Processing of Soft Magnetic Fine Powders Directly From As-Spun Partial Crystalline Fe77Ni5.5Co5.5Zr7B4Cu Ribbon via Ball Mill Without Devitrification

Abstract

A key property for high-frequency inductive applications is the magnetic softness, ideally characterized by complete reversibility of the hysteresis curve. Magnetic softness of alloys depends on the dimensional ratio of grain size to the correlation length. The random anisotropy model predicts the optimum magnetic softness if the crystallite size is well below the correlation length. Based on this theory, a common strategy in processing is to obtain extremely fine nanoscale grains via rapidly solidified alloys, followed by a controlled vacuum anneal. As is well known, this conventional approach requires two demanding experimental conditions: 1) an extremely high quench rate, >105 K/s, and 2) a high vacuum anneal near 600 °C to avoid oxidation. Furthermore, the completely amorphous ribbon is extremely elastic and mechanically strong and is not easily made into the fine powder form for net-shaping and 3-D printing of the soft magnets. Therefore, annealing to induce crystallization is a necessary step before using any conventional powdering techniques, such as ball milling. In this article, we report a new processing strategy of making Fe77Ni5.5Co5.5Zr7B4Cu powders directly from the as-spun ribbons without any crystallization annealing step. This is achieved by melt spinning of the alloy at a relatively low quenching rate, resulting in partially crystallized ribbons that deform plastically, a behavior that is in sharp contrast to its amorphous counterpart. The ribbon samples are ball milled into fine powders by tungsten carbide (WC) at various time intervals. Magnetization measurements show extremely low coercive field (Hc) of 0.51 Oe for the powder that is ball milled for 60 min. However, Hc consistently increases with ball milling time, as the samples are strain-hardened, introducing more magneto anisotropy. The coercive fields of these samples are easily recovered during the high-temperature magnetization measurement at 900 K, and an Hc of 3.78 Oe is obtained upon recovery. This strategy paves a new way for straightforwardly making soft magnetic powders while avoiding high quenching rate requirement and the possibility of oxidation during annealing.