Indentation Schmid factor and orientation dependence of nanoindentation pop-in behavior of NiAl single crystals
Indentation Schmid factor and orientation dependence of nanoindentation pop-in behavior of NiAl single crystals
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DOI：
10.1016/j.jmps.2011.04.003
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发表时间：
2011-06
影响因子：
5.3
通讯作者：
Tianlei Li;Yanfei Gao;H. Bei;E. George
中科院分区：
文献类型：
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作者：
Tianlei Li;Yanfei Gao;H. Bei;E. George
仪器纳米压痕技术已被广泛用于表征材料的小尺度力学行为。纳米压痕过程中的弹塑性转变通常由测量的载荷-位移曲线中的突然位移爆发（pop-in）来表示。在无缺陷的单晶中，突入被认为是均匀位错成核的结果，因为与突入载荷相对应的最大剪切应力接近材料的理论强度，并且因为突入应力的统计分布与热激活的均匀位错成核过程的预期一致。本文研究了这一过程是否受到除分解剪应力外的晶体学和应力分量的影响。采用Stroh形式和二维傅里叶变换，推导了弹性各向异性固体在赫兹接触下的解析应力场，从而确定了压痕施密德因子，即最大分解剪应力与最大接触压力的比值。对不同表面法向的b2结构NiAl单晶进行了纳米压痕实验。之所以选择这种材料，是因为它在室温下以{1 10}<0 0 1>滑移变形，从而避免了部分位错成核的复杂性。实验数据与基于压痕施密德因子的理论预测的弹射载荷方向依赖关系吻合较好。压痕方向接近< 1.1 >时，弹出负载最低，而接近< 1.1 >时，弹出负载最高。在纳米压痕中，由于垂直于滑移面的应力分量的大小通常与分解的剪切应力相当，我们发现均匀位错成核的压力敏感性不能通过弹出试验来确定。我们的统计测量基本上证实了均匀位错成核的热活化模型。也就是说，除了接近<0 0 1>的压痕方向外，在本研究考虑的所有压痕方向上提取的活化能对分解剪应力的依赖关系几乎是相同的。由于在< 0.1 >附近测得非常高的弹出载荷，这意味着在弹出处有很大的接触面积，因此在这些方向中激活先前存在的位错的可能性更高，这可以解释< 0.1 >附近的差异。
Instrumented nanoindentation techniques have been widely used to characterize the small-scale mechanical behavior of materials. The elastic–plastic transition during nanoindentation is often indicated by a sudden displacement burst (pop-in) in the measured load–displacement curve. In defect-free single crystals, the pop-in is believed to be the result of homogeneous dislocation nucleation because the maximum shear stress corresponding to the pop-in load approaches the theoretical strength of the materials and because the statistical distribution of pop-in stresses is consistent with what is expected for a thermally activated process of homogeneous dislocation nucleation. This paper investigates whether this process is affected by crystallography and stress components other than the resolved shear stress. A Stroh formalism coupled with the two-dimensional Fourier transformation is used to derive the analytical stress fields in elastically anisotropic solids under Hertzian contact, which allows the determination of an indentation Schmid factor, namely, the ratio of maximum resolved shear stress to the maximum contact pressure. Nanoindentation tests were conducted on B2-structured NiAl single crystals with different surface normal directions. This material was chosen because it deforms at room temperature by {1 1 0}<0 0 1> slip and thus avoids the complexity of partial dislocation nucleation. Good agreement is obtained between the experimental data and the theoretically predicted orientation dependence of pop-in loads based on the indentation Schmid factor. Pop-in load is lowest for indentation directions close to <1 1 1> and highest for those close to <0 0 1>. In nanoindentation, since the stress component normal to the slip plane is typically comparable in magnitude to the resolved shear stress, we find that the pressure sensitivity of homogeneous dislocation nucleation cannot be determined from pop-in tests. Our statistical measurements generally confirm the thermal activation model of homogeneous dislocation nucleation. That is, the extracted dependence of activation energy on resolved shear stress is almost the same for all the indentation directions considered in this study, except for those close to <0 0 1>. Because very high pop-in loads are measured for orientations close to <0 0 1>, which implies a large contact area at pop-in, there is a higher probability of activating pre-existing dislocations in these orientations, which may explain the discrepancy near <0 0 1>.