Phase stability and defect structure of the C15 phase in the Hf–V–Nb system

Abstract

The phase stability and defect structure in the C15 HfV₂-based Laves phase were studied by a combined experimental and theoretical investigation. It was found that C15 HfV₂ can be alloyed with a large amount of Nb (about 20 at.%), where the Nb atoms occupy both Hf and V sites, and the lattice parameters of the ternary C15 phases containing the same Nb content decrease with increasing V to Hf ratio. The extended defects observed in the ternary C15 phase are extrinsic {111} stacking faults, {111}〈112〉 annealing twins, and faceted ∑ = 3 (〈111〉, 70.53°) grain boundaries. Examination of the extrinsic {111} stacking faults revealed that synchroshear may occur in this C15 phase. The total and l-projected density of electronic states of C15 HfV₂ were obtained by first-principles total energy and electronic structure calculations. The phase stability, Nb site occupancy and compositional dependence of the lattice parameter of the C15 phase in the Hf—V—Nb system are qualitatively explained by geometric and electronic structure considerations.