Mines Paris Tech, MAT, Centre des Matériaux, CNRS UMR 7633, BP 87, 91003 Evry, France
Philos Trans A Math Phys Eng Sci. 2015 Mar 28;373(2038). doi: 10.1098/rsta.2014.0131.
The size and the character (low and large angle, special boundaries, tilt and twist boundaries, twins) of the grain boundaries (GBs) in polycrystalline materials influence their strength and their fracture toughness. Recent studies devoted to nanocrystalline (NC) materials have shown a deviation from the Hall-Petch law. Special GBs formed by Σ3 twins in face-centred cubic metals are also known to have a strong effect on the mechanical behaviour of these metals, in particular their work-hardening rate. Grain orientation influences also crack path, the fracture toughness of body-centred cubic (BCC) metals and the fatigue crack growth rate of microstructurally short cracks. This paper deals both with slip transfer at GBs and with the interactions between propagating cracks with GBs. In the analysis of slip transfer, the emphasis is placed on twin boundaries (TBs) for which the dislocation reactions during slip transfer are analysed theoretically, experimentally and using the results of atomic molecular simulations published in the literature. It is shown that in a number of situations this transfer leads to a normal motion of the TB owing to the displacement of partial dislocations along the TB. This motion can generate a de-twinning effect observed in particular in NC metals. Crack propagation across GBs is also considered. It is shown that cleavage crack path behaviour in BCC metals is largely dependent on the twist component of the GBs. A mechanism for the propagation of these twisted cracks involving a segmentation of the crack front and the existence of intergranular parts is discussed and verified for a pressure vessel steel. A similar segmentation seems to occur for short fatigue cracks although, quite surprisingly, this crossing mechanism for fatigue cracks does not seem to have been examined in very much detail in the literature. Metallurgical methods used to improve the strength of the materials, via grain boundaries, are briefly discussed.
多晶材料中晶界的大小和特征（小角度和大角度晶界、特殊晶界、倾斜和扭转晶界、孪晶）会影响其强度和断裂韧性。最近对纳米晶（NC）材料的研究表明，这些材料偏离了霍尔-佩奇定律。在面心立方金属中由Σ3 孪晶形成的特殊晶界也被认为对这些金属的力学性能有很强的影响，特别是对它们的加工硬化率有影响。晶粒取向也会影响裂纹路径、体心立方（BCC）金属的断裂韧性和微观短裂纹的疲劳裂纹扩展速率。本文既涉及晶界的滑移传递，也涉及扩展裂纹与晶界的相互作用。在滑移传递的分析中，重点是孪晶界（TB），对其滑移传递过程中的位错反应进行了理论、实验和利用文献中发表的原子分子模拟结果的分析。结果表明，在许多情况下，这种传递会导致 TB 沿法线方向运动，这是由于部分位错沿 TB 发生了位移。这种运动可以产生在 NC 金属中特别观察到的去孪晶效应。还考虑了裂纹在晶界处的扩展。结果表明，BCC 金属中解理裂纹路径行为在很大程度上取决于晶界的扭转分量。讨论并验证了一种涉及裂纹前缘分段和存在晶间部分的这些扭曲裂纹扩展的机制，该机制适用于压力容器钢。尽管令人惊讶的是，这种疲劳裂纹的穿越机制似乎尚未在文献中进行非常详细的研究，但对于短疲劳裂纹似乎也会发生类似的分段。简要讨论了通过晶界提高材料强度的冶金方法。
