复杂载荷环境下镍基单晶高温合金力学性能的厚度尺寸效应

Nickel-based single crystal superalloys is the key material for modern gas turbine cooling blades. To improve operating efficiency, the thickness of the advanced blade is as low as 0.2mm, which leads to great differences between the mechanical behaviors of the blades and those of the standard test specimen. Experiments have shown that the creep life of the thin thickness specimen is 30%-60% lower than that of the standard specimen. This is defined as thickness size-dependent effect which causes unsafty in the analysis of the blade based on the traditional data. So far studies have been limited to the creep behavior under constant temperature, and the influence factors and the mechanisms have not been revealed. The thickness size-dependent effect of the mechanical behaviors in nickel-based single crystal superalloys will be studied in the project with the consideration of the specialty of the material and the working conditions. The crystal orientation of the material, termperature, stress, environment and fatigue/creep loading conditions will be taken into accout. From the analysis of the macro influnce factors, the quantitative relations between the macromechanical properties and the thickness will be established. The microstructure and its evolution will be studied both experimentally and theoretically using dislocation dynamics modelling and the phase field methods. The micro-mechanisms of the thickness size-dependent effect will be revealed. The micro controlling factors will be quantitatively extracted as representative variables to describe microstructure evolutions influenced by the thickness. Based on the micro-mechanisms, the consititutive model and life prediction model will be established which could describe the thickness size-dependent effect under service loading conditions. The finite element program will be finished. The research of this project can reveal the micro-mechnisms of the thickness size-dependent effect. The models developed can be used directly in engineering applications, therefore this study is not only theoretically importand but also widely applicable.

镍基单晶高温合金是先进航空发动机冷却叶片的关键材料，高效冷却通道的设计使得叶片局部最薄可达0.2mm，现有试验表明材料蠕变寿命比常规标准件普遍降低30%-60%，产生显著的厚度尺寸效应，导致用传统数据进行叶片分析偏于危险。国内外对其研究限于恒温蠕变性能，且没有明确揭示影响因素及内在机理。本项目拟针对材料特点和服役工况，研究晶体取向、温度、应力、环境及疲劳/蠕变交互作用等复杂载荷下材料力学性能的厚度尺寸效应：建立厚度与宏观性能之间的定量关系；从材料的细微观组织结构及其演化出发，结合位错动力学模拟和相场计算等理论分析，揭示厚度尺寸效应产生的细微观机理；提取控制厚度尺寸效应的细微观特征量；把特征量和晶体塑性理论相结合，建立本构模型和寿命模型；完成相应的有限元程序开发。该研究可揭示厚度尺寸效应产生的细微观机理，建立的模型可直接应用于工程冷却叶片的强度和寿命分析，有重要的理论研究价值和工程应用前景。

目前，镍基单晶高温合金仍是航空发动机高压涡轮叶片的唯一材料，由于发动机进口温度的不断提高所采取的冷却措施，使得整个叶片为空心薄壁结构。本项目针对这种薄壁结构特征，研究其厚度尺寸效应。采用大块块体材料、铸造不同厚度材料以及真实冷却叶片材料，进行了不同环境、温度、应力状态的广泛试验研究，采用扫描电镜和投射电镜对试样的微观组织形貌及其演化进行了详细分析。 采用单胞模型有限元分析、分子动力学模型分析以及相场模拟计算进行理论分析，对试验现象进行模拟和分析，进一步揭示厚度尺寸效应的机理。定量分析结果表明，直接铸造不同厚度确实会引起微观结构的不同，这种不同表现在强化相的体积分数、尺寸、概率分布等特征。在高温低应力载荷下，厚度尺寸效应较为严重，随着试件厚度的减小蠕变寿命减小。定量分析发现，蠕变寿命和厚度成指数关系，蠕变断裂应变和厚度的平方根的倒数成线性关系。在高温环境下，氧化不仅能引起承载面积的减小而且引起微观结构的变化，出现了无强化相和强化相体积分数明显减少的微观结构。而镍基单晶合金之所以具有优异的高温性能，主要和它的规则的强化相/基体相结构以及强化相体积分数相关。定量分析发现无强化相和强化相体积分数减少这两个微观结构的发展为时间的对数函数。在此基础上，考虑厚度引起的材料微观结构的变化及损伤特点，建立了蠕变本构模型、疲劳-蠕变本构模型及相应的寿命模型，并结合商用有限元软件ABAQUS，实现了模型的程序化。采用不同壁厚的薄壁圆筒模拟冷却叶片进行分析，该模型能够对镍基单晶高温合金的厚度尺寸效应进行表征和预测。