Microstructure Effects on 49Fe49Co-2V: Examining the Tradeoff between Magnetic and Mechanical Performance

Several lots of 49Fe49Co-2V Hiperco® 50A bar (Hiperco is a tradename of Carpenter Technologies Inc., Reading PA) were investigated in a heat treatment study followed by magnetic BH testing and tensile test evaluations. Optical microscopy, electron probe microanalysis (EPMA), SEM/EDS, electron backscatter diffraction (EBSD), and X-ray diffraction (XRD) were used to characterize microstructure before and after heat treatments. Microstructural analysis indicates that decreasing the amount of a bcc phase with higher vanadium content, known as alpha-2, correlates to the improvement in magnetic properties. The reduction in alpha-2 occurs simultaneously with other annealing effects such as grain growth and decreasing dislocation density. Importantly, longer heat treatment times also reduce the batch-to-batch variability in magnetic performance. Mechanical properties are also affected by the heat treat process with ductility improving relative to the as-received (as-machined) condition. Internal residual strain from bar processing and, most importantly, surface machining damage, cause low elongation in the as-received material. During heat treatment, the internal residual strain and near-surface machining damage are eliminated and ductility is improved, despite a higher degree of crystallographic ordering in the heat-treated condition (which typically lowers ductility). The yield strength appears to be only slightly reduced by the longer heat treatment times in the temperature range studied. Understanding the role of microstructure enables pathways to improve the performance of solenoids, transformers, electric motors employing this soft magnetic alloy.

SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525