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

γ钛铝化物（γ-TiAl合金）具有ll_0型结构，由于其低密度、高比强度和高刚度，是未来燃气涡轮航空发动机和汽车发动机的候选材料。然而，在空气中，众所周知，在1123 K以上的温度下，铝化钛的氧化速度更快；因此，抗氧化性成为TiAl合金在高温（可能高于1073 K）下使用的关键因素。TiAl合金的涂层是抗高温氧化的关键。近年来，钛铝化物的抗氧化涂层方法包括铝化物包覆涂层、Cr_2_3、SiO_2和Cr粉末包覆涂层、MCrAlY（M=Ni, Fe, Co）合金包覆涂层、硅化物/陶瓷包覆涂层、物理气相沉积涂层和WO_3粉末流化床包覆涂层。在提高抗氧化性的涂层工艺中，填料胶结是一种非常简单的工艺，在燃气轮机部件中仍然是一种广泛使用的技术。特别是铝化物涂层是最具记录的包覆工艺，用于生产镍基高温合金的高温保护涂层。另一方面，D0_<22>型结构的Al_3Ti由于铝含量高，其抗氧化性能较好。利用扩散包渗铝技术在TiAl合金上成功制备了Al_3Ti相的抗氧化涂层。但由于Al_3Ti的低对称晶体结构和四方D0_<22>结构，在常温下极易脆性；因此，它作为TiAl合金涂层材料的应用受到了限制。为了提高（Al，X）_3 Ti （X=Ni, Fe, Cu, Mn, Cr， Ag和Pd等）合金的延展性，近年来对其进行了大量的研究，主要集中在形成立方Ll_2晶体结构上，希望Ll_2结构具有高度的对称性和足够数量的滑移系统以实现均匀变形。本文作者最近报道了具有Ll_2结构的Ti-67Al-8Cr （mol %）合金在室温下具有一定的固有弯曲延展性，而具有较高铬含量的Ll_2- Ti-61Al-14Cr合金的弯曲延展性更强，塑性应变可达0.9%。为了提高γ-TiAl合金的抗氧化性能，采用填料胶结或浆液法在Ti-43Al-5Cr（Ll_0）合金基体上涂覆Ll_2-(Al,Cr)_3Ti层。将基体浸泡在不含活化剂的90% Ti- 61al - 14cr （Ll_2）合金粉末或Ti、Al和Cr粉末的混合物中，然后在1423 K下，在~ 10^<-4> Pa的真空条件下加热1 ~ 96 h，形成涂层。通过铝、钛和铬的表面扩散和体扩散，可以有效地形成γ-TiAl合金涂层。该涂层由两个扩散层组成，一个是外部的ll_2相层，一个是内部的ll_0相层。在这些层中，成功地建立了抗氧化涂层的梯度合金成分和显微组织。与未涂覆的TiAl合金相比，涂层合金的抗氧化性能有显著提高。少

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