Abstract

Despite extensive investigations conducted on magnetic Fe-Co-V alloys, some aspects of microstructural evolution during annealing of cold rolled sheets have not yet been clarified yet. Various techniques such as dilatometry, X-ray diffraction (XRD), transmission and scanning electron microscopy (TEM, SEM) as well as ThermoCalc predictions were employed to study ordering, precipitation and austenite transformation in an 86% cold-rolled 40Fe-50Co-10 V alloy. Dilatometric results revealed atomic ordering initiated from disordered martensite at 350 °C, while XRD patterns ascertained the completion of the process at 600 °C and its disappearance close to 750 °C. X-ray and electron diffraction patterns exhibited the B2 type ordered structure in this temperature range. Rod-shaped precipitates with HCP crystalline structure and the constant composition of (Co,Fe)₃ V were detected by SEM within the deformed ferrite grains after annealing in the temperature range of 500–700 °C. The percentage of the precipitates decreased by raising the annealing temperature and they vanished above 700 °C, in accordance with ThermoCalc prediction. Austenite grains formed below 750 °C had an ordered L1₂-type structure as revealed by TEM and XRD patterns. A combination of different techniques demonstrated that the two-phase ferrite-austenite region exists in the temperature range of 495–840 °C. Vanadium partitioning into austenite during isothermal holding in addition to austenite grain size were evaluated as the main parameters affecting austenite stability during quenching from temperatures between 500 and 800 °C. As a final result, most of the existing disagreements and ambiguities in previous works could be explained.