Formation of mesoscale roughening in 6022-T4 Al sheets deformed in plane-strain tension
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Formation of Mesoscale Roughening in 6022-T4 Al Sheets Deformed in Plane-Strain Tension Y.S. CHOI, H.R. PIEHLER, and A.D. ROLLETT Mesoscale surface-roughening evolution in 6022-T4 Al sheets was investigated using plane-strain tension. The formation of grain-scale hills and valleys and their relation to the morphologies and corresponding orientations of surface grains after deformation were examined experimentally. These observations were analyzed using various approaches based on the Schmid and Taylor crystal plasticity models. It was observed that surface grains with and without slip bands tend to form valleys and hills, respectively, wherever these two types of grains are adjacent to each other along the planestrain tension direction. When the sample was pulled along the transverse direction, the formation of hills and valleys by unbanded and banded grains was more lineally organized in the plane-strain (rolling) direction than in the sample that was pulled along the rolling direction (RD). Slip banding and valley formation were principally observed in the surface grains with either very few (1 to 2) slip systems of high Schmid factors or with low Taylor factors, in contrast to nonslip-banded and hill-forming surface grains. Quantitative analysis using correlation coefficients showed that the Schmid factor provided slightly better agreement than the Taylor factor in predicting the slip-banding (and valley-forming) and nonslip-banding (and hill-forming) behaviors of surface grains. In addition, measures that quantify the image qualities of electron backscattered diffraction (EBSD) patterns for selected surface grains suggested that the slip-banded and valley-forming grains contain less lattice distortion than the nonslip-banded and hill-forming grains, despite the larger strains experienced by these grains. This indicates that dislocations in the slip-banded grains move out of the surface to create deformation without lattice distortion. Plastic interactions between specific neighboring grains are central to the formation of mesoscale surface roughening.
I. INTRODUCTION
DURING plastic deformation of sheet metals, surface or near-surface grains have an additional degree of freedom in deformation, compared to interior grains, because of the relaxed deformation constraint at the free surface. In general, individual plastic responses of the less-constrained grains on the surface or in the subsurface layer generate different surface heights, producing surface perturbations referred to as mesoscale roughening. Mesoscale, in the present study, was defined to be an observation window covering several grains to dozens of grains, such that contributions of individual grains to the surface roughening must be considered. Some Al sheets tested have shown the inherent grain-scale roughening behavior or “orange-peel” effect. It has been known that numerous factors, such as grain-size distribution, crystallographic texture, crystal structure, etc., influence the topographical features of mesoscale sur
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