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Sintered $Nd_{15-x}Dy_xFe_{77}B_8$ magnets with x=0, 0.5, 1.0, and 1.5 have been prepared by employing the conventional powder metallurgical processes. For a magnet with x=0, the effects of post-sintering heat-treatment conditions on the coercivity and microstructure have been investigated by means of transmission electron microscopy, X-ray diffraction, and magnetic measurements. The magnets investigated were composed of the hard magnetic $Nd_2Fe_{14}B$, B-rich ($Nd_{1+\varepsilon}Fe_4B_4$) and Nd-rich phases. The structures of $Nd_2Fe_{14}B$ and B-rich phases did not change upon heat-treatment, while two kinds of Nd-rich phases with different crystal structures were observed according to the cooling rates after post-sintering heat-treatments. The {\boldmath$fcc$} Nd-rich phase has been observed in both slow ($4.5\,^\circ\!C$/min) and rapid cooling (about $200\,^\circ\!C$/min) conditions, but the {\boldmath$dhcp$} Nd-rich phase has been found only in the specimens rapidly cooled after the post-sintering heat-treatment. It is also found that the magnets containing the {\boldmath$dhcp$} Nd-rich phase exhibit the higher coercivity values. Both TEM and X-ray diffraction analyses have shown that part of the Nd-rich phases undergo the reversible {\boldmath$fcc$} $\leftrightarrow$ $\boldmath$dhcp$} transformation according to varying cooling rates and that the change of coercivity with heat-treatments and cooling rates in the sintered $Nd_{15}Fe_{77}B_8$ magnet is associated with the reversible {\boldmath$fcc$} $\leftrightarrow$ {\boldmath$dhcp$} transformation of the Nd-rich phase.
From the variation of X-ray spectra for the specimens with different thermal history, it was observed that the peaks of the $Nd_2Fe_{14}B$ phase (space group $P4_2$/mnm) were more intense and sharp in the magnets with higher coercivity, which were cooled rapidly after post-sintering heat-treatments, while the integrated intensity of each peak was found to be almost constant. This shows that the degree of ordering is increased by rapid cooling after post-sintering heat-treatments. It appears that the increased degree of ordering is contributing to the coercivity enhancement.
The effects of Dy contents (x) and heat-treatment temperature on the magnetic properties of sintered $Nd_{15-x}Dy_xFe_{77}B_8$ magnets have been also investigated. All the $Nd_{15-x}Dy_xFe_{77}B_8$ magnets showed the maximum coercivity values for the post-sintering heat-treatment temperature of $600\,^\circ\!C$. The maximum energy product increased slowly up to $650\,^\circ\!C$, and then decreased sharply at $700\,^\circ\!C$. The coercivity of $Nd_{15-x}Dy_xFe_{77}B_8$ magnets increased with increasing x independent of the heat-treatment temperatures. The increasing rates were found to be 3. 51 and 3.47 kOe/at\%Dy for the as-sintered and heat-treated ($600\,^\circ\!C \times 1 hr \times$ quenching) conditions, respectively. The addition of Dy to $Nd_{15}Fe_{77}B_8$ resulted in the significant enhancement of coercivity, but the remanence and maximum energy product decreased somewhat increased with Dy content from 0 to 1.5 at\%.
As in the ternary $Nd_{15}Fe_{77}B_8$ magnet, the $Nd_{15-x}Dy_xFe_{77}B_8$ magnets were composed of the $(Nd,Dy)_2Fe_{14}B$, B-rich and Nd-rich phase, and no new phase was observed even for a magnet with x=1.5. The Curie temperatures of $Nd_{15-x}Dy_xFe_{77}B_8$ magnets were found to be constant as $312\,^\circ\!C$ regardless of x. Wavelength dispersive spectrometry (WDS) analysis showed that the ratio of Dy to Nd in the $(Nd, Dy)_2Fe_{14}B$ phase was higher than the Nd-rich phase. This result suggests that the increase in coercivity with Dy addition (x) may be due to the enhanced anisotropy field by partitioning Dy in Nd site of the hard magnetic $(Nd,Dy)_2Fe_{14}B$ phase.
IV-1-4.
$Nd_{15}Fe_{77}B_8$ 소결 자석은 TEM 및 X-ray를 이용한 미세 구조 관찰 결과 열처리 조건에 관계없이 $Nd_2Fe_{14}B$, B-rich 및 Nd-rich의 세가지 상으로 이루어져 있었다. 그러나, 보자력은 소결 후 열처리 및 급냉에 의하여 크게 증가하였으며, 반복 열처리 시 급냉과 서냉을 번갈아 행함에 따라 보자력은 가역적으로 증가 또는 감소하였다. 이러한 보자력의 변화 거동은 dhcp 구조의 Nd-rich상의 생성 및 소멸과 일치하였으며, 열처리 후 급냉에 의하여 보자력이 증가된 시편은 fcc구조의 Nd-rich 상이외에도 dhcp 구조의 Nd-rich 상을 포함하고 있었다.
열처리에 따른 보자력의 변화에 영향을 미치지 않는 것으로 알려진 $Nd_2Fe_{14}B$ 상에서는 뚜렷한 구조의 변화는 관찰되지 않았다. 그러나, $Nd_2Fe_{14}B$ 상에 의한 X-ray peak들의 회절 강도가 열처리와 급냉에 의하여 크게 증가하였으며, 반복 열처리 시 급냉과 서냉을 행함에 따라 회절 강도의 증가 및 감소는 dhcp Nd-rich 상의 경우와 같이 가역적이었다. 이는 열처리와 급냉에 의하여 $P4_2/mnm$ 구조인 $Nd_2Fe_{14}B$ 상을 이루는 원자들의 규칙화 정도(degree of ordering)가 증가하였기 때문인 것으로 여겨지며, 이러한 규칙화의 증가는 anisotropy field의 증가를 가져와 $Nd_{15}Fe_{77}B_8$ 소결 자석의 보자력 증가에 기여했을 것으로 생각된다.
IV-2-5. 요약
$Nd_{15-x}Dy_xFe_{77}B_8$ 소결 자석은 Dy의 함량 x에 관계없이 $600\,^\circ\!C$의 열처리 온도에서 최대 보자력이 얻어졌으며, x의 증가에 따라 보자력은 소결 상태에서는 3.51 kOe/at\%Dy의 비율로, $600\,^\circ\!C$에서 열처리하였을 때는 3.47 kOe/at\%Dy의 비율로 크게 증가하였다. 또한, $(BH)_{\max}$는 Br이 최대인 $650\,^\circ\!C$의 열처리 온도에서 가장 높은 값이 얻어졌다.
미세 조직 관찰 결과 Dy의 함량이 가장 높은 x=1.5인 자석에서도 새로운 상의 형성은 관찰되지 않았으며, 첨가된 Dy는 $(Nd,Dy)_2Fe_{14}B$, B-rich 및 Nd-rich 의 세 가지 상 모두에 존재하였다. WDS를 이용한 조성 분석 결과 강자성 $(Nd,Dy)_2Fe_{14}B$ 상의 Dy/Nd의 비가 Nd-rich 상에서 보다 높게 나타났다. 따라서, Dy 첨가에 의한 보자력의 증가는 Dy가 강자성 $(Nd,Dy)_2Fe_{14}B$ 상의 Nd 자리에 치환되어 anisotropy field를 증가시켰기 때문으로 생각된다.
