Asymmetric cross rolling: a new technique for alleviating orientation-dependent microstructure inhomogeneity in tantalum sheets

Abstract

New rolling technique, i.e. asymmetric rolling combined with cross rolling is adopted to produce Ta sputtering targets in this study. Electron backscatter diffraction (EBSD) analysis suggests that {111} and {100} deformed grains distribute alternately along normal direction in cross rolling (CR) and asymmetric cross rolling (ACR) samples. Misorientation angle distribution indicates that severe orientation-dependent grain fragmentation exists in the CR sample, which is also confirmed by kernel average misorientation and grain reference orientation deviation-hyper. Grain average misorientation (GAM) and distribution of geometrically necessary dislocations (GNDs) suggest that the effect of increasing shear strain introduced by asymmetric rolling on deformation microstructure is mainly reflected in the {100} grains, which is further verified by orientation-dependent microhardness values. The computation of Schmid factor indicates that slip within {100} grains in the ACR sample is easier, and the system with higher Schmid factor can alone accommodate the majority of plastic strain. Transmission electron microscopy (TEM) reveals that dense dislocation walls (DDWs) are formed within the {100} deformed grains in the ACR sample, while only sparse dislocation lines can be observed in the CR sample. X-ray line profile analysis (XLPA) displays that ACR can significantly increase the stored energy of the {100} deformed grains and thus weaken the orientation-dependent stored energy distribution. The enhanced recrystallization ability of the {100} grains in the ACR sample facilitates homogenization of the annealing microstructure.