Evolution mechanism of platelet-shaped grains in Nd-Fe-Co-Ga-B ribbons during annealing

The Nd13.65Fe73.55B5.6Co6.6Ga0.6 ribbons melt-spun at 25 m/s formed a microstructure of mixed platelet-shaped and equiaxed grains after pressureless annealing at 700 °C. With the help of the Nd-Ga-rich molten liquid phase at the grain boundaries, some equiaxed grains preferentially grow along the (110) plane of the Nd2(Fe, M)14B phase into platelet-shaped grains with a width of about 100 nm and an aspect ratio of 6–8:1, while other equiaxed grains grow to 150–450 nm. The driving force for the growth of equiaxed grains into platelet-shaped grains is mainly the temperature field rather than the press field. The amorphous grain boundaries with a width of 5.5 ± 4.5 nm in the as-spun ribbons discharged Nd, Co, and Ga and absorbed Fe during the crystallization process, forming a Nd-Ga-rich and Fe-poor grain boundary phase with a width of 1–2 nm. The crystallization of Nd2(Fe, M)14B grains improves the coercivity of the ribbons, but the reduced Fe content and the random orientation of the grains decrease the remanence and magnetization of the ribbons. The ribbons annealed at 700 °C have the highest coercivity, increased by 54.2% compared to the as-spun ribbons.