The transformation characteristics of the beta phase in some copper-zinc alloys.

A metallographic study has been made of the two diffusional transformations which occur during isothermal tempering of metastable beta' brass (38.1-43.4 at. % zinc). For each composition times to start of visible precipitation have been located and the T.T.T diagrams consist of two intersecting 'C' curves. A rod-like (?) precipitate is associated with the upper temperature reaction and a plate-like (alpha[1]) precipitate with the lower temperature reaction. The upper temperature limit for a1 plate formation (beta[s]) decreases with increasing zinc content. alpha[1] plate growth ceases if plates initially formed in the lower region are tempered above the beta[s] temperature supporting the limiting nature of the beta[s] temperature. Hotstage microscopy confirms that the alpha[1] plates grow slowly accompanied by a shape change similar to that of the subzero martensite. The habit {2 11 12} varies little with composition and temperature as predicted by the phenomenological theory of martensite formation. It is concluded that the structure of these plates is f.c.c. with internal faulting along {111} f.c.c. planes. Electron probe microanalysis has confirmed that plate growth proceeds with solute enrichment of the beta matrix. Consequently the process is concluded to be bainitic. The a rods have been confirmed by X-ray and electron probe microanalysis to be f.c.c. and of approximately equilibrium composition. The growth is accompanied by a shape change, indicating the presence of a shear component. The morphology together with the kinetics of transformation have been studied by high temperature filming techniques. Sympathetic nucleation of a rods from alpha[1] plates, on tempering below the betas temperature has been investigated, and the nucleation sites related to the substructure of the alpha[1] plates. The effect of prior deformation and ternary gold additions on the transformation characteristics have been considered. Deformation favours the nucleation of the bainitic product and increases the growth rate of a rods. Gold substituting for copper fails to significantly influence the transformation characteristics. Hardening has been found to only accompany the precipitation of alpha[1] plates and a mechanism to account for this has been proposed