Effects of second phases on the pulverization of Nb₃Al-base alloys by hydrogenation

Some Nb₃Al-base alloys are known to be pulverized by holding in hydrogen atmosphere. The pulverization by hydrogenation is expected to be applied to preparation of fine powder with high quality and low cost. The purpose of this study is to investigate the mechanism of pulverization by hydrogenation of Nb₃Al-base alloys with or without second phases. Alloys prepared were Nb₃Al single phase alloy (Nb-21 mol%Al alloy: Nb-21Al), and two phase alloys including Nb solid solution (Nb-16 mol%Al alloy: Nb-16Al) and Nb₂Al (Nb-28 mol%Al alloy: Nb-28Al). They were heat-treated at 1473 K for 86.4 ks in a vacuum atmosphere. After surface treatment, hydrogenation was carried out under hydrogen pressure from 0 to 3.4 MPa at 313 K and 353 K. Powder formed by hydrogenation was observed using a scanning electron microscope. Identification of phases and measurement of lattice constants before and after hydrogenation were carried out by X-ray diffraction. It is clearly seen that both the two phase alloys, Nb-16Al and Nb-28Al, are pulverized by hydrogenation. On the other hand, the single phase alloy, Nb-21Al, is not pulverized under the experimental conditions. Then, it is assumed that the existence of the second phases accelerates the pulverization by hydrogenation. Both lattice constants and volumes increase remarkably through hydrogenation and no hydric compound is recognized. It is concluded that the pulverization of Nb-16Al and Nb-28Al is caused by large strain energy generated by the difference in lattice expansions between Nb₃Al and Nb solid solution, and between Nb₃Al and Nb₂Al in pulverization by hydrogenation. In the case of single phase alloy of Nb-21Al, little strain energy is generated through hydrogen absorption, that leads to difficulty in pulverization under the experimental conditions.