Oxidation behavior of a high Si content Al-Si composite coating fabricated on Ti-6Al-4V substrate by mechanical alloying method

An Al-Si coating was successfully fabricated on Ti-6Al-4V alloy substrate by means of mechanical alloying method. The coating was prepared with Al-33.3 wt% Si powder mixture. The coating had a composite structure and its thickness was about 350 μm. The mass gain and the oxidation rate of the substrate at 850 °C were largely decreased because of the as-synthesized coating. The oxidized coating had a multilayered structure, which included an Al diffused layer, a Ti5Si4 interlayer, an inner TiAl3 layer, an outer Ti-Al-Si alloy layer and an oxide layer from the inner substrate to the top coating surface. During oxidation, the coating was melted and gradually formed the ternary Ti-Al-Si alloy with the diffusion of Ti from the substrate to the coating. The Al diffused layer formed with the adequate interdiffusion of the coating and the substrate. The Ti5Si4 interlayer and the inner TiAl3 layer formed owing to their low formation energy at the elevated temperature. The outer Ti-Al-Si alloy layer mainly included Ti(AlxSi1-x)2 phase, which formed due to the decrease of Al after the formation of the TiAl3 layer and the oxide layer. The direct oxidation of the outer layer promoted the formation of a continuous oxide layer on the coating surface. These layers could retard and prevent the inward diffusion of oxygen at high temperature. The as-synthesized Al-Si coating could effectively improve the oxidation resistance of the Ti-6Al-4V substrate.