Influence of cooling rate on the magnetic properties of Hf–Co–Fe–B melt-spun alloy

In the present work, Hf2Co9.5Fe1.5B melt-spun (MS) alloy is synthesized by employing melt spinning at different wheel speeds viz. 16, 20, 24 and 28 m/s to study the effect of quenching on the thermal, structural, microstructural and magnetic properties. The phase purity and the magnetic behaviour of the MS ribbons are highly dependent on the cooling rate that is controlled by altering the tangential wheel speed during melt spinning. Cooling rates are found to increase with increase in wheel speed with a concurrent decrease in the ribbon thickness owing to the increase in the heat transfer coefficient at the thermal contact. The best phase purity and the magnetic properties are found for the ribbons melt-spun at 28 m/s. This could be attributed to the high cooling rate 2.3 × 107 K/s causing crystallization of hard magnetic Hf2Co11B phase leading to refined grain size. A maximum coercivity (HC) ~ 2.18 kOe, remanence ratio (Mr/Ms) ~ 0.61, an appreciable magnetic energy product (BH)max ~ 3 MGOe observed in the MS ribbons at 28 m/s illustrates the critical role of wheel speed in the enhancement of permanent magnetic properties in a single-step without annealing. XRD patterns reveal that the alloy was found to crystallize in orthorhombic Hf2Co11B in addition to cubic Co and Hf6Co23 phases. FE-SEM analysis is carried out to realize the grain morphology and phase identification. The current work exhibits the efficacy of rapid quenching by melt spinning as an effective technique in the development of high-performance Hf2Co9.5Fe1.5B rare-earth-free permanent magnet alloy for future energy applications in the high-temperature regime.