Trans. Nonferrous Met. Soc. China 25(2015) 803−810 Microstructure and high temperature oxidation resistance of Si−Y co-deposition coatings prepared on TiAl alloy by pack cementation process Yong-quan LI1,2, Fa-qin XIE2, Xiang-qing WU2 1. School of Materials Science and Engineering, Beifang University of Nationalities, Yinchuan 750021, China; 2. School of Aeronautics, Northwestern Polytechnical University, Xi’an 710072, China Received 31 March 2014; accepted 10 December 2014 Abstract: In order to improve the high temperature oxidation resistance of TiAl alloy, Y modified silicide coatings were prepared by pack cementation process at 1030, 1080 and 1130 °C, respectively, for 5 h. The microstructures, phase constitutions and oxidation behavior of these coatings were studied. The results show that the coating prepared by co-depositing Si−Y at 1080 °C for 5 h has a multiple layer structure: a superficial zone consisting of Al-rich (Ti,Nb)5Si4 and (Ti,Nb)5Si3, an out layer consisting of (Ti,Nb)Si2, a middle layer consisting of (Ti,Nb)5Si4 and (Ti,Nb)5Si3, and a γ-TiAl inner layer. Co-deposition temperature imposes strong influences on the coating structure. The coating prepared by Si−Y co-depositing at 1080 °C for 5 h shows relatively good oxidation resistance at 1000 °C in air, and the oxidation rate constant of the coating is about two orders of magnitude lower than that of the bare TiAl alloy. Key words: TiAl alloy; Si−Y co-deposition coating; microstructure; high temperature oxidation resistance; pack cementation process 1 Introduction In recent years, Ti−Al based intermetallics have received significant attention, since they possess low density, high specific strength and low cost compared to currently used nickel-based super alloys [1−3]. It is estimated that, with the use of TiAl alloy in aero-engine, potential component weight savings of over 50% can be achieved over conventional super alloys [4]. However, the high temperature oxidation resistance of TiAl alloys is relatively poor because they can hardly form a continuous Al2O3 scale, but the non-protective TiO2 and Al2O3 mixed scale [5]. This becomes a “notable” problem that impacts the performances of TiAl alloys [6]. Thus, a protective coating is necessary for the high- temperature applications of TiAl alloy. Hence, a number of techniques including ion implantation, plasma spray and sputtering deposition have been studied in the past decades to improve the oxidation resistance of TiAl alloy [7]. Among these techniques, the pack cementation, which is a combination of high volume, low capital and operating cost and applicable for a wide range of shapes and sizes, has been widely accepted and used in a broad range of Foundation item: Project (2014JZ012) supported by the Natural Science Program for Basic Research in Key Areas of Shaanxi Province, China Corresponding author: Fa-qin XIE; Tel/Fax: +86-29-88494371; E-mail: fqxie@nwpu.edu.cn DOI: 10.1016/S1003-6326(15)63666-4 applications to enhance the surface properties of nickel- based superalloys and steel components with great success [8], but that was seldom reported on TiAl alloy. Due to the formation of a protective SiO2 scale with sufficient low growth rate at high temperature, silicide coatings were extensively employed to improve the surface properties of nickel and Ti components [9]. However, the brittleness of silicide coatings always leads to cracking and spalling. Therefore, modified elements such as Al, B, Zr and Y. have been employed to develop the silicide coatings. Among these elements, Y has been reported to be beneficial in reducing the grain size and improving the ductility of the coatings [10−13]. In this work, Si−Y co-deposition coatings were prepared on a TiAl alloy by halide activated pack cementation (HAPC) technique. The structures and the phase constituents of the Si−Y co-deposition coatings were studied. The oxidation behaviors of both TiAl substrates and coatings at 1000 °C were also investigated. 2 Experimental The TiAl alloy used in this work was supplied by the State Key Laboratory of Solidification Processing of
