Formation and subsequent phase evolution of metastable Ti-Al alloy coatings by kinetic spraying of gas atomized powders

High temperature application of modern Ti-alloys is often limited by their insufficient oxidation resistance at temperatures exceeding 650 °C. One way to overcome this obstacle is application of an overlay coating made of TiAl-based alloys. In this study kinetically sprayed coatings of Ti-46Al-8.5Nb-1Ta (at%) on a commercial alloy IMI-834 were investigated. The feedstock powder fabricated by gas atomization consisted of non-equilibrium α-Ti and β-Ti phases. Heat treatment of the powder at 750 °C triggered transformation to the equilibrium ordered γ-TiAl and α₂-Ti₃Al phases. Mechanical properties of both the as-atomized and the heat treated powders were tested, showing that the as-sprayed one possesses lower hardness and higher ductility. Since such properties of the as-atomized powder are more suited for kinetic spraying, the as-atomized powder was used for coating fabrication. The as-sprayed coatings inherited the non-equilibrium phases of the atomized powder with significant deformation. The coatings were annealed at 750 °C and immediate transformation to a refined microstructure containing the γ-TiAl and α₂-Ti₃Al phases was observed in < 20 min. Longer annealing time resulted in significant grain growth and formation of defect-free grains.