Gradient Microstructure of FeCr30Co8 Hard Magnetic Alloy Subjected to Plastic Deformation by Complex Loading

Abstract

Magnetically hard Fe-Cr-Co-based alloys are distinguished by their good ductility, excellent magnetic properties and low cost. Their superior magnetic properties are obtained by magnetic treatment and multistage tempering, which results in spinodal decomposition of the solid solution into the isomorphous α1 and α2 phases. However, the α1+α2 microstructure causes a reduction in the plasticity and strength of the material. It can often be advantageous for permanent magnets to maintain fine magnetic properties throughout their volume along while retaining good mechanical properties only in the subsurface layer. To improve the mechanical properties of the latter, FeCr30Co8 samples were deformed in tension combined with torsion. Loading was applied at 750°C, which ensured that the conditions for superplastic deformation were fulfilled. Here, we present the results of microstructure investigations of the samples treated in the aforementioned manner. Observations of the longitudinal section of the samples showed the formation of a gradient microstructure with the maximum grain refinement in the surface layer and the characteristic rotation of the elongated α phase grains from positions nearly perpendicular to the tension axis at the surface to positions tilted at approximately 45º to the tension axis inside the material. Deformation at superplastic conditions also activated precipitation of the σ intermetallic phase, particularly in the areas of highest deformation. The refinement of the microstructure and precipitation of the σ-phase resulted in a significant increase in hardness at the surface of the FeCr30Co8 samples.