Study On Plasma Surface Chromizing Of TiAl Alloy And Its Effects On Oxidation Resistance

TiAl alloys are superior high temperature structural materials for aerospace vehicle due to their low density,high specific strength and specific rigidity.However,insufficient high temperature oxidation resistance of TiAl alloy is still a problem.Surface treatment is one of the most effective solutions which usually improve the oxidation resistance of TiAl alloy by forming a protective oxide film on its surface,such as Al₂O₃,Cr₂O₃,etc.In this study,chromising of TiAl alloys was conducted by double glow plasma surface alloying technique and the optimized chromising layer was obtained by investigating the processing parameters systematically.The alloyed layer showed gradient composition profile and strong cohesion with the substrate.Fretting and ball-on-disc sliding wear tests were carried out at room temperature and high temperature respectively.The oxidation kinetics of TiAl and the chromising layer on TiAl surface were observed in air at 650℃,900℃,1000℃and the oxidation mechanisms were discussed.The process of particles colliding under double glow plasma surface alloying was simulated to calculate the sputtering and absorbing of alloy element and the surface vacancies concentration under ion bombardment.The research results show that:1.The thickness of alloyed layer under the optimization parameters reached 60μm;the layer had good adhesion with substrate and the composition changed gradually with the depth.TiAlCr alloyed layer performed well on room temperature fretting and high temperature sliding due to the changing of wearing mechanism brought by the surface alloying processing.2.At the early stage oxidation of TiAl at 900℃,θ-Al₂O₃ was formed in the scale and then changed intoα-Al₂O₃,which caused the germination and growth of cracks during the process.As a result,the oxidized layer would easily be damaged as the oxidation time went by.3.The Cr element functioned in two ways:1)firstly,protective Cr₂O₃ film was formed and this film possessed self-repairing characteristic as the oxidation overreached the Cr alloyed layer;2)when the alloyed layer was almost consumed and the content of Cr was too low to form Cr₂O₃ film,Cr took place the position of Ti in TiAl alloy.It enhanced the activity of Al and lowered that of Ti,facilitating the formation of α-Al₂O₃ directly on the surface.4.Particles' colliding was simulated with SRIM2003 software,and the obtained sputtering yield of target and absorbing of substrate were highly comparable with experiment results.The sputtering yield reached nearly 1.0%.5.With the bombarding of Cr⁺,Ar⁺,the vacancies concentration of TiAl on the surface was about 0.2～0.5×10^-4%,and the depth reached 10(?).