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

Abstract

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.

Crystallographically degenerate B2 precipitation in a plastically deformed fcc-based complex concentrated alloy

“…The latter observation suggests that the secondary twins have been intersected by other deformation entities (e.g. dislocations, fine-scale tertiary twins) during the cold-rolling process [18,19]. A careful examination of the secondary twins (dotted rectangle in Figure 4(d)) also implies that they have emanated from a wider variant (#1).…”

“…A careful examination of the secondary twins (dotted rectangle in Figure 4(d)) also implies that they have emanated from a wider variant (#1). Such twin intersections denote extensive strain-hardening [18,19], which locally alters the matrix orientation. Evidence of such lattice misorientation can be seen in the 'smearing/streaking' of the primary fcc and twin reciprocal lattice reflections (inset Figure 4(e)).…”

Ductility Sensitivity to Stacking Fault Energy and Grain Size in Cu–Al Alloys

“…[18] Note that there is a plateau when the tensile true strain is larger than 0.45, which is supposed to be induced by twin intersections. [19] Above all, the monotonic increase of ductility with decreasing the SFE is mainly resulted from the continuous improvement of strain-hardening capability of the Cu-Al alloys.…”

“…[19][20][21] In contrast, fully recrystallized twinning-induced plasticity steels and Cu-Al alloys with grain sizes smaller than 1 μm have been fabricated by cold rolling and annealing process, [18,22,23] which makes it feasible to study the ductility and deformation mechanisms from the finegrained regime to the coarse-grained regime.…”

A TEM Investigation on Precipitation Behavior of AEREX350 Superalloy

“…MP35N, the original member of the MP family, is commercially strengthened by cold working [1,2]. Due to chemical composition of AEREX TM 350, the alloy benefits from precipitation hardening and solid solution strengthening as well as cold working and can be used in higher temperatures [3].…”