Anomalies in the response of V, Nb, and Ta to tensile and shear loading: Ab initio density functional theory calculations

摘要: Ab initio density functional calculations of the response V, Nb, and Ta to tensile shear loading have been performed. We find that behavior all three metals V group under large strains differs considerably from reported for other body-centered cubic (bcc) transition (Mo, W, Fe). Under unaxial $⟨100⟩$ loading, Nb undergo a bifurcation tetragonal an orthorhombic deformation path, associated with instability, before reaching stress maximum. The strongly reduces ideal strengths 11.5 GPa 12.5 Nb. For point coincides maximum 13.6 along path so metal fails strain by not cleavage. stress-strain curves calculated ${110}⟨111⟩$ ${211}⟨111⟩$ slip systems are asymmetric; both 6.5 (5.5) 7.8 (6.0) Hence only half as bcc Mo but much larger than expected on basis low moduli shows intermediate between these two groups metals, strength 7.1 (6.5) GPa, in excellent agreement experimental estimate nanoindentation experiments. saddle-point structure determining is special body-centered-tetragonal structure. This identical identified uniaxial tension. also will fail tension cleavage, shear.