Flow stress analysis of ultrafine grained AA 1050 by plane strain compression test

Plane strain compression (PSC) test was used to study the flow stress of ultrafine grained commercially pure aluminum at large strains. AA 1050 sheets were processed by various Accumulative Roll-Bonding (ARB) cycles up to 10 cycles as the initial specimens for the test. An approach was developed to measure the coefficient of friction and to suppress its effect on the results. It is shown that as a result of having an anisotropy parameter (R-value) of less than one, Von-Mises tensile strengths are significantly higher than PSC strengths. Comparing these strengths, the R-value as an average anisotropy parameter of rolling and transverse directions is estimated for the ARBed sheets, where it is decreased by 6 ARB cycles and increased by the following 8–10 cycles. Estimated R-values are used for drawing the flow curves based on the Hill's 1948 anisotropic plasticity. While the flow curves display a steady state flow stress for all samples, a flow softening is observed at the beginning of the curves for 8 and 10-cycle specimens up to strain of 0.3. It is revealed that despite evidence for shear banding, the softening has a microstructural reason due to different strain rates during pre-straining by ARB and PSC. In other words, since the grain refinement during ARB up to 8 and 10 cycles has exceeded the smallest possible grain size of the PSC test, post-ARB deformation of these two specimens result in a microstructural revolution toward the preferred steady state conditions along with a gradual work softening.