Phase transformations and properties of concentration-inhomogeneous magnetic materials based on the Fe–30%Cr–27%Co system

The phase and structural transformations of a powder magnetically hard alloy of the system Fe–30%Cr–27%Co–1%Si–0,07%B with a metastable α1+α2 phase composition, a high content of cobalt, and a high level of magnetic properties were studied. The density and coefficient of variation of the concentration of the main elements of the sintered blanks at the level of deformable analogs are achieved by sintering in the α-phase with contact melting in the presence of a «vanishing» liquid phase formed due to the addition of silicon and boron ferroalloys. A kinetic approach to the development of a competitive hard magnetic alloy with a high proportion of the strongly magnetic phase is proposed. The effect of boron additions on the incubation period of the formation of an undesirable σ-phase and the temperature range of the concentration stratification of the α-solid solution on the strongly magnetic α1-phase and the weakly magnetic α2-phase were established. Optical microscopy, X-ray phase analysis and differential scanning calorimetry were used to determine temperature and time parameters of heat treatment of the alloy, including hardening, thermomagnetic treatment (TMT) and final aging, providing the required combination of Hc and Br by increasing the stability of the metastable α-phase up to 20 minutes in the interval temperatures of spinodal decomposition α → α1 + α2. The greatest increase in magnetic properties after TMT, observed at the 1st and 2nd steps of final aging, is related to the decomposition of the α-solid solution and the formation of subgrain boundaries. Elements of the obtained structure are characterized by submicron and nanometric sizes, which correlate with the research results on deformable alloys based on the Fe – Cr – Co system. The anisotropic α1+α2-structural state achieved by thermomagnetic treatment provided an increase in the magnetic properties of the studied 30H27KSR powder alloy to 30 % and the squareness coefficient of the magnetic hysteresis loop equal to 0,82 .

powder magnets, liquid phase sintering, Fe – Cr – Co, spinodal decomposition, metastable phase, σ-phase, borides, anisotropic structure, thermomagnetic treatment

1. Kaneko H., Homma M., Nakamura K. New ductile permanent magnet of Fe—Cr—Co system. AIP Conf. Proc. «Magnetism and Magnetic Materials». 1971. No. 5. P. 1088—1092. http://doi.org/10.1063/1.2953814.

2. Homma M., Okada M., Minowa Т., Horikoshi E. Fe—Cr— Co permanent magnet alloys heat-treated in Ridge region of the miscibility Gap. IEEE Trans. Magn. Vol. Mag.17. No. 6. 1981. P. 3473—3478. http://doi.org/10.1109/ TMAG.1981.1061702.

3. Zhukova E.Kh., Malinina R.I., Zhukov D.G., Shubakov V.S., Menushenkov V.P. Heat treatment and magnetic properties of cold-deformed alloy 30KH15K2MT. Met. Sci. Heat Treat. 2014. Vol. 56. No. 1—2. P. 74—77. http:// doi.org/10.1007/s11041-014-9706-0.

4. Okada M., Thomas G., Homma M. Microstructure and magnetic properties of Fe—Cr—Co alloys. IEEE Trans. Magn. 1978. Vol. Mag.-14. No. 4. P. 245—252. http://doi. org/10.1109/TMAG.1978.1059752.

5. Miller M.K., Camus P.P., Hetherington M.G. Atomic level characterization of the morphology of phases in chromindur magnetic alloys. In: Magnetic materials: microstructure and properties: Mater. Res. Soc. Symp. Proc. 1991. Vol. 232. P. 59—64. http://doi.org/10.1557/PROC-232-59.

6. Zhen Liang, Sun Xue yin, Xu Cheng yan, Gao Run sheng, Xu Ren gen, Qin Lu chang. Magnetic anisotropy in Fe— 25Cr—12Co—1Si alloy induced by external magnetic field. Trans. Nonferr. Met. Soc. China (Engl. ed.). 2007. Vol. 17. No. 2. P. 346—350. http://doi.org/10.1016/S10036326(07)60096-X.

7. Antsiferov V.N., Peshcherenko S.N., Shatsov A.A. Diffusion homogenization of powder materials of the system Fe— Ni—Cr—M. Izvestiya vuzov. Chernaya metallurgiya. 1987. No. 9. P. 65—68 (In Russ.).

8. Fedorchenko I.M., Frantsevich I.N., Radomysel’skii I.D. Powder metallurgy. Materials, technology, properties, applications. Kiev: Nauk. dumka, 1985 (In Russ.).

9. Szymura S., Sojka L. The effect of silicon on the structure and properties of Fe—Cr—Co permanent magnet alloys. J. Magn. Magn. Mater. 1986. No. 53. P. 379—389. http:// doi.org/10.1016/0304-8853(86)90184-8.

10. Sidorova G.V., Korneev V.P., Milyaev I.M., Kovalenko L.V., Efimenko S.P. Investigation of structural changes in the Fe— Cr—Co alloy at the initial stage of the formation of a highly coercive state. Metally. 1997. No. 6. P. 90—92 (In Russ.).

11. Gol’dshtein M.I., Grachev S.V., Veksler Yu.G. Special steel. Moscow: MISIS, 1999 (In Russ.).

12. Milyaev I.M., Prutskov M.E., Laisheva N.V., Milyaev A.I., Yusupov V.S. Kinetics of the σ-phase formation in Fe— Cr—Co hard magnetic alloys. Metally. 2010. No. 11. P. 1053—1055. http://doi.org/10.1134/S0036029510110108.

13. Bannykh O.A., Budberg P.B., Alisova S.P. State diagrams of binary and multicomponent iron-based systems. Moscow: Metallurgiya, 1986 (In Russ.).

14. Kut’in A.B., Zabil’skii V.V. Structure, properties and destruction of structural steels. Ekaterinburg: UrO RAN, 2006 (In Russ.).

15. Savitskii A.P. Liquid phase sintering systems with interacting components. Novosibirsk: Nauka, 1991 (In Russ.).

16. Bobrova S.N., Vakutin A.P., Timokhova A.P., Shatsov A.A. Sintering of iron—silicon—boron magnetically soft steels. Powder Metall. Met. Ceram. 1998. No. 37. P. 487—494. http://doi.org/10.1007/BF02675809.

17. Kubaschewski O. Iron-binary phase diagrams. Berlin: Springer, 1982. http://doi.org/10.1007/978-3-662-08024-5.

18. Al’tman A.B., Gladyshev P.A., Rostanaev I.D. Technical magnetic properties of soft metal-ceramic alloys. In: Elektrotekhnicheskie metallokeramicheskie izdeliya. Moscow: Informelektro, 1965. P. 22—31 (In Russ.).

19. Gorelik S.S., Skakov Yu.A., Rastorguev L.N. X-ray and electron-optical analysis. Moscow: MISIS, 1994 (In Russ.).

20. Shatsov A.A. Powder materials of the Fe—Cr—Co system. Met. Sci. Heat Treat. 2004. No. 46. P. 152—155. http://doi. org/10.1023/B:MSAT.0000036668.48856.02.

21. Ushakova O.A., Dinislamova E.H., Gorshenkov M.V., Zhukov D.G. Structure and magnetic properties of Fe— Cr—Co nanocrystalline alloys for permanent magnets. J. Alloys Compd. 2014. Vol. 586. P. 291—293. http://doi. org/10.1016/j.jallcom.2012.12.076.

22. Sun X.Y., Xu C.Y., Zhen L., Lu L.X., Qin L.-C. Spinodal decomposition in Fe—25Cr—12Co—1Si alloy under a 100 kOe magnetic field. J. Magn. Magn. Mater. 2006. No. 306. P. 69—72. http://doi.org/10.1016/j.jmmm. 2006.02.107.