| Ti Al alloy has the following characteristics: low density, high specific strength, high temperature creep resistance and oxidation resistance, which made them has been widely used in the aerospace, engineering, manufacturing, and other fields. Wherein the directionally solidified Ti Al alloy is specially used in some special fields due to its microstructure was refined and aligned. However, due to the high chemical activity under high temperature molten state of Ti Al alloy, which very easy to react with the mold material, causing to alloy castings produced all kinds of casting defects, these defects directly affect the quality and performance of the Ti Al castings. This makes the production cost of Ti Al alloy becomes higher, and the subsequent treatment process will also be more complicated, which become an urgent problem.In this paper, by changing the of Al and Nb contents extent in castings with different coatings, the metal-coating interface, chemical composition, phases at interface and microhardness variation from surface to the inside of samples is evaluated respectively, then get the corresponding coatings of descending interface reaction extent is: Zr O2>Al2O3>Y2O3. This shows the degree of interfacial reaction not only related to thermodynamic stability of the coating material, but also impacted by its solubility in high temperature melt to a large extent. In addition, the increase of Al contents in the melt made the melt activity weakened, which reduced the degree of interfacial reaction, thereby the interface reaction layer thickness are following reduced. Unfortunately, the variation law of Nb contents in the melt are contrast to of Al contents. In the interfacial reaction process, the existence of phases dissolution and elements diffusion at the interface made a series of new phases and partial solid solution were formed at the interface after solidification, resulting in the formation of hardness gradient at the casting surface.This paper also obtained the standard free formation energy decreasing order of three coatings by standard free energy formula at 1873 K is: Zr O2>Al2O3>Y2O3. This indicates that the stability of Y2O3 is best and hardly breaking down; Al2O3 followed, and Zr O2 is most easily decomposed, which consistent with experimental results. The Meidema generated thermal model was used to calculate the Ti-Al-Nb ternary system activity, the results showed that in the range of 46%-50%, when the Al contents is greater, the value of the corresponding system activity are reduced, which showed substantially a linear relationship and described that addition of Al elements can reduce the activity of the alloy melt within this range. The results also showed that in the range of 2%-9%, the greater the Nb contents, the greater the value of the corresponding system activity, which showed approximately quadratic exponential relationship and described that addition of Nb elements can significantly increase the activity of the alloy melt within this range.On the basis of experiments and calculations, the Materials Studio software is used to execute the molecular dynamics simulations of interface reaction between Ti Al alloys and different coatings. Discover module have been used to calculate the combination energy between Ti Al alloys and different coatings to determine the extent of reaction, and the simulation results are well consistent with the experimental results. Besides, oxygen atom diffusion coefficient of alloys with different coatings were calculated by Forcite module, the results showed that the maximum oxygen atom diffusion coefficient corresponding to the Al2O3, and the remaining two coatings corresponding fairly. This indicates that chemical reaction dominated the interfacial reaction of alloy with Al2O3, and the other two corresponding materials are diffusion-based model. |
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