Multi-scale Examination of the Microstructure Evolution of AZ31 During Multi-pass Cross-Rolling

Sheet specimens of alloy AZ31 were cross-rolled to equivalent strains of 0.05, 0.10, 0.20, 0.30, 0.40, 0.56, and 0.77. The microstructure evolution was examined using a combination of optical metallography, electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), and X-ray diffraction. The results revealed significant activity of twinning and basal slip. The twins were mainly of the contraction and double-twin (contraction-extension) types. In addition to the micron-scale (1 to 5 μm) twins observed on EBSD patterns, nano-scale twins were observed by TEM. The nano twins had a width of less than 0.20 μm and existed either as individual/isolated twins or as twin-bundles that are several microns thick. The number of nano twin-bundles increased with increasing strain. Twinning appears to play a key role in strain localization leading to the formation of shear bands. The repeated strain-path changes during cross-rolling delayed the onset of shear banding compared to specimens in which the rolling direction was unchanged.