Evolution of dislocations and grain boundaries during multi-axial forging of tantalum

Abstract

Presented here is the evolution of microstructure of Ta during multi-axial forging, with a specific understanding of how grain boundary movement and their interaction occurs during deformation. Coarse grained (CG) tantalum was multi-axially forged till 3, 6, and 9 passes in a closed channel die to refine grain size and increase its strength. The grain size was refined from 64 μm to 760 nm, the dislocation density increased by an order of magnitude, the Vickers hardness increased from 92 to 223 HV, and the yield strength at room temperature increased from 178 to 653 MPa. It was found that the contribution of GBs to strengthening was higher than that of dislocations. Mechanisms for GB structure evolution during deformation were studied using molecular dynamics (MD) simulations. GB mobility was high for low angle GBs (LAGBs) and reduced with increasing tilt angle, after which it took on low values for high angle GBs (HAGBs). The mobility of specific HAGBs were due to the formation of ledges/disconnections and dissociation into other GB types. MD results showed that the decrease in fraction of HAGBs during initial deformation can be attributed to the dissociation of HAGBs with low mobility into GBs with higher mobility.