Diffusion Research in BCC Ti-Al-Ni Ternary Alloys

Interdiffusion in BCC phase of Ti-Al-Ni ternary system was investigated at 1473 K (1200 °C) by employing the diffusion-couple technique. The raw composition profiles resulting from interdiffusion treatment and retrieved from EMPA were first analytically represented by error function expansion (ERFEX), and the ternary interdiffusion and impurity diffusion coefficients were then extracted by the Whittle-Green and generalized Hall methods, respectively. The obtained main interdiffusion coefficients D~AlAlTi and two cross coefficients, i.e. D~AlNiTi and D~NiAlTi, were found to increase with increasing composition of diffusing species, whereas the values of D~NiNiTi show no noticeable compositional dependence. The impurity diffusivities D~Al(Ti - Ni)∗ and D~Ni(Ti - Al)∗ increase with decreasing the Ni and Al compositions, respectively. The results imply that Al diffusion in β Ti-Al-Ni alloys would occur via an ordinary vacancy diffusion mechanism, whereas Ni diffusion, at least one order magnitude faster than Al, very likely benefits from interstitial diffusion as Fe and Co anomaly diffuse in BCC Titanium alloys.