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The Superplasticity of Hard Magnetic Alloy Fe-23wt.%Co-30%Cr with Submicrocrystalline Structure

Alexander Korznikov 1Zbigniew Pakieła 2Galia F. Korznikova 1Krzysztof 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


Fe-23wt.%Co-30%Cr alloy is a precipitation hardening alloy used industrially as a hard magnetic material. Magnets are usually produced from this alloy by casting, as mechanical deformation and machining are hampered by the presence of brittle intermetallic σ phase in the temperature range 700-1050oC. However it is known, that variation of the size and morphology of intermetallic phases in commercial alloys enhances plasticity. Previously it was reported, that severe plastic deformation at room temperature of Fe-23wt.%Co-30%Cr alloy by high-pressure torsion results in a partial transformation of the initial phases and the formation of a nanocrystalline structure. The nanocrystalline samples exhibited increased strength and plasticity in comparison with coarse grained samples. The drawback of the samples processed by means of high-pressure torsion is their small thickness (less than 1 mm) whereas deformation at elevated temperatures in superplastic conditions enables to produce massive samples.
The purpose of this work was to study the evolution of the structure and mechanical properties of the Fe-23wt.%Co-30%Cr alloy during superplastic deformation at elevated temperatures. Maximum value of elongation (about 700%) was found at strain rate about 10-3s-1 in the temperature range 900-950oC corresponding to γ+σ phase domain. Tensile tests at different heating rates before deformation revealed significant increase of elongation at high value of heating rate. This increase can result from a less thickness of quasi-eutectoid lamellae formed at high heating rate. Subsequent deformation leads to transformation of lamellar quasi-eutectoid to a microduplex structure with a mean size of grains of both phases comparable to the lamellae thickness. Thus, the higher was the rate of preliminary heating, the smaller the mean size of γ and σ grains (less than 1 μm), what in turn increased plasticity of material.


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Presentation: poster at E-MRS Fall Meeting 2003, Symposium G, by Alexander Korznikov
See On-line Journal of E-MRS Fall Meeting 2003

Submitted: 2003-05-07 16:08
Revised:   2009-06-08 12:55