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Atomistic scale behaviors of crack propagation in nanocrystalline bcc iron

机译：纳米晶体BCC铁裂纹繁殖的原子规模行为

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Nanocrystalline materials have an extensive application in engineering due to their excellent performance on strength and toughness. This work carries out molecular dynamics simulation to identify the atomistic scale behaviors of crack propagation in nanocrystalline bcc iron, where the effect of grain size less than 15 nm is examined. The crack, instead of the grain boundary, is found to play a major role in the engineering stress-strain behavior. The maximum stress of the cracked model is no longer consistent with the inverse Hall-Petch relationship. Based on three-dimensional investigation, the diverse propagation behaviors on different cross sections along crack front are revealed, and the mechanism of intergranular decohesion is clarified. Through inter-granular decohesion, the fast brittle cleavage on one cross section can accelerate the ductile propagation on the other cross sections. With a decrease in grain size, the intergranular decohesion effect weakens, and the crack propagation becomes more ductile. The threshold for crack ductile growth also increases with grain size decrease, which is very different from the inverse Hall-Petch relationship. The nanocrystalline iron with a small grain size has lower tensile strength but higher crack ductile growth threshold. Besides, the convictive evidence of asymmetrical propagation in nanocrystalline bcc iron is found. The asymmetrical propagation behavior is determined by the different abilities of leading partial dislocation nucleation on the asymmetrically distributed atomic close-packed planes. Since the grain orientation is random, the atoms are usually distributed asymmetrically along the crack plane. The asymmetrical behavior dominates the crack propagation inside grain for nanocrystalline bcc iron.
机译：由于它们对强度和韧性的优异性能，纳米晶体材料具有广泛的工程应用。该工作进行了分子动力学模拟，以鉴定纳米晶体BCC铁中裂纹繁殖的原子尺度行为，其中检查晶粒尺寸小于15nm的效果。发现裂缝而不是晶界，在工​​程应力 - 应变行为中发挥着重要作用。裂纹模型的最大应力不再与逆霍尔辅助关系保持一致。基于三维调查，揭示了沿裂缝前沿的不同横截面上的不同传播行为，阐明了晶间脱粘的机制。通过颗粒状脱粘，一个横截面上的快速脆性裂解可以加速其他横截面上的延性传播。随着晶粒尺寸的降低，晶间腐蚀效果削弱，并且裂纹繁殖变得更加延展性。裂缝延性生长的阈值也随着晶粒尺寸的降低而增加，这与逆霍尔竖起关系非常不同。具有小粒度的纳米晶铁具有较低的拉伸强度，但裂缝韧性生长阈值较高。此外，发现了纳米晶体BCC铁中不对称繁殖的定罪证据。不对称传播行为由在不对称分布的原子封闭式平面上的前导部分位错成核的不同能力决定。由于晶粒取向是随机的，因此原子通常沿着裂缝平面不对称地分布。不对称行为占据纳米晶体BCC铁内部颗粒内的裂纹繁殖。

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