Effect of pressless heat treatment on the magnetic performance and microstructure of hot-deformed Nd-Fe-B magnet

Pressless heat treatment (PHT) at different temperatures was carried out on the samples cut off from the hot-deformed (HD) Nd-Fe-B magnets. It showed remarkably influence on both the remanence and coercivity of the HD samples. When the PHT temperature was higher than 850 °C, the HD samples exhibited a drastic remanence increase from 13.58 kGs to 14.45–14.69 kGs and dramatic coercivity decrease. This declined coercivity could be raised by 3 kOe when the HD sample experienced a second stage of PHT at 700 °C for 2 h. The texture of the HD samples with enhanced remanence was evaluated by X-ray diffractometry (XRD), suggesting the texture improvement after the PHT at 900 °C and 1080 °C. The microstructures of these HD samples were examined by scanning electron microscopy (SEM), which revealed the inherited crystalline orientation during the growth of nanosized Nd₂Fe_12.8Co_1.2B platelets into microsized grains after PHT. Backscatting electron (BSE) SEM and energy dispersive spectroscopy (EDS) revealed the aggregation of Nd-rich phase both at ribbon boundary and grain boundary at the PHT temperature of 900 °C, which plays an important role in the dramatic decrease of coercivity. The BSE-SEM further revealed that the increase of coercivity during the secondary stage of PHT at 700 °C was ascribed to the improved distribution of aggregated Nd-rich phase.