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

Abstract

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 88.3h (x=0), 71.1h (x=10) and 16.2h (x=17.5) at 673K. In contrast, directionally solidified (DS) Nb30Ti25Hf10Co35 exhibits a substantial enhancement of hydrogen permeability and embrittlement resistance. Typically, DS samples solidified at 1μm/s show the highest permeability of 4.83×10−8molH2m−1s−1Pa−0.5 at 673K, 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 120h at 673K. 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.