Mössbauer Study on the Crystallization Process of α-Fe/Nd₂Fe₁₄B type Nanocomposite Magnet Alloy

Abstract

In order to investigate the crystallization process of an α-Fe/Nd₂Fe₁₄B type nanocomposite magnet alloy, Mössbauer study has been carried out on Nd₈Fe₈₀Co₄Nb_1.5B_6.5 alloy ribbons prepared by melt spinning and post-heat-treatment at 600, 650, 700 and 760°C. The relative mass fractions of the α-Fe, Fe₃B, Nd₂Fe₁₄B and the intergranular phases, in crystallization process, were estimated from Mössbauer line-intensities. As-quenched alloys are practically amorphous states and start to crystallize at 600°C, but the crystallized grains are still so fine as to be observed with X-ray diffraction method and the Mössbauer spectroscopy. Around 650°C, α-Fe, Nd₂Fe₁₄B and metastable Fe₃B phases are formed definitely and the amorphous phase decreases drastically but still remains as the intergranular phase. With increasing temperature, metastable Fe₃B phase converts into α-Fe, Nd₂Fe₁₄B, and a part of the intergranular phases also crystallize to form α-Fe and Nd₂Fe₁₄B . At 760°C being the optimum heat-treating temperature to obtain the alloy with the best magnetic properties, about 10 mass% of the intergranular phases is considered to remain between the grains of α-Fe and Nd₂Fe₁₄B.