Mechanical properties of neodymium magnets

In the process of applying neodymium magnets, we usually focus on their excellent magnetic properties. However, in different working conditions and use environments, the mechanical properties of neodymium magnets cannot be ignored. For example, in the process of motor manufacturing, neodymium magnets need to have sufficient mechanical properties to withstand mechanical stresses during assembly, which may come from compressive forces during assembly or the mechanical clamping force of fixing the magnets. In order to ensure that the magnets do not suffer damage under these conditions, engineers pay special attention to the hardness and flexural strength of the magnets.

In addition, in high-speed rotating motors, strong centrifugal forces, accelerations, and high-frequency vibrations can affect neodymium magnets. If exposed to these extreme conditions for a long time, neodymium magnets may wear out or even break, which will directly affect the operational efficiency and service life of the motor. Therefore, the mechanical properties of neodymium magnets play a crucial role in ensuring the long-term stable operation of the motor.

Due to the heat generated by motors and generators during operation, neodymium magnets must also be able to withstand dimensional changes caused by temperature differences. This thermal expansion and contraction can produce internal stresses within the magnet, affecting its structural stability. Therefore, the thermal expansion coefficient and thermal conductivity of the magnet are also important parameters to consider during design.

Internal structure determines mechanical properties of neodymium magnets

Mechanical properties of neodymium magnets
Unit cell of Nd2Fe14B (P42/mnm space group). The c/a ratio in the figure is exaggerated to emphasize the puckering of the hexagonal iron nets.

Neodymium magnets, also known as neodymium-iron-boron magnets, are tetragonal crystals composed of neodymium, iron, and boron (Nd2Fe14B). The neodymium element in neodymium magnets forms a hexagonal lattice with the boron element, and the iron element is encapsulated inside, forming a hexagonal close-packed structure. This structure has a high packing density and a small lattice constant, which gives neodymium magnets a high hardness.

1. Hardness:

The high hardness of neodymium magnets mainly comes from their internal crystal structure, which has high crystallinity and tight arrangement of atoms. This structure enables neodymium magnets to exhibit high hardness when subjected to external forces due to the strong interatomic interactions.

2. Resilience:

Due to the strong bonding in the crystal structure of neodymium magnets, when external forces are applied to the magnets, the bonds between atoms are difficult to slip, resulting in stress concentration and the formation of microcracks. These microcracks easily spread inside the neodymium magnets, eventually leading to the fracture of the magnets. Therefore, neodymium magnets have high brittleness and are prone to fracture. This means that when neodymium magnets are subjected to rapid or impact loads, they are prone to breakage rather than bending or deformation.

3. Plasticity:

Material plasticity refers to the property of a solid undergoing deformation under external forces and maintaining the deformation.  It mostly refers to the property of rubber, plastic, or most metals that can change shape at room temperature or after heating.

The characteristics of neodymium magnets make it unsuitable for traditional plastic deformation processing methods such as forging, stamping, or drawing.

Due to poor plasticity, sintered neodymium magnets can only be machined by cutting and grinding. This also makes it difficult to manufacture and process oversized sintered neodymium magnets, which are prone to problems such as cracking and edge collapse.  The larger the magnet size, the lower the yield in the processing step.

Adding other metal elements has a certain impact on the strength of sintered NdFeB.

When a certain amount of Ti, Nb, or Cu is added, the impact fracture toughness of the permanent magnet is slightly improved.

When a small amount of Co is added, the bending strength of the permanent magnet is increased.

Three commonly used mechanical performance indicators

HardnessHvFracture toughness(MPa.m1/2)Impact toughness(KJ/m2)Flexural strength/MPa
Sintered Neodymium Magnet400-6002.2-5.527-47150-350

As a typical brittle material, three indicators are commonly used to describe the mechanical properties of neodymium magnets:

1. Fracture toughness:

It describes the resistance of brittle materials to the propagation of flaws under an applied stress, and its unit is MPa·m1/2. The testing of the fracture toughness of a material requires the use of tensile testing machines, stress sensors, extensometers, signal amplification dynamic strain gauges, etc. In addition, the sample should be made into a thin sheet shape.

Mechanical properties of neodymium magnets
Mechanical properties of neodymium magnets 11

2. Impact toughness:

The pendulum impact tester is a device used to test the behavior of materials under impact loading. It evaluates the impact toughness of materials by measuring the energy change of the pendulum before and after striking the sample.

Impact toughness refers to the ability of a material to absorb plastic deformation work and fracture work under impact loading, reflecting the internal micro-defects and impact resistance of the material. The unit is J/m2. The measured value of impact strength is too sensitive to the size, shape, processing accuracy, and test environment of the sample, and the measurement dispersion is relatively large, requiring multiple experiments.

3. Flexural strength:

The three-point bending method is a commonly used experimental method for measuring the fracture resistance of materials under bending loads. In this test, a rectangular cross-section specimen is placed on two supports, and pressure is applied to the center point of the specimen, causing it to bend until it breaks. By measuring the force, specimen size, and support spacing, the bending strength of the magnet can be calculated, usually in MPa units.

Due to the possible internal defects such as pores or cracks in sintered neodymium magnets during the manufacturing process, these defects can become stress-concentrated points during testing, resulting in fracture. Therefore, through testing, the internal quality of the magnet and the advantages and disadvantages of the sintering process can be evaluated. By optimizing the sintering process and material formulation, the overall mechanical properties of neodymium magnets can be improved, making them more suitable for various engineering applications.

Improvement method of mechanical properties of neodymium magnet

1. Add rare earth element

image 2
Mechanical properties of neodymium magnets 12

(1) The content of Nd has a certain influence on the strength of neodymium magnets. Generally, the higher the content of Nd, the higher the strength of the magnet.

(2) By adding a suitable amount of low-melting-point metals such as Cu and Ga, the distribution of grain boundary phases can be improved, enhancing the toughness of the magnet.

(3) Adding high-melting-point metals such as Zr, Nb, and Ti can form precipitates at grain boundaries, refine grains, and inhibit crack propagation, which helps improve strength and toughness. However, excessive addition of high-melting-point metals can cause the hardness of the magnetic material to be too high, seriously affecting processing efficiency.

2. Grain refinement

The refinement of the particle size of the raw material powder can result in the refinement of the grain size in the final sintered neodymium magnet.

Back scattered images of microstructure and topography of grains. (a) A microstruc
ture; (b) B microstructure; (c) A grains; (d) B grains

The B sample with grain refinement has significantly higher bending strength and Vickers hardness than the A sample, and the coercivity has also been improved.

(1) Grain refinement increases the coercivity of the magnet from 1115.7 kA/m to 1339.3 kA/m, while the remanence remains almost unchanged.

(2) The average grain size of the sintered magnet decreased from 8.57 μm to 4.76 μm.

(3) Grain refinement increases the bending strength of the magnet from 422.98 MPa to 546.70 MPa.

(4) The Vickers hardness increased from 577.30 HV to 642.49 HV.

Fracture topography of (a) Sample A and (b) Sample B

The general view is that the microscopic mechanism of sintered NdFeB fracture is mainly intergranular fracture, and the distribution of neodymium-rich phases in the grain boundary region is the main concern for improving mechanical properties. It can be seen from the figure that there are more coarse grains in the fracture surface of conventional magnets, while the grains of refined magnets are relatively uniform and densely combined.

The bonding strength between the main phase grain and the Nd-rich phase is one of the important factors determining the strength of the material. After grain refinement, the volume fraction of the Nd-rich phase remains unchanged, but its distribution becomes more uniform, and the grain boundary itself is strengthened, changing the crack propagation path. This helps reduce defects such as edge damage and cracking caused by processing.

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