Orginal Article

# Effect of Different Temperatures on He Atoms Behavior inα-Fe with and without Dislocations

WANG Jin, YU Liming, LI Chong, HUANG Yuan, LI Huijun, LIU Yongchang

State Key Lab of Hydraulic Engineering Simulation and Safety, School of Materials Science and Engineering,Tianjin University, Tianjin 300354, China

Abstract

The requirement of meeting rapidly growing demand for energy while maintaining environmentally friendly has been motivating the hot research on thermonuclear fusion. One of the key issues in future fusion reactors is that structural materials, especially fusion device first wall material, will suffer from He cumulative effects and atomic displacements from radiation cascades. Such harsh service conditions lead to the formation of He bubbles, which are responsible for severe degradation of the structural materials (e.g., swelling, embrittlement, loss of ductility etc.). It is thus essential to further understand the formation of He bubbles and hardening characteristics for the development of future nuclear materials. In this work, the behaviors of He segregation and tensile deformation have been investigated by molecular dynamics (MD) simulations in α-Fe with and without dislocations (dislocation densities are 0 and 3.36×1011 cm-2, respectively ) and at the annealing temperatures of 300 and 600 K with 0.1%He (atomic fraction) injection. The results show that during the process of 300 K annealing, the effect of dislocation is rather weak, and He atoms are easier to form small He clusters by self-trapping. The size of He clusters and the number of dislocation loops are lower. Furthermore, higher temperature can notably intensify He diffusion, and the size of He clusters and the number of dislocation loops both increase at 600 K. In the process of tensile deformation, dislocations can notably accelerate small He clusters to develop into larger He bubbles, which leads to lower yield stress and strain. In addition, at 300 K, the model mainly occurs to brittle fracture and the dislocations density is lower. At 600 K, larger He bubble can promote dislocation multiply and enhance the deformability. Therefore, there exhibits a better plasticity in the model.

Keywords： α-Fe; ; dislocation ; temperature ; He ; molecular dynamics

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WANG Jin, YU Liming, LI Chong, HUANG Yuan, LI Huijun, LIU Yongchang. Effect of Different Temperatures on He Atoms Behavior inα-Fe with and without Dislocations[J]. Acta Metallurgica Sinica, 2019, 55(2): 274-280 https://doi.org/10.11900/0412.1961.2018.00190

## 1 原子模型与方法

Fig.1   Cross sections of models A (a) and B (b) (Atoms are colored according to their common neighbor analysis (CNA). The bcc atoms are colored in blue, and the distorted structure atoms (for example dislocations) are colored in white)

Table 1   Geometrical dimensions of models A and B

Modelx / nmy / nmz / nmNumber of dislocationsNumber of atoms
A19.832.44.650256000
B19.832.44.652255720

## 2 模拟结果与讨论

### 2.1 低温300 K

2.1.1 He原子偏聚行为 图2a和b分别为A和B模型在300 K下经过5 ns退火弛豫后He原子偏聚平衡构型图。图中仅仅显示He原子和位错。可以看出,对于A模型,当温度为300 K时,He间隙原子由于较低的溶解度和移动能[22,23],发生扩散和聚集成团,如图2a所示。同时,可发现He团簇释放出2个新间隙位错环,位错环的类型均为1/2<111>[33],这与以往实验观察结果一致[15,34]。对于B模型,由于含有预置位错,一些He原子会偏聚到位错处[8,28,35],但仍有大量He团簇存在于基体中,未发现新生成的位错环,如图2b所示。图2c为2个模型中He团簇尺寸分布条形图。其中,单个He团簇内He原子数目代表He团簇的尺寸,单个He团簇中He原子数目越多,代表He团簇尺寸越大。A模型中He团簇尺寸变化为1~21,B模型中He团簇尺寸变化为1~16。从图2c可见,A模型中He团簇最大尺寸比B模型大,这主要是由于B模型中位错的存在,He原子优先偏聚到位错处,降低了基体中形成大尺寸He团簇的概率。尽管存在He原子向位错的偏聚,但偏聚的He原子数目较少,A和B模型中大部分He原子均以He团簇弥散分布在基体中,且尺寸较小。

Fig.2   Exemplary snapshots of He clusters distributions at 300 K for model A (a) and model B (b), and the size distribution histogram in these two models (c)

2.1.2 拉伸变形中应力和位错演化行为 图3a和b分别给出了A和B模型在拉伸应变过程中的应力-应变曲线和位错密度-应变曲线。如图3a所示,当温度为300 K时,2种模型均先经历弹性变形,达到应力最大值,之后发生脆性断裂,几乎无塑性变形阶段。对于A模型,当应变ε=0.105,最大应力值σ=17.58 GPa;对于B模型,当ε=0.08,最大应力值σ=14.55 GPa;可见,A模型的临界应力应变值均大于B模型。与之相对应,如图3b所示,2种模型的位错密度值先增加后降低,对于A模型,当ε=0.11,达到最大位错密度值28.37×1011 cm-2;对于B模型,当ε=0.105,最大位错密度值为22.02×1011 cm-2。可见,在整个变形过程中,A和B模型的位错密度值均较低。

Fig.3   Stress-strain (σ-ε) curves (a) and dislocation density-strain curves (b) of models A and B at 300 K

Fig.4   Evolutions of atomistic configurations for model A (a~c) and model B (d~f) with increasing strains at 300 K (a) ε=0.095 (b) ε=0.105 (c) ε=0.17 (d) ε=0.08 (e) ε=0.095 (f) ε=0.17

### 2.2 高温600 K

2.2.1 He原子偏聚行为 图5a和b分别为A和B模型在600 K下经过5 ns退火弛豫后He原子偏聚平衡构型图。可以看出,当温度为600 K时,He原子也发生扩散和聚集成He团簇,He团簇尺寸明显比300 K时大,这主要是由于温度升高引起He原子热扩散行为加剧[25,26]造成的。对于A模型,如图5a所示,基体中He团簇释放出4个新间隙位错环;对于B模型,如图5b所示,He团簇释放出3个间隙位错环。这均比300 K时生成的位错环数目增多,而且偏聚到预置位错的He原子也明显增加。从图5c可知,He原子偏聚达到稳定状态后,A模型中He团簇尺寸变化为1~24,B模型中He团簇尺寸变化为1~30。A和B模型的最大He团簇尺寸均比300 K时要大,而且B模型的增加更明显,这主要是随着温度升高,He原子更容易偏聚到位错处,因而形成了较大尺寸的He团簇。

Fig.5   Exemplary snapshots of He clusters distributions at 600 K for model A (a) and model B (b), and the size distribution histogram in these two models (c)

2.2.2 拉伸变形中应力和位错演化行为 图6a和b分别为A和B模型在拉伸应变过程中的应力-应变曲线和位错密度-应变曲线。可以看出,随着温度提高到600 K,2种模型均先经历弹性变形,达到应力最大值,然后进行塑性变形,最后断裂失效。如图6a所示,对于A模型,当ε=0.075时,最大应力值σ=13.27 GPa;对于B模型,当ε=0.07时,最大应力值σ=12.82 GPa;可见,A模型的临界应力应变值仍大于B模型,并且它们均低于300 K时的临界应力应变值。进一步观察图6a可以发现,B模型的应力应变曲线存在明显的屈服平台。另外,由图6b可知,2种模型的位错密度均较高,对于A模型,当ε=0.12时,位错密度达到最大值85.66×1011 cm-2;对于B模型,当ε=0.20时,最大位错密度值为116.7×1011 cm-2。这均与300 K时的应力-应变曲线和位错密度-应变曲线不同,可能与温度促进He团簇生成和提高位错的可移动性有关。

Fig.6   Stress-strain curves (a) and dislocation density-strain curves (b) of models A and B at 600 K

Fig.7   Evolutions of atomistic configurations of models A (a~c) and B (d~f) with increasing strains at 600 K (a) ε=0.08 (b) ε=0.11 (c) ε=0.17 (d) ε=0.08 (e) ε=0.11 (f) ε=0.30

## 3 结论

(1) 注入0.1%He时,低温300 K下位错对He扩散偏聚行为影响较小,He团簇主要弥散分布在基体中,尺寸较小。在拉伸变形过程中,He团簇较难转变为空洞,强度较高。之后,多空洞合并发生脆性断裂,塑性较差。

(2) 高温600 K时,位错对He扩散偏聚行为影响较大,He团簇尺寸较大。偏聚到位错处的He团簇极易合并长大成He泡,这些He泡展现了较好的延展性和可变形性,因此在拉伸变形过程中,强度较低但塑性较高。

The authors have declared that no competing interests exist.

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