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

Abstract

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 α 2-Ti 3Al phase forms between the end materials and exhibits deeper penetration in the α 2 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 α 2 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 D 0 = ( 7.56 ± 7.14 ) × 10 − 5 m 2 / s and an activation enthalpy of Q = ( 255.6 − 8.3 + 8.9 ) kJ / mol .