Grain boundary dislocations in NdFeB magnets are closely related to their microstructure, deformation mechanisms, and performance regulation. Dislocations are linear defects in crystals; their distribution, motion, and interaction within NdFeB magnets directly affect the mechanical properties, magnetic properties, and deformation behavior of the magnet.
Meaning of Grain Boundary Dislocations
Dislocations are line defects resulting from local mismatches in the atomic arrangement within a crystal. In NdFeB magnets, grain boundaries are the regions at and around the interfaces between grains of the main phase (Nd₂Fe₁₄B). At the grain boundaries, the atomic arrangement is irregular and a large number of dislocations exist.
This is because during the production of NdFeB, grain boundary dislocations are generated in large quantities at the grain boundaries due to differences in grain orientation (orientation deviation) or phase boundary mismatch; in addition, the migration of atoms during grain boundary diffusion also introduces new dislocations at the grain boundaries.
Distribution of Grain Boundary Dislocations
Grain boundaries (low-angle grain boundaries) can themselves be regarded as geometric structures composed of a series of dislocations. Under high-temperature or high-strain conditions (such as high-temperature compression), grain boundary sliding triggers grain rotation and generates geometrically necessary dislocations (local misorientation) near the grain boundaries, leading to changes in dislocation density.
Distribution of dislocations at the interface between the grain boundary and the main phase: At the interface between the main phase and the grain boundary phase, interface dislocations form due to differences in lattice constants (misfit degree). For example, when the epitaxial layer of a grain (such as the Tb-rich shell layer) undergoes lattice distortion, internal stress is generated at the interface, accompanied by a distribution of dislocations.
Distribution of dislocations within the grains: Within the grain interiors (especially in regions near the grain boundaries), local misorientation accumulates, and its distribution agrees well with the theoretical spacing of geometrically necessary dislocations, indicating that dislocations are significantly distributed within the grains (particularly near the grain boundaries).
Effects of Grain Boundary Dislocations on Magnet Performance
| Mechanical Properties: Grain boundaries are the main path for crack propagation. An appropriate amount of grain boundary dislocations and an optimized grain boundary phase, such as a continuously distributed Nd-rich phase, can improve the grain boundary bonding strength, hinder crack propagation, and enhance the bending strength and fracture toughness of the magnet; however, a high dislocation density at the grain boundaries can induce microcracks, increasing the brittleness of the magnet and deteriorating the magnet’s mechanical properties.
| Coercivity: Due to the irregular atomic arrangement at the grain boundaries, the diffusion activation energy is low, making them fast channels for element diffusion. For example, heavy rare earth elements Tb/Dy diffuse along the grain boundaries to form a “core-shell” structure that effectively pins reverse magnetic domains, thereby enhancing coercivity. However, the high defect density (dislocations) at the grain boundaries may also serve as nucleation sites for reverse magnetization domains; if the grain boundary defects are excessive or the grain boundary bonding is weak, the coercivity may decrease.
| Corrosion Resistance: Due to composition segregation at the grain boundaries (such as the Nd-rich phase), the electrochemical activity is relatively high, making them preferential sites for corrosion—commonly referred to as grain boundary corrosion. The more grain boundary defects there are, the more easily corrosion micro-cells form, reducing the corrosion resistance of the magnet; optimizing the grain boundary structure and reducing grain boundary defects can improve corrosion resistance.
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