Preparation And High Temperature Oxidation Resisitance Of Ti₅Si₃/Al₂O₃ Composite Diffcusion Coating On γ-TiAl-based Alloys Surface

γ-TiAl-based alloys are high-temperature lightweight alloy with high specific strength,high hardness,low density and excellent creep resistance.It is widely used in energy-saving,lightweight aerospace,automotive and other fields.However,the high-temperature oxidation resistance ofγ-TiAl-based alloys above 800°C is insufficient,which limits their application as high-temperature structural materials.Preparing Ti₅Si₃/Al₂O₃composite diffusion coating on the surface ofγ-TiAl-based alloy can improve its high-temperature oxidation resistance.This article used Al₂O₃-SiO₂powder to successfully prepare Ti₅Si₃/Al₂O₃composite diffusion coating on the surface of Ti-48Al-2Cr-2Nb（Ti4822）alloy through vacuum heat treatment.Researched the high-temperature oxidation behavior of Ti4822 alloy prepared by powder metallurgy,the formation mechanism of Ti₅Si₃/Al₂O₃composite diffusion coating and the high-temperature oxidation behavior of this composite diffusion coating.The tissue structure and phase characteristics of the coating were analyzed,as well as the phase composition and tissue structure characteristics of the coating and substrate alloy oxide layer.The following conclusions were drawn:（1）Ti4822 alloy with metallurgical defects on the surface was prepared using powder metallurgy method.The presence of surface defects increases the actual contact area between the alloy and O,resulting in an increase in unit area oxidation weight gain and oxidation rate.Oxide clusters were generated at the surface defect sites,consisting of aggregated and irregularly distributed mixture of Ti O₂and Al₂O₃.（2）Ti4822 alloy was embedded in Al₂O₃-SiO₂powder and subjected to thermal vacuum treatment.After being held at 1000°C for 5 hours,a Ti₅Si₃/Al₂O₃composite diffusion coating was successfully prepared on the surface of the alloy through thermal diffusion.The coating maintained a layered structure with an outer layer of Ti₅Si₃,a middle layer of Al₂O₃,and an inner layer of Ti₅Si₃.（3）During the heat treatment process,a displacement reaction occurs between SiO₂in the Al₂O₃-SiO₂powder and Ti elements in the substrate,resulting in the formation of Ti₅Si₃phase.Free O reacts with Al elements in the substrate to form Al₂O₃at the bottom of the coating,while Al₂O₃in the powder is continuously covered by newly formed Ti₅Si₃phase and distributed on the surface.These two parts of Al₂O₃are squeezed by newly formed Ti₅Si₃phase towards the middle part of the coating to form an area enriched with Al₂O₃.（4）The Ti₅Si₃/Al₂O₃composite diffusion coating has an oxidation weight gain of only2.39 mg/cm²per unit area after being oxidized for 100 hours at 900°C,while the Ti₅Si₃coating has an oxidation weight gain of 2.93 mg/cm²per unit area and the Ti4822 alloy has a weight gain of 10.30 mg/cm²per unit area.The average thickness of the oxide layer on the Ti4822 alloy is 1.5 times that of the Ti₅Si₃/Al₂O₃composite diffusion coating.Preparing a Ti₅Si₃/Al₂O₃composite diffusion coating on the surface of a Ti4822 alloy can improve its high-temperature oxidation resistance.（5）A layered structure is formed on the surface of Ti₅Si₃/Al₂O₃composite diffusion coating,with both outer and inner layers composed of Ti O₂,Al₂O₃,and SiO₂,while the middle layer is composed of Al₂O₃.This relatively dense Al₂O₃layer can effectively prevent the diffusion of external O elements and substrate Ti elements.