Local lattice instability analysis on mode I crack tip in hcp-Mg: Unstable mode for crack propagation vs. dislocation emission
Local lattice instability analysis on mode I crack tip in hcp-Mg: Unstable mode for crack propagation vs. dislocation emission
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DOI：
10.1016/j.commatsci.2017.02.004
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发表时间：
2017-04
影响因子：
3.3
通讯作者：
K. Yashiro
中科院分区：
文献类型：
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作者：
K. Yashiro
作为讨论原子弹性刚度Bijα=Δσiα/Δεj引起的局域形变的系列研究之一，对Hcp镁中的基裂纹、棱柱形裂纹和金字塔形裂纹进行了各种分子动力学模拟，并通过每个原子点的第一本征值BijαΔεj=ηαΔεi及其特征向量主轴{Δεi}讨论了它们不同的形变行为.在线性断裂力学中，尽管单裂纹和周期裂纹的情况不同，但基础裂纹和金字塔裂纹表现为脆性断裂，裂纹尖端应力与σyy=KIc/2πr的大小一致。在基面裂纹中，ηα(1)和lt；0原子的主轴明显呈现出相对于裂纹面的对称形变模式。金字塔形裂纹在30°倾斜的相邻金字塔平面上扩展，ηα(1)和lt；0原子出现在裂纹尖端周围，对应的主轴通常垂直于新的裂纹表面。另一方面，柱状裂纹表现为位错发射和尖端钝化。在应力-应变峰值或位错发射开始之前，ηα(1)<；0原子在裂纹尖端附近广泛地以蝴蝶状出现。在这些ηα(1)<；0区没有明显的结构变化，但材料性质或局部刚度与周围介质明显不同，导致裂纹面上的应力分布明显偏离连续介质理论。这些原子的主轴不是在晶体取向上，而是在与加载轴成45°的方向上，这种差异可能会阻止结构的变化。位错发射的触发点被位于尖端表面、沿滑移方向取向的ηα(1)和lt；0原子的主轴俘获。
As a series study discussing local deformation by the atomic elastic stiffness, B ij α= Δ σ i α/Δ ε j, various molecular dynamics simulations are performed on the basal, prismatic and pyramidal cracks in hcp Mg; and their different deformation behaviors are discussed by the 1st eigenvalue of B ij α Δ ε j= η α Δ ε i and their principal axis of the eigenvector {Δ ε i} at each atom point. The basal and pyramidal cracks show brittle cracking and the crack tip stress coincides with the magnitude of the σ yy= K IC/2 π r in the linear fracture mechanics, despite of the different conditions of single crack vs. periodic crack array. In the basal crack, the principal axes of η α (1)< 0 atoms clearly show symmetrical deformation mode against crack plane. The pyramidal crack propagates in the 30° tilted adjacent pyramidal plane, and η α (1)< 0 atoms emerge around the crack tip and the corresponding principal axes are often normal to the new crack surface. On the other hand, the prismatic crack shows dislocation emission and tip blunting. η α (1)< 0 atoms widely emerge in butterfly shape around crack tip before the stress–strain peak or the onset of dislocation emission. There is no remarkable structural change in these η α (1)< 0 region; however, the material property or local stiffness is clearly different from the surrounding media so that the stress distribution on the crack plane remarkably deviates from the continuum theory. The principal axes of these atoms are not in the crystal orientation but in 45° against loading axis; this difference possibly prevents the structural change. The trigger of dislocation emission is captured by the principal axis of η α (1)< 0 atoms, which is located on the tip surface and oriented in the slip direction.