Zhou Lei, Guo Ya-Fang
Institute of Engineering Mechanics, Beijing Jiaotong University, Beijing 100044, China.
Materials (Basel). 2015 Aug 13;8(8):5250-5264. doi: 10.3390/ma8085250.
In this work, the plastic deformation mechanisms responsible for mechanical properties and fracture toughness in nanotwinned (NT) magnesium is studied by molecular dynamics (MD) simulation. The influence of twin boundary (TBs) spacing and crack position on deformation behaviors are investigated. The microstructure evolution at the crack tip are not exactly the same for the left edge crack (LEC) and the right edge crack (REC) models according to calculations of the energy release rate for dislocation nucleation at the crack tip. The LEC growth initiates in a ductile pattern and then turns into a brittle cleavage. In the REC model, the atomic decohesion occurs at the crack tip to create a new free surface which directly induces a brittle cleavage. A ductile to brittle transition is observed which mainly depends on the competition between dislocation motion and crack growth. This competition mechanism is found to be correlated with the TB spacing. The critical values are 10 nm and 13.5 nm for this transition in LEC and REC models, respectively. Essentially, the dislocation densities affected by the TB spacing play a crucial role in the ductile to brittle transition.
在这项工作中，通过分子动力学（MD）模拟研究了纳米孪晶（NT）镁中负责力学性能和断裂韧性的塑性变形机制。研究了孪晶界（TBs）间距和裂纹位置对变形行为的影响。根据裂纹尖端位错形核的能量释放率计算，左边缘裂纹（LEC）模型和右边缘裂纹（REC）模型在裂纹尖端的微观结构演变并不完全相同。LEC扩展以韧性模式开始，然后转变为脆性解理。在REC模型中，裂纹尖端发生原子脱粘以产生新的自由表面，这直接导致脆性解理。观察到韧性到脆性的转变，这主要取决于位错运动和裂纹扩展之间的竞争。发现这种竞争机制与TB间距相关。在LEC和REC模型中，这种转变的临界值分别为10 nm和13.5 nm。本质上，受TB间距影响的位错密度在韧性到脆性转变中起关键作用。
