Effect of magnetism on precipitation of Cu in bcc Fe: Ab-initio based modeling
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Effect of magnetism on precipitation of Cu in bcc Fe: Ab-initio based modeling O.I. Gorbatov1, A.V. Ruban2, P.A. Korzhavyi2, Yu.N. Gornostyrev1,3 1 Institute of Quantum Materials Science, Ekaterinburg 620107, Russia 2 Royal Institute of Technology (KTH), SE-100 44 Stockholm, Sweden 3 Institute of Metal Physics, Ural Division RAS, Ekaterinburg 620041, Russia ABSTRACT Theoretical modeling of the decomposition in bcc Fe-Cu alloys has been performed using a combined approach which includes ab-initio calculations of the effective cluster interactions and statistical-mechanical (Monte Carlo) simulations. We showed that the effective Cu-Cu and Cuvacancy interactions in the bcc Fe matrix have a strong dependence on the global magnetic state of iron. As a result, all the related thermodynamic properties of the alloys (such as solubility limit and diffusivity) are expected to have a pronounced non-Arrhenius temperature behavior, originated from variation of the global magnetization with temperature. We find that strong Cuvacancy interactions in the bcc Fe matrix lead to a remarkable effect of vacancies on the Cu precipitation and significantly modify the alloy decomposition kinetics under irradiation.
INTRODUCTION Alloying is a major approach to improving the mechanical properties of low-carbon steels. However, the resulting fracture toughness does not always meet the strict requirements imposed on materials for modern industrial applications. The pronounced strengthening effect of copper precipitates in steels [1,2,3] results in a good combination of strength, toughness, and weldability making Cu-alloyed steels suitable for applications as construction materials for natural gas pipelines, shipbuilding, etc. Precipitation of copper in iron and steels has been studied extensively in the past [4,5,6,7], however the mechanism of this phenomenon, as well as the factors controlling the formation of nanometer-sized copper inclusions, are still under debate (see, e.g., Refs. [8,9]). Copper precipitation is strongly accelerated in Fe-based alloys under irradiation, due to high concentration of non-equilibrium vacancies. It has been established that the decomposition of supersaturated Fe-Cu-based solutions (such as Fe-Cu, Fe-Cu-Mn, Fe-Cu-Ni) occurs in several stages [4,6,10]. First, Cu-rich precipitates (having the bcc structure and containing 50-70% of copper) appear [7,11,12]. Nanometer-sized Cu-rich particles produce dispersion strengthening of the steel while preserving high ductility and fracture toughness. Next, upon growing in size during annealing, the precipitates transform into the fcc structure of Cu through an intermediate close-packed phase with the 9R crystal structure, after which the fracture toughness of the alloy rapidly decreases. The pronounced effect of bcc-Cu precipitates on the mechanical properties of α-Fe is stimulating researchers to study the structural, thermodynamic, and elastic properties of the alloy. Experimental studies of Fe-Cu solid solutions are difficult because of the low solubility of Cu in α-Fe
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