Effect of helium on irradiation-induced hardening of iron: A simulation point of view

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Abstract

Irradiation-induced hardening and loss of ductility of ferritic/martensitic materials envisaged for future fusion reactor is still difficult to understand. In particular, helium (He), produced by transmutation by the fusion neutrons of 14 MeV, is known to impact mechanical properties, but its effect at the microstructural level is still unclear. Molecular dynamics simulations of the mobility of an edge dislocation in iron (Fe) are performed to study the effect of He, either as a gas in solid solution or in cavities. Obstacle to the dislocation, the cavity, in the form of a void or a He bubble, is compared to a a0 [1 0 0] dislocation loop, all being 2 nm in size. Results show that He in solid solution up to 1.0 at.% has a little effect on the dislocation mobility. Conversely, the cavities and the a0 [1 0 0] dislocation loop are strong obstacles to the passage of the edge dislocation. Interestingly He bubbles present a lower obstacle strength than voids for low He contents, while for high He content the bubble promotes loop punching, which induces a strong resistance to the passage of the dislocation.

Introduction

Irradiation-induced defects and transmutation elements produced in the future fusion reactor first wall materials by the impinging 14 MeV fusion neutrons will substantially impact their plasticity. He in particular is known to have detrimental effects on the macroscopic mechanical behaviour of ferritic/martensitic steels [1]. Despite numerous experimental works over several decades, the basic mechanisms underlying these effects are still unclear. The investigation of those by numerical simulations appears nowadays instrumental in expanding our knowledge on this matter. At the basis of this modelization is a hierarchical multiscale approach that encompasses several numerical simulation methods including molecular dynamics (MD) simulations, spanning over many space and time scales. Such an approach substantially increases our ability to predict the long-term behaviour and performance of materials under irradiation.

MD simulations in Fe-based samples of the interaction of a given defect and a dislocation, the vector of plasticity, are performed to quantify numerically those effects. Nowadays, MD simulations based on the embedded atom method approach [2] allows the investigation at the atomic scale with relatively extended sample sizes. For example, it allows the necessary sample sizes needed to simulate the movement and interaction of a dislocation with structural defects. Previous works, for instance Osetsky and Bacon [3] and Wirth et al. [4] have demonstrated the interaction of a moving dislocation with a void in Fe and with a stacking fault tetrahedron in Cu, respectively, using MD simulations.

In this work, the effect of He on the plasticity of Fe is explored. Firstly, the effect of He in solid solution on the mobility of an edge dislocation in Fe is studied. Secondly, the effect He as a bubble is investigated, and the resulting obstacle strength is deduced. For comparison purposes the effect of a void and a dislocation loop with a Burgers vector a0 [1 0 0], commonly observed in irradiated ferritic/martensitic steels, is investigated. Finally, the effect of the content of He in the cavity on the mobility of the edge dislocation in Fe is analyzed.

Section snippets

Simulation methods

The dislocation is described using anisotropic elasticity of the continuum. It provides a good description of the dislocation down to the core region, as was proven by a comparison with high resolution TEM [5], [6]. The formalism of Stroh [7] was selected, as it provides a fairly convenient mathematical description, used successfully in, for example, the weak beam TEM image simulation of dislocations [8]. In this work we derived the code DISLOC. It allows implementing any type of dislocation

Results

Fig. 2 shows the resulting flow stress as a function of strain rate at 10 K for an edge dislocation in pure Fe without defects. As expected the flow stress decreases with decreasing strain rate. The stress at the point of yielding, at minute strain levels, indicates the critical stress necessary to move the dislocation. At and below a strain rate value 3.0 × 10−8 fs−1 the yield stress saturates, to a value of about 20 MPa. It corresponds to a macroscopic strain rate of 3.0 × 107 s−1 and, for the given

Summary

Molecular dynamics simulation of the effect of He on the mobility of an edge dislocation in Fe was performed at 10 K. In our simulations the Peierls stress for an edge dislocation in pure Fe is about 20 MPa. It appears that He in solid solution has no significant effect on the yield stress. There is some effect on the flow stress at 1.0 at.% He. The 2 nm void presents an obstacle strength of 590 MPa to the edge dislocation. A 2 nm sessile a0 [1 0 0] loop appears to be also a strong obstacle, at 440 MPa,

Acknowledgements

J. Marian, M.J. Caturla, D. Rodney, B. Singh, J. Evans are thanked for fruitful discussions. EFDA is thanked for financial support (task TW3_TTMS_007_D10). The Paul Scherrer Institute is acknowledged for the overall use of the facilities.

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    Present address: Department of Chemical and Materials Engineering, The University of Auckland, Auckland City, New Zealand.

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