Microstructure and Phase Evolution Mechanism in Hot-pressed and Hot-deformed Nd-Fe-B Magnets with Nd85Cu15 Addition

Abstract

Chemical composition and structure characteristics of the intergranular phases in Nd-Fe-B magnets play a key role in regulating the magnetic properties. However, it has still been a challenge to understand the evolution mechanism of grain boundary phases in hot-pressed and hot-deformed Nd-Fe-B magnets due to the complex phase composition and structure. In this work, grain boundary modification using Nd85Cu15 alloy has been applied to hot-pressed and hot-deformed Nd-Fe-B magnets. The structure evolution mechanism of the grain boundary phase during hot-pressing was studied by TEM analysis, which shows that an overall reaction, Nd + Nd26Ga7Co10 + Nd2Fe14B → Nd3Co + Nd5Fe2B6 + Nd6(Fe,Co)13Ga, occurs at 600°C in hot-pressed magnet with Nd85Cu15 addition. When the hot-deformation was conducted at 675°C abnormal grain growth was observed in hot-deformed magnets with or without Nd85Cu15 addition. In contrast, as the hot deformation temperature decreased to 630 °C, the uniform and ultra-fine Nd2Fe14B grains formed in the Nd85Cu15 doped magnet, which shows a significantly improved coercivity of 2.03 T and a squareness factor of 0.93.