Constant true strain rate simple compression tests were conducted on annealed, polycrystalline samples of ␣-brass and MP35N, and the evolution of the true stress ()-true strain (ε) response was documented. From these data, the strain hardening rate was numerically computed, normalized with shear modulus (G), and plotted against both ( Ϫ 0 )/G ( 0 being the initial yield strength of the alloy) and ε. Such normalized plots for ␣-brass and MP35N were found to be almost identical to each other, and revealed four distinct stages of strain hardening: stage A, with a steadily decreasing strain hardening rate up to a true strain of about Ϫ0.08; stage B, with an almost constant strain hardening rate up to a true strain of about Ϫ0.2; stage C, with a steadily decreasing strain hardening rate up to a true strain of about Ϫ0.55; and a final stage D, again with an almost constant strain hardening rate. Optical microscopy and transmission electron microscopy (TEM) were performed on deformed samples. The results suggested that stage A corresponded to stage III strain hardening (dynamic recovery) of higher stacking fault energy (SFE) fcc metals such as copper. The onset of stage B correlated with the first observation of deformation twins in the microstructure. Further straining in stage B was found to produce clusters of parallel twins in an increasing number of grains. Stage C correlated with the development of severe inhomogeneity of deformation within most grains, and with the development of significant misorientation between the twin/matrix interface and the {111} plane in the matrix of the grain, i.e., the matrix/twin interface lost coherency with continued deformation. Stage D correlated with extensive formation of secondary twins that resulted in twin intersections in many grains. Early in stage D, some strain localization in the form of shear bands was observed. Although formation of these shear bands had no detectable effect on the macroscopic strain hardening rate, it did correlate with a marked change in texture evolution. Based on these experimental observations, we have developed and presented a physical description of the microstructural phenomena responsible for the various strain hardening stages observed in low SFE fcc alloys.

This article investigates the microstructural variables influencing the stress required to produce deformation twins in polycrystalline fcc metals. Classical studies on fcc single crystals have concluded that the deformation-twinning stress has a parabolic dependence on the stacking-fault energy (SFE) of the metal. In this article, new data are presented, indicating that the SFE has only an indirect effect on the twinning stress. The results show that the dislocation density and the homogeneous slip length are the most relevant microstructural variables that directly influence the twinning stress in the polycrystal. A new criterion for the initiation of deformation twinning in polycrystalline fcc metals at low homologous temperatures has been proposed as ( tw Ϫ 0 )/G ϭ C(d/b) A , where tw is the deformation twinning stress, 0 is the initial yield strength, G is the shear modulus, d is the average homogeneous slip length, b is the magnitude of the Burger's vector, and C and A are constants determined to have values of 0.0004 and Ϫ0.89, respectively. The role of the SFE was observed to be critical in building the necessary dislocation density while maintaining relatively large homogeneous slip lengths.

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