Degradation of the Strength of a Grain Boundary of Ni-Base Superalloys Under Creep-Fatigue Loading at Elevated Temperature
Degradation of the Strength of a Grain Boundary of Ni-Base Superalloys Under Creep-Fatigue Loading at Elevated Temperature
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高温蠕变疲劳载荷下镍基高温合金晶界强度的退化
DOI：
10.1115/imece2020-24473
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发表时间：
2020
期刊：
影响因子：
--
通讯作者：
Miura Hideo
中科院分区：
文献类型：
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作者：
Takahashi Yukako;Luo Yifan;Ishihara Kenta;Suzuki Shujiro;Miura Hideo
晶界强度的劣化通过使用扫描电子显微镜中的微拉伸试验来测量。本文用电子背散射衍射分析法研究了镍基合金617和625在高温蠕变-疲劳过程中晶界结晶度的变化。从分析中获得的图像质量（IQ）值（其指示缺陷的总密度）被应用于结晶度的定量评价。可以清楚地观察到，缺陷的积累发生在晶界，这是垂直于加载方向，并由晶粒与施密德因子的差异很大。从试样上切取不同结晶度的双晶试样，利用微拉伸试验系统测量了双晶试样的强度。可以确认，随着IQ值的降低，换言之，随着空位和位错等各种缺陷的总密度的增加，晶界的强度单调地降低约50%。另一方面，晶粒的有效屈服应力随着IQ值的减小而单调增加。这是因为这些缺陷的总密度的增加抑制了位错的运动，换句话说，抑制了塑性变形。因此，有三个独立的强度，两个晶粒的强度和晶界，其中包括双晶体试样。由于晶粒的强度增加，在同一时间，晶界随着IQ值的降低单调下降，它被证实存在临界IQ值，在该值下，双晶试样的断裂模式从常规的穿晶断裂向沿晶断裂转变。
The degradation of the strength of a grain boundary was measured by using a micro tensile test in a scanning electron microscope. The change of the crystallinity of grain boundaries during creep-fatigue tests of Ni-base alloy such as Alloy 617 and 625 at elevated temperatures was monitored by electron back-scatter diffraction analysis. The image quality (IQ) value obtained from the analysis, which indicates the total density of defects, was applied to the quantitative evaluation of the crystallinity. It was clearly observed that the accumulation of defects occurred at grain boundaries which were perpendicular to the loading direction and consisted of grains with large difference of Schmid factor. Bicrystal specimens with different crystallinity were cut from the tested samples and the strength of the bicrystal specimens were measured by using the micro tensile test system. It was confirmed that the strength of a grain boundary decreased monotonically by about 50% with the decrease of IQ value, in other words, the increase in the total density of various defects such as vacancies and dislocations. On the other hand, the effective yielding stress of grains increased monotonically with the decrease of the IQ value. This is because the increase in the total density of these defects suppresses the movement of dislocations, in other words, plastic deformation. Therefore, there were three independent strengths, the strength of two grains and that of a grain boundary which consisted of the bicrystal specimen. Since the strength of grains increased, at the same time, that of a grain boundary decreased monotonically with the decrease of the IQ value, it was confirmed that there was critical IQ value at which the fracture mode of a bicrystal specimen changed from conventional transgranular cracking to intergranular cracking under the application of uniaxial tensile load.