Titanium tracer diffusion in grain boundaries of α-Ti, α 2-Ti 3Al, and γ-TiAl and in α 2/γ interphase boundaries

Abstract

Grain boundary (GB) self-diffusion in α-Ti and titanium aluminides (α 2-Ti 3Al and γ-TiAl) was systematically investigated as a function of temperature and composition of the corresponding alloys. 44Ti 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 3Al (Rüsing 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 3Al 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 3Al of stoichiometric composition and even by two orders of magnitude larger than in α 2-Ti 68Al 32 at similar temperatures. Interphase boundary diffusion of 44Ti was studied in the two-phase Ti 48Al 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 3Al alloy. This feature is explained by the specific atomistic structure of the α 2/γ interphase boundary.