Effects of quench rate on the microstructure in melt-spun Nd–Fe–B alloys

Abstract

Electron microscope studies have shown significant changes in the microstructure of the Nd₁₄Fe₈₀B₆ melt-spun ribbons, depending on the quenching conditions. The average grain size of about 20 nm near the wheel side, increasing to 500–700 nm near the free side, can be observed for ribbons of the thickness of about 25–30 μm, indicating strong internal variations in quench rate across the ribbon thickness. For low quenching rates different morphologies of the crystalline phase Nd₂Fe₁₄B are observed. Equiaxed grains near the wheel side of the ribbon become columnar grains or even dendritic grains near the free side. Texturing effects in the equiaxed grains are observed only for grains larger than 30 nm and disappear for very large equiaxed grains (about 500 nm), as well as for dendritic structures. Most evident texture is observed in columnar grains, which crystallized with the c-axis perpendicular to the ribbon plane.