SHEAR RESPONSE OF GRAIN BOUNDARIES WITH HYDROGEN DEFECTS
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摘要: 针对4个α-Fe对称倾斜晶界，采用分子静力学考察了4个晶界中H原子偏析能的分布特征，并采用分子动力学方法研究了晶界内植入不同数量H原子对其在室温条件下剪切行为的影响。H原子通过随机方式植入界面内，利用植入H原子数量与晶界面积的比值来定义H原子面密度ρ。在含H原子晶界剪切行为分析过程中，重点考察了在不同H原子密度ρ下，4个晶界的初始塑性临界应力和晶界迁移位移的变化趋势以及4个晶界在加载过程中的微观变形机理。研究表明：晶界内的H原子偏析能明显偏低，4个晶界附近的H原子会自发向晶界内偏析；随着植入H原子数量的逐渐增多，晶界的初始塑性临界应力和后续变形阶段应力均会降低。晶界内植入H原子会从本质上改变晶界的微观变形机理，进而影响晶界在外载荷条件下的迁移属性。与不含H原子晶界的变形机理对比发现，加载过程中晶界的微结构会发生剧烈的演化，H原子的扩散和团簇化效应会导致晶界内出现纳米孔缺陷。Abstract: The segregation energy distributions of hydrogen in four α-Fe symmetric tilt grain boundaries (GBs) are analyzed by using molecular statics (MS), and then the shear responses of four GBs embedded with different number of hydrogen atoms at the room temperature are investigated by using molecular dynamics (MD) methods. To facilitate our quantitative analysis, the hydrogen density ρ is defined as the ratio between the number of hydrogen atoms and the GB area. At different hydrogen densities, the variations of initial critical stress of GB plasticity and GB migration displacements are analyzed, and the micro-deformation mechanisms in each GB with the presence of hydrogen atoms are analyzed as well. It is found that the hydrogen segregation energies are generally lower in GB than those inside grain, which lead four GBs to absorb hydrogen atoms in the vicinity of GBs. With the increase of hydrogen density ρ the critical stress of incipient plasticity as well as the flow stress could be reduced. Moreover, the micro-deformation mechanisms of fours GBs with hydrogen atoms are different from those of GBs without hydrogen atoms. In particular, presence of hydrogen atoms remarkably affects GB migration velocity. Thus, GB with hydrogen atoms may undergo a pure sliding deformation instead of the shear-coupling deformation for GB without hydrogen atoms. Meanwhile, in contrast to GBs without hydrogen atoms, the micro-structures of GB with hydrogen atoms drastically evolve upon loading. In addition, the diffusion and agglomeration of hydrogen atoms may lead to the formation of nanovoid in GBs.
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Key words:
- grain boundary /
- hydrogen /
- segregation energy /
- shear behavior
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图 3 $\sum $9(114) 和$\sum $11(332) 内部氢原子偏析能分布趋势(全部结构图为[110]方向投影图)
Figure 3. Distributions of hydrogen segregation energies in GBs $\sum $9(114) and $\sum $11(332)
图 4 氢原子偏析能随晶界面法向方向距离变化趋势
Figure 4. Variations of hydrogen segregation energies in four GBs vs. distance from GBs
图 5 剪切载荷作用下不含H原子晶界的应力-应变曲线(4个晶界的初始塑性加载点分别用A，B，C和D标注)
Figure 5. Stress vs. strain curves of four GBs without hydrogen during the shear
图 6 剪切载荷作用下的不含H原子晶界迁移位移曲线; 晶界的迁移发生在晶界的初始塑性加载点A, B, C和D
Figure 6. GB displacements of four GBs without Hydrogen during the shear
图 7 不同剪切载应变下$\sum $11(332) 晶界的变形构型图(全部结构图为[110]方向投影图)
Figure 7. Snapshots of GB $\sum $11(332) at the different shear strains (all structures are viewed along [110] direction)
图 8 $\sum $11(332) 晶界耦合剪切变形机理(黑色和白色原子处于两个相邻的(110) 面)
Figure 8. Mechanism of shear-coupling deformation in GB $\sum $11(332)(black and white atoms are on the two successive (110) planes)
图 9 含有不同面密度H原子晶界在剪切载荷作用下的应力-应变曲线
Figure 9. Stress vs. strain curves of four GBs for different hydrogen densities in them
图 10 晶界初始塑性临界应力与不同H原子面密度之间的关系曲线
Figure 10. The variations of the critical stress corresponding to the incipient plasticity of four GBs vs. the hydrogen densities
图 11 H原子密度为0.18 Å$^{ -2}$且剪应变分别为0.0和0.15时，4个晶界的原子结构图(灰色和红色原子分别对应Fe和H，绿色带状区域用来跟踪晶界变形过程)
Figure 11. Structures of four GBs at shear strains 0.0 and 0.15 for Hydrogen density of 0.18 Å$^{ -2}$ (Fe and hydrogen atoms are colored in grey and red, the green strip is used to track the GB deformation)
图 12 植入H原子对$\sum $9(114) 和$\sum $11(113) 晶界迁移位移的影响
Figure 12. Migration displacements of GBs $\sum $9(114) and $\sum $11(113) for different hydrogen densities
表 1 晶界的几何和物理参数
Table 1. The geometrical and physical properties of four GBs
表 2 每个晶界中植入的H原子密度对应的H原子数量$N^{\rm H}$
Table 2. The number of hydrogen atoms placed in each GB for a specified hydrogen density
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