SIMULATING IRRADIATION-INDUCED DISLOCATION LOOPS IN ZIRCONIUM ALLOYS: UNDERSTANDING THEIR STRUCTURE, FORMATION AND EFFECT ON X-RAY DIFFRACTION LINE PROFILES.

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Abstract

Zr components in nuclear power reactors have their service life reduced by irradiation damage caused by energetic neutron bombardment. This bombardment produces point defects, which evolve into dislocation loops that contribute to a non-uniform growth phenomenon called irradiation-induced growth (IIG). At low irradiation doses loops inhabiting the prismatic planes with in basal plane Burgers vectors, known as <a> loops, populate the crystal. At higher doses, loops with Burgers vectors that have a c-component appear and these correlate with a rapid increase in IIG. This study is part of a wider effort to improve understanding of irradiation-induced dislocation loops so that solutions can be found to negate their influence on IIG. Accurate quantification of dislocation loops in irradiated Zr is of vital importance to evaluating IIG resistant candidate alloys. One way of doing this is via line profile analysis (LPA) of X-ray diffraction (XRD) profiles. However, attempts to do this have been hampered by the presence of asymmetric features in the low intensity ranges of the peaks, which complicated fitting of the profiles. We refer to these asymmetric features as 'humps'. By simulating dislocation loops we were able to gain insights into them that would have been difficult or impossible to acquire experimentally. To this end, we studied the structure and energetics of irradiation induced dislocation loops in Zr using atomistic simulations. The second half of this project was dedicated to determining the origin of humps and to correlating their characteristic features to defect populations. We achieved this by using our simulated defect populations to produce theoretical XRD profiles relating to the populations and then analysing the resulting peaks. Additionally, we developed a novel technique for extracting the humps from the XRD profiles. Our study of loop structure and energetics enabled us to make several determinations: at radii of less than 3.2 nm, c-component loops will have lowest energy when they are pure edge loops and above this radii they reduce their energy by transforming into a loop containing an intrinsic stacking fault; <a> loops form on the 1st prismatic loop as edge loops, shear to create 1st order prismatic sheared loops and then rotate to inhabit the 2nd order prismatic plane as edge loops; considering energy alone, ellipticity is the same for interstitial and vacancy <a> loops; Interstitial <c/2 + p> loops are energetically feasible; below a threshold loop diameter, high strain lobes emanating from the bounding dislocation line strongly overlap, preventing the loop's strain field from extending far into the crystal. With regards to our study of humps we made several conclusions: planar defects such as dislocation loops generate humps because they change the inter-planar spacing in their locality; humps appear on different sides of the relevant Bragg reflection depending on their character, with vacancy loops causing humps on the low 2theta side and interstitial loops causing humps on the high 2theta side; there is a minimum loop diameter beneath which no hump is generated; the integrated intensity beneath a hump correlates to the strained volume fraction and this could be used to develop a technique to correlate hump size to defect density.

Keyword(s)

Atomistic Modelling; Dislocation Loops; Irradiation Damage; Line Profile Analysis; Microstructural Evolution; X-ray Diffraction; Zr

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