EFFECTS OF 2ND PHASES ON THE PULVERIZATIO N OF NB3AL-BASE ALLOYS BY HYDROGENATION

Some Nb3Al-base alloys are known to be pulverized by holding in hydrog en atmosphere. The pulverization by hydrogenation is expected to be ap plied to preparation of fine powder with high quality and low cost. Th e purpose of this study is to investigate the mechanism of pulverizati on by hydrogenation of Nb3Al-base alloys with or without second phases . Alloys prepared were Nb3Al single phase alloy (Nb-21 mol%Al alloy: N b-21Al), and two phase alloys including Nb solid solution (Nb-16 mol%A l alloy: Nb-16Al) and Nb2Al (Nb-28 mol%Al alloy: Nb-28Al). They were h eat-treated at 1473 K for 86.4 ks in a vacuum atmosphere. After surfac e treatment, hydrogenation was carried out under hydrogen pressure fro m 0 to 3.4 MPa at 313 K and 353 K. Powder formed by hydrogenation was observed using a scanning electron microscope. Identification of phase s and measurement of lattice constants before and after hydrogenation were carried out by X-ray diffraction. It is clearly seen that both th e two phase alloys, Nb-16Al and Nb-28Al, are pulverized by hydrogenati on. On the other hand, the single phase alloy, Nb-21Al, is not pulveri zed under the experimental conditions. Then, it is assumed that the ex istence of the second phases accelerates the pulverization by hydrogen ation. Both lattice constants and volumes increase remarkably through hydrogenation and no hydric compound is recognized. It is concluded th at 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 b y 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.