HYDRIDE FORMATION IN 2-PHASE (TI3AL+TIAL) TITANIUM ALUMINIDES DURING CATHODIC CHARGING AND ITS DISSOCIATION

A (TiAl)H-x hydride which has a tetragonal crystal structure with latt ice parameters a=0.452 nm and c=0.326 nm (c/a=0.721) has been observed in Ti-42Al, Ti-45Al and Ti-50Al (at.%) two-phase (Ti3Al (alpha(2))+Ti Al (gamma)) titanium aluminides by cathodic charging in a 5% H2SO4 sol ution. Cracks or pits are also observed within the gamma phase regions in the two-phase (alpha(2)+gamma) coexisting grains (such as lamellar grains) but not within the alpha(2) phase or single gamma grains. Wei ghts of the samples decrease with increasing charging time owing to th e crack or pit formation, and this is more drastic in the Ti-50Al allo y than in the Ti-42Al and Ti-45Al alloys. The hydride is thermally sta ble at temperatures up to about 550 K (277 degrees C) and dissociates completely at temperatures between 673 K (400 degrees C) and 723 K (45 0 degrees C).