Effect of cooling rate on microstructure and magnetic properties of Nd60Fe30Al10 hard magnetic alloys

The effect of cooling rate on microstructure, thermodynamic and magnetic properties of Nd60Fe30Al10 alloys prepared by different methods, i.e., arc melting, centrifugal casting and melt spinning, was studied. Although the samples present different morphology due to different applied cooling rates, the existence of an 'amorphous phase' is confirmed through X-ray diffraction and transmission electron microscopy. All samples are hard magnetic at room temperature with coercivity values of 0.2-0.32 T. From the observation of the domain structure, the 'amorphous phase' is found to be responsible for the high coercivity. Thermomagnetic curves and isothermal differential scanning calorimetry measurements suggest that the 'amorphous phase' in fact consists of two separated amorphous phases with different compositions. This 'amorphous phase' is very similar to the A1 phase observed in Nd-rich Nd-Fe binary alloys. The formation of such an A1-like amorphous phase is discussed in terms of the positive heat of mixing between Nd and Fe and the metastability of the NdFe2 intermetalic phase