Correlation between the microstructure studied by X-ray line profile analysis and the strength of high-pressure-torsion processed Nb and Ta

Niobium and tantalum are two body centered cubic metals with very different elastic anisotropy. The Az = 2 × c44/(c11−c12) constant for Nb and Ta is 0.51 and 1.58, respectively. The submicron grain-size state of the two refractory metals was produced by the method of high-pressure torsion with different pressure values of 2 and 4 GPa for Nb, and 4 and 8 GPa for Ta, and two different deformations of 0.25 and 1.5 rotations, respectively, with equivalent strains of up to ∼40. The dislocation density and the grain size were determined by high-resolution diffraction peak-profile analysis. The beam size on the specimen surface was 0.2 × 1 mm, allowing the sub-structure along the radius of the specimen to be characterized. The strength of the two metals was correlated with the dislocation density and the grain size. It is found that, though the grain size is well below 100 nm, the role of dislocations in the flow stress of these two metals is significantly greater than that of the grain size.