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

Abstract 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 Nd2Fe12.8Co1.2B 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.