A study on diffusion couples of Ti and polysynthetically twinned (PST) Ti-Al: II. Interdiffusion results

Interdiffusion in multi-phase diffusion couples of polycrystalline Ti and polysynthetically twinned (PST) Ti-49.3 at.% Al, with the diffusion direction parallel to the lamellar planes, is investigated in the temperature range 973-1173 K. A reaction zone (RZ) of the α2-Ti3Al phase forms between the end materials and exhibits deeper penetration in the α2 lamellae than in the primary γ lamellae. Direct measurements of the RZ thickness reveal a parabolic growth of the RZ, indicating a diffusion-controlled growth macroscopically. Concentration profiles from the Ti, through the RZ, into the PST γ and α2 lamellae are measured by X-ray spectroscopy in a transmission electron microscope. Deviations from a diffusion-controlled composition profile indicate some extent of interface-controlled growth. The interdiffusion coefficients are found to be largely independent of composition with a temperature dependence that obeys the Arrhenius relationship with a pre-exponential factor of D0=(7.56±7.14)×10−5m2/s and an activation enthalpy of Q=(255.6−8.3+8.9)kJ/mol.