稀土协同作用影响镍基高温合金成分稳定性的微观机理

It is difficult to control the final compositions of the nickel-based superalloys with rare earth elements during the process of investment casting, which brings down the properties of anti-oxidation and anti-creep and thus the production rate of finished casting turbines. One of main reasons is due to the interfacial reactions between alloys and moulds. These reactions can induce inclusions often being oxides of reactive elements which are difficult to remove. In the present project, it is aimed to study the role of rare earth elements during the interfacial reactions between the superalloys and moulds, where advanced technique of in-situ trace characterisation by synchrotron radiation X-ray fluorometric analysis is to be applied. It is to systematically investigate the microstructure and formation order of new phases for the cases of adding single rare earth element as well as adding two rare earth elements, via experimental measurements and theoretical modeling including thermodynamics and kinetics. The relationship between rare earth elements, mould materials and interfacial microstructure is to be analysed in order to find the mechanism for synergetic effect of rare earth elements during the interfacial reactions. The main reason for the stability of the composition of nickel-based superalloys is to be determined, which can provide theoretical and experimental guidance for solving the problem of processing stability of the investment casting for nickel-based superalloys.

精密铸造含稀土元素的镍基高温合金最终成分难以控制，削弱其抗氧化及抗蠕变性能，降低铸件（叶片）成品率，其主要原因之一为合金与铸型间发生界面反应，在铸件表面形成无法打磨的黏砂（通常为活性元素氧化物）。通过前期研究发现，同时添加镧钇两种稀土元素较之单独添加一种稀土元素能提高稀土元素整体收得率及稳定性，但相关机理尚不明确。因此，项目针对稀土元素在镍基高温合金铸造过程中参与界面反应的问题，采用先进同步辐射X射线荧光分析无损原位痕量微区观测等技术，结合相平衡热力学、扩散动力学，从实验测定与理论计算两方面，系统性地研究添加稀土元素时界面处元素分布演化规律、合金与铸型界面形态转化机制，建立新相种类及其生成序列模型，探索合金成分与界面微观组织结构间的关联，从而揭示高温合金与铸型间界面反应中稀土元素协同作用影响成分稳定性的微观机理，为解决精密铸造镍基高温合金稳定性难题提供理论与实验新思路。

精密铸造含稀土元素的镍基高温合金最终成分难以控制，削弱其抗氧化及抗蠕变性能，降低铸件（叶片）成品率，其主要原因之一为合金与铸型间发生界面反应，在铸件表面形成无法打磨的黏砂（通常为活性元素氧化物）。项目针对稀土元素在镍基高温合金铸造过程中参与界面反应的问题，完善了含稀土元素的（La,Ce,Y,Pr,Nd,Er）的Ni-Cr-Al基高温合金体系热力学数据库，系统研究单一添加Y和复合添加Y+La的高温合金与Al2O3基型壳间的界面反应情况，通过对合金与型壳间界面处的形貌、组织观察和界面反应产物测定，了解稀土元素间的协同作用，并进行热力学计算分析其作用机理。复合添加Y+La的镍基单晶高温合金与Al2O3型壳间的界面反应情况相对于单一添加Y的合金更加轻微；复合添加Y-La合金界面反应产物为Y3Al5O12 (YAG)和LaAlO3 (LaAP)，单一添加Y的合金界面反应产物是YAG；通过热力学计算分析得知复合添加Y+La合金界面反应更轻微的原因是：Y、La与Al2O3反应会优先生成LaAP，Y元素溶解在LaAP中而不是与Al2O3发生界面反应。并且复合添加Y+La合金的抗氧化性能也更好，氧化增重速率小，合金表面氧化层薄。明确了合金成分，特别是稀土元素与界面微观组织结构间的关联，揭示高温合金与铸型间界面反应中稀土元素协同作用影响成分稳定性的微观机理，为解决精密铸造镍基高温合金稳定性难题提供理论与实验新思路。