ROLE OF GRAIN BOUNDARIES OF HYDRIDE PRECIPITATION IN TITANIUM.

Experimental data show that hydrogen, present in Ti in concentrations as low as 100-200 ppm by weight, precipitates as TiH//2 platelets along specific habit planes of the alpha titanium grain boundaries. The intergranular hydrides are found to be anisotropic, i. e. , they are found to precipitate along certain grain-boundary segments while not along others. The distribution and the orientation of these hydride platelets are significantly altered when they precipitate in the presence of an applied stress. A study of the effect of grain boundary density, and hence grain size, leads to the conclusion that for the same hydrogen concentration, smaller grain size will tend to suppress the precipitation of TiH//2 along grain boundaries. The enhancement for larger grain size Ti is primarily attributable to a change in the texture of titanium so as to increase the interfacial areas of those types of boundaries favorable for TiH//2 precipitation.