Surface Aluminizing Treatment And High-Temperature Oxidation Of Ti Alloy

The Ti alloy is regarded as a new type and light metal material and is widelyused to aviation and spaceflight, ocean and engineer, oil and chemistry industry andmedical treatment because of low-density, light-weight, high-strength and corrosion-resistant characters. However, its insufficient oxidation resistance has greatlylimited its practical application, the layer formed on Ti alloy is not a continuousand dense Al2O3 scale, but TiO2 or mixture TiO2 + Al2O3 when it is worked atelevated high temperature and in aggressive environment, which can notsufficiently increase its oxidation resistance of Ti alloy.At present, two attempts have been made to find a method to improve theoxidation resistance of Ti alloy either by microalloying or by surface treatment.The oxidation resistance of Ti alloy may be improved to a certain extent by theaddition of alloying elements such as Nb, Mo, Ta, Cr, Si or W. however, theimprovement of oxidation resistance by addition of these alloying elements is notenough for practical applications, and these elements can both have a deleteriouseffect on the mechanical properties of material and increase its cost. Therefore,surface treatments are thought to be more attractive to improve the oxidationresistance of Ti alloy.There are many surface treatments such as MCrAlY overlay coating,aluminizing, silicide/ceramics, pre-oxidizing at low oxygen pressure, which canimprove the oxidation resistance of Ti alloy. Of the many coatings processes, packcementation seems to be quilt promising because its process is relatively simplyand its equipment need not very high and widely to be used. The pack aluminizingof Ti alloy produces a Al3Ti coating layer, and due to a sufficient amount of Al inthe coating layer, stable and continuous Al2O3 is formed as it is exposed to hightemperature oxidation atmosphere. However, the Al3Ti coating layer that formed on Ti alloy is quilt brittle and,thus, easily spalls with cracks under mechanical and thermal stresses duringaluminizing process and oxidation. The elements of coating layer/substrate ofteninterdiffuse during the oxidation. The addition of ternary alloying elements such asNi, Fe, Cu, Mn, Ag or Cr are confirmed to change the crystal structure of Al3Tifrom low symmetry DO22 to high symmetry LI2 which can improve its ductility. The addition of a layer of diffusion barrier at the interface of coating/substratecan decrease the diffusion rate of elements between the coating and substrate,which can prolong its working lifetime of coating. There also leaves some unsolvedquestions about them such as how to effectively solve the adherence andinterdiffusion of coating/substrate and the effect of each part of pack mixturesduring the aluminizing, in spittle of that the methods can improve its oxidationresistance to some extent. Based on these questions, the major research efforts ofthe present study are as follows:(1) Preoxidizing treatment at low oxygen pressure improves the oxidationresistance of Ti alloy. The surface of Ti alloy with preoxidizing treatment is covered with a continuousoxide, which can effectively decrease the diffusion rate of oxygen, titanium,aluminium atoms, and thus decrease the rate of oxidation; the Ti alloy withpreoxidizing treatment also improves the adherence between the coating andsubstrate, which can improve the oxidation resistance of Ti alloy.(2) The aluminide coating with dispersant Y2O3 formed on Ti alloy improvesthe oxidation resistance. The aluminide coating with dispersant Y2O3 is formed on the surface of Ti alloy,due to the addition of Y2O3, the aluminide coating not only improves the oxidationresistance, but also increases the adherence between the coating and substrate.(3) The preoxidizing treatment improves the ductility of aluminide coatingformed on Ti alloy. To resolve the problem that the aluminide coating easily appears cracks duringthe aluminizing, the preoxidizing treatment before aluminizing can transform thecrystal structure of coating from the brittle phase to compressive phase to improvethe ductility of coating layer, and the preoxidizing treatment can decrease thediffusion rate between the coating layer and substrate, thus it improves theoxidation resistance.(4) The effect of dispersant in the aluminizing is discussed by changing thedispersant in the pack mixture and the aluminizing mechanism. To make clear the effect of dispersant in the aluminizing, the aluminizing ismade by changing the dispersant of the pack mixture, the results show that thedispersant does not take part into the aluminizing process, but also greatly affectthe phase in the aluminide coating layer, the crystal style of Al2O3, the diffusionmechanism of the aluminide coating layer during the high-temperature oxidation. In a word, the research of the present study both improves the oxidationresistance of Ti alloy by most cheap way and discusses the aluminizing mechanismby changing the dispersant in the aluminizing process, which not only more deeplyknows the oxidation law of Ti alloy and the practical application in the field ofhigh-temperature oxidation, but also more deeply know the aluminizingmechanism. At the same time, it establishes a certainly basic knowledge of theoryand application in high-temperature oxidation and protection of Ti alloy andaluminizing mechanism. The author also put forward to some discussing resultsabout the traditional aluminizing mechanism and some referenced and use for...