Quaternary Nb-Hf-Co-Fe alloy with superior hydrogen permeation properties over a wide temperature range

Abstract

Group 5 metals and their alloys are expected to become a new generation of hydrogen-permeable membranes to replace commercial palladium-based alloys. Recently, we synthesized a new Nb-Hf-Co alloy hydrogen permeation membrane, and the Nb30Hf35Co35 alloy has good mechanical properties and permeability. However, the development of alloys with higher hydrogen permeability is a challenge for these Nb-based membranes. To this end, Nb30Hf35Co35−xFex (0 <x < 35) alloys were developed and their microstructure and hydrogen permeation properties were systematically studied by means of SEM, XRD and TEM, among other techniques. When the Fe content is lower than 10 at%, Nb30Hf35Co35−xFex alloys are composed completely of a eutectic structure. With increasing Fe content, changes in microstructure and constituent phases are observed. In particular, a large amount of new impurity phases (Hf2Fe, Fe2Nb, FeNb and Fe0.7Hf0.3) is formed when the Fe content in the alloys is higher than 20 at%. With these changes, the hydrogen permeability (Φ) of these alloys first increases and then decreases. The Nb30Hf35Co30Fe5 alloy exhibits the largest hydrogen permeability at 673 K, at 3.41 × 10−8 mol H2 m−1 s−1 Pa−0.5, or 2.2 times that of palladium metal under the same conditions. The increase in the Φ value is mainly due to the increase in hydrogen diffusion coefficient (K) rather than hydrogen solubility (D). The present work demonstrates that Nb-Hf-Co-Fe quaternary alloys are expected to become a more promising hydrogen permeable material compared with traditional ternary alloys.