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Enhanced formation of <1 0 0> and <1 1 1> interstitial loops by helium clustering in bcc iron
Materials Letters 190 (2017) 260–262
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Enhanced formation of <1 0 0> and <1 1 1> interstitial loops by helium clustering in bcc iron L. Yang a,⇑, H. Liu a, H.L. Zhou a, S.M. Peng b, X.G. Long b, X.S. Zhou b, X.T. Zu a, F. Gao c a
School of Physical Electronics, University of Electronic Science and Technology of China, Chengdu 610054, China Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, China c Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109, USA b
a r t i c l e
i n f o
Article history: Received 21 December 2016 Received in revised form 3 January 2017 Accepted 5 January 2017 Available online 7 January 2017 Keywords: Defects Microstructure Atomistic simulation He bubble Bcc iron
a b s t r a c t The nucleation and growth of helium (He) clusters in bcc iron (Fe) are simulated using molecular dynamics method. The self-interstitial atoms (SIAs) can be emitted from the He clusters, and the evolution of these SIAs depends on the spatial distribution of He clusters. A 1/2<1 1 1> interstitial loop is presented from the emitted SIAs for the two He clusters separated far away from each other, but a <1 0 0> interstitial loop is formed at special conditions. A new mechanism for the formation and growth of a <1 0 0> interstitial loop in bcc Fe due to He clustering is proposed. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction A large number of experimental and computer simulation studies have focused on the irradiation damage of Fe-based bcc materials [1–4]. Especially, modelling the effects of He bubble growth on self-defects in bcc Fe and Fe-alloys at an atomistic level was carried out a few decades ago [5–9]. Up to now, only 1/2<1 1 1> interstitial dislocation loops (IDLs), as induced by the He clustering, were reported in bulk Fe and Fe alloys in the simulations [5–8]. However, after He implantation, two sets of IDLs, with Burgers vectors of 1/2<1 1 1> and <1 0 0>, were identified using transmission electron microscopy in oxide dispersion strengthened ferritic steel [10]. Two types of loops (1/2<1 1 1> and <1 0 0> loops) were observed in experiments. These two types of loops have different characteristics, and thus have different effects on the mechanical properties of materials. The previous molecular dynamics (MD) results are in contrast to the experimental observations, which will affect the reliability of simulations, thus affecting further simulations at larger scales. Furthermore, ferritic/martensitic steels and alloys have been used for structural materials in current nuclear fission reactors and proposed as candidate first wall materials in future fusion energy facilities. The behaviour of He in irradiated
⇑ Corresponding author. E-mail addresses: [email protected] (L. Yang), [email protected] (F. Gao). http://dx.doi.org/10.1016/j.matlet.2017.01.024 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
materials is of interest because high concentrations of He created by transmutation is known to induce the formation of He bubbles with the existing and radiation-induced defects, and significantly degrade the mechanical properties of the first wall structural materials. A general hypothesis for this process is that the dislocationloop punching represents an important mechanism for the growth of He bubbles. Thus, we focus on the formation of IDLs during the clustering of He in a-Fe. In this letter, we present the direct observation of the formation and growth of <1 0 0> and <1 1 1> IDLs during the formation of He bubbles in MD simulations, and analyze the condition for the formation of <1 0 0> IDLs. We found that the formation of <1 0 0> loops in a-Fe is enhanced by the pressure field between He clusters. The results provide new insights into the formation of two types of IDLs and link experiments and simulations in a-Fe and ferritic alloys subject to He implantation or in an irradiation environment.
2. Simulation methods In this work, the Fe-Fe, Fe-He and He-He interactions are described by the interatomic potentials of Ackland et al. [11], Gao et al. [12] and Aziz [13], respectively. A modified version of the MOLDY computer code is employed, and all simulations are carried out with periodic boundary conditions and constant volume. A box of 250,000 Fe atoms (50a0 50a0 50a0, where a0 is 0.28553 nm) is applied with a time step of 1 fs. At the first step,
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L. Yang et al. / Materials Letters 190 (2017) 260–262
two small He clusters, each containing four He atoms, are introduced into the simulation box with different separation distances and spatial relationships. It should be noted that a He cluster with four He atoms is the smallest size that can emit SIAs at high temperatures [8], and thus each cluster is initiated to contain four He atoms. However, He atoms are continuously inserted one by one at the centre of each of the two He clusters. After each He atom insertion, the configuration is quenched to 0 K, followed by a temperature rescaling to 300 K and an additional 100 ps annealing. The configuration is then quenched back to 0 K and the atomic-level pressure of each atom Pi is calculated using the following equation:
Pi ¼
1 xx zz r þ ryy i þ ri ; 3 i
ð1Þ
The stress tensor r with ab component (a, b 2 x, y, z) can be expressed as ab
ri
" ! # b a 1 X @Vðr ij Þ @ Uðqi Þ @ Uðqj Þ @ Wðr ij Þ r ij rij ¼ þ þ ; @ qj 2Xi j–i @r ij @ qi @r ij r ij
ð2Þ
where Xi is the volume of atom i, V(rij) is a pair-potential term and P U(qi) is an embedding energy term, qi ¼ j Wðrij Þ, W(rij) is a local density part, rij is the vector distance between atoms i and j. Subsequently, a similar procedure is repeated by adding the next He until to the desirable number. Interstitials and vacancies are identified and counted using a displacement analysis method [14]. 3. Results and discussion Firstly, the two He cluster seeds are allocated along a [0 0 1] direction with different distances: 10a0, 8a0 and 6a0. For the distance of 10a0, with increasing He atoms, some SIAs are created that form either <1 1 0> dumbbells or <1 1 1> crowdions at one side of the He clusters. After inserting 62 He atoms to each He cluster, a <1 1 1> interstitial cluster with 28 SIAs are formed near the lower He cluster, but a SIA cluster mixing with <1 1 1> and <1 1 0> SIAs are attached to the top He cluster. The further increase of He atoms in the He clusters induces the glide of the top SIA cluster to another SIA cluster along a <1 1 1> direction, forming a large SIA loop with 55 SIAs, as shown in Fig. 1. It is clear that the SIA cluster is a 1/2<1 1 1> loop. For a separation distance of 8a0 between the two He cluster seeds, a mixing interstitial cluster consisting of <1 1 1> and <1 0 0> SIAs are formed as consequence of SIA emission from the
[001] [010] Fig. 1. Configurations of He clusters with 63 He atoms on the (1 0 0) plane in bulk Fe. The inset shows defects in the region marked by the purple circle, which is viewed normal to a <1 1 1> direction. The black spheres are Fe atoms or SIAs in the inset, and red and green spheres present He atoms and vacancies, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
[001] [010]
SIA
He
V
Fig. 2. Pressure (GPa) distribution of He atoms and SIAs for the two He cluster with 61 He atoms. Large and small spheres are SIAs and He atoms, respectively. Different colors of the spheres represent different pressure levels. Green squares are vacancies. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
He clusters. Fig. 2 shows the quenched pressure distribution of He atoms and SIAs for the two He clusters with 61 He atoms each. It is noted that a <1 0 0> SIA cluster with 11 SIAs is formed, as indicated by the black circle. Furthermore, over 50 <1 1 1> SIAs are distributed at the periphery of the <1 0 0> SIA cluster, i.e., a <1 0 0> junction is formed [4]. Fig. 2 illustrates that the pressure of the top He cluster is higher than that of the lower He cluster, about 2.0 GPa higher per He atom. Especially, the pressure of the <1 0 0> SIAs is obviously lower than that of the <1 1 1> SIAs. These results suggest that the formation of a perfect <1 0 0> SIA loop may be associated with the existing large pressure in the crystal. Thus, we further decrease the distance between the two He cluster seeds to 6a0. As expected, a <1 0 0> IDL that consists of 23 SIAs is produced during the growth of the He clusters. In order to understand the formation mechanism of the <1 0 0> SIA loop, several snapshots of the pressure distribution of He atoms and SIAs at different stages of adding He atoms into the clusters are shown in Fig. 3. After adding seven He atoms to each He cluster, the top He7 cluster emits a Fe atom, forming a He7V cluster and a SIA. The initially formed SIA appears to be a <1 1 1> crowdion, but soon transfers to a <1 1 0> dumbbell, which is the lowest energy configuration of SIAs [11]. The SIA moves around the He cluster as the next He atom is added into the He clusters. In Fig. 3(b), most SIAs move to relocate at the middle region between the two He clusters after adding 13 He atoms. Three SIAs between the two He clusters transfer to <1 0 0> dumbbells, but the other SIAs remain as either <1 1 1> or <1 1 0> configurations. At the joint region between the two He clusters, more SIAs transfer to <1 0 0> dumbbells when He atoms are continuously inserted into the two He clusters, but SIAs collected at the peripheries of the two He clusters are still <1 1 0> or <1 1 1> configurations, which can be seen in Fig. 3(c). A <1 0 0> IDL with 24 SIAs is formed after insertion of 37 He atoms. With adding more He atoms, the <1 0 0> SIA loop grows and becomes larger. However, the emission of SIA loops does not occur even until each He cluster includes 63 He atoms, which may be attributed to the low mobility of the <1 0 0> loop at 300 K and its strong binding with the He clusters. Experimental results revealed that small <1 0 0> loops exhibited one-dimensional (1D) glide motion at temperatures higher than approximately 770 K [15]. Fig. 3 indicates that the pressure of He atoms decreases as the He clusters grow from 7 to 21 atoms, but remains almost constant from 21 to 37 atoms. The results indicate that the SIA IDL formed in
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L. Yang et al. / Materials Letters 190 (2017) 260–262
(a) 7He
[001]
[001]
[001] [100] [010]
(c) 37He
(b) 13He
[100
[100]
[010]]
[010]
SIA
He
V
Fig. 3. Pressure (GPa) distribution of He atoms and SIAs: (a) 7 He, (b) 13 He and (c) 37 He in each He cluster, where the representations of spheres are the same as those in Fig. 2.
the simulations probably depends on the separation distance between the two He cluster seeds. The value of 6a0 is likely a critical distance that enhances the formation of <1 0 0> IDLs. We further studied the growth of two He clusters that are orientated along [1 1 1] and [1 1 0] directions with an initial distance of 60. However, the <1 0 0> SIA loop is not produced. We continually decreased the distance between the two He cluster seeds to 5.5a0. Only as the two He clusters are located along the [1 1 0] direction, a <1 0 0> cluster with 6 SIAs is formed as the number of He atoms in each cluster increases to 13. These results suggest that the formation and evolution of SIAs are associated with not only the specific space distribution of the He clusters, but also the distance between the He clusters. It is likely that the pressure fields produced by the He clusters respond to the fate of SIA loop formation. In this work, two types of loops are observed at different conditions, which closes the gap between the experimental and previous MD simulation results. Moreover, the transformation from a 1/2<1 1 1> loop formation to a <1 0 0> loop formation observed in the present study provides one of the important mechanisms for the formation of <1 0 0> loops in bcc metals, and the results may have important impacts on the defect evolution in bcc Fe with the presence of helium in an irradiation environment. 4. Conclusion Using MD simulations, we have studied the emission, the accumulation and loop formation of SIAs with the growth of two He clusters in a-Fe. A 1/2<1 1 1> or a <1 0 0> IDL can be formed from the emitted SIAs, which depends on the separation distance of the He clusters and their orientation. The pressure fields created
by the He clusters provide a new mechanism for the formation and growth of a <1 0 0> IDL in bcc Fe. Acknowledgements L. Yang and X.T. Zu are grateful for the support by the National Natural Science Foundation of China — NSAF (Grant No: U1430109). References [1] A.E. Ward, S.B. Fisher, J. Nucl. Mater. 166 (1989) 227. [2] L. Zhang, C.C. Fu, G.H. Lu, Phys. Rev. B 87 (2013) 134107. [3] H.X. Xu, R.E. Stoller, Y.N. Osetsky, D. Terentyev, Phys. Rev. Lett. 110 (2013) 265503. [4] J. Marian, B.D. Wirth, J.M. Perlado, Phys. Rev. Lett. 88 (2002) 255507. [5] D. Stewart, Y. Osetskiy, R. Stoller, J. Nucl. Mater. 417 (2011) 1110. [6] A. Caro, J. Hetherly, A. Stukowski, M. Caro, E. Martinez, S. Srivilliputhur, L. Zepeda-Ruiz, M. Nastasi, J. Nucl. Mater. 418 (2011) 261. [7] S.H. Guo, B.E. Zhu, W.C. Liu, Z.Y. Pan, Y.X. Wang, Nucl. Instr. Meth. B 267 (2009) 3278. [8] L. Yang, F. Gao, R.J. Kurtz, X.T. Zu, S.M. Peng, X.G. Long, X.S. Zhou, Acta Mater. 97 (2015) 86. [9] L. Yang, F. Gao, H.L. Heinisch, R.J. Kurtz, D. Terentyev, X.T. Zu, Acta Mater. 82 (2015) 275. [10] J. Chen, P. Jung, W. Hoffelner, H. Ullmaier, Acta Mater. 56 (2008) 250. [11] G.J. Ackland, M.I. Mendelev, D.J. Srlolovitz, S. Han, A.V. Barashev, J. Phys.: Condens. Matter 16 (2004) s2629. [12] F. Gao, H.Q. Deng, H.L. Heinisch, R.J. Kurtz, J. Nucl. Mater. 418 (2011) 115. [13] R.A. Aziz, A.R. Janzen, M.R. Moldover, Phys. Rev. Lett. 74 (1995) 1586. [14] L. Yang, X.T. Zu, Z.G. Wang, H.T. Yang, F. Gao, H.L. Heinisch, R.J. Kurtz, J. Appl. Phys. 103 (2008) 063528. [15] K. Arakawa, M. Hatanaka, E. Kuramoto, K. Ono, H. Mori, Phys. Rev. Lett. 96 (2006) 125506.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Enhanced formation of <1 0 0> and <1 1 1> interstitial loops by helium clustering in bcc iron L. Yang a,⇑, H. Liu a, H.L. Zhou a, S.M. Peng b, X.G. Long b, X.S. Zhou b, X.T. Zu a, F. Gao c a
School of Physical Electronics, University of Electronic Science and Technology of China, Chengdu 610054, China Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, China c Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109, USA b
a r t i c l e
i n f o
Article history: Received 21 December 2016 Received in revised form 3 January 2017 Accepted 5 January 2017 Available online 7 January 2017 Keywords: Defects Microstructure Atomistic simulation He bubble Bcc iron
a b s t r a c t The nucleation and growth of helium (He) clusters in bcc iron (Fe) are simulated using molecular dynamics method. The self-interstitial atoms (SIAs) can be emitted from the He clusters, and the evolution of these SIAs depends on the spatial distribution of He clusters. A 1/2<1 1 1> interstitial loop is presented from the emitted SIAs for the two He clusters separated far away from each other, but a <1 0 0> interstitial loop is formed at special conditions. A new mechanism for the formation and growth of a <1 0 0> interstitial loop in bcc Fe due to He clustering is proposed. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction A large number of experimental and computer simulation studies have focused on the irradiation damage of Fe-based bcc materials [1–4]. Especially, modelling the effects of He bubble growth on self-defects in bcc Fe and Fe-alloys at an atomistic level was carried out a few decades ago [5–9]. Up to now, only 1/2<1 1 1> interstitial dislocation loops (IDLs), as induced by the He clustering, were reported in bulk Fe and Fe alloys in the simulations [5–8]. However, after He implantation, two sets of IDLs, with Burgers vectors of 1/2<1 1 1> and <1 0 0>, were identified using transmission electron microscopy in oxide dispersion strengthened ferritic steel [10]. Two types of loops (1/2<1 1 1> and <1 0 0> loops) were observed in experiments. These two types of loops have different characteristics, and thus have different effects on the mechanical properties of materials. The previous molecular dynamics (MD) results are in contrast to the experimental observations, which will affect the reliability of simulations, thus affecting further simulations at larger scales. Furthermore, ferritic/martensitic steels and alloys have been used for structural materials in current nuclear fission reactors and proposed as candidate first wall materials in future fusion energy facilities. The behaviour of He in irradiated
⇑ Corresponding author. E-mail addresses: [email protected] (L. Yang), [email protected] (F. Gao). http://dx.doi.org/10.1016/j.matlet.2017.01.024 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
materials is of interest because high concentrations of He created by transmutation is known to induce the formation of He bubbles with the existing and radiation-induced defects, and significantly degrade the mechanical properties of the first wall structural materials. A general hypothesis for this process is that the dislocationloop punching represents an important mechanism for the growth of He bubbles. Thus, we focus on the formation of IDLs during the clustering of He in a-Fe. In this letter, we present the direct observation of the formation and growth of <1 0 0> and <1 1 1> IDLs during the formation of He bubbles in MD simulations, and analyze the condition for the formation of <1 0 0> IDLs. We found that the formation of <1 0 0> loops in a-Fe is enhanced by the pressure field between He clusters. The results provide new insights into the formation of two types of IDLs and link experiments and simulations in a-Fe and ferritic alloys subject to He implantation or in an irradiation environment.
2. Simulation methods In this work, the Fe-Fe, Fe-He and He-He interactions are described by the interatomic potentials of Ackland et al. [11], Gao et al. [12] and Aziz [13], respectively. A modified version of the MOLDY computer code is employed, and all simulations are carried out with periodic boundary conditions and constant volume. A box of 250,000 Fe atoms (50a0 50a0 50a0, where a0 is 0.28553 nm) is applied with a time step of 1 fs. At the first step,
261
L. Yang et al. / Materials Letters 190 (2017) 260–262
two small He clusters, each containing four He atoms, are introduced into the simulation box with different separation distances and spatial relationships. It should be noted that a He cluster with four He atoms is the smallest size that can emit SIAs at high temperatures [8], and thus each cluster is initiated to contain four He atoms. However, He atoms are continuously inserted one by one at the centre of each of the two He clusters. After each He atom insertion, the configuration is quenched to 0 K, followed by a temperature rescaling to 300 K and an additional 100 ps annealing. The configuration is then quenched back to 0 K and the atomic-level pressure of each atom Pi is calculated using the following equation:
Pi ¼
1 xx zz r þ ryy i þ ri ; 3 i
ð1Þ
The stress tensor r with ab component (a, b 2 x, y, z) can be expressed as ab
ri
" ! # b a 1 X @Vðr ij Þ @ Uðqi Þ @ Uðqj Þ @ Wðr ij Þ r ij rij ¼ þ þ ; @ qj 2Xi j–i @r ij @ qi @r ij r ij
ð2Þ
where Xi is the volume of atom i, V(rij) is a pair-potential term and P U(qi) is an embedding energy term, qi ¼ j Wðrij Þ, W(rij) is a local density part, rij is the vector distance between atoms i and j. Subsequently, a similar procedure is repeated by adding the next He until to the desirable number. Interstitials and vacancies are identified and counted using a displacement analysis method [14]. 3. Results and discussion Firstly, the two He cluster seeds are allocated along a [0 0 1] direction with different distances: 10a0, 8a0 and 6a0. For the distance of 10a0, with increasing He atoms, some SIAs are created that form either <1 1 0> dumbbells or <1 1 1> crowdions at one side of the He clusters. After inserting 62 He atoms to each He cluster, a <1 1 1> interstitial cluster with 28 SIAs are formed near the lower He cluster, but a SIA cluster mixing with <1 1 1> and <1 1 0> SIAs are attached to the top He cluster. The further increase of He atoms in the He clusters induces the glide of the top SIA cluster to another SIA cluster along a <1 1 1> direction, forming a large SIA loop with 55 SIAs, as shown in Fig. 1. It is clear that the SIA cluster is a 1/2<1 1 1> loop. For a separation distance of 8a0 between the two He cluster seeds, a mixing interstitial cluster consisting of <1 1 1> and <1 0 0> SIAs are formed as consequence of SIA emission from the
[001] [010] Fig. 1. Configurations of He clusters with 63 He atoms on the (1 0 0) plane in bulk Fe. The inset shows defects in the region marked by the purple circle, which is viewed normal to a <1 1 1> direction. The black spheres are Fe atoms or SIAs in the inset, and red and green spheres present He atoms and vacancies, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
[001] [010]
SIA
He
V
Fig. 2. Pressure (GPa) distribution of He atoms and SIAs for the two He cluster with 61 He atoms. Large and small spheres are SIAs and He atoms, respectively. Different colors of the spheres represent different pressure levels. Green squares are vacancies. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
He clusters. Fig. 2 shows the quenched pressure distribution of He atoms and SIAs for the two He clusters with 61 He atoms each. It is noted that a <1 0 0> SIA cluster with 11 SIAs is formed, as indicated by the black circle. Furthermore, over 50 <1 1 1> SIAs are distributed at the periphery of the <1 0 0> SIA cluster, i.e., a <1 0 0> junction is formed [4]. Fig. 2 illustrates that the pressure of the top He cluster is higher than that of the lower He cluster, about 2.0 GPa higher per He atom. Especially, the pressure of the <1 0 0> SIAs is obviously lower than that of the <1 1 1> SIAs. These results suggest that the formation of a perfect <1 0 0> SIA loop may be associated with the existing large pressure in the crystal. Thus, we further decrease the distance between the two He cluster seeds to 6a0. As expected, a <1 0 0> IDL that consists of 23 SIAs is produced during the growth of the He clusters. In order to understand the formation mechanism of the <1 0 0> SIA loop, several snapshots of the pressure distribution of He atoms and SIAs at different stages of adding He atoms into the clusters are shown in Fig. 3. After adding seven He atoms to each He cluster, the top He7 cluster emits a Fe atom, forming a He7V cluster and a SIA. The initially formed SIA appears to be a <1 1 1> crowdion, but soon transfers to a <1 1 0> dumbbell, which is the lowest energy configuration of SIAs [11]. The SIA moves around the He cluster as the next He atom is added into the He clusters. In Fig. 3(b), most SIAs move to relocate at the middle region between the two He clusters after adding 13 He atoms. Three SIAs between the two He clusters transfer to <1 0 0> dumbbells, but the other SIAs remain as either <1 1 1> or <1 1 0> configurations. At the joint region between the two He clusters, more SIAs transfer to <1 0 0> dumbbells when He atoms are continuously inserted into the two He clusters, but SIAs collected at the peripheries of the two He clusters are still <1 1 0> or <1 1 1> configurations, which can be seen in Fig. 3(c). A <1 0 0> IDL with 24 SIAs is formed after insertion of 37 He atoms. With adding more He atoms, the <1 0 0> SIA loop grows and becomes larger. However, the emission of SIA loops does not occur even until each He cluster includes 63 He atoms, which may be attributed to the low mobility of the <1 0 0> loop at 300 K and its strong binding with the He clusters. Experimental results revealed that small <1 0 0> loops exhibited one-dimensional (1D) glide motion at temperatures higher than approximately 770 K [15]. Fig. 3 indicates that the pressure of He atoms decreases as the He clusters grow from 7 to 21 atoms, but remains almost constant from 21 to 37 atoms. The results indicate that the SIA IDL formed in
262
L. Yang et al. / Materials Letters 190 (2017) 260–262
(a) 7He
[001]
[001]
[001] [100] [010]
(c) 37He
(b) 13He
[100
[100]
[010]]
[010]
SIA
He
V
Fig. 3. Pressure (GPa) distribution of He atoms and SIAs: (a) 7 He, (b) 13 He and (c) 37 He in each He cluster, where the representations of spheres are the same as those in Fig. 2.
the simulations probably depends on the separation distance between the two He cluster seeds. The value of 6a0 is likely a critical distance that enhances the formation of <1 0 0> IDLs. We further studied the growth of two He clusters that are orientated along [1 1 1] and [1 1 0] directions with an initial distance of 60. However, the <1 0 0> SIA loop is not produced. We continually decreased the distance between the two He cluster seeds to 5.5a0. Only as the two He clusters are located along the [1 1 0] direction, a <1 0 0> cluster with 6 SIAs is formed as the number of He atoms in each cluster increases to 13. These results suggest that the formation and evolution of SIAs are associated with not only the specific space distribution of the He clusters, but also the distance between the He clusters. It is likely that the pressure fields produced by the He clusters respond to the fate of SIA loop formation. In this work, two types of loops are observed at different conditions, which closes the gap between the experimental and previous MD simulation results. Moreover, the transformation from a 1/2<1 1 1> loop formation to a <1 0 0> loop formation observed in the present study provides one of the important mechanisms for the formation of <1 0 0> loops in bcc metals, and the results may have important impacts on the defect evolution in bcc Fe with the presence of helium in an irradiation environment. 4. Conclusion Using MD simulations, we have studied the emission, the accumulation and loop formation of SIAs with the growth of two He clusters in a-Fe. A 1/2<1 1 1> or a <1 0 0> IDL can be formed from the emitted SIAs, which depends on the separation distance of the He clusters and their orientation. The pressure fields created
by the He clusters provide a new mechanism for the formation and growth of a <1 0 0> IDL in bcc Fe. Acknowledgements L. Yang and X.T. Zu are grateful for the support by the National Natural Science Foundation of China — NSAF (Grant No: U1430109). References [1] A.E. Ward, S.B. Fisher, J. Nucl. Mater. 166 (1989) 227. [2] L. Zhang, C.C. Fu, G.H. Lu, Phys. Rev. B 87 (2013) 134107. [3] H.X. Xu, R.E. Stoller, Y.N. Osetsky, D. Terentyev, Phys. Rev. Lett. 110 (2013) 265503. [4] J. Marian, B.D. Wirth, J.M. Perlado, Phys. Rev. Lett. 88 (2002) 255507. [5] D. Stewart, Y. Osetskiy, R. Stoller, J. Nucl. Mater. 417 (2011) 1110. [6] A. Caro, J. Hetherly, A. Stukowski, M. Caro, E. Martinez, S. Srivilliputhur, L. Zepeda-Ruiz, M. Nastasi, J. Nucl. Mater. 418 (2011) 261. [7] S.H. Guo, B.E. Zhu, W.C. Liu, Z.Y. Pan, Y.X. Wang, Nucl. Instr. Meth. B 267 (2009) 3278. [8] L. Yang, F. Gao, R.J. Kurtz, X.T. Zu, S.M. Peng, X.G. Long, X.S. Zhou, Acta Mater. 97 (2015) 86. [9] L. Yang, F. Gao, H.L. Heinisch, R.J. Kurtz, D. Terentyev, X.T. Zu, Acta Mater. 82 (2015) 275. [10] J. Chen, P. Jung, W. Hoffelner, H. Ullmaier, Acta Mater. 56 (2008) 250. [11] G.J. Ackland, M.I. Mendelev, D.J. Srlolovitz, S. Han, A.V. Barashev, J. Phys.: Condens. Matter 16 (2004) s2629. [12] F. Gao, H.Q. Deng, H.L. Heinisch, R.J. Kurtz, J. Nucl. Mater. 418 (2011) 115. [13] R.A. Aziz, A.R. Janzen, M.R. Moldover, Phys. Rev. Lett. 74 (1995) 1586. [14] L. Yang, X.T. Zu, Z.G. Wang, H.T. Yang, F. Gao, H.L. Heinisch, R.J. Kurtz, J. Appl. Phys. 103 (2008) 063528. [15] K. Arakawa, M. Hatanaka, E. Kuramoto, K. Ono, H. Mori, Phys. Rev. Lett. 96 (2006) 125506.