Effective grain pinning revealed by nanoscale electron tomography

Abstract

The grain pinning behavior of TiC particles in a rapidly solidified MRE-Fe-B (MRE = Nd + Y + Dy) nanocrystalline hard magnet was studied using electron tomography (ET). The 3D reconstruction overcomes the inherent 2D nature of conventional transmission electronmicroscopy (TEM) to reveal how this grain boundary phase controls the nanoscale structure in the rapidly solidified alloy. The 3D reconstruction was performed on the optimally annealed alloy (750 °C/15 min) with hard magnetic properties of Mr = 8.1 kGs, Hc = 6.2 kOe, (BH)max = 11.2 MGOe measured at 300 k. The sampled volume, 425 × 425 × 92.5 nm3, contains more than 20 grains of the RE2-14-1 phase and more than 70 TiC nanoparticles. The TiC grains’ shapes depend on their sizes and locations along the grain boundary. Most of the TiC particles are oval or short rod like shapes and range from 5 nm to 10 nm. TiC particles less than 10 nm formed between adjacent 2-14-1 grains, while the largest ones formed at triple junctions. There are ∼1.7 × 108 TiC particles within a 1 mm3 volume in the alloy. This accounts for the strong grain boundary pinning effect, which limits grain growth during annealing.

Magnetic Hardening of CeFe11Ti and the Effect of TiC Addition

“…Diffraction profiles among the samples with or without 11 Ti + x TiC WHERE x = 0, 3, AND 6 AFTER 5 MIN ANNEALING the existence of multiple competing binary and ternary phases in the vicinity of the stoichiometry of interest [11], and has not been well understood. With our experimental findings, literature reports of similar studies on Nd 2 Fe 14 B [12]- [18] and binary and ternary phase diagrams involving Ce, Fe, Ti, and C, we argue phenomenologically that TiC would solidify first and provides nucleation seeds for the undercooled liquid, making it easier for the liquid melt to crystallize into fine nanograins of 1:12. However, the rapid crystallization of 1:12 phase causes recalescence, which raises the temperature so that local temperature would surge above the phase transition temperature of 1:12 phase.…”