DEVELOPMENT OF LIGHT WEIGHT Ll_2-Al_3Ti ALLOYS AND FORMATION OF GRADED OXIDATION-RESISTANT LAYER FOR TiAl ALLOYS.

点击翻译按钮获取中文摘要

Gamma titanium aluminides (γ-TiAl alloys), having an Ll_0-type structure, are candidate materials for use in future gas turbine aero-engines and automotive engines because of their low density, high specific strength and high stiffness. In air, however, it is well known that titanium aluminide oxidizes at a more rapid rate at temperatures above 1123 K; therefore, the oxidation resistance becomes a critical factor for TiAl alloys to be used at high temperatures (perhaps above 1073 K). Coatings for the TiAl alloy are essential to high temperature oxidation resistance. The recent approaches toward oxidation resistant coatings for titanium aluminides include the aluminide pack coatings, pack coatings with Cr_2_3, SiO_2 and Cr powders, MCrAlY(M=Ni, Fe, Co) alloy coatings, silicides/ceramics coatings, physical vapor deposition coatings, and coatings used a fluidized bed with WO_3 powder. Among the coating processes to improve oxidation resistance, pack cementation is a very simple process an … More d remains a widely used technique for gas turbine components. Especially aluminide coatings are the best documented pack processes used to produce high-temperature protective coatings for nickel-based superalloys.On the other hand, the oxidation resistance of Al_3Ti, having a D0_<22>-type structure, appears to be superior owing to its high aluminum content. Oxidation resistant coatings based on the Al_3Ti phase have been successfully produced on TiAl alloys by diffusion pack aluminizing. But the Al_3Ti is extremely brittle at ambient temperature because of its low-symmetry crystal structure and tetragonal D0_<22> structure; therefore, its application as a coating material for TiAl alloys has been limited. To improve its ductility, recently, much work on the (Al,X)_3 Ti (X=Ni, Fe, Cu, Mn, Cr, Ag and Pd etc.) alloys has been focused on the formation of the cubic Ll_2 crystal structure, with the hope that the Ll_2 structure is highly symmetrical and may have a sufficient number of slip systems for homogeneous deformation. The present authors recently reported that Ti-67Al-8Cr (in mol %) alloy with the Ll_2 structure does possess some intrinsic bend ductility at ambient temperature, and that Ll_2- Ti-61Al-14Cr alloy (with higher chromium content) was more ductile in bending, with a plastic strain of up to 0.9% being recorded.To improve the oxidation resistance of γ-TiAl alloys, Ti-43Al-5Cr(Ll_0) alloy substrates were coated with the Ll_2-(Al,Cr)_3Ti layer by pack cementation or slurry method. Coatings were formed by immersing the substrate in a mixture consisting typically of 90% Ti-61Al-14Cr(Ll_2) alloy powders without activators, or slurry with Ti, Al, And Cr powders, and then heating in an alumina boat under a vacuum of 〜10^<-4> Pa for 1 〜 96 h at 1423 K. The coating layer for γ-TiAl alloys was effectively formed by surface diffusion and bulk diffusion of aluminum, titanium and chromium from pack alloy powders. The coating consisted of the two diffusion layers, an outer Ll_2-phase layer and an inner Ll_0-phase layer. In these layers, the graded alloy composition and microstructures for the oxidation resistant coating was successfully established. The coated alloy showed a remarkable improvement in the oxidation resistance over the uncoated TiAl alloys. Less