Evaluation of growth kinetics of aluminide coating layers on Ti-6Al-4V alloys by pack cementation and the oxidation behaviours of the coated Ti-6Al-4V alloys

Abstract

Ti-6Al-4V(Ti64) alloy has excellent mechanical properties and corrosion resistances, but poor oxidation properties at high temperatures. It is necessary to improve the oxidation resistances of the alloy in order to expand its application in the aerospace and automotive fields. This study aimed to improve oxidation resistances through diffusion coatings on the surface of the alloy. Aluminide coatings on Ti64 alloys have been investigated via pack aluminium process at temperature ranges of 400–800 °C. Al and/or TiAl3 layers were formed at the interface between the Ti64 alloy and the Al coating layer, when the Ti64 alloys were heat-treated at 400 and 500 °C. TiAl3 layer was manufactured on Ti64 alloys at 600, 700 and 800 °C. The growth of the coated TiAl3 layer followed diffusional growth manner, and the estimated value of the nucleation barrier was the lowest among the intermediate phases of the Ti-Al system. The coating kinetics were discussed together with free energy diagram observations. The activation energy for growth of the TiAl3 layer was estimated as ~82 kJ/mol in the experimental temperature ranges. At the same time, isothermal oxidation tests of the bare Ti64 alloy and the aluminized Ti64 alloys were performed at 1000 °C. By confirming the diffusion pathway of the Ti-Al-O ternary system, the oxidation behaviours and resistances of the alloys were compared and analysed.