Hardening mechanisms in underaged ordered and disordered Cu 3Au-Co alloy single crystals

Abstract

Precipitation hardening of ordered and disordered Cu 3Au by coherent f.c.c. Co precipitates was investigated by compression testing of oriented single crystal specimens of stoichiometric Cu 3Au alloys containing 0.64 and 1.50 at.% Co. Pairs of specimens were aged at 450°C for various aging times, after which one sample of each pair was heat-treated for the purpose of ordering the Cu 3Au matrix. The samples were then mechanically tested at 77, 183, 300 and 373K. Transmission electron microscope examination of sections cut parallel to the primary slip plane showed that the dislocations in the underaged samples, with which this paper is concerned, are predominantly screw in character. Using this observation, and measurements of the Co particle sizes by a superparamagnetic method, the data were analyzed in terms of several theories of precipitation hardening. The best quantitative agreement is obtained with a modified version of the Gerold and Haberkorn theory of coherency hardening, assuming that the contributions of the matrix, both ordered and disordered, and the precipitates are additive. Several other ways of analyzing the data are presented, and the merits of each are discussed; for example, it is shown that the contribution of modulus hardening could be significant. It is concluded, nevertheless, that coherency hardening is the major strengthening mechanism in the underaged samples, although this conclusion is qualified by the fact that the theories used to analyze the data have recognized shortcomings, which are discussed.