Fracture behaviors of niobium alloys by hydrogenation and its application for fine powder fabrication

Hydrogen-induced cracking and pulverization was investigated for Nb-Ti alloys consisting of a single-phase Nb solid solution. Multiple cracks were propagated in Nb-Ti alloys when they were annealed at 1473 K and cooled in a hydrogen atmosphere, suggesting that annealing in a hydrogen atmosphere at 1473 K activates surface, and cooling under a hydrogen atmosphere to room temperature enables Nb to absorb hydrogen. Hydrogenated Nb-Ti alloys containing 0 to 10 at % Ti were composed of a brittle Nb-hydride phase of the face centered orthorhombic ß-NbH, while alloys containing 15 to 18 at.% Ti were of ß-NbH and bcc Nb₂H, leading to lattice expansion of the metal lattice. Therefore, we propose that hydrogen-induced cracking and pulverization in Nb-Ti alloys occurs in the following sequence: hydrogen absorption, local strain generation due to hydride formation, embrittlement of the Nb-hydride phase, and crack nucleation and propagation. Brittle fragments of Nb-Ti alloys produced by hydrogen-induced cracking were beneficial for further refinement of particle size. Additionally, a friability of hydrogenated Nb-Ti alloys was enhanced by increasing Ti content and lowering the process temperature; and fine Nb-Ti alloy powders having good size uniformity could be produced using hydrogenation and mechanical milling procedures. Dehydrogenation at a temperature higher than 724 K leads to full hydrogen desorption. Therefore, hydrogenation - ball milling - dehydrogenation procedures can be used as a promising method to fabricate fine and contamination-free Nb-Ti alloy powders.