单晶高温合金γ/γ′双相氧化腐蚀机理的第一性原理研究

The development tendency of higher thrust-weight ratio and higher front inlet temperature in aero-engines, as well as the marine atmosphere, require better oxidation and corrosion performances of the hot component materials at high temperature. Two-phase single crystal superalloys have many special features in their microstructures and corrosion behaviors. As a result, their hot corrosion mechanisms are difficult to analyze by applying the existing oxidation models of two-phase alloys and need to be studied urgently. Moreover, few attentions were paid on the relationships between the compositions as well as the bonding features of the two phases and their hot corrosion behaviors. In terms of the difficulties in providing effective evidences in hot corrosion diffusion through experimental approaches, this project is intended to investigate the effects of bonding features and micro-alloying elements on the hot corrosion diffusion process and to clarify the essential reasons of the hot corrosion behavior differences between the two phases by using first-principles calculations. The site-preferences and solid solubility of the micro-alloying elements like Mo, Re, Cr and Ta, as well as the diffusion activation energies and diffusion coefficients of O and S in the two phases would be calculated and analyzed. Multiple micro-alloying elements and their concentrations would be involved in the physical calculation models simultaneously. The related study results in this project would provide experimental and theoretical supports for the hot corrosion performance design of single crystal superalloys.

航空发动机向高推重比和高前进口温度的发展趋势与海洋气氛服役环境，要求发动机热端部件材料具有良好的抗高温氧化腐蚀性能。双相单晶高温合金具有独特的微观组织与氧化腐蚀行为特征，难以应用现有的双相合金氧化腐蚀模型分析其热腐蚀微观机理，且较少关注双相组织的成分、键合特征与其热腐蚀行为间的联系，亟待深入研究。针对试验手段无法在原子尺度上为热腐蚀扩散提供有效信息的难点问题，本项目拟采用第一性原理计算方法，在添加多种微合金化元素条件下，计算Mo、Re、Cr和Ta等微合金化元素在单晶高温合金双相组织中的原子占位、固溶度以及O和S原子在双相组织中的扩散激活能及扩散系数，分析多种微合金化元素在γ′-Ni3Al中的原子占位规律与机理，研究双相组织键合特征以及微合金化元素对热腐蚀扩散过程的影响，并结合熔盐环境下的热腐蚀试验进一步探究合金发生相选择性氧化腐蚀行为的本质原因，为合金的耐热腐蚀性能设计提供试验与理论依据。

针对双相单晶高温合金独特的微观组织与高温氧化腐蚀行为特征，本项目采用第一性原理计算方法深入研究了双相组织成分、键合特征与合金氧化腐蚀行为间的联系，并结合验证性试验重点探究了多种合金化元素在双相组织中的存在形式、固溶度、强化机理及对原子扩散行为的影响规律与机理。首先，基于合理的计算模型与反位置缺陷形成机制，确定Mo、Re、Cr和Ta等元素在单一及共合金化情况下均优先占据γ′-Ni3Al中的Al位，且与最近邻宿主Ni/Al原子间存在明显增强的化学键合，从而有效强化γ′-Ni3Al相。其次，利用精确muffin-tin轨道理论与相干势近似相结合的方法，获得了0K及高温下Mo元素在γ-Ni无序固溶体相中的固溶度，揭示了Mo在17.5 at.%浓度以下对γ-Ni相的持续固溶强化作用，且高Mo含量使Ni基固溶体在高温下具有更低的体模量下降值与更小的热膨胀系数。在确定合金化元素原子与单一空位在γ-Ni/γ'-Ni3Al相界面上的稳定占位基础上，进一步发现Re元素对近邻Ni原子在相界面处的迁移具有一定的阻碍作用，而Mo和Ta元素的影响则甚微。此外，结合试验中所发现的适量添加Ce或Dy微合金化元素可有效提高单晶高温合金高温抗氧化性能的新现象，通过第一性原理计算确定了Ce元素主要偏聚于γ-Ni相中，且对近邻Ni原子在γ-Ni相中的迁移具有明显的阻碍作用，从而为揭示上述试验现象的微观机理奠定了基础。本项目所取得的成果，不仅为阐明双相组织单晶高温合金高温氧化腐蚀行为的物理本质提供了原子尺度上的信息，而且还为澄清其微观强化机理与新型单晶高温合金的成分设计提供了试验与理论依据。