Atomistic modelling of irradiation-induced microstructure evolution in Fe alloys

QC 2024-05-23

Abstract

The nuclear reactors of the future require materials that are exceptionallyresistant to irradiation-induced degradation. This study presents a theoreticalframework, combining density functional theory and interatomic potentialmethods, to predict microstructural evolution in Fe-based alloys and oxides(Al2O3) subjected to damaging irradiation. Our research employs a powerfulcreation-relaxation algorithm to simulate defect formation and microstructuredevelopment under intense irradiation.We present the pioneering implementation of a first-principles quantummechanical approach for directly modelling the microstructural evolution ofmagnetic materials and ceramics under irradiation. A crucial aspect of studiesinvolves investigating the influence of the spatial distribution of Frenkel-pairs(FPs) on the microstructural evolution in Fe. Our findings reveal that spa-tially localised FP distributions, replicating low-energy transfer irradiationconditions, predict a significantly more moderate microstructure developmentcompared to uniform distributions. This highlights the importance of consid-ering the FP distribution for an accurate prediction of the formation andgrowth of the dislocation segments under low-energy irradiation conditions.Furthermore, first-principles calculations suggest that irradiation-inducedexcess energy can trigger polymorphism in bcc Fe, leading to magnetic insta-bilities, localised structural constriction, and ultimately local phase transfor-mations. Consequently, under extreme conditions, α-Fe undergoes local trans-formations into three-dimensional, non-parallel C15 Laves phase structureswith highly close-packed stacking and internal short-range ferromagnetism.Notably, the inclusion of antiferromagnetic chromium in bcc Fe significantlyenhances the stability of C15 interstitial clusters in concentrated FeCr alloys.Beyond these structural insights, the investigation delves into the intricateinterplay between atomic constituents and their profound impact on the non-linear magnetic properties of FeCr systems under irradiation. A striking cor-relation emerges, revealing that the chromium content directly influences theappearance of swelling, a key phenomenon following irradiation-induced dam-age. Increasing the chromium content mitigates irradiation-induced swellingby approximately 40%, compared to pure Fe, highlighting the profound effectof alloying in Fe-based alloys.In addition, our first-principles simulations of irradiation-induced damagein bcc FeCrAl and hcp Al2O3 predict that while there are relatively small dif-ferences in total defect number densities among bcc Fe and its alloys, there aresignificant discrepancies in defect concentrations between these bcc structuresand hexagonal Al2O3. Notably, the surviving FP content in alumina is seventimes higher than that recorded for FeCrAl alloys. Consequently, the differ-ent build-up of surviving damage in Fe alloys and alumina leads to diverselevels of swelling in the irradiated materials, with a remarkable three timeshigher swelling observed in alumina upon reaching a saturation state after anirradiation dose of approximately 1 displacement per atom (dpa). Further-more, our observations of amorphous phase formation in damaged corundumalumina, as predicted in this study, corroborate that there are significantirradiation-induced effects in alumina.These findings not only deepen our fundamental understanding of theresponses of structural materials to irradiation, but also pave the way foradvanced materials engineering with potential applications in near-future nu-clear reactor components.

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