Synthesis And Properties Of Re-Fe Based Bulk Metallic Alloys

As a new type of magnetic material, Fe-based bulk metallic glasses (BMGs) have been widely used as high frequency transformers and sensors. However, Fe-based magnetic amorphous alloys are usually limited to small size due to low glass forming ability (GFA), which restricts their widespread applications. Therefore, in order to expand their industrial applicability, search for advanced Fe-based BMGs possessing high GFA along with superior magnetic properties is essential. In addition, copper mold casting followed by one-step annealing is a new technique to fabricate Re-Fe based bulk nanocomposite magnets. This method provides an economical and simple approach to produce miniature nanocomposite permanent magnets. Nowadays, researchers are trying to fabricate large size bulk nanocomposite magnets with high magnetic properties by using this technique. In this work, Fe-based bulk amorphous magnetic alloys with improved size were synthesizied by copper mold casting. The thermal stability, structure, magnetic properties and mechanical properties of the alloys were studied systematically. On the basis, the large size Nd-Fe nanocomposite permanent magnets along with high hard magnetic properties were developed. The influence law of annealing treatment on the microstructure and magnetic properties of the magnets was discussed. The main results are summarized as follows:The Fe-Mo-B-Dy and (Fe, Co)-Mo-B-Dy BMG system were developed by doping Dy and Co elements to Fe72Mo4B24 base alloy. It was found that Dy and Co were favorable for the improvement of GFA of the Fe72Mo4B24 alloy. With the addition of Dy and Co, the critical size of the Fe72Mo4B24 amorphous alloy was increased from 65μm to 3 mm. In addition, the dynamics studies showed that the activation crystallization energy of the Fe-Mo-B-Dy and (Fe, Co)-Mo-B-Dy alloy system was much higher than that of Fe-Mo-B system, which indicated that the addition of Dy and Co to the alloy could enhance the thermal stability and anti-crystallization ability. The new developed Fe-Mo-B-Dy and (Fe, Co)-Mo-B-Dy alloys also exhibited good magnetic and mechanical properties. For example:the saturation magnetization and coercivity of the (Fe72Mo4B24)96Dy4 BMG with 2 mm in diameter were 78.81 Am2/kg and 0.4 kA/m, respectively. The [(Fe0.7Co0.3)72Mo4B24]94Dy6 BMG showed high compressive strength and micro hardness with the values of 3540 MPa and 1185 kg/mm2. Nanocomposite Nd9Fe6522B22Mo4, Nd5Fe64B23Mo4Y4 and permanent magnets with good magnetic properties were fabricated by copper mold casting and subsequent annealing. The magnetic properties of as-cast Nd9Fe65B22Mo4 BMG with the size of 1×5×40 mm3 were changed from soft to hard after annealing treatment. The good magnetic properties with Br=0.56 T, iHc=920.7 kA/m and (BH)max=50.2 kJ/m3 were achieved by annealing the Nd9Fe65B22Mo4 amorphous precursor at 983 K for 10 min. Adding 4 at.% Y or replacing Mo with 1.0 at.% Zr can increase the critical size of Nd-Fe-B-Mo alloy to 2 mm, which indicated that proper Y or Zr doping can improve the GFA of the Nd9Fe65B22Mo4 alloy effectively. In addition, it was found that the magnetic properties of the magnets were also enhanced by adding Y and Zr elements. The (BH)max and Br for the annealed Nd5Fe64B23Mo4Y4 magnet were improved to 57.3 kJ/m3 and 0.6 T with keeping iHc value of 764.2 kA/m at 1013 for 10 min. The iHc of Nd7Fe67B22Mo3Zri magnet was increased to 1110.4 kA/m after annealing Nd7Fe67B22Mo3Zr1 amorphous precursor at 1013 for 10 min. The XRD and TEM studies revealed that the annealed magnet microstructure contained nanoscale grains of soft and hard magnetic phases whereby soft phase grain surrounded well to the adjacent hard phase grain with almost homogenous distribution in the composite microstructure. The improvement of magnetic properties in the magnet was presumed to be related to the refinement of the microstructure and uniform distribution of phase grains, which gave a rise to the exchange interactions between magnetic grains.The annealing temperature dependence of the structure and magnetic properties of Nd9Fe65B22Mo4 and Nd5Fe64B23Mo4Y4 magnets was investigated systematically. It was found that the Br,iHc and(BH)max increased first then decreased with the increasing of annealing temperature. When the annealing temperature was low, due to incomplete crystallization, the residual amorphous phase hindered the exchange coupling between magnetic grains, which led to low magnetic properties. However, over-high temperature annealing treatment induced phase grain coarsening which deteriorated the magnetic properties of the magnets. It was concluded that, the magnetic properties of the magnet were strongly dependent on the amount, size and distribution of magnetically soft and hard grains.