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

Changes in microstructures and hydrogen permeability by long-time annealing (168 h) at 1073, 1173 and 1173 K for eutectic Nb30Hf35Co35 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 × 10−8 to (0.82-1.5) × 10−8 mol H2 m−1 s−1 Pa−0.5 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 Hf2Co 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 (1/4-1/2) times than that for the as-cast one. It is thus concluded that the lower permeability by annealing for eutectic Nb30Hf35Co35 is mainly attributed to the significant reduction of hydrogen diffusivity after annealing.