High coercivity microcrystalline Nd-rich Nd–Fe–Co–Al–B bulk magnets prepared by direct copper mold casting

•2 mm hard magnetic Nd25Fe40Co20Al15−xBx rods were prepared by direct casting.•High coercivity of 1.78 T was achieved in x=11 sample after heat treatment.•Small grains are responsible for the significant increase in HC after annealing.•Nd2Fe14B grains with two different sizes lead to two-step demagnetization process.

High coercivity Nd25Fe40Co20Al15−xBx (x=7–15) hard magnets were prepared by a simple process of injection casting. Different from many previous investigations on nanocomposite compositions, the magnets in this work contain hard magnetic Nd2(FeCoAl)14B, Nd-rich, and Nd1+ε(FeCo)4B4 phases. The magnetic properties, phase evolution, and microstructure of the as-cast and annealed magnets were investigated. As the boron content increased from 7 to 11 at%, the intrinsic coercivity Hcj of the as-cast magnet increased from 816 to 1140 kA/m. The magnets annealed at 750 °C have shown more regular and smaller grains than the as-cast alloys, especially for the x=11 alloy. The high intrinsic coercivities for the annealed alloys with x=8~11 result from the presence of small-sized grains in the microstructure. The highest Hcj of 1427 kA/m was obtained for the heat treated alloy with x=10. This work provides an alternative approach for preparing fully dense Nd-rich bulk hard magnets with relatively good properties.