Strain hardening regimes and microstructural evolution during large strain compression of low stacking fault energy fee alloys that form deformation twins

Abstract

Constant true strain rate simple compression tests were conducted on annealed, polycrystalline samples of α-brass and MP35N, and the evolution of the true stress (cr)-true strain (e) response was documented. From these data, the strain hardening rate was numerically computed, normalized with shear modulus (G), and plotted against both (σ - σ0)/G (σ0 being the initial yield strength of the alloy) and e. Such normalized plots for α-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 -0.08; stage B, with an almost constant strain hardening rate up to a true strain of about -0.2; stage C, with a steadily decreasing strain hardening rate up to a true strain of about -0.55; 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) fee 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 fee alloys.