusion process continues to the position where the concentration gradient becomes approximately level. It has become evident that, if the Zn concentration difference, C _αβ -C _α0 is smaller than a critical value, which is estimated to lie between 5·5 and 6·8 wt.%, it is possible to obtain two-phase bicrystals that do not contain twinning faults, where C _αβ is the Zn concentration at the α : α + β equilibrium at the diffusion temperature and C _α0 the initial concentration of Zn in an α-brass crystal before diffusion. The generation of misfit dislocations to accommodate the change of the lattice parameter arising from a steep concentration gradient in the α-phase at the initial stage of diffusion, is assumed to be responsible for the twin nucleation : consequently, it is interpreted that the twin formation plays an important role in accommodating the change of lattice parameter in the diffusion zone of the α-phase. Some observations of the substructure by an etch-pitting method are also presented to supplement understanding of the microstructure of the diffusion couples produced by the present diffusion process.