The evolution of texture and microstructure uniformity in tantalum sheets during asymmetric cross rolling

Abstract

The evolution of texture and microstructure uniformity in tantalum (Ta) sheets for sputtering target applications is analyzed in detail across the thickness during asymmetric cross rolling (ACR). Three samples with different strains, i.e. 60%, 70% and 80% are obtained via ACR processing. X-ray diffraction suggests that the increase of strain during ACR results in the randomization of deformation texture across the sample thickness due to the penetration of shear strain into the center. Electron backscatter diffraction results indicate that the increasing strain in ACR can alleviate region-dependent microstructure inhomogeneity. This is also confirmed by the distributions of Vickers hardness and geometrically necessary dislocations. Taylor model analysis along with strain contouring maps suggest that relatively low and centralized number of active slip systems in the 80% sample effectively reduces strain concentrations and thus homogenizes the average shear strain of most active slip system in different grain orientations. Upon annealing, nuclei with random orientations can grow evenly from the deformed matrix in the 80% sample because of relatively homogeneous grain fragmentation and random deformation texture. These contribute to uniform and fine grain size combined with random crystallographic orientations after the completion of recrystallization.