Nominal boron content modulates phase evolution and core-shell formation in Nd-Y-Fe-B alloys

In Nd-Y-Fe-B alloys, Y substitution promotes the formation of core-shell grains, but the mechanism governing shell-thickness regulation remains insufficiently understood. In this work, Nd-Y-Fe-B amorphous precursors with different nominal B contents are used in a staged two-step annealing treatment to clarify the relationship between B-dependent phase evolution and subsequent Nd-rich shell formation. The results show that first-step annealing at 800 °C promotes the precipitation of B-containing metastable constituents. Their subsequent decomposition is accompanied by distinct rare-earth (RE) redistribution and changes in the chemical composition of grain-boundary (GB) phases. After the 800-1080 °C treatment, both B-lean and B-rich alloys develop clear Nd-rich shells, although the shell thickness and RE distribution depend on the nominal B content. In contrast, after first-step annealing at 1000 °C, the B-lean alloy undergoes prior decomposition of B-containing metastable constituents and extensive grain coarsening. These changes result in a GB phase composition that is unfavorable for continued shell growth during the second-step annealing, producing only a nanoscale shell. The B-rich alloy, by comparison, retains a more favorable phase-evolution pathway and GB phase composition conducive to continued shell growth, leading to a submicrometer-thick shell and a broad compositionally graded transition layer. The formation of Nd-rich shells generally contributes to coercivity enhancement, whereas excessive shell growth accompanied by a broadened transition layer can reduce the anisotropy gradient and weaken domain-wall pinning. These results demonstrate that nominal B-content design provides an effective strategy for controlling metastable phase evolution, shell morphology, and magnetic properties in Nd-Y-Fe-B alloys.