Microstructure evolution and nitriding mechanism of Ti-6Al-4 V alloy in alumina-based refractories

Abstract

The aims of this study were to explore the nitriding mechanism of Ti-6Al-4 V alloy in Al₂O₃-based refractories. Al₂O₃-based composite refractories were fabricated using Ti-6Al-4 V and sintered alumina as main materials, followed by nitriding at different temperatures in a nitrogen atmosphere. Phase and microstructure evolution of the products was analyzed by X-ray diffraction and field-emission scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy. The results reveal a multi-stage nitridation process of Ti-6Al-4 V at different temperatures. At 900°C, nitridation of the Ti-6Al-4 V surface results in the formation of a (Ti,V)N solid solution, while Al and V migrate inward, forming intermetallic compounds Ti₃Al and a-Ti (Ti_1-x-yV_xAl_y) with rich Al and V. At 1100°C, nitridation of Ti₃Al produces intermediate products like Ti₂AlN or Ti₃AlN, with continued inward migration of Al or V to form the intermetallic α₂ phase of (Ti_1-xV_x)_3yAl_y in the center of the alloy particles. At 1300°C, decomposition of Ti₂AlN or Ti₃AlN results in the outward migration of Al atoms and the presence of AlN on the outer layer of Ti-6Al-4 V particles, thus forming a hollow structure. Finally, at 1500°C, the (Ti,V)N solid solution grains grow as a result of further diffusion and dissolution of nitrogen.