小尺度下$C$15 CaAl$_2$ Laves相的断裂
Fracture of the $C$15 CaAl$_2$ Laves phase at small length-scales
AI总结：
本研究通过微柱劈裂和微悬臂梁弯曲实验首次测定$C$15 CaAl$_2$ Laves相的断裂韧性，发现其韧性受解理面控制，{011}面优先断裂，并借助原子模拟揭示晶格捕获和动力学机制。
摘要：
立方$C$15 CaAl$_2$ Laves相是Mg-Al-Ca合金中一种关键的脆性金属间化合物析出相。尽管共存相的力学性能知识对于改进合金设计至关重要，但由于需要微型化测试，其断裂韧性尚未通过实验研究。本文采用微柱劈裂和微悬臂梁弯曲实验测定CaAl$_2$的韧性。研究发现，通过柱劈裂得到的韧性值约为1 $MPa\cdot\sqrt m$，且该值对样品热处理、制备所用的离子束、微柱直径及表面取向基本不敏感。借助电子通道衬度成像和背散射衍射辅助的纳米压痕实验，观察到断裂主要发生在{011}晶面上。对模型$C$15 Laves相的原子级断裂模拟表明，{011}解理面相对于能量上更有利的{111}面的优先选择是由晶格捕获和动力学控制断裂所致。使用矩形微悬臂梁弯曲测试（其中缺口平面与最接近的{112}解理面偏离约8°，与最近的{001}、{011}和{111}面偏离大于20°），测得韧性约为2 $MPa\cdot\sqrt m$，同时电子显微镜观察到裂纹路径的显著偏离，表明优先结晶学解理面决定了该材料的韧性。进一步使用缺口与解理面精确对齐的五边形微悬臂梁进行的研究显示，不同低指数晶面具有相似的断裂韧性值。本文呈现的结果是对$C$15 CaAl$_2$ Laves相断裂韧性的首次详细实验研究，可从裂纹面和裂纹前沿依赖的断裂韧性角度加以理解。
英文摘要：
The cubic $C$15 CaAl$_2$ Laves phase is a crucial brittle intermetallic precipitate in Mg-Al-Ca alloys. Although knowledge of the mechanical properties of coexisting phases is essential for improved alloy design, the fracture toughness is not yet studied experimentally due to the need for miniaturised testing. Here, micropillar splitting and microcantilever bending are used to experimentally determine the toughness of CaAl$_2$. It is found that the toughness value of ~1 $MPa\cdot\sqrt m$ from pillar splitting is largely insensitive to sample heat treatment, ion beam used for fabrication, micropillar diameter, and surface orientation. From nanoindentation supported by electron channelling contrast imaging and backscatter diffraction, fracture is observed to take place mostly on {011} planes. Atomistic fracture simulations on a model $C$15 Laves phase showed that the preference of {011} cleavage planes over the more energetically favourable {111} is due to lattice trapping and kinetics controlling fracture. Using rectangular microcantilever bending tests where the notch plane was misoriented to the closest possible {112} cleavage plane by ~8°, and the closest {001}, {011} and {111} plane by >20°, a toughness of ca. 2 $MPa\cdot\sqrt m$ was determined along with the electron microscopy observation of significant deviations of the crack path, demonstrating that preferential crystallographic cleavage planes determine the toughness in this material. Further investigation using pentagonal microcantilevers with precise alignment of the notch with the cleavage planes revealed similar fracture toughness values for different low-index planes. The results presented here are the first detailed experimental study of fracture toughness of the $C$15 CaAl$_2$ Laves phase, and can be understood in terms of crack plane and crack front dependent fracture toughness.
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