Ab-initio modeling of Fe-Mn based alloys and nanoclusters
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Ab-initio modeling of Fe-Mn based alloys and nanoclusters Peter Entel, Denis Comtesse, Heike C. Herper, Markus E. Gruner, Mario Siewert, Sanjubala Sahoo and Alfred Hucht Faculty of Physics and CeNIDE, University of Duisburg-Essen, 47048 Duisburg, Germany ABSTRACT New methods in steel design and basic understanding of the novel materials require large scale ab initio calculations of ground state and finite temperature properties of transition metal alloys. In this contribution we present ab initio modeling of the structural and magnetic properties of XYZ compounds and alloys where X, Y = Mn, Fe, Co Ni and Z = C, Si with emphasis on the Fe-Mn steels. The optimization of structural and magnetic properties is performed by using different simulation tools. In particular, the finite-temperature magnetic properties are simulated using a Heisenberg model with magnetic exchange interactions from first-principles calculations. Part of the calculations are extended to the nanoparticle range showing how ferromagnetic and antiferromagnetic trends influence the nucleation, morphologies and growth of Fe-Mn-based nanoparticles. INTRODUCTION The γ-phases of Fe and Mn exist only in a limited region at high temperatures. However, when mixed together to form Fe-Mn alloys, the γ-phase regions of Fe and Mn are opened up so that the fcc structure is stable over a broad range of compositions (0.3 < x < 0.6) down to low temperatures. This is of technological importance regarding the austenitic Fe-Mn steels which contain additional elements like C and Si in small amounts, for example, see [1]. The alloys Fe100-xMnx undergo fcc-bcc (γ-α) and fcc-hcp (γ-ε) transformations at about 0 < x < 10 at.% and 15 < x < 30 at.%, respectively (for discussion of structure and magnetism and further references, see [2, 3]) . The martensite product phase in the region 10 < x < 15 at.% is mixed α+ε and ε+γ. The γ-α transformation in Fe-Mn is very similar to that in Fe-rich Fe-Ni alloys where the transformation is from the high-temperature close-packed fcc structure to the less-close packed bcc structure which shows the stabilizing factor of ferromagnetic correlations in α-Fe-Ni at low temperatures. This also demonstrates that the martensitic transformations in Fe alloys are closely related to their magnetic properties. In order to understand this relationship, we plot in Fig.1 the martensitic (austenitic) transformation temperatures together with the magnetic transformation temperatures of Fe-Mn and Fe-Ni alloys. The martensitic transformations occur for both systems on the Fe-rich side. For the Fe-rich Fe-Mn alloys the α-phase is largely quenched at low temperatures due to the antiferromagnetic correlations in the γ- and ε-phases while Fe-rich Fe-Ni alloys prefer a bcc ground state (the fcc structure is metastable). However, regarding the γ-phase and γ-α transformation in Fe-Mn, ferromagnetic correlations in the γ-phase are relevant. This is also because in the Fe-rich alloys with x < 10 at.% Mn the anti-Invar behavior at high temperatures requires the presen
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