The hot deformed Nd-Fe-B permanent magnets have attached much attention due to their good magnetic properties, excellent thermal stability, desirable corrosion and strong fracture toughness. The microstructure, crystallographic alignment, magnetic domain evolution, magnetic properties and magnetic hardening mechanism are studied for the hot deformed Nd-Fe-B permanent magnets. Three distinct regions, namely fine-grain (FG), coarse-grain (CG) and large-grain (LG) regions were identified in the hot deformed Nd-Fe-B magnets. Further investigations indicate that the heterogeneous microstructure plays a key role in determining the magnetic reversal and magnetic properties of the hot deformed magnets. The FG regions are composed of fine and uniform plate-like grains with strong c-axis crystal texture. The FG regions provide the strong pinning force of the magnetic domains, leading to high coercivity and good squareness of the demagnetization curve. The CG regions are composed of partially aligned equiaxed grains, with sub-micron size diameters. Most reversal domains grow from the CG region and propagate into the FG region, resulting in magnetic reversal and contribute to the observed deviations from squareness in the demagnetization curves. The LG regions are composed of randomly aligned equal-axis grains with micron size diameters. Most grains in the LG regions are multi-domains, which reverse even in their remanent magnetization states, lowering the remanence of the magnets.
By changing the technical parameter, the optimization of microstructure and magnetic properties of the hot deformed Nd-Fe-B magnets are realized. The proportion of FG, CG and LG regions change from 78.2 %, 20.0 % and 1.8 % to 88.7 %, 11.0 % and 0.3 %, respectively. Correspondingly, the magnetic properties have also been improved. The Jr and (BH)max of the optimized HD magnets are 14.23 kGs and 50.70 MGOe, which are 6.3 % and 16.7 % higher than the original HD magnets, respectively.