Atomistic simulation of the effects of hydrogen on the mobility of edge dislocation in alpha iron
Atomistic simulation of the effects of hydrogen on the mobility of edge dislocation in alpha iron
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DOI：
10.1007/s10853-007-2364-5
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发表时间：
2008-02
影响因子：
4.5
通讯作者：
S. Taketomi;R. Matsumoto;N. Miyazaki
中科院分区：
文献类型：
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作者：
S. Taketomi;R. Matsumoto;N. Miyazaki
尽管对氢脆机制进行了广泛的研究，但并不是所有的氢对材料性能的影响都被阐明。氢的作用之一是增强塑性局部化，被称为氢增强局部化塑性(HELP)机制[1]。在氢气环境下对位错进行了原位观察，发现在环境池中加入氢气后，位错间距减小[2，3]。这样的塑性局部化在大量的材料和滑移系统中观察到[4]。因此，这些实验结果被认为是有帮助的有力证据。虽然实验观察只表明在氢气环境下位错之间的距离减小，但这种反应的确切原因仍然不清楚。弹性分析表明，氢屏蔽了边缘位错周围的弹性应力，导致位错局部化[5]。然而，在那个特定的分析中，氢的浓度非常高。此外，与从头算结果相反，这些分析没有考虑到位错核上的氢积累，后者表明位错核与氢有很强的结合[6]。此外，从头算研究表明，氢降低了纯铝的Peierls应力。在这项研究中，分析是基于原子模型组成的阿尔法铁，以研究氢对边缘位错的迁移率的影响。首先，基于Wen等人拟合的嵌入原子方法(EAM)势，精确地估计了刃位错周围的氢分布[7]。其次，用微缩弹性带方法[8，9]研究了刃位错运动的能垒。最后，用分子静力学(MS)方法计算了氢存在下刃型位错周围的应力场。分析模型的几何形状和晶体取向如图1所示。滑移发生在(112)面上，滑移方向沿[111]轴。据报道，这种滑移系统对氢脆很敏感[10]；此外，根据弹性理论，只有产生流体静力的边缘成分才有望与氢原子相互作用。为了在模型中引入刃位错，去掉了图1中所示的原子平面，然后用共轭梯度法(CG)松弛了结构。该体系含有8,054个Fe原子，x、y、z方向的晶胞尺寸分别为11.05、4.91和2.02 nm。在x和z方向上应用周期边界条件。在该系统中，位错密度约为0.018 nm-2。本研究采用了温等人提出的Fe-H体系的EAM势[9]，与以前提出的原子间势[11]相比，它被认为是最可靠的原子间势[11]。完美体心立方晶格中的氢占据位可分为两个位：四面体位(T位)和八面体位(O位)。据报道，T位是α铁中氢原子最稳定的位置[12]。然而，刃位错周围的晶体结构不是
Despite extensive investigations concerning hydrogen embrittlement mechanisms, not all the effects of hydrogen on material properties have been clarified. One of the effects of hydrogen is an enhancement of plasticity localisation and is known as the hydrogen enhanced localised plasticity (HELP) mechanism [1]. An in situ observation of the dislocations under a hydrogen gaseous environment was performed using a transmission electron microscope (TEM), which revealed the reduction of the distance between dislocations when hydrogen gas was added into the environmental cell [2, 3]. Such a plasticity localisation is observed in a large number of materials and slip systems [4]. Therefore, these experimental results are considered to be powerful evidence of HELP. Although the experimental observations show only the reduction of the distance between dislocations under a hydrogen gaseous environment, the precise reason for this reaction is still unclear. Elasticity analyses suggest that hydrogen shields the elastic stress around edge dislocations and results in dislocation localisation [5]. However, hydrogen concentration was extremely high in that particular analysis. Furthermore, the analyses could not take into account the hydrogen accumulation at dislocation core, in contrast with the results of the ab initio study, which show that the dislocation core has strong binding to hydrogen [6]. Moreover, the ab initio study showed that hydrogen lowers the Peierls stress for pure aluminium. In this study, analyses are conducted based upon atomistic models composed of alpha iron in order to investigate the effect of hydrogen on the mobility of edge dislocations. First, the hydrogen distribution around an edge dislocation is accurately estimated based on the embedded atom method (EAM) potential fitted by Wen et al.[7]. Second, the energy barriers for edge dislocation motion are investigated using the nudged elastic band (NEB) method [8, 9]. Finally, the stress fields around the edge dislocation in the presence of hydrogen are calculated using the molecular statics (MS) method. The geometrical shapes and crystallographic orientations of the analysis model are shown in Fig. 1. Slip occurs on the (112) plane, and the slip direction is along the [111] axis. This slip system is reported to be sensitive to hydrogen embrittlement [10]; moreover, according to the theory of elasticity, only an edge component that yields hydrostatic stress is expected to interact with hydrogen atoms. In order to introduce an edge dislocation into the model, the atomic plane shown in Fig. 1 is removed, and the structure is then relaxed by the conjugate gradient (CG) method. The system contains 8,054 Fe atoms, and the unit cell size is 11.05, 4.91 and 2.02 nm in the x, y and z directions, respectively. Periodic boundary conditions are applied in the x and z directions. In this system, dislocation density is approximately 0.018 nm-2. The EAM potential for the Fe–H system proposed by Wen et al.[9], which is reported to be the most reliable interatomic potential compared to previously proposed interatomic potentials [11], is adopted in this study. The hydrogen occupation site in a perfect bcc lattice may be characterised into two sites: a tetrahedral site (T-site) and an octahedral site (O-site). The T-site is reported to be the most stable site for a hydrogen atom in alpha iron [12]. However, the crystallographic structure around an edge dislocation does not