In sintered Nd-Fe-B based magnets, coercivity value is only a small fraction of the theoretical value. Considerable efforts were made in various investigations on studying methods of coercivity improvement. The coercivity of sintered Nd-Fe-B based magnets strongly depends on their thermal history. Since the nucleation of reverse domain preferentially occurs at grain boundaries, the increase of intrinsic coercivity caused by post-sintering heat treatment has been related to the structural changes of the Nd-rich phase existing along inter-grain boundaries. This work is concerned with the structural changes of Nd-rich phase according to heat treatments and elucidating the relationship between coercivity and the structure of Nd-rich phase in the sintered $Nd_{15}Fe_{77}B_8$ magnets. The effects of post-sintering heat treatment conditions on the coercivity and crystal structure have been investigated by means of transmission electron microscopy, X-ray diffraction and magnetic measurements.
The effect of varying heat treatment temperatures from 300℃ to 800℃ on coercivity was examined. It was found that coercivity had a peak value in the vicinity of 600℃. The coercivity of the as-sintered sample was 4.3kOe, while the coercivity increased to 8.3kOe after the heat treatment at 600℃ for 1 hour followed by rapid cooling with Ar injection. On the other hand, the slowly cooled(4.5℃/min) specimens showed poor coercivity value(4.2kOe). It was identical with that of as-sintered sample.
The magnets investigated were composed of the hard magnetic $Nd_2Fe_{14}B$, B-rich($Nd_{1+\varepsilon}Fe_4B_4$) and Nd-rich phase. It was reported that the coercivity of sintered Nd-Fe-B based magnet is controlled by nucleation of reverse domain. In this sintered Nd-Fe-B magnets, thin Nd-rich phase layer have been observed along the $Nd_2Fe_{14}B$ grain boundaries. The crystal structure of Nd-rich phases were typical facecentered cubic crystal. It was confirmed by selected area diffraction patterns. However the Nd-rich phases in the sample which underwent heat treatment (at 600℃ for 1hour) were not simple FCC structure. The extra spots were found within the FCC major spots. They were super lattice reflections. There were not any compounds corresponding to that patterns in Nd-Fe-B ternary system. So it can be suggested that the system was quarternary rather than ternary system. Oxygen was introduced as the fourth element and among the compounds in Nd-Fe-B-O quaternary system, body centered cubic, $Nd_2O_3$(a=1.108nm) with Ia$\bar{3}$ space group coincide with that phase. The $Nd_2O_3$ with BCC(as-sintered condition) transformed into hexagonal close packed structure in the vicinity of 600℃. The heat treatment at 600℃ promotes the formation of HCP phase. This fact makes it possible to reach the conclusion that the FCC Nd-rich phase in the $Nd_{15}Fe_{77}B_8$ magnets transformed into BCC with Ia$\bar{3}$ super lattice during the heat treatment at 600℃ and the HCP structure stable above 600℃ exist even at room temperature. The coercivity enhancement by heat treatment at 600℃ for 1 hour followed by rapid cooling is suggested to aid the transformation of the Nd-rich phase into HCP structure.
