Investigation On High Temperature Oxidation Resistance Of TiAl/Cr And Ti Al₂/Cr Nano-multilayer Films

Ti₃Al alloys have integrated properties of lightweight,high strength,high-temperature resistance,and good creep resistance,and are considered ideal lightweight structural materials in the temperature range of 650～700°C for aero-engines and supersonic vehicle skins.Although Ti₃Al alloys exhibit good oxidation resistance due to their relatively high aluminum content,severe oxidation can still occur at high temperature due to the formation of loose and porous mixed oxidation products of Ti O₂and Al₂O₃,which limit its direct service in high-temperature oxidation environments.With the rapid development of composite reinforcement technology and the demand for equipment for high-temperature short-time environments,the increasing service temperature of Ti₃Al-based materials will further aggravate their high-temperature oxidation and make higher demands on their oxidation resistance.Applying anti-oxidation coatings is an effective way to solve this problem.Firstly,the design of surface protective coating should meet the requirements of high-temperature service environment,and secondly,it should have a good interface matching with the substrate to ensure its high reliability in service.In addition,the"Cr effect"in Ti₃Al alloys has a positive effect on the improvement of high temperature oxidation resistance.Considering these factors,TiAl and TiAl₂coatings,are selected as the anti-oxidant modulation layers for the surface protective coatings owing to their high compatibility and matching with Ti₃Al substrates,and Cr nanolayers are alternately introduced（to induce and promote the formation of continuous and dense Al₂O₃barrier layer by the"Cr effect"）to construct TiAl/Cr and TiAl₂/Cr nano-multilayer films to improve the high-temperature oxidation resistance of Ti₃Al alloys.Based on the above analysis,TiAl/Cr and TiAl₂/Cr nano-multilayer films with different Cr thicknesses(l _Cr)were constructed on Ti₃Al alloy by magnetron sputtering technology and heat treated at 800°C,900°C and 1000°C for 1h to investigate the effects on microstructure,mechanical properties,and high-temperature oxidation resistance.The results are as follows.1.TiAl/Cr nano-multilayer films with different Cr thicknesses(l _Cr)were prepared on Ti₃Al substrates by magnetron sputtering technology and heat treated at 800°C,900°C and 1000°C for 1h,the effect of Cr nanolayer on the microstructure,mechanical properties,and high-temperature oxidation behaviors have been investigated.High hardness and fracture toughness are achieved in the 45 nm-TiAl/10 nm-Cr nano-multilayer film when the amorphous TiAl and nanocrystalline Cr are used as the modulation layer to construct the TiAl/Cr nano-multilayer film.During the isothermal oxidation process of TiAl monolayer film at 800°C,900°C and 1000°C,the strong competitive growth of Ti O₂and Al₂O₃oxidation products hinders the formation of the continuous dense Al₂O₃oxide layer,resulting in a thickness of 1.52μm（800°C）,1.74μm（900°C）and 2.00μm（1000°C）.Compared with the TiAl monolayer film,the TiAl/Cr nano-multilayer film exhibited better oxidation resistance,and achieved the best oxidation resistance in TiAl/Cr films when the Cr layer thickness was 10 nm,and the lowest oxide layer thickness of 357 nm was obtained at 800°C.This can be attributed to the fact that the Cr₂O₃formed by the preferential oxidation of the Cr sublayer at high-temperature oxidation can induce the formation of a continuous dense（Al,Cr）₂O₃oxide layer on the surface,which effectively avoids the competitive growth of Ti O₂and Al₂O₃in TiAl monolayer.The formation of continuous and dense layered oxidation products induced by the Cr layer can effectively block the diffusion of O and greatly improve the oxidation resistance of the TiAl/Cr nano-multilayer films.2.In order to further verify whether the"Cr effect"can play an oxidation-inducing role in the TiAl₂system with higher aluminum content-to promote the formation of a continuous dense oxide layer,TiAl₂/Cr nanoparticles with different Cr thickness(l _Cr)were prepared by magnetron sputtering and heated at 800°C,900°C and 1000°C for 1h.Compared with the TiAl/Cr system,the TiAl₂/Cr multilayer film with the same Cr layer thickness exhibits higher oxidation resistance,and the oxide layer thickness was only 198nm at 800°C.This was due to the increase in the Al content at high temperatures resulting in an increase in the diffusion rate of Al,and the film surface is more likely to form a continuous and dense Al₂O₃barrier layer.Similar to the oxidation results of TiAl/Cr nano-multilayer film,during the oxidation process of TiAl₂/Cr nano-multilayer film,the Cr nanolayer induces the preferential formation of continuous and dense（Al,Cr）₂O₃layer,and finally produces（Al,Cr）₂O₃/Ti O₂/（Al,Cr）₂O₃/Ti O₂layered oxidation sequence,which indicates that the"Cr effect"still works in the TiAl₂/Cr nano-multilayer film,which can effectively improve the oxidation resistance of TiAl₂.