Micromechanisms Responsible in Deformation of Nickel-Base Superalloy Single Crystals under Multiaxial Loading at High Temperatures

经过几十年的实施，镍基单晶高温合金在燃气轮机热段中发挥的作用仍然至关重要。该类材料耐高温领域的进一步进展只能通过以下三方面的相互作用来实现：(1)接近现实的实验设置；(2)提供可靠实验数据的表征方法；(3)使用这些数据作为输入的寿命和变形预测模型。在该项目中，申请人希望对高温合金单晶蠕变性能的进步、理解和改进做出贡献。为此，将对标准试样和带有圆形缺口的试样进行受控高温蠕变试验，并对其进行表征，圆形缺口会产生技术上相关的多轴应力条件。具体而言，基于位错的实验技术应应用扫描透射电子显微镜（STEM）模式下新开发的位错分析方法来描述ni基高温合金单晶LEK 94高温蠕变行为的微观机制。蠕变试验将在1000°C以上的温度下对缺口试样进行。重点将集中在以下几点：(1)涉及变形的微观机制对应力状态多轴性的依赖；(2)圆形缺口的存在对高温蠕变行为的影响；(3)模拟工具与实验数据的验证。生成并汇编一组高温下多轴应力状态下的蠕变试验数据。评估的数据将包括单个相内的位错类型和密度，以及由多轴性引起的微观结构演变将被确定并与位错活动相关联。这些数据对于正确验证用于模拟单晶高温合金蠕变行为的本构方程至关重要，这将作为最后一步实施。

The role played by Ni-based single crystal superalloys in hot sections of gas turbines remains essential after decades of implementation. Further progress in the area of high temperature resistance of this materials class can only be achieved through the interaction between (1) close-to-reality experimental setups, (2) characterization methods that provide reliable experimental data and (3) life and deformation prediction models that use this data as input.In the proposed project the applicant wishes to contribute to the advancement, the understanding, and thus the improvement of creep performance of superalloy single crystals. To this end, controlled high temperature creep tests will be performed and characterized on standard specimens and on specimens with a circular notch, which generates technically relevant multiaxial stress conditions. Specifically, experimental dislocation-based techniques shall be conducted by applying a newly developed methodology for dislocation analysis in the scanning transmission electron microscopy mode (STEM) to describe the micromechanisms responsible for high temperature creep behavior of the Ni-based superalloy single crystal LEK 94. Creep tests will be conducted at temperatures above 1000°C on the notched specimens. Focus will be set on the following points: (1) the dependence of micromechanisms involved in deformation on the multiaxiality of the stress state, (2) the effect of the presence of a circular notch on high temperature creep behavior and (3) the validation of simulation tools with experimental data obtained. Experimental data from a set of creep tests in the high temperature regime under multiaxial stress state will be generated and compiled. The data evaluated will comprise dislocation types and densities within the individual phases and microstructural evolution induced by multiaxiality will be determined and correlated with the dislocation activity. This data is essential for proper validation of constitutive equations intended to model creep behavior of monocrystalline superalloys, which will be implemented as a last step.