Microstructure and superconductivity of some C15 laves phase alloys: V 2 Hf, V 2 Hf .51 Nb .49 and V 2 Hf .51 Nb .21 Ti .28

Effect of different heat treatments on the lattice structure, microstructure and superconductivity of the C15 phase materials: V 2 Hf, V 2 Hf .51 Nb .49 and V 2 Hf .51 Nb .21 Ti .28 are investigated. X-ray, optical microscopy and SEM investigations show the distribution of C15 phase filaments (10-25μ) in a bcc matrix for the V 2 Hf .51 Nb .49 and V 2 Hf .51 Nb .21 Ti .28 materials. The composition of the filaments is sensitive to heat treatment and the maximum ratio of V to Hf, Nb and Ti atoms is obtained after annealing the V 2 Hf .51 Nb .21 Ti .28 at (1200°C;6h). T c increases and the Vicker's hardness decreases after the substition of Nb and Ti for the Hf in V 2 Hf compound. Maximum T c and H c2 (8K) values of 9.76K and 9.4 Tesla are also observed for the annealed (1200°C;6h) V 2 Hf .51 Nb .21 Ti .28 alloy. Anomalies in (R vs. T) and ( \chi_{m} vs. T) curves are observed for the homogeneous V 2 Hf (annealed 900°C;70h) at ∼ 130K and 150K respectively, whereas multifilamentary annealed (900°C;70h) V 2 Hf .51 Nb .49 and V 2 Hf .51 Nb .21 Ti .28 do not show such anomalies. T c values of annealed (900°C;70h) V 2 Hf, V 2 Hf .51 Nb .49 and V 2 Hf .51 Nb .21 Ti .28 are 8.89K, 9.07 and 9.71K respectively. It is concluded that for these alloys the lattice instability is microstructure dependent and does not influence the superconducting parameters.