Changes in microstructures and hydrogen permeability of Nb30Hf35Co35 eutectic alloy membranes by annealing

Abstract

a b s t r a c tChanges in microstructures and hydrogen permeability by long-time annealing (168 h) at 1073, 1173 and 1173 K for eutectic Nb 30 Hf 35 Co 35 alloy are investigated. After annealing at 1073 K and 1173 K for 168 h, the fully lamellar eutectic microstructure in as-cast state has disappeared and turned to a duplex microstructure. Correspondingly, the hydrogen permeability at 673 K, decreases from 3.3 Â 10 À8 to (0.82e1.5) Â 10 À8 mol H 2 m À1 s À1 Pa À0.5 after annealing. The annealed (a-) sample at 1273 K for 168 h is brittle and susceptible to hydrogen embrittlement that results from the appearance of the additional Hf 2 Co phase. The hydrogen solubility coefficient (K) for each a-samples at 673 K is almost the same as that for the as-cast sample, while the hydrogen diffusion coefficient (D) for the a-samples are (1/4e1/2) times than that for the as-cast one. It is thus concluded that the lower permeability by annealing for eutectic Nb 30 Hf 35 Co 35 is mainly attributed to the significant reduction of hydrogen diffusivity after annealing.