A numerical and experimental approach to compare the effect of sample thickness in small in-situ SEM and large ex-situ tensile testing in Alloy 709
A numerical and experimental approach to compare the effect of sample thickness in small in-situ SEM and large ex-situ tensile testing in Alloy 709
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DOI：
10.1016/j.matchar.2021.111614
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发表时间：
2021-12-11
影响因子：
4.7
通讯作者：
Rabiei, Afsaneh
中科院分区：
文献类型：
--
作者：
Lall, Amrita;Bowen, Paul;Rabiei, Afsaneh
ASTM拉伸试验标准定义了具体的样品尺寸要求，而不考虑材料的晶粒尺寸。然而，试验的样本量要求应与其减小的横截面积内的晶粒数量一起考虑。这对于原位扫描电子显微镜（SEM）拉伸测试尤其重要，因为它们必须在较小的样品上进行。在这项研究中，一个全面的实验和数值评估的试样厚度的影响（和所产生的横截面内的晶粒数）进行原位SEM拉伸试验（亚毫米厚的样品）和异位拉伸试验（0.68-5.9毫米厚的样品），并与有限元模拟结果进行比较。所有试验均在室温下进行，结果与试样厚度内的晶粒数相关。拉伸试验结果表明，即使0.2%屈服应力和拉伸强度对于当前的厚度范围（横截面中的晶粒数从13变化到118）没有变化，颈缩机制也存在差异。拉伸强度后，较薄的样品经历更多的剪切破坏和对角局部颈缩，而较厚的样品经历更多的扩散颈缩，这表明边缘处的剪切破坏面积减少，样品中心处的压痕拉伸破坏面积增加。有限元分析结果补充了实验结果，显示共轭局部剪切带的形成上，下表面上的0.68毫米厚的样品和剪切带结合，形成奇异剪切带在较厚的样品。这些结果也证实了在较薄的样品上进行的原位SEM拉伸试验的有效性，只要所需的最小数量的晶粒存在于横截面内。
ASTM standards for tensile tests define specific sample size requirements regardless of grain size of the material. However, sample size requirements for testing should be considered in conjunction with the number of grains within its reduced cross-sectional area. This is particularly important for in-situ Scanning Electron Microscope (SEM) tensile tests, as they have to be conducted on smaller samples. In this study, a comprehensive experimental and numerical evaluation of the effect of specimen thickness (and the resulting number of grains within the cross-section) were conducted using in-situ SEM tensile test (on submillimeter thick samples) and ex-situ tensile tests (on samples of 0.68-5.9 mm thickness) and the results are compared with FEM simulations outcome. All tests were conducted at room temperature and the results are correlated to the number of grains within the thickness of specimens. The tensile test results indicated that even though the 0.2% proof stress and the tensile strength do not vary for the current range of thicknesses (with number of grains in the cross-section varying from 13 to 118), a difference in necking mechanism exists. Post tensile strength, thinner samples undergo more shear failure and diagonal localized necking whereas thicker samples experience more diffuse necking indicated by decreased area of shear failure at the edges and increased area of dimpled tensile failure at the center of the sample. FEM results complement the experimental findings by showing the formation of conjugated localized shear bands on the upper and lower surface in the 0.68 mm thick sample and shear bands combining to form singular shear bands in thicker samples. These results also confirm the validity of the in-situ SEM tensile tests conducted on thinner samples as long as the required minimum number of grains exist within the cross-section.