Spatial Distribution of the Dislocation Density and the Strength of Nb and Ta Deformed by High-Pressure-Torsion Determined by X-Ray Peak Profile Analysis

Niobium and Tantalum are two bcc metals with very different elastic anisotropy. The Az=2xc44/(c11-c12) constants for Nb and Ta are 0.51 and 1.58, respectively. Submicron grain-size state of the two refractory metals is produced by the method of high-pressure-torsion (HPT) with different pressures 2 and 4 GPa, and two different deformations of 0.25 and 1.5 rotations, respectively. The grain size, the dislocation density and the screw or edge type of dislocation character are investigated by high resolution diffraction peak-profile analysis. The beam size on the specimen surface is 0.2x1 mm allowing to characterize the sub-structure along the radius of the specimen. The strength of the samples is determined by microhardness testing. The sub-structure parameters are correlated with the strength data. The deformation mechanisms of the two different metals are discussed in terms of the different deformation modes, the elastic anisotropy and the dislocation structure.