Titanium tracer diffusion in grain boundaries of alpha-Ti, alpha(2)-Ti3Al,and gamma-TiAl and in alpha(2)/gamma interphase boundaries

Grain boundary (GB) self-diffusion in alpha -Ti and titanium aluminides (al pha (2)-Ti3Al and gamma -TiAl) was systematically investigated as a functio n of temperature and composition of the corresponding alloys. Ti-44 GB diff usion in alpha -Ti of high purity was studied in the broad temperature inte rval from 840 to 1120 K. The results can be interpreted in terms of 'normal ' vacancy-mediated GB diffusion with a ratio of GB and volume diffusion act ivation enthalpies Q(gb)/Q approximate to 0.62. Though Ti bulk self-diffusi on is practically independent on composition in the phase Ti3Al (Rusing J, Herzig Chr. Intermetallics 1996;4:647), Ti GB self-diffusion reveals a dist inct compositional dependence - with increasing Al content on the Al-rich s ide of alpha (2)-Ti3Al the GB diffusivity systematically decreases. On the other hand, the GB diffusion data in gamma -TiAl reveal practically no comp ositional dependence. The comparison of the absolute values shows that the Ti GB diffusivity in gamma -TiAl is by about one order of magnitude larger than in alpha (2)-Ti3Al Of stoichiometric composition and even by two order s of magnitude larger than in alpha (2)-Ti68Al32 at Similar temperatures. I nterphase boundary diffusion of Ti-44 was studied in the two-phase Ti48Al52 alloy with samples providing an unidirectional lamellar alpha (2)/gamma st ructure. The absolute values of the alpha (2)/gamma interphase diffusivity turned out to be very similar to the GB diffusion data in the Al-richest al pha (2)-Ti3Al alloy. This feature is explained by the specific atomistic st ructure of the alpha (2)/gamma interphase boundary. (C) 2001 Elsevier Scien ce Ltd. All rights reserved.