Phase Transformations and Properties of Concentration-Inhomogeneous Magnetic Materials Based on the Fe–30% Cr–27% Co System

Abstract

The phase and structural transformations of a powder hard magnetic alloy of the Fe–30% Cr–27% Co–1% Si–0.07% B system with a metastable α1 + α2 phase composition, elevated cobalt content, and high level of magnetic properties are studied. The density and variation coefficient of the concentration of main elements of sintered billets at a level of deformable analogs are attained 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 fraction of a strong magnetic phase is proposed. The influence of boron additives on the incubation period of the formation of the undesirable σ phase and the temperature range of the concentration stratification of the α solid solution into the strong magnetic α1 phase and weak magnetic α2 phase are established. Optical microscopy, X-ray phase analysis, and differential scanning calorimetry are used to determine the temperature–temporal parameters of the thermal treatment of the alloy, including quenching, thermomagnetic treatment (TMT), and final aging, which provide the required combination of Hc and Br due to an increase in stability of a metastable α phase up to 20 min in a temperature range of spinodal decomposition α → α1 + α2. The largest increase in magnetic properties after TMT observed at the first and second stages of final aging is associated with the supplementary decomposition of the α solid solution and the formation of subgrain boundaries. Elements of the formed structure have submicron and nanometer sizes that correlate with the results of studying the deformable alloys based on the Fe–Cr–Co system. The anisotropic α1 + α2 structural state attained by thermomagnetic treatment provides an increase in characteristics of magnetic properties of the studied 30Kh27KSRA powder alloy to 30% and squareness ratio of the magnetic hysteresis loop of 0.82.