Grain Boundary Diffusion is an important new advance in Neodymium magnet production technology.
Heavy Rare Earth Elements (HREE) are delivered directly to critical areas of the magnet’s microstructure. These areas are where HREE are the most effective at increasing the properties of the magnet to withstand high opposing magnetic fields at high temperatures. HREE have been greatly reduced in the Main Phase in favor of Neodymium -conserving these valuable metals and enhancing magnetic strength and Coercivity.
Grain Boundary Diffusion (GBD) increases the Coercivity of Neodymium magnets while conserving the Remanence (Br) due to the unique grain structure of Neodymium magnets.
HREE Dysprosium (Dy) and Terbium (Tb) have been used for decades in the manufacture of Neodymium magnets. They increase the resistance of these magnets to demagnetization under two primary conditions: 1) High temperatures, and 2) External magnetic fields that oppose the magnet.
These HREE enter the microstructure as a substitute for Neodymium. They are heavier, melt at a higher temperature, and make for a higher Curie temperature than Neodymium, and they improve the magnet’s high-temperature properties.
Using traditional technology for over three decades, manufacturers of NdFeB magnets added HREE to the magnets to increase Coercivity.
But this came at a price, because while the HREE increased Coercivity, they also reduced Remanence (Br).
So – for the last three decades, the application designer has had to choose –at some level- between Remanence and Coercivity.
It’s a compromise between magnetic field strength (Br) on the one hand, and resistance to demagnetization (Hcj) on the other.
This technology leaves designers with a current technical limit of [BHMax + Hcj <70] and the magnet’s performance decreases with increasing temperature rating.Remanence (Br) means remainder. It is a term used to describe the magnet’s own remaining magnetic field strength after the external magnetizing field has been removed.
The reason why designers have had to make this uncomfortable choice is that the traditional way of adding HREE has been to add it to the entire alloy at an early stage of the production cycle. This insures that the HREE will be distributed evenly throughout the melt.
Grain Boundary Diffusion (GBD) Technology changes all that.
Introduction of HREE via Grain Boundary Diffusion leads to the microstructure see above. The main phase is visible as the dark-colored grains. The Neodymium-rich phase is seen as large white grains, and the grain boundaries are the very thin, faint white areas.
When diffusion occurs, the HREE feed the microstructure preferentially through the grain boundaries. The areas of the microstructure that do need HREE get them. The areas that don’t need it don’t get it.
The demand for High-Coercivity Neodymium magnets has created additional demand for the Heavy Rare Earth Elements (HRE’s) Dysprosium and Terbium. The supply of these HRE’s has not increased in accordance with the demand, which has at times caused concern.
The automotive industry is expected to be a particularly significant source of demand in the coming years with the electrification of the automobile. It has been widely reported that automakers will all be producing many models of electric and hybrid automobiles.
Almost all electrified cars are expected to use significant quantities of Neodymium magnets in these motors due to the cost benefits, efficiency and miniaturization these magnets provide.
Automotive electric motors generate a lot of heat when they create high-power output, so the magnets must be able to perform well at high temperatures. Traditional technology magnets must have Dysprosium and/or Terbium added to them to withstand these temperatures and the opposing magnetic fields that go with high power output.
Automotive isn’t the only market using large amounts of Neodymium Magnets. Offshore wind power generators are using amounts of magnets measured in tons-per-turbine. And offshore wind power is growing faster than onshore wind power as onshore locations become fully developed.
Rare Earth Elements are fairly expensive and comprise a large portion of the cost of Neodymium magnets.
Over the last decade, there have been significant price fluctuations in all of the Rare Earth Elements.
The prices of Dysprosium and Terbium are significantly higher than the price of PrNd –the main rare earth component of Neodymium magnets.
This makes HREE a significant cost driver at current prices. If rare earth prices fluctuate, HREE could be an even bigger factor.
Using traditional technology, demand for HREE is expected to continue to increase, so technical solutions have been investigated for reducing or eliminating the demand for HREE in Neodymium magnets.
Grain Boundary Diffusion has been under investigation and development for a number of years. It is now entering mass production at BJMT and is a successful method of drastically reducing the use of HREE while still achieving Coercivity requirements.
When GBD is used synergistically with traditional metallurgical practices like grain refinement and grain modification, magnetic properties can be increased while using cheaper and more plentiful metals.
GBD is employed along with other metallurgical methods. Some of these methods have been effective at eliminating the use of Dysprosium and Terbium for certain grades of Neodymium magnets and reducing the need for these HREE’s in other grades.
Summary: GBD offers the following improvements to NdFeB magnets:
In order to effectively make Neodymium magnets by GBD, it is first necessary to upgrade the whole metallurgical and manufacturing approach.
The first step is to reduce the size of the individual grains in the microstructure.
A reduction in grain size creates a reduction in the ratio of volume-to-grain-boundary-phase surface area, since volume is cubic and surface area is a square.
This process requires the use of advanced equipment and great attention to multiple process factors
Secondly, the grain structure must be refined. This means that the size of the individual grains must be regulated so the grain size distribution is narrowed. So the grains are now almost the same size.
When grain size is kept uniform, magnetic domains will also be kept uniform. This creates uniformity of properties in the entire magnet.
Grain refinement is achieved largely by the use of metallurgical additives that are known to refine the microstructure.
The third step is to make the shape of the grains as similar as possible. This has a similar effect on the magnetic domains mentioned above in the section on Grain Refinement.
Making the shape uniform also makes the magnetism uniform by making each magnetic domain as similar as possible.
These three steps 1) Grain Size Reduction 2) Grain Refinement and 3) Grain Modification create synergies that reduce the need for HREE because they now increase the ratio of Grain Boundary Phase/Base Phase.
These steps have been a part of standard metallurgy practice in many alloy systems for decades. All they do is help Neodymium Magnet materials engineers increase the ratio of Grain Boundary Phase-to-Main Phase which reduces and in many cases eliminates the need for HREE.
For example, in Neodymium magnets, these techniques are successful at reducing and eliminating HRE for the lower temperature grades of magnets.
Grades M and H previously required approximately 1- 3% Dysprosium before these changes were made. Now both grades are achievable with zero HREE due to 1) Grain Size Reduction 2) Grain Refinement and 3) Grain Modification.
Higher temperature grades that previously required over 7% Dysprosium now require less than 3% Dy.
Materials engineers place a coating of HREE on the surface of the magnet. The magnet is then heated to a temperature high enough for diffusion to occur.
The HREE then diffuse into the grain boundaries and then into the magnet’s microstructure.
The HREE penetrates to a depth of typically 5+ mm per side, so there are thickness limitations to this technique. Technical staff members at BJMT can assist with the design of the magnetic circuit to maximize the use of the GBD magnet’s enhanced capabilities as well as dealing with the limitations of the penetration depth.
Grain Boundary Diffusion has done a fine job of reducing the need for HREE in Neodymium magnets, but it has provided another very important benefit to industry. GBD allows for the creation of new grades of magnets that were never before possible.
The previous upper limit of a figure of merit known as BHMax + Hcj has now increased. The previous maximum sum was 70. With GBD, BHMax + Hcj can go as high as 80, bringing an improvement of over 18% to this important figure.
These advances in magnet technology bring new advantages to the designer. The magnets now have enhanced high-temperature performance, and they bring new opportunities to designers.