Titanium tracer diffusion in grain boundaries of α-Ti, α2-Ti3Al, and γ-TiAl and in α2/γ interphase boundaries

Abstract Grain boundary (GB) self-diffusion in α-Ti and titanium aluminides (α 2 -Ti 3 Al and γ-TiAl) was systematically investigated as a function of temperature and composition of the corresponding alloys. 44 Ti GB diffusion in α-Ti of high purity was studied in the broad temperature interval 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 activation enthalpies Q gb / Q ≈ 0.62. Though Ti bulk self-diffusion is practically independent on composition in the phase Ti 3 Al (Rusing J, Herzig Chr. Intermetallics 1996;4:647), Ti GB self-diffusion reveals a distinct compositional dependence — with increasing Al content on the Al-rich side of α 2 -Ti 3 Al the GB diffusivity systematically decreases. On the other hand, the GB diffusion data in γ-TiAl reveal practically no compositional dependence. The comparison of the absolute values shows that the Ti GB diffusivity in γ-TiAl is by about one order of magnitude larger than in α 2 -Ti 3 Al of stoichiometric composition and even by two orders of magnitude larger than in α 2 -Ti 68 Al 32 at similar temperatures. Interphase boundary diffusion of 44 Ti was studied in the two-phase Ti 48 Al 52 alloy with samples providing an unidirectional lamellar α 2 /γ structure. The absolute values of the α 2 /γ interphase diffusivity turned out to be very similar to the GB diffusion data in the Al-richest α 2 -Ti 3 Al alloy. This feature is explained by the specific atomistic structure of the α 2 /γ interphase boundary.