Microstructural Observation of Ordered β-Ta2H in Hydrogenated Tantalum

Abstract

SATOSHI SEMBOSHI, TOYOHIKO J. KONNO, NAOYA MASAHASHI, and SHUJI HANADA The microstructural changes accompanying the formation of the ordered b-Ta 2 H phase in pure tantalum (Ta) by hydrogenation have been investigated to elucidate fundamentals for hydrogeninduced 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 b-Ta 2 H phase having a pseudo-tetragonal structure. Furthermore, the transformation of the ordered b-Ta 2 H 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 b-Ta 2 H phases, strain accommodation, microcrack nucleation at the boundaries, and concentric crack propagation.