Microstructural Observation of Ordered β-Ta₂H in Hydrogenated Tantalum

The microstructural changes accompanying the formation of the ordered β-Ta₂H phase in pure tantalum (Ta) by hydrogenation have been investigated to elucidate fundamentals for hydrogen-induced cracking and pulverization. Concentric cracks as well as a number of microcracks with relieflike straight line contrasts on {001}_bcc planes of a Ta grain are introduced by annealing and cooling in a hydrogen atmosphere. These contrasts correspond to boundaries between grains of the ordered β-Ta₂H phase having a pseudo-tetragonal structure. Furthermore, the transformation of the ordered β-Ta₂H lattice leads to an anisotropic misfit in the boundaries between the Ta matrix and the adjacent hydride grains and between hydrides with different variants. Thus, the hydride formation brings about strain large enough to trigger multiple cracks. Hence, the hydrogen-induced cracking in Ta plates occurs in the following sequence: hydrogen absorption, formation of the ordered β-Ta₂H phases, strain accommodation, microcrack nucleation at the boundaries, and concentric crack propagation.