镍基单晶高温合金疲劳行为的原位透射电镜研究

Fatigue behavior of Ni⁃based single crystal superalloy based on in⁃situ TEM study

疲劳破坏是镍基单晶高温合金涡轮叶片常见的失效形式之一,研究循环变形过程中位错组态的演化行为对于理解和提升叶片服役寿命具有重要的意义。本文借助透射电子显微镜和原位力学样品台,研究了[110]取向的ReneN5单晶高温合金亚微米柱在室温压-压循环载荷下的缺陷结构演变和疲劳损伤机理。结果表明,γ/γ′两相位错的循环不可逆滑移是亚微米柱变形损伤的主要来源,两相结构使其抵抗循环变形的能力存在差异,二者在循环载荷下的位错演化行为明显不同:强度较低的γ相通道中观察到“循环愈合”现象,预先存在的高密度位错在循环压应力和映像力的作用下逃逸至自由表面而湮灭,γ相强度提高;在基体“循环愈合”后,反相畴界剪切机制主导了γ′相的疲劳变形过程,超位错于表面形核并在γ′相内部运动以继续产生塑性。γ/γ′两相位错动态演化过程与单晶高温合金循环变形行为存在显著关联,在高温合金塑性变形机制研究中,原位透射电镜表征技术展现出潜在的应用前景。

Nickel⁃based single crystal superalloys(Ni⁃SXs)are widely used in turbine blades.However,they suffer from the fatigue damage.The prediction method for their fatigue life require to understand the damage mechanism.On one hand,the differences in dislocation movement cause different deformation behaviors of oriented superalloys.On the other hand,it is quite difficult to characterize the dislocation behaviors at the early stage of fatigue by traditional method.Few studies have been conducted on the real⁃time dynamic process of fatigue dislocations.In the present work,the low cycle fatigue(LCF)behaviors of[110]oriented Ni⁃SXs were investigated at room temperature through in⁃situ TEM technique.Here,we demonstrate that the cyclic slip irreversibility of theγ/γ′dislocations is the main reason for damage,and the two⁃phase structure makes it different in the ability to resist cyclic loading.Cyclic healing was observed in theγmatrix with lower strength,and the pre⁃existing dislocations escaped to the free surface under the cyclic stress and image forces,with increasingγmatrix strength.After cyclic healing,the APB shearing dominated the fatigue deformation inγ′precipitates.The super⁃dislocations nucleated on the surface and move inside theγ′precipitate to generate plasticity.The correlation between the dynamic evolution process of the dislocations and the cyclic deformation behavior of the Ni⁃SXs was explained in this work.