The grain alignment degree distribution of NdFeB magnets is a key microstructural feature that determines the macroscopic magnetic properties of the magnet. Generally, the smaller the alignment degree distribution, the better the magnetic properties.
Characteristics of the Alignment Degree Distribution
| Distribution characteristics: Ideally, the grain alignment follows a normal distribution, and as the texture strengthens, the distribution curve becomes concentrated toward the preferred orientation, such as the easy-axis direction of NdFeB, with a narrowing of the distribution range.
| Dynamic changes during the sintering process: In the early stage of sintering, grains mainly grow along the pressing direction, and the alignment distribution is relatively large. As sintering proceeds, the grain orientation gradually becomes parallel to the magnetic field direction, and the distribution gradually narrows. Subsequently, due to grain coalescence and growth, the alignment degree may fluctuate slightly, but the overall distribution tends to stabilize.
| Concentration with increased deformation/pressing: During hot deformation or high-field pressing, grains rotate, elongate, and align along the preferred direction. The alignment degree distribution decreases significantly as the deformation amount or magnetic field strength increases, with the distribution range concentrating toward the preferred orientation.
Factors Influencing the Alignment Degree Distribution
| Preparation process and magnetic field: The alignment magnetic field strength is the core influencing factor. The stronger the magnetic field, the more concentrated the grain alignment and the smaller the distribution. In addition, the greater the hot deformation amount (e.g., hot pressing, hot deformation), the more concentrated the grain alignment along the pressing direction (c-axis) and the smaller the distribution.
| Initial powder state: The particle size and morphology of the powder affect the alignment degree distribution. When the particle size is moderate, the change in alignment degree is small, whereas sizes that are too large or too small may cause uneven alignment distribution. The initial alignment state of the powder, whether isotropic or anisotropic, also determines the initial baseline of the distribution.
| Composition and microstructure: The rare-earth content (liquid phase amount) and the addition of heavy rare earths (such as Dy) have a relatively small effect on the alignment degree distribution, but the distribution of grain boundary phases (such as the ratio of continuous grain boundary phases to triple-junction grain boundary phases) can indirectly affect the rotation and alignment distribution of grains.
| Sintering and deformation process: The pressing direction in the early stage of sintering, the magnetic field application time during sintering, and the temperature all influence the rotation and growth of grains, thereby changing the alignment distribution. During high-temperature compressive deformation, local misorientation of grains can also cause differences in the alignment distribution in localized regions.
Effects of the Alignment Degree Distribution on Magnetic Properties
| Effect on remanence (Br): The smaller the alignment degree distribution (the stronger the texture), the more consistent the magnetization direction of the grains, the smaller the self-demagnetization effect, and the higher the remanence. High alignment degree is positively correlated with high remanence.
| Effect on coercivity (Hcj): The alignment degree distribution directly affects the reversal magnetization process. In regions with low alignment degree (large distribution), reversal domains nucleate more easily, becoming weak points for magnetization reversal and leading to a decrease in coercivity. In regions with high alignment degree, reversal domains exhibit “instantaneous propagation,” resulting in higher coercivity. Therefore, reducing the alignment degree distribution and improving the overall alignment degree is an important way to enhance the coercivity of the magnet.
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