Effect of Processing Conditions on Microstructure and Property of Multiphase V-Ti-Ni Alloys for Hydrogen Purifying

The method of separation and purification of hydrogen from a mixed gas based on the permeation of hydrogen through a dense metallic membrane appears as an attractive mean of producing high purity hydrogen at a large scale. V-based alloy membranes with bcc structure are of great interest for hydrogen separation applications due to their low cost and high permeability. The hydrogen flux of the membrane is proportional to its hydrogen permeability and inversely proportional to its thickness. Therefore, V-based alloys should be fabricated in the form of large and thin membranes with the least possible thickness. Rolling process is presently regarded as the most promising route to a large scale fabrication of hydrogen permeable metal membranes. The refractory nature of the most prospective bcc alloys, and their potentially complex compositions, restrict the fabrication techniques which can be applied to form the alloy into a thin foil. In this work, thin sheets of V55Ti30Ni15 alloy were produced by a thermo-mechanical treatment consisting in successive heat treatment, rolling and annealing treatment, and the effect of microstructures resulting from different processing conditions on hydrogen permeability, have been investi-gated for the multiphase V55Ti30Ni15 alloy. Precipitation of NiTi particles from V-matrix of V55Ti30Ni15 alloy during heat treatment, reduces the volume fraction of V-matrix contributing mainly to hydrogen permeation, which results in the decreasing of hydrogen permeability. The microstructure of the alloy after heat treatment evolved into a fibrous/lamellar microstructure during hot-rolling deformation, and a significant reduction in hydrogen permeability accompanied this deformation. Subsequent annealing decreased the dislocation density and increased hydrogen permeability. Dislocations have a great impact on hydrogen permeability due to their ability to trap diffusing hydrogen.