Kuo, Chun-Hsin (2010)
Ph.D. thesis, University of Birmingham.
Restricted to Repository staff only until 31 December 2020.
Alloy compositions near to stoichiometric Nd2Fe14B produced under normal casting conditions, with subsequent isothermal heat treatments and with sintering have been characterised. The possible mechanisms for removing the soft magnetic phase, free α-Fe, via additions of 0.6 at% of Zr or 1.0 at% of ZrB2 have been investigated by EPMA (Electron Probe Microanalysis) and TEM (Transmission Electron Microscopy). 1.0 at% of ZrB2 has been found to be very effective in suppressing the formation of free α-Fe dendrites during solidification, but the excessive ZrB2-type needles formed in the NdFeB alloy may have a deleterious effect on the magnetic properties. However, the coercivity of the NdFeB sintered magnets can be improved to some extent by addition of Zr or ZrB2 to the composition Nd12.68Fe77.91-80.91B6.41-8.41Zr0-1 (at%) due to the smaller and more uniform grains of the Nd2Fe14B phase (~6.5 μm) in the sintered magnets. Both SEM (equipped with WDX) and TEM (equipped with EDX) have been employed to investigate ZrB2 needles in Nd12.68Fe77.91-80.91B6.41-8.41Zr0-1 (at%) alloys. It is found that suppression of free α-Fe dendrites in both Zr-containing NdFeB alloys resulted from different solidification routes. Since the sequences of formation of the ZrB2-type needles in Zr-containing alloys are different, the Nd2Fe14B grain growth is inhibited via different mechanisms. Meanwhile, evidence for the high temperature solubility of Fe in ZrB2 needles is found for both Zr-containing NdFeB alloys. Chemical analysis shows that the ZrB2-type needles in the NdFeB alloys are composed mainly of B (~70 at%) and Zr (~20 at%) with a small but significant amount of Fe (~7 at%), a formula which can be expressed as (Zr1-x, Fex)B2 (x~0.3).
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