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Spinodal decomposition in the Fe-Cr-Co ternary system

Name
Boburjon
Surname
Mukhamedov
Scientific organization
Materials Modeling and Development Laborotory (NUST "MISIS")
Academic degree
PhD student
Position
Engineer
Scientific discipline
Physics & Astronomy
Topic
Spinodal decomposition in the Fe-Cr-Co ternary system
Abstract
Using the exact muffin-tin orbitals (EMTO) method based on the density functional theory (DFT) we have studied the tendency toward the spinodal decomposition of bcc Fe-Cr-Co solid solution. The ferromagnetic and the paramagnetic states were considered. Calculations of the lattice parameters, mixing enthalpy and Curie temperature were performed for both magnetic states. The calculations predicted that in the ferromagnetic state an increase Co and Cr content increases the tendency of Fe-Cr-Co alloys to spinodal decomposition, while in the paramagnetic state the alloys are stable
Keywords
Density functional theory, first-principles calculations, Fe-Cr-Co system, phase stability, spinodal decompositon, mixing energy.
Summary

Fe-Cr-Co hard magnetic materials have a unique combination of high magnetic properties with a corrosion stability, ductility and toughness. High mechanic properties make it possible to subject these alloys to cool and hot mechanic treatment and produce magnets with a different shape and size.

Hard magnetic properties in Fe-Cr-Co alloys can be reached by the formation of modulated structure during the spinodal decomposition into two isostructural phases: a (Fe-Co)-rich ferromagnetic phase (a1) and a Cr-rich paramagnetic phase (a2). A combination of thermomagnetic treatment and step-aging allows to reach a high-coercive state in these alloys [1]. Duration of these treatments strongly depends on alloys chemical compositions. For instance, in binary Fe-Cr alloys the high-coercive state  have not been achieved as a result of slow atomic diffusion due to low decomposition temperature (710…790K) [2]. Thermal treatment methods are very sensitive to Co and Cr concentration, and depending on the concentration of alloys thermal treatment might last from 10 [1] to 100 [3] hours. In this respect, a knowledge of fundamental thermodynamic properties of Fe-Cr-Co system is highly desirable for the design of new magnetic alloys.

Unfortunately, there is quite limited experimental information about the thermodynamic properties of Fe-Cr-Co alloys. At the same time, the first-principles simulations allow one to obtain reliable description of the mixing thermodynamics of alloys with substitutional disorder, including magnetic alloys.

We have performed first-principles electronic structure and total energy calculations in the framework of the density functional theory (DFT) using the exact muffin-tin orbitals (EMTO) method combined with the coherent potential approximation (CPA) [4]. The disordered local moment (DLM) approximation was used for the description of the paramagnetic alloys [5]. In order to consider the effect of increasing temperature we have used the mean-field approximation in the configurational entropy calculations.

According to the calculations of Curie temperature the addition of Co (Cr) in Fe-Cr system the tendency to increase (decrease) in Curie temperature is observed, which does not contradict to the experimental data.

Starting by the results of the mixing energy calculations, in the ferromagnetic state FexCryCoz solid solution decomposes spinodaly into two phases: (Fe-Co)-rich α1-phase and Cr-rich α2-phase, which is in a good agreement with experiment [1]. The local tendency to decomposition increases with the increasing of Co and Cr concentration.

So we can conclude that the theoretical modeling makes it possible to predict the most favorable decomposition directions and the chemical compound of the decomposition products.

 

Reference

[1] H. Kaneko, M. Homma, Y. Nakamura, “New Ductile Permanent Magnet of Fe-Cr-Co”, AIP Conf. Proc., No. 5, 1088 – 1092 (1971).

[2] E.Z.Vintaikin, V.Yu. Kolontsov, E.A. Medvedev, “Low temperature region of Fe-Cr phase diagram” Izv. AN SSSR. Metal., No. 4,  169 – 172 (1969)

[3] M.L. Green, R.C. Sherwood, G. Chin, “Low cobalt CrCoFe and CrCoFex permanent magnet alloys”, IEEE Trans. On Magn., V. 16, No. 5, 1053 – 1055 (1980)

[4] L. Vitos, Computational Quantum Mechanics for Materials Engineers:The EMTO Method and Applications (Springer-Verlag, London, 2007).

[5] B. L. Györffy, A. J. Pindor, J. B. Staunton, G. M. Stocks, and H. Winter, J. Phys. F: Met. Phys.915, 1337 (1985).