| ?-TiAl alloy was considered to be the most valuable and potential new material for elevated temperature,which exhibit valuable properties i.e.relatively low density,high specific strength and temperature strength,good creep resistance.It is hoped that it can replace nickel-based high temperature material to reduce the weight of aeroengine components.However,the main limiting factor influencing their potential use is an insufficient oxidation resistance at temperatures over 750?.In order to solve this problem,Ta-W alloy coating was prepared on the surface of ?-TiAl alloy by double glow plasma alloying technology.The oxidation mechanism of ?-TiAl matrix and Ta-W alloy coating was analyzed by constant temperature oxidation test at 800? and 900?.The oxygen adsorption was studied by first principles to further explore the oxidation mechanism.The optimum alloying process parameters were determined by orthogonal test,and the Ta-W alloy coating with smooth and compact surface,fine grain,no obvious defects and good bonding with the substrate was obtained.The total thickness of the modified layer was about 25?m,of which the deposition layer was about 15?m,the diffusion layer was about 10?m.XRD phase analysis showed that a single body-centered cubic structure was formed on the surface of the alloy layer,and W element was completely dissolved in Ta,which played a role of solid solution strengthening.The average microhardness of the matrix and the Ta-W alloy layer were 348.3HV0.1 and 1021.3HV0.1,respectively.The microhardness of the alloy layer was about 2.93 times of that of the matrix.Nanoindentation measurements showed that Ta-W alloy coating has high nano-hardness,modulus of elasticity and resistance to plastic deformation.The high temperature oxidation resistance of ?-TiAl matrix and Ta-W alloy specimens at 800? and 900? was investigated by comparison of isothermal oxidation tests.Alternate oxidation of Ti and Al occurs in the oxidation process of the substrate,and the mixed oxide films of titanium dioxide and alumina were formed.The oxidation products of Ta-W alloyed samples at the initial stage of oxidation were Ta2O5,(Ta,O),Ta O2 and WO3.With the increase of oxidation time,the elements of Ta and W were continuously consumed,and the oxidation products were mainly titanium dioxide and Al Ta O4.When the substrate was oxidized at 800?,the oxidation curve presents the law of “linear-parabolic-linear-parabolic” cyclic alternation,and at 900?.When the substrate was oxidized at 900?,the oxidation curve presents the law of “parabolic-linear” rule.At this time,the protective effect of the oxide film on the substrate was almost zero.The oxidation curves of Ta-W alloyed samples show parabolic shape when they are oxidized at 800? and 900?.The mixed oxide film formed by the alloy layer has high oxidation resistance at high temperature.The first-principles method was used to study the alloying and oxidation mechanism.The calculation results showed that the Al site was preferentially occupied in the matrix of ? phase when Ta and W elements were doped.By calculating the vacancy formation of Ti and Al before and after Ta W doping,it can be found that the doping of Ta and W was beneficial to the formation of Ti and Al vacancies,the diffusion ability of elements in the alloy layer was enhanced,and the formation of Ta-W alloy layer with high solid solubility in the alloying process was facilitated.The results of O adsorption calculation showed that the O atoms in the substrate of ?-TiAl were more easily adsorbed on the fcc-Al and hcp-Al sites with more Ti atoms around it,which increases the formation rate of Ti-O bond.The change of the adsorption potential of O on the ?-TiAl(111)surface after Ta W doping proves that Ta W doping can inhibit the internal diffusion of O.The electronic structure analysis shows that the activity of the adsorbed O increases with Ta,W and Al atoms,while the interaction with Ti atoms weakens.Therefore,Ta W doping was very beneficial to improve the oxidation resistance of ?-TiAl alloy. |
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