Microstructure And Properties Of Al And Al-si Pack Cementation Diffusion Coating On Zirconium-Titanium Alloys

Titanium and titanium alloys have been widely used in chemical industry,aerospace components and military fields due to their high specific strength,good corrosion resistance,stable mechanical properties at medium temperature and low density.However,the poor high temperature oxidation resistance of titanium and titanium alloys limits their further application as high temperature components of aero-engine.In this paper,Al coating and Si-Al coating were prepared on the surface of Ti-30Zr-6Al-4V（wt.%）alloy by powder embedding method,respectively.The phase composition,surface morp hology and microstructure of the coatings were characterized,and the high temperature oxidation resistance and mechanism of the different coatings were explored.The results show that three kinds of aluminized coatings with 15%,25%and 35%Al were prepared at 1000℃/4 h,and their thicknesses are about 110μm,165μm and 235μm,respectively.Aluminized layer is divided into two layers:permeable layer and transition layer.The main composition of permeable layer is Al₃Ti phase,and the main composition of transition layer is Ti-Al phase and Ti₃Al phase.Four Si-Al coatings with different proportions of permeating agents were prepared at 1000℃/4 h.When the Al content in the penetrating agent is 25%,the Si content is 5%,8%,10%,15%,and the coating thickness is about 55μm,30μm,25μm,and 14μm,respectively,the whole is mainly aluminized.When the Si content is 5%,8%and 10%,the permeating layer is divided into outer layer and transition layer.The outer layer is mainly composed of Al₃Ti phase,and the transition layer is Al-Ti and Ti₃Al.Ti₅Si₃ silicides are dispersed inside the outer layer.With the increase of Si content,the more silicides are gathered and the closer to the surface of the coating.When the Si content is 15%,the permeation layer is a single layer,the permeation layer is Al₃Ti,TiAl,Ti₃Al from the outside to inside,and the silicide Ti₅Si₃ is very close to the surface of the coating.The total thickness of the aluminosilicate layer becomes thinner with the increase of Si content.Zr and Si exist in the form of solid solution in Ti-Al compounds.The oxidation behavior of Al coating and Si-Al coating in air at 800℃and 900℃for90 h was studied by isothermal frequency conversion cyclic oxidation experiment.The results show that after 90 h of oxidation at 800℃and 900℃,the matrix alloy with a size of 12 mm×12 mm×3 mm is completely oxidized and exists in the form of powder or fragmentsis,and the weight gain is 266.67 mg·cm^-2.After oxidizing at 800℃for 90 h,both Al coating and Si-Al coating can improve the high temperature oxidation resistance of the matrix alloy.The oxidation resistance of aluminized coating is the best when the Al content is 15%,and the oxidation gain is 1.32 mg·cm^-2.The high temperature oxidation resistance of 5%Si+25%Al coating is the best,and the oxidation gain is 0.12 mg·cm^-2.After oxidizing at 900℃for 90 h,the Al coating has good oxidation resistance.The oxidation resistance of Si-Al coating can be improved only at the initial oxidation stage.When the Al content in the aluminized coating and the Si content in the silicon-aluminum co-permeated coating increases,the high-temperature oxidation resistance decreases.