Substantial enhancement of hydrogen permeability and embrittlement resistance of Nb30Ti25Hf10Co35 eutectic alloy membranes by directional solidification

As-cast Nb30Ti35−xHfxCo35 (x=0, 10, and 17.5) alloys consist of a fully lamellar eutectic structure, where the substitution of Ti by Hf induces higher hydrogen solubility and permeability, but poorer embrittlement resistance. Different as-cast alloys were found to be subject to embrittlement failure after hydrogen permeation for 88.3 h (x=0), 71.1 h (x=10) and 16.2 h (x=17.5) at 673 K. In contrast, directionally solidified (DS) Nb30Ti25Hf10Co35 exhibits a substantial enhancement of hydrogen permeability and embrittlement resistance. Typically, DS samples solidified at 1 μm/s show the highest permeability of 4.83×10−8 mol H2 m−1 s−1 Pa−0.5 at 673 K, which is 1.72 times that of its as-cast counterpart and more than three times that of pure Pd. This sample does not fail during hydrogen permeation for 120 h at 673 K. The high permeability and large embrittlement resistance of DS samples are attributed to their modified eutectic microstructure, which induces a high overall hydrogen diffusivity and a high tolerance to lattice expansion. The present work demonstrates that the directional solidification technique has a high potential to produce high-performance eutectic alloy membranes for hydrogen separation.