镍基单晶高温合金由于具有优异的力学性能而广泛应用于喷气发动机的涡轮叶片中。尤其是随着涡轮进气口温度的提高，现代喷气发动机采用薄壁空心叶片结构和气膜孔冷却技术提高叶片冷却效率，而薄壁结构和气膜孔对镍基单晶的服役性能有较大的影响。与此同时，由于镍基单晶合金消除了晶界，在材料中引入了各向异性的影响，因此晶向对其力学性能也具有重要的影响。在实验研究方面，由于宏观实验难以给出镍基高温合金的微观失效机理，因此研究者借助SEM开展了原位实验研究。但是，由于SEM特殊的实验环境和对稳定性的要求，使得材料的原位高温力学测量存在诸多挑战。鉴于此，本文开展了扫描显微环境中原位高温力学测量系统与高温变形测量技术的研究，在此基础上研究了壁厚、第二晶向和气膜孔对镍基单晶高温合金拉伸力学性能的影响机制。主要研究内容和结论如下：研制了一套在SEM中可实现从室温-1000℃的原位高温拉伸、疲劳力学性能测量系统，自主设计了热电子屏蔽单元并施加反向电压，使得该测量系统可以在扫描显微环境中实现被测材料室温-1000℃的原位清晰成像；用水蒸气诱导铂膜重塑法制备了平均直径为100nm均匀且致密的高温变形载体，实验验证了该变形载体在高温下的稳定性，并证明其可在高温条件下实现亚微米尺度分辨的位移场测量。完成了不同壁厚条件下[100]第二晶向和[110]第二晶向镍基单晶DD6合金的原位单轴拉伸实验，发现不同第二晶向DD6合金均存在薄壁效应，即延伸率随厚度减小而减小。结合原位观测结果和晶体塑性有限元计算，发现自由表面效应是镍基单晶薄壁效应产生的主要机制。完成了不同壁厚条件下[100]第二晶向和[110]第二晶向带气膜孔DD6合金的原位单轴拉伸实验，发现[100]第二晶向带气膜孔DD6合金存在薄壁效应，结合原位观测结果和晶体塑性有限元计算发现，随着带气膜孔试件厚度的减小，气膜孔附近材料平面外应力约束因子减小，从而导致较薄试件更接近平面应力状态，承受塑性变形能力减小；对于[110]第二晶向带气膜孔试件则不存在薄壁效应，结合原位观测结果和晶体塑性有限元计算发现，较厚的试件开动的不同滑移系塑性滑移量相差较大，导致变形不协调，而较薄的试件变形相对均匀，致使试件延伸率对试件厚度的变化不敏感。
Nickel-based single-crystal superalloys are widely used in turbine blades of jet engines due to their excellent mechanical properties. Especially with the increase of turbine inlet temperature, thin-walled hollow blade structure and air film cooling technique have been applied to improve the cooling efficiency of the modern jet engines. However, the thin-walled structure and the film-cooled holes have great influence on the service performance of nickel-based single superalloys. At the same time, the effect of anisotropy is introduced in the nickel-based single-crystal superalloy due to the elimination of grain boundaries. Therefore, the crystal orientation also has an important influence on its mechanical properties. As for the experimental study of the mechanical behavior of nickel-based superalloys, researchers have carried out in-situ experimental studies with the help of a scanning electron microscope (SEM) because it is difficult to give the microscopic failure mechanism of nickel-based alloys by macroscopic experiments. However, due to the special experimental environment and stability requirements of the SEM, the development of in-situ high-temperature mechanical measurements faces many challenges. In view of this, this paper carried out a study on the in-situ high-temperature mechanical measurement system and high-temperature deformation measurement techniques in a scanning microscope environment, on the basis of which the mechanisms of the influence of thickness, secondary orientation and film-cooled holes on the tensile mechanical properties of nickel-based single superalloys were investigated. The main research contents and results are as follows:A system for in-situ measurement of high temperature tensile and fatigue mechanical properties from room temperature to 1000°C in an SEM has been developed. By using a self-designed thermo-electronic shielding unit and applying reverse voltage, the measurement system can achieve clear imaging of the material under test from room temperature to 1000°C in a scanning microscope environment. The suitable conditions for the preparation of uniform and dense high-temperature deformation carriers with an average diameter of about 100 nm by water vapor-induced platinum film remodeling were investigated, and the stability of the deformation carriers at high temperatures was verified, which proved that they could achieve submicron-scale resolved displacement field measurements at high temperatures.In-situ tensile experiments of the nickel-based single-crystal superalloy DD6 with different thicknesses with [100] and [110] secondary orientations were completed, and it was found that the thin-wall effect existed in all the samples with different secondary orientation, i.e., the elongation decreased with decreasing thickness. Combining the in situ observations and crystal plasticity finite element analysis, the free surface effect was found to be the mechanism of the thin-wall effect in nickel-based single crystals.In-situ tensile experiments were completed for [100] and [110] secondary orientation DD6 alloys with a hole under different thickness conditions. It was found that the thin-wall effect existed in [100] secondary orientation DD6 alloys with a hole. Combining the in-situ observation results and crystal plasticity finite element calculations, it was found that the out-of-plane stress constraint factor of the material near the hole decreases as the decrease of specimen thickness, resulting in a thinner specimen was closer to the in-plane stress state and less able to withstand plastic deformation. However, the thin-wall effect was not present in DD6 alloys with [110] secondary orientation. Combining the results of in-situ observation and crystal plasticity finite element calculation, it was found that the difference in plastic slip of different slip systems of the thicker specimen was rather large, leading to uncoordinated deformation, while the thinner specimens were relatively uniformly deformed, so the difference of the elongation of specimens with different thicknesses was not significant.
