Microstructure And Performance Of Al-Y Co-Deposition Coating On The Surface Of TiAl Alloy

In order to improve the high-temperature performance of TiAl alloy and improve its failure caused by friction and thermal shock,Al-Y coating was prepared by pack cementation process.SEM,EDS and XRD were used to analyze the influence of different activators,different holding time and different content of rare earth elements on the microstructure of the co-deposition coatings,and the forming mechanism of the pack cementation coating has been revealed in this paper.Moreover,the high temperature oxidation resistance,hot corrosion performance,thermal shock resistance,friction and wear resistance of both TiAl alloy and Al-Y coating were evaluated,and the failure mechanism was also clarified.The results show that:The Al-Y coatings prepared with different activators,different contents of rare earth elements and different holding times have similar structures,however the thickness,density and adhesion force between the coatings and alloy were significantly different.The coatings prepared by NH₄Cl have compact structure and no cracking,but the coatings prepared by Na F have more diffusion holes and penetrating cracks,meanwhile the coatings prepared by Al Cl₃·6H₂O have dendritic penetrating cracks,which indicates that the bonding force of the coating itself is poor;The addition of rare earth Y element(2 wt.%)can increase the thickness of the permeable layer,but excessive Y element(4 wt.%)can inhibit the formation of Al gaseous halide and hinder the diffusion of Al element,resulting in the phenomenon that the thickness of the coating decreases instead of increasing with the addition of Y element.After considering various factors,the preparation process of Al-Y co-deposition coating on TiAl alloy surface is optimized as follows:A pack mixture with a composition 15Al-6NH₄Cl-2Y₂O₃-77Al₂O₃(wt.%),co-deposition temperature:930?,co-deposition time:2 h.Coating prepared by this process have a good metallurgical combination with the alloy and a composite structure with a thickness of 53?m,mainly composed of a TiAl₂outer layer wrapped with Al₂O₃,a TiAl₃middle layer and a TiAl₂inner layer.Thermal shock experiment at 1000?show that oxidation occurs to a certain extent in both TiAl alloy and Al-Y coating during thermal shock.Visual cracks appear in TiAl alloy after 31 times of thermal shock,and the mixed oxide layer formed after oxidation is not protective which is easy to peel off and produce"notch".Once the"notch"is generated,the crack will continue to expand under the impact stress,and finally penetrated through the alloy;Al-Y coating is also oxidized after 43 thermal shocks,and a uniform dense Al₂O₃protective oxide layer is formed on the surface with strong anti-peeling ability,so the incubation period of crack is longer,but some cracks will develop into penetrating cracks after 43 times of thermal shock.Moreover,the crack does not propagate laterally at the interface and the structure of Al-Y co-deposition coating is keep intact without peeling off during the thermal shock process,which means the Al-Y coating prepared by pack cementation process can improve the thermal shock resistance of TiAl alloy to a certain extent.Friction and wear tests of TiAl alloy and Al-Y coating with GCr15 ball show that the friction coefficient of TiAl alloy increases rapidly and then fluctuates in the range of0.45-0.62,while the friction coefficient of Al-Y coating increases slowly,and the value of friction coefficient tends to be gentle after 45 min,remaining at about 0.39 with a small amplitude,indicating that Al-Y co-deposition coating has obvious wear reduction effect;Wear trace analysis shows the wear mechanism of TiAl matrix is ploughing and abrasive,with obvious weight loss after wear,meanwhile Al-Y coating show obvious weight gain and does not exhibit obvious wear,which indicates that the Al-Y co-deposition coating can improve the friction and wear resistance of TiAl alloy effectively.High temperature oxidation experiments of both TiAl alloy and Al-Y coating show that:the loose Ti-rich oxide outer layer and TiO₂+Al₂O₃secondary outer layer formed on the surface of TiAl matrix during the oxidation process can not hinder the internal diffusion of O element,resulting in the repeated peeling of oxide film,and eventually leading to catastrophic oxidation.Moreover,dense Al₂O₃film is formed on the Al-Y coating during oxidation,which hinders the internal diffusion of O element and effectively improves the high-temperature oxidation resistance of the alloy.With the prolonging of oxidation time,the inner/outer diffusion of Al and the outer diffusion of Tilead to the peeling off of the oxide film,but the remaining permeable layer still maintains good metallurgical bonding with the alloy and still have certain protective properties.Thermal corrosion performance under the mixed molten salt system of 25%Na Cl+75%Na₂SO₄(wt.%)of both TiAl alloy and the coating show that the lamellar?₂-Ti₃Al phase in TiAl alloy was selectively corroded with O and S medium at the initial stage of corrosion,resulting in serious spalling of the scale and catastrophic corrosion.However,Al-Y coating can form dense Al₂O₃film at the early stage of hot corrosion,which effectively protects the coating.With the extension of hot corrosion time,the coating gradually cracked under the action of corrosion stress and element diffusion inside and outside.The formation of cracks becomes a"channel"for diffusion of S and O elements.At this time,The TiAl₃phase in the coating continues to decompose,providing Al elements to the crack,forming dense Al₂O₃to compensate the crack,delaying the internal diffusion rate of S and O,and significantly improving the thermal corrosion resistance of TiAl alloy.