Microstructure and oxidation behavior of B Al layers on Ti-6Al-4V alloy by REO-boriding and aluminizing at 700 °C, 800 °C, and 900 °C

A high-temperature antioxidant layer on Ti-6Al-4V alloy was prepared by a two-step method of REO-boriding followed by aluminizing (REOs are rare-earth oxides, which are Nd2O3, Gd2O3, and Tm2O3). The microstructure and oxidation behavior of the layer were investigated at temperatures of 700 °C, 800 °C, and 900 °C in air. The findings unveiled that the layer significantly enhances the antioxidant capacity of Ti-6Al-4V alloy. After cyclic oxidation at 900 °C for 100 h, the average weight gain of the layer is only one fortieth of that of the matrix. TiAl3 in the layer can react with O2 to form a dense continuous Al2O3 layer, which effectively inhibits the diffusion of O2 to the matrix. During the cyclic oxidation, the thicker TiAl2, TiAl, and Ti3Al diffusion layers are formed sequentially between TiAl3 layer and matrix, and TiAl2 layer can also provide the good oxidation resistance.