Interdiffusion in multi-phase diffusion couples of polycrystalline Ti and polysynthetically twinned Ti–49.3at%Al was studied at 973 to 1173K, with the diffusion direction being parallel to the lamellar planes. It was found that a reaction zone of the α2-Ti3Al phase formed between the end materials, and exhibited a deeper penetration in the α2 lamellae than in the primary γ lamellae. Direct measurements of the reaction-zone thickness revealed parabolic growth of the reaction-zone; thus indicating diffusion-controlled growth. The concentration profiles from the Ti, through the reaction-zone and into the polysynthetically twinned γ and α2 lamellae, were measured by means of X-ray spectroscopy in a transmission electron microscope. Deviation from a diffusion-controlled composition profile indicated some degree of interface-controlled growth. The interdiffusion coefficients (figure 48) were largely independent of composition, and could be described by:

D (m2/s) = 7.56 x 10-5 exp[-255.6(kJ/mol)/RT]

L.Pan, D.E.Luzzi: Intermetallics, 2006, 14[1], 68-74

Table 289

Diffusivities of O and N in Ti-25Al and Ti-24Al-15Nb at 1300K