A new mechanism of loop formation and transformation in bcc iron without dislocation reaction

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Abstract

Structure and kinetics of dislocation loops in α-Fe is an active field in material science, due to their implications on fundamental understanding as well as application of structural materials in irradiation environments. Recent computer simulations provoke new conceptions, which call for experimental verification. The present investigation reports transmission electron microscopy of small interstitial dislocation loops (2.5–10 nm diameters) in bcc iron, irradiated with 25 MeV α-particles at 573 K up to 0.13 dpa. The observed 〈1 0 0〉 and ½〈1 1 1〉 loops have habit planes of (1 0 0), and (1 1 0), (1 1 1) and (2 1 1), respectively. Furthermore it is observed that loops also contain ½〈1 1 1〉{2 1 1} and 〈1 0 0〉{1 0 0} components which are considered as intermediate stages of transformation of ½〈1 1 1〉 loops to 〈1 0 0〉. Based on these observations, a new mechanism of loop formation and transformation by self-interstitial atoms aggregation is proposed, with concurrent molecular dynamic simulations supporting the kinetic feasibility of the proposed process.

Introduction

A vital interest in a thorough understanding of loop formation in irradiated materials comes from its technical implications: dislocation loops as a result of aggregation of point defects and small defect clusters generated during irradiation have a decisive influence on mechanical properties, e.g. hardening and embrittlement, of structural materials. Furthermore they have a strong impact on other properties such as swelling, irradiation creep and precipitation by changing the kinetics of point defects. Pure iron and ferritic steels which are base-materials for structural materials have bcc structure. As close packed crystallographic planes are missing in bcc, the atomic configurations and detailed reaction kinetics of dislocations observed in fcc materials cannot be directly adopted. In the last decades, a number of experimental [1], [2], [3], [4], [5] and computer simulation studies [6], [7], [8], [9], [10], [11] investigated the formation of dislocation loops in Fe-based bcc materials, but left important questions still unsolved. For example the commonly observed interstitial loops in irradiated bcc materials have Burgers vectors b = ½〈1 1 1〉 while in iron and Fe-based ferritic steels a significant fraction of loops with 〈1 0 0〉 are formed during irradiation at elevated temperatures. A mechanism for the formation of loops with 〈1 0 0〉 in addition to the energetically (for most potentials) more favourable b = ½〈1 1 1〉 was first proposed by Eyre and Bullough [12], with a modification given in Ref. [13]. Both models are based on the classical dislocation reaction theory obeying Kirchhoff’s law for the Burgers vector [14], but are at variance with recent experimental in situ observations [15], [16]. One of these [15] reported that a change of Burgers vector of loops occurred without contact with another external dislocation loop. The other experiment demonstrated that larger loop absorb small ones by an absorption reaction [16]. Furthermore, a complete explanation for the relative probability of forming loops with b = 〈1 0 0〉 to b = ½〈1 1 1〉 in irradiated iron is still not possible. On the other hand, recent calculations demonstrated that the temperature dependence of elastic constants seriously influences the ratio of 〈1 0 0〉 to ½〈1 1 1〉 loops in Fe [17]. This could give a thermodynamic reason for the formation of 〈1 0 0〉 loops but cannot account for the kinetic process. In the present paper, based on our experimental findings of new dislocation loop structures, an alternative mechanism of loop formation by self-interstitial atom aggregation is proposed. Feasibility of the proposed process is discussed in the light of concurrent molecular dynamics (MD) simulation.

Section snippets

Experimental

For the present study, 0.3 mm thick discs were cut from a rod of high purity polycrystalline Fe (99.99 + %), supplied by Goodfellow. The discs were annealed at 1073 K for 1 h and finally mechanical polished from both sides down to 100 μm. Irradiation was performed by He-implantation at the cyclotron of CEMHTI/CNRS, Orleans. Details of the experimental set up are given in Ref. [18]. The 100 μm specimens were 3D-homogeneously implanted with 25 MeV α-particles passing through a magnetic scanning system

Results

Small dislocation loops were observed by TEM after irradiation. The loops were approximately circular with diameters from 2.5 to 10 nm and had Burgers vectors of ½〈1 1 1〉 and of 〈1 0 0〉. In agreement with previous experiments [1], 〈1 0 0〉 loops had habit planes of (1 0 0), while the habit planes of the ½〈1 1 1〉 loops were determined to {1 1 0}, {1 1 1}, {2 1 1} and to compositions of them, even some with planes between {1 1 0} and {1 1 1}. Loops consisting of ½〈1 1 1〉{2 1 1} and 〈1 0 0〉{1 0 0} components are firstly

MD simulation and discussion

Computer simulation technique was employed to assess the formation of defect clusters, starting from single defects which are beyond the resolution limit of TEM, and their eventual transformation to loops. Earlier computer simulations have already shown, that in contrast to small foreign interstitial atoms, SIA clusters do not occupy the most spacious interstitial positions in the lattice, but will be squeezed between two regular atoms on the closest-packed lattice direction (〈1 1 1〉 in bcc), a

Conclusion

The change of Burgers vector of small loops from ½〈1 1 1〉 to 〈1 0 0〉 in Fe-materials can possibly result from the re-arrangement and reorientation of crowdion clusters, without reactions with other dislocations.

Acknowledgements

The authors are grateful to the team at CEMHTI/CNRS, Orleans, France for operation of the cyclotron. The work was performed within the Swiss Generation IV Program.

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