As its name suggests, sintered NdFeB is an alloy material based on the compound Nd₂Fe₁₄B, which is formed from neodymium (Nd), iron (Fe), and boron (B). However, sintered NdFeB is not a single-phase material. Its microstructure consists of three main phases:
- the Nd₂Fe₁₄B phase, which is the primary and functional phase,
- a boron-rich phase (also known as the Nd₁.₁Fe₄B₄ phase), and
- a neodymium-rich phase, often referred to as the rare-earth-rich phase.
Among these, the Nd₂Fe₁₄B phase dominates and serves as the core phase responsible for the magnet’s magnetic properties.
Most rare-earth (RE) elements can form compounds with iron and boron in the general formula RE₂Fe₁₄B, which is the fundamental phase of sintered rare-earth iron–boron permanent magnets. This phase typically accounts for 96–98% of the total volume of a sintered NdFeB magnet.
Although all RE₂Fe₁₄B compounds share the same crystal structure, their magnetic properties vary significantly depending on the rare-earth element involved. As a result, partially substituting neodymium with other rare-earth elements in sintered NdFeB magnets can effectively modify specific magnetic properties.
Effect of Heavy Rare Earth Dy Substituting for Nd
- Significant Improvement in Coercivity
The compound Dy₂Fe₁₄B has an anisotropy field (Hₐ) that is approximately 2.14 times higher than that of Nd₂Fe₁₄B. As a result, replacing a small portion of neodymium with dysprosium (Dy) can significantly increase the magnet’s coercivity (Hc).
In theory, for every 1 at.% of Nd replaced by Dy, the intrinsic coercivity (Hcj) can increase by about 11.4 kA/m. In practical applications, however, the actual coercivity improvement depends on the presence and interaction of other alloying elements and the overall microstructure.
- Reduction in Magnetic Polarization (Js), Remanence (Br), and Maximum Energy Product
Dy has a lower magnetic moment than Nd. Therefore, substituting Dy for Nd leads to a reduction in magnetic polarization (Js).
Theoretically, each 1 at.% Dy addition reduces Js by approximately 90 mT, which in turn lowers both the remanence (Br) and the maximum energy product ((BH)max) of the magnet.
- Improved Temperature Stability of Br and (BH)max
Although Dy reduces Br and (BH)max at room temperature, it also lowers the temperature coefficients of these parameters. This means magnets containing Dy exhibit better magnetic stability at elevated temperatures, making them more suitable for high-temperature applications.
Cost Consideration
It is important to note that heavy rare earth elements such as Dy are expensive. Adding Dy significantly increases the raw material cost of sintered NdFeB magnets. Therefore, Dy content must be carefully optimized to achieve the required coercivity and thermal performance while balancing cost and magnetic performance.
Effect of Heavy Rare Earth Tb Substituting for Nd
Adding terbium (Tb) to sintered NdFeB magnets to partially replace neodymium produces effects similar to those of dysprosium (Dy) substitution. However, Tb₂Fe₁₄B has an even higher anisotropy field (Hₐ) than Dy₂Fe₁₄B, making Tb more effective at increasing coercivity.
That said, Tb is much scarcer than Dy in rare-earth ores and is therefore significantly more expensive. As a result, its use is typically limited to applications that require extremely high coercivity, such as high-temperature or high-reliability environments.
Effect of Gd and Ho Substituting for Nd
Among heavy rare earth elements, gadolinium (Gd) has the largest natural reserves. Gd can form the compound Gd₂Fe₁₄B, which exhibits lower magnetic polarization (Js) and lower anisotropy field (Hₐ) compared with Nd₂Fe₁₄B. However, it has the highest Curie temperature (T₍C₎) among rare-earth iron–boron compounds.
Because Gd is relatively abundant and lower in cost, some manufacturers add it in the form of gadolinium–iron alloy to partially replace Nd in an effort to produce lower-cost sintered NdFeB magnets. From a long-term perspective, however, substituting Nd with Gd offers limited practical benefit and may be considered inefficient. If Gd later proves to be critical for more advanced applications, such usage could represent a non-recoverable resource loss.
Substituting holmium (Ho) for Nd presents similar effects and similar concerns, combining reduced magnetic performance with questionable long-term resource efficiency.
What Buyers Should Know
- Dy and Tb are used primarily to increase coercivity, especially for magnets operating at high temperatures or in demagnetizing environments (such as EV motors, servo motors, and aerospace applications).
- Tb is more effective than Dy, but it is also much more expensive and far less abundant. It is usually reserved for critical, high-performance applications only.
- Adding heavy rare earth elements always increases material cost and reduces remanence and energy product, so they should be used only when necessary.
- Substituting Gd or Ho for Nd may reduce cost in the short term, but offers limited magnetic benefits and raises concerns about long-term resource efficiency.
In practice, the optimal approach is to use the minimum amount of heavy rare earth required to meet performance needs, balancing cost, performance, and long-term material sustainability.