MCrAlY涂层对单晶中γ'相消耗及TCP相生成影响的模拟研究

Modelling Study about the Influence of MCrAlY Coatings on the Degradation of γ' Phase and the Formation of TCP Phases in a Single Crystal

高温下,MCrAlY涂层与高温合金单晶基材之间会发生元素互扩散,引起显微组织结构变化,往往造成单晶中具有析出强化作用的γ'相的消耗和对力学性能有害的拓扑密堆(Topological closed packed, TCP)相的生成。本研究采用扩散模拟计算的方法,设计了6种MCrAlY模型涂层,分析Co、Cr、Al主元素对DD6单晶基材显微组织的影响规律。研究结果表明,基材中γ'相消耗区在靠近涂层-基材界面处形成,该区的形成主要与Co、Cr元素内扩散相关。基材中TCP相则在更深位置的γ'相富集区内生成,γ'相富集区与Al元素内扩散有关。另外,本文着重分析了涂层成分对γ'相消耗区深度及TCP相生成含量与深度的影响。

At high temperature, elemental interdiffusion occurs between MCrAlY coating and single crystal superalloy, causing microstructural changes typically with the degradation of precipitation-strengthening γ' phase and the formation of topological close-packed (TCP) phases which are harmful to mechanical properties. In this study, diffusion modelling calculations were performed and 6 MCrAlY model coatings were designed aiming at analyzing how the coating main elements Co, Cr and Al affect the microstructural evolution in DD6 single crystal. The results show that the γ' phase depletion zone was formed just beneath the coating-substrate interface. The inward diffusion of Co and Cr was responsible for that. TCP phases were generated in a γ' phase rich zone which was formed under the γ' phase depletion zone. The formation of γ' phase rich zone was related to Al inward diffusion. In addition, this paper mainly analyzed the influence of coating composition on those microstructural changes.Table 1 shows the composition of the 6 designed MCrAlY model coatings. The composition of DD6 single crystal substrate used was Ni-5.6Al-9Co-4.4Cr-2Mo-0.6Nb-2Re-7.3Ta-8W (wt.%). The diffusion model, as shown in Fig. 1 (taking Coat A as an example), contained 100 um thick coating (0~100 um) and 1000 um thick substrate (100~1100 um) with the interface at distance=100 um. ThermoCalc/DICTRA software (ThermoCalc AB, Sweden) was used to simulate the diffusion process at 1100 oC for 100 h. In the diffusion model,α(BCC_B2),β(BCC_ B2#2),γ(FCC_L12),γ'(FCC_L12#2),μ(MU_PHASE),σ(SIGMA) phases were included.Fig. 2 shows that TCP phases were formed in DD6 substrate after ~300 h ageing process at ~1050 oC without coating influence. The composition in the TCP forming zones had higher content of W, Mo and Re. As presented in Fig. 3(a) and (b), the calculated equilibrium microstructure of DD6 was made of γ/γ' with some amount of μ and σ phases below 850 oC. But when increasing the content of W, Mo and Re, the amount of TCP phases and their critical forming temperature were both increased. Therefore, the formation of TCP phases in Fig. 2 could be due to the higher amount of W, Mo and Re elements in those zones.Taking Coat A as an example, Fig. 4 gives the results of the interdiffusion of elements between coating and substrate. As shown in Fig. 4(a) and (b), coating elements Co, Cr and Al diffused inwards to the substrate. Al diffused faster than Cr and Co. Substrate elements diffused outwards to the coatings and the diffusion rate (according to their diffusion depths) can be ranked as Ta/Nb (fastest), Mo, W, and Re. Due to the lost of Al in the coating,β phase has been highly depleted (Fig. 4(c)). In the substrate, a γ' depletion zone was formed just beneath the coating-substrate interface, following which a γ' rich zone (at ~ distance=150 um) was produced. In this γ' rich zone, TCP phases (μ and σ) was formed.Fig. 5 helps to analyze how the γ' depletion zone and TCP phases were formed. It seems that the inward diffusion of Co and Cr caused the depletion of γ' phase in the substrate since their concentration in this zone changed a lot comparing to the original composition;in addition, Cr and Co was not γ' stabilizers. The deeper formed γ' rich zone contained higher amount of Al and Ta due to the inward diffusion of these two elements. As the inward diffusion of Al produced higher fraction of γ', the amount of γ phase decreased. As the result of that, some heavy elements such as W, Mo and Re would precipitate to form TCP phases (Fig. 5b).To analyze the coating composition effect on the γ' depletion and TCP phase formation, Fig. 6 was made. Fig. 6a shows that increasing Co and Cr amount (A to B) increased the depth of γ' depletion, while increasing Al content did not. Increasing Al content caused the increase of γ' phase amount in the γ' rich zone. Obviously, a Ni based coating (D) had better ability to hold γ/γ' microstructure in substrate than a Co based one (F). Increasing coatings' Cr or Al amount can stimulate TCP phases (amount and depth) according to the results in Fig. 6b and c.