激光熔覆制备单晶MCrAlY涂层微结构调控及高温氧化行为研究

Single crystal MCrAlY coatings can reduce the property mismatch between the coating and single crystal superalloy substrate, and thus improve the component life time. Our previous study indicated that laser cladding can allow the epitaxial growth of a single-crystalline structure on a single crystal superalloy substrate, which makes the deposition of a single-crystal MCrAlY coating be technically feasible. However, the main limit to achieving a single crystalline structure in the deposition material is the possibility of a columnar to equiaxed solidification transition occurring when the solidification front approaches the surface. So, it is necessary to study the dendrite-growth rule to obtain a single crystal coating produced by laser cladding. Based on the previous research results, the present study will investigate the relation between the cladding processing parameters and the dendrite growth in the corresponding γ'-constituted MCrAlY coating through both experimental investigation and numerical computation. The thermodynamic and dynamic of the columnar to equiaxed transition during epitaxial deposition will also be analyzed, and the microstructure-processing map for the solidification of the multi-component MCrAlY coating alloy will be proposed. According to these results, the microstructure control strategy of the single crystal MCrAlY coating during laser cladding process will be suggested. Furthermore, the selective oxidation behavior and failure mechanisms of the single crystal MCrAlY coating at high temperatures will be investigated. The results obtained in the present study are expected to instruct the development of the single crystal MCrAlY coating deposition.

单晶MCrAlY涂层可缓解涂层与单晶高温合金基体之间的性能不匹配，提高叶片的服役寿命。前期研究结果表明，利用激光熔覆技术可以在单晶高温合金基体上外延生长单晶组织，这使制备MCrAlY单晶涂层成为可能。本项目拟在已有研究工作的基础上，以γ'单相组成的MCrAlY涂层材料为研究对象，采用实验验证和数值模拟相结合的方法，探明激光熔覆工艺参数与MCrAlY涂层枝晶生长行为和显微组织的响应关系，分析涂层外延生长过程中柱状晶向等轴晶转变这一激光熔覆关键问题的热力学和动力学，构建熔覆工艺参数与涂层微观组织结构的内在关系图，提出涂层微观组织结构的控制策略。在此基础上，研究激光熔覆MCrAlY涂层的高温氧化行为，阐明该MCrAlY涂层的损伤过程和失效机理，为今后发展单晶MCrAlY涂层奠定理论基础。

传统MCrAlY涂层是多晶组织，与单晶高温合金基体之间存在组织性能的不匹配，影响单晶叶片的服役寿命。发展与单晶基体晶体学取向一致的单晶MCrAlY涂层有利于提高叶片的服役寿命。激光熔覆技术是一种制备单晶MCrAlY涂层非常有潜力的方法。鉴于此，本项目以γ'单相组成的MCrAlY涂层材料为研究对象，通过试验表征和计算模拟相结合的方法，研究了激光熔覆工艺参数与MCrAlY涂层枝晶生长行为和显微组织的响应关系，分析了涂层外延生长过程中柱状晶向等轴晶转变这一激光熔覆关键问题的影响因素，提出了涂层微观组织结构的控制策略。同时研究了激光熔覆MCrAlY涂层的高温氧化行为，分析了该MCrAlY涂层的损伤失效过程。研究结果表明，激光熔覆制备的单晶MCrAlY涂层中下部为二次枝晶不发达的微细枝晶组织，涂层顶部存在转向枝晶层。转向枝晶层的厚度与凝固过程中温度梯度和基板取向息息相关，而温度梯度主要受熔覆工艺参数的影响。通过合理控制激光熔覆工艺参数和单晶合金基板的取向可避免转向枝晶的形成，制备出无转向枝晶层的单晶MCrAlY涂层。所得单晶MCrAlY涂层在1000℃时的高温氧化时，氧化初期，涂层表面同时形成了针状亚稳态θ-Al2O3和小块状稳态α-Al2O3氧化物。随着氧化时间的推移，发生了θ-Al2O3向稳态α-Al2O3的转变。而后，氧化膜/涂层界面处形成了尖晶石类氧化物。在氧化膜厚度达到6~7μm时，氧化膜发生了大面积的剥落。剥落后的涂层表面形成了新的氧化物NiO。本项目的研究结果在深入认识激光熔覆技术及涂层损伤失效机理方面具有一定的科学意义，可为单晶MCrAlY涂层的制备及应用提供一定的理论指导。然而，在研究过程中发现，如果将激光熔覆制备单晶涂层技术推广应用到单晶涡轮叶片表面涂层制备，该技术还存在一些待解决的问题，如制备效率低、工艺窗口小、对工装夹具要求高等。

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