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Formation of ultrafine gradient structure in hard magnetic alloy Fe-30%Cr-8%Co

Alexander Korznikov 1Zbigniew Pakiela 2Galia Korznikova Krzysztof J. Kurzydlowski 2

1. Institute for Metals Superplsticity Problems, Russian Academy of Sciences, Ufa, Russian Federation
2. Warsaw University of Technology, Faculty of Materials Science and Engineering (InMat), Wołoska 141, Warszawa 02-507, Poland


The alloys of Fe-Cr-Co system belong to the group of hard magnetic materials of dispersion-hardening class. Low cobalt alloys alongside with economy of expensive cobalt offer a number of advantages. The most important of those is an open single α phase area. This feature allows the method of strain ageing to be applied for obtaining high coercivity state. It permits anisotropic permanent magnets to be produced by means of conventional metallurgical techniques including: quenching for fixing metastable α solid solution, preliminary ageing for decomposition of the solid solution into spherical α1 precipitations and α2 matrix, intermediate uniaxial deformation for elongation and orientation of α1 precipitations, giving rise to magnetic anisotropy, final thermal treatment.
The need for uniaxial cold deformation by 60-70% on the intermediate stage of formation of high coercivity state imposes serious limitations on this method due to the low deformation ability of the alloy in the preliminarily aged state. However it is known that grain size refinement enhances plasticity of brittle alloys. One way to refine the structure in bulk cylindrical billets without change in shape is hot torsion. Deformation by hot torsion allows the attainment of large plastic straining without failure. Due to inhomogeneity of deformation distribution along the radius, more refined structure is formed in the periphery parts of the billet causing plasticization of outer layers.
Using Fe-30%Cr-8%Co alloy it is shown that in a preliminarily aged state the room temperature plasticity of the material enhances due to the formation of gradient submicrocrystalline structure in a cylindrical billet by means of rotation in the temperature interval corresponding to dynamic recrystallisation. Owing to gradient structure the uniaxial deformation at room temperature was carried out without failure of the billet by means of grooved rolling to achieve magnetic anisotropy.


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Presentation: poster at E-MRS Fall Meeting 2005, Symposium I, by Alexander Korznikov
See On-line Journal of E-MRS Fall Meeting 2005

Submitted: 2005-03-24 08:08
Revised:   2009-06-07 00:44