Long-term oxidation behavior of silicon-aluminizing coating with an in-situ formed Ti₅Si₃ diffusion barrier on γ-TiAl alloy

Highlights

• A Ti (Al, Si)₃ silicon-aluminizing coating was prepared on γ-TiAl alloy.

• The coating significantly improved the oxidation resistance of γ-TiAl alloy.

• The formation of Ti₅Si₃ precipitates decreased outward diffusion of Ti.

• An in-situ Ti₅Si₃ diffusion barrier effectively inhibited the interdiffusion.

Abstract

In the present study, a novel silicon-aluminizing diffusion coating composed of uniform Ti (Al, Si)₃ phase was manufactured on γ-TiAl alloy via post heat-treatment of cold-sprayed Al-40Si (wt.%) coating. The high temperature oxidation resistance of the diffusion coating was evaluated under 950 ^◦C for 1000 h. During the oxidation process, Ti₅Si₃ precipitations with network-like structure were shaped in the inner of the diffusion coating, which acted as getters for Ti and promoted the formation of Al₂O₃ scale. Meanwhile, an in-situ Ti₅Si₃ diffusion barrier was also formed, which inhibited the interdiffusion between the coating and substrate, especially the fast depletion of Al in the diffusion coating and thus improved the long-term oxidation resistance of γ-TiAl. The microstructure evolution, formation of the in-situ diffusion barrier and high temperature oxidation resistance mechanisms of the diffusion coating were discussed in detail.

Introduction

TiAl-based alloys are inherently characterized by their low densities, high specific strength, high stiffness [1], [2], appropriate strength retention and high creep resistance under moderately elevated temperatures [3], [4], [5], which makes them ideal for various high-temperature structural applications in automobile, aerospace and gas turbine industries [6], [7], [8]. However, unprotected rutile (TiO₂) or TiO₂/Al₃O₂ mixed oxide scale is usually formed on the surface of TiAl alloys at high temperature, which greatly limited their widespread applications [9], [10], [11]. In order to make TiAl alloys served at elevated temperatures, development of high temperature oxidation resistance coatings on TiAl alloys is important [12], [13], [14], [15].

To ameliorate oxidation protection of TiAl alloys, researchers have studied numerous coating systems, such as aluminide coatings [16], [17], [18], [19], [20], [21], silicide coatings [22], [23], [24], Ti-Al-Cr-Y(-Si) coatings [25], [26], TiAlSiN coating [12], MCrAlY (M = Co, Ni or combination) coatings [27], [28], and Ni-Al coatings [29], [30]. These coatings can raise the oxidation protection of TiAl alloys by forming a continuously protective Al₂O₃ or SiO₂ scale [31]. However, interdiffusion between these coatings and TiAl alloys is inevitable at high temperature, which often produces phase degradation and expedites coating failure prematurely, and induces mechanical properties deterioration of TiAl alloys [32], [33].

Firstly, the outward diffusion of Ti in TiAl alloys would destroy integrity or adhesion of oxide scale by forming non-protective TiO₂ [7], [32]. Secondly, the inward diffusion of Al would lead to depletion of Al in the coating, which in return promotes the degradation of the coating and thus decreases the coating’s long-term oxidation resistance [32], [34]. Finally, interdiffusion would lead to the formation of harmful and brittle phases in TiAl alloys. For example, brittle Laves phases can be formed between the TiAlCr/MCrAlY coatings and TiAl alloys after high temperature exposure, which inevitably degrade the mechanical properties of TiAl alloys [35]. Therefore, inhibiting the interdiffusion between the coatings and TiAl alloys is meaningful for improving the long-term oxidation resistance of the coating and preventing the mechanical properties degradation of TiAl alloys.

Interdiffusion can be retarded by intentionally preparing a diffusion barrier to the atoms’ passage [36]. For MCrAlY/TiAl or Ti₂AlNb system, an Al/Al₂O₃ [28], Mo/Cr [35], Ni-Re [37] or Cr₂O₃ [38] layer has been introduced similar to the MCrAlY/superalloy system. High temperature exposure tests revealed that the above diffusion barriers would increase oxidation resistance of the MCrAlY coating to some degree. However, these externally introduced diffusion barriers result in the increasing structural complexity, adding process expense, and high tendency of coating to peel off due to the poor bonding strength at the interface [32], [36].

Another way is to design the coating forming an in-situ diffusion barrier by making full use of the interdiffusion of substrate and coating. Moreover, the interdiffusion products must be stable, dense, continuous, and atoms from the coating and substrate have low diffusivity in them [36]. For instance, Li et al. [36] informed that a dense and continuous Zr₂Si-rich diffusion barrier was in-situ formed because of the interfacial reactions between Si and Zr in FeCrAlMoSiY-coated Zr system, leading to a better oxidation resistance.

Recently, cold spray, which utilizes high velocity micron-sized powders to form metal or metal matrix composite coatings, has been widely developed to deposit various coatings [21], [22]. Inspired by the above research, basing the high affinity between Si and Ti [39], in this paper, a novel silicon-aluminizing diffusion coating was produced on γ-TiAl alloy by post heat-treatment of cold-sprayed Al-40Si (wt. %) coating. Results indicated that an in-situ Ti₅Si₃ diffusion barrier was formed, which restrained the interdiffusion between the silicon-aluminizing diffusion coating and substrate, and thus enhanced the long-term oxidation resistance performance of the coating. The microstructure evolution, formation of the in-situ diffusion barrier, and oxidation resistance mechanisms of the silicon-aluminizing diffusion coating were discussed in depth.