Study Of Microstructure & Properties Of Nd₂Fe₁₄B/(α-Fe, Fe_3.5B) Nanocomposites Prepared Under Extreme Conditions

In this paper, Nd2Fe14B/(Fe3.5B,α-Fe) nanocomposite magnets were prepared by annealing Nd-low and B-high amorphous Nd4.5Fe77.5B18 alloy under high pressure. Microstructure transformation and magnetic properties were studied by employing x-ray diffraction (XRD) analysis, transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). The annealing temperatures were selected to be 650, 700 and 750℃, the used pressure ranges from 0 to 6 GPa, and the annealing time is 10 min. We found that the application of high pressure has a significant influence on the phase transformation. First, the Nd2Fe14B phase was observed in the sample annealed at 650℃under high pressure, but not in the sample annealed at 650℃under normal pressure, suggesting strongly that the high pressure decreases the crystallization temperature of Nd2Fe14B phases. Second, microstructure analyses show that the samples annealed under high pressure have a much more homogenous microstructure with smaller grain size. The average grain sizes of theα-Fe, Nd2Fe14B and Fe3.5B phases in the samples annealed at 700℃under normal pressure are 22.9 nm, 20.1 nm and 24.9 nm , respectively, while those in the samples annealed under 6 GPa are 12.3 nm, 11.5 nm, and 13.9 nm, respectively. The magnetic properties of the sample annealed under high pressure are fairly good and the optimum magnetic energy product (BH)max is as high as 8.948 MGOe. According to the dependence of grain sizes on pressure, the activation volume of atomic diffusion in the growth is derived. At 650℃, the activation volumes for the growth ofα-Fe, Nd2Fe14B and Fe3.5B phases are determined to be 0.17 ?, 0.08 ? and 0.06 ?, respectively, where ? is the mean atomic volume of the alloy. This demonstrates that the growth of nanocrystals is controlled by a collective atom diffusion mechanism. High-pressure-torsion-deformation (HPTD) induced nanocrystallization ofα-Fe phase in amorphous Nd9Fe85B6 alloy at room temperature has been investigated. The grain size of the crystallizedα-Fe phase is less than 10 nm. At the number of revolution n=1 the volume fraction ofα-Fe phase is 36%. At n=8, the volume fraction ofα-Fe phase can be even as high as 49 %. For comparison, the largest volume fraction ofα-Fe phase produced by annealing the amorphous alloy at high temperature is only about 18% (at T=500℃, t= 10 min). The possible mechanisms account for the nanocrystallization induced by HPTD at room temperature is discussed.