Abstract
Microstructural, magnetic and mechanical properties of Nd6.8Fe63.3Nb4.0Co1.3Cr1.1Zr0.9B22.6 nanocrystalline hard magnets produced by rapid solidification technique and subsequent thermo-magnetic annealing have been investigated in this study. X-ray diffraction analysis revealed that the as-cast Nd6.8Fe63.3Nb4.0Co1.3Cr1.1Zr0.9B22.6 magnet has 60% Nd2Fe14B, 16% α-Fe, 9% Fe3B and 15% amorphous phases, whereby, Nd2Fe14B being the majority phase. Thermomagnetic annealing induces a complete crystallization of Nd2Fe14B, α-Fe, Fe3B and amorphous phases and establishes ideal volume fractions between phases leading to improvement in magnetic properties. Scanning and transmission electron microscopy show nano-scaled hard Nd2Fe14B grains surrounded with soft α-Fe and Fe3B grains. The Nd6.8Fe63.3Nb4.0Co1.3Cr1.1Zr0.9B22.6 magnet annealed at 993 K for 15 min shows values of coercivity iHc, remanence Br and maximum energy product (BH)max up to 861 kA/m, 0.68 T and 81.9 kJ/m3, respectively. Compressional stress strain measurements elucidated that mechanical properties of alloys are affected by the morphology of phases as well as crystal structure. As-cast Nd6.8Fe63.3Nb4.0Co1.3Cr1.1Zr0.9B22.6 magnet demonstrates the hardness (Hv) of 960 ± 20 and compressive stress (σf) of 1270 ± 10 MPa, which tend to decrease by thermo-magnetic annealing process.
Highlights
Nanocrystalline exchange-coupled R2Fe14B (R= Nd, Pr)/α-Fe (Fe3B) hard magnets have gained ample interest because of their unique phase construction, high remanence, less rare earth contents and maximum energy product up to 800 kJ/m3 predicted by models.[1,2,3,4] Magnetic properties in Nd2Fe14B (R= Nd, Pr)/α-Fe (Fe3B) magnets depend critically on the grain size, chemistry of grain boundary phase and volume fraction of phases.[5]
High Br is attributed to the elongation of magnetic phase grains in applied field direction during thermomagnetic annealing treatment
Nanocrystalline Nd6.8Fe63.3Nb4.0Co1.3Cr1.1Zr0.9B22.6 hard magnets in rod form have been produced through injection casting followed by thermomagnetic annealing
Summary
Nanocrystalline exchange-coupled R2Fe14B (R= Nd, Pr)/α-Fe (Fe3B) hard magnets have gained ample interest because of their unique phase construction, high remanence, less rare earth contents and maximum energy product up to 800 kJ/m3 predicted by models.[1,2,3,4] Magnetic properties in Nd2Fe14B (R= Nd, Pr)/α-Fe (Fe3B) magnets depend critically on the grain size, chemistry of grain boundary phase and volume fraction of phases.[5] The R2Fe14B (R= Nd, Pr)/α-Fe (Fe3B) hard magnets have been fabricated by mechanical alloying, deposition technique and rapid solidification process using melt-spinning or copper mold casting techniques.[6,7,8] Researchers have produced magnetic products in the form of ribbons, rods, sheets, tubes and foils by the combination of rare earth metal (Nd, Pr, Y)-transition metal (Fe, Co, Nb, Zr, Ti)-metalloid (C, B).[9,10,11,12,13] hard magnets with an energy product of 60-160 kJ/m3 have been prepared, but, it is far less than the predicted value.[14] Literature review indicates that non-optimum microstructures (e.g., grain size, volume fraction, phase distributions, etc.) and lack of preferred orientation (texture) of phases cause the low magnetic properties
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