Strain dependence of deformation and recrystallization microstructure homogeneity in clock-rolled tantalum sheets

Microstructure and crystallographic texture are the key factors that determine the sputtering target properties. Clock rolling plays an important role in improving the microstructure homogeneity, but the effect of strain during rolling on deformation and recrystallization behavior is not clear. Thus, high-purity tantalum (Ta) plates were 135° clock rolled to 70% and 87% reduction and then annealed at various temperatures to observe the microstructure evolution. Texture and microstructure in the center layer of the rolled and annealed Ta sheets were characterized via optical microscope (OM), X-ray diffraction (XRD), electron backscatter diffracting (EBSD) and transmission electron microscope (TEM). The results displayed that significant microstructure difference existed between 70% and 87% sample. Grain average misorientation value of {111} grains {〈111〉//normal direction (ND)} in the 70% sample was considerably higher than that in the 87% sample, suggesting a more heterogeneous grain fragmentation. Schmid factor (SFrolling) and Taylor model analysis of {111} grains in the 70% sample demonstrated that the slip was easier, and the system with higher SFrolling could alone accommodate the majority of plastic strain, contributing to the formation of micoshear bands. Upon annealing, the sample rolled 70% recrystallized more quickly, owing to strong {111} deformed texture, and severe microstructure subdivision and great stored energy within {111} grains. The {111} texture is very strong and grain size distribution was not uniform after the completion of recrystallization. However, after annealing of sample rolled 87%, smaller average grain size and variation, and relatively homogeneous texture distribution can be obtained