烧结NdFeB的显微组织分析及高矫顽力烧结NdFeB的研制
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摘要
本文通过金相显微分析、扫描电子显微镜和能谱分析、X射线衍射分析、红外测氧分析、密度及磁性能测试等手段研究了烧结NdFeB的铸锭组织、烧结组织及影响其磁性能的各种因素。并用传统的粉末冶金的方法研究了高矫顽力烧结NdFeB的制备，制备出了B_r=1.229T，H_(cj)=12.87kOe，(BH)_(max)=35.2MGOe和B_r=1.162T，H_(cj)=17.24kOe，(BH)_(max)=32MGOe等性能的磁体。研究发现：
良好的烧结NdFeB的铸锭组织是细小的板条状Nd2Fe_(14)B主相被富钕相均匀的分开，不存在块状的富钕相、α-Fe及其他非磁性夹杂相。α-Fe呈树枝状和颗粒状析出，成分中稀土总量越低、冷却速度越慢、浇铸温度越高，α-Fe越容易析出。甩带法制备的NdFeB铸锭的显微组织细而均匀。
NdFeB的烧结组织中存在成分和形貌不同的富钕相；主相晶粒均匀性越好，颗粒形状越规则，其磁性能越好。晶粒的异常长大严重地恶化烧结体的磁性能，粉末颗粒的尺寸差异越大、烧结温度越高、时间越长、磁粉含氧量越低，晶粒越容易出现异常长大。晶粒度对磁体性能的影响很大，晶粒越细，磁体的磁能积和矫顽力都越高。
粉末形状的不规则性和不均匀性对烧结组织和磁性能影响很大。气流磨制得的粉末形貌规则性、颗粒的尺寸分布较差：球磨制得的粉末形状更差，存在长条状的大颗粒，甚至出现了没有完全破碎的大块。得出了一种取向度计算方法，该方法得出的取向度值与X射线衍射谱所反映的取向情况比较相符。氧含量与工艺过程密切相关，一般来讲，气流磨+防氧化剂制粉可显著降低氧含量，提高磁性能。
加入一些合金元素可以改善烧结NdFeB的磁性能。Dy的加入对提高矫顽力非常显著，加入3.8％Dy后，矫顽力可达18kOe。Nb加入到合金中形成了NbFe和Nb_2Fe_3两种类型的化合物，在一定程度上阻碍了α-Fe的析出，细化了烧结体的晶粒。Cu主要分布在富钕相中，有时与Co伴生，而Co主要进入了主相。在制粉时加入Dy_2O_3，可以改善烧结体的矫顽力，阻碍了颗粒的长大。
The microstructure of ingots and sintered magnets of NdFeB and the effective factors on the magnetic properties have been investigated by means of optical metallurgical analysis, SEM, EDAX, X-ray diffraction analysis, determination of oxygen by infrared analysis and the measurement of density and magnetic properties. The sintered NdFeB magnets with high coercivity have been prepared by traditional powder metallurgy, and the magnets with Br=l. 229T, H.j=12. 87kOe, (BH)粁=35. 2MGOe and Br=l. 162T, Hcj=17. 24kOe, (BH)MI=32MGOe have been gained.
The results are as following:
The ideal microstructure of NdFeB ingots is composed of fine Nd2Fe,4B lath uniformly divided by Nd-rich phase, without block Nd-rich phase, a -Fe and other nonmagnetic phases. The lower the total rare earth is, the slower the cooling rate is and the higher the casting temperature is, the easier a -Fe with dendritic and granular character separates out. The microstructure of NdFeB ingots prepared by strip cast is fine and uniform.
Nd-rich phase with difference composition and form exits in NdFeB sintered magnets. The better the matrix distributes and the more regular the grains are, the higher the magnetic properties. The abnormal grain growth deteriorates the magnetic properties seriously, which has close relation with the size distribution of powder, the sintering temperature and time and the oxygen content, etc. The grain size has important effects on the magnetic properties. The finer the grain is, the higher the magnetic properties.
The regularity and size distribution of powders have great effects on the magnetic properties. The powder produced by jet milling has bad regularity and size distribution, which are worse for the powder produced by ball milling. A method calculating alignment degree has been gained, the results achieved by which agrees with the alignment reflected by X-ray diffraction patterns. The oxygen content has close relations with the process. Generally speaking, the oxygen content can be reduced notably by jet milling with additions against oxide.
The additions of some elements can improve the magnetic properties of sintered
NdFeB. When 3. 8%wt Dy is added, the coercivity can reach 18kOe. The addition of Nb forms the compounds: NbFe and Nb2Fe3, prevents the formation of a -Fe and refines the grains. Cu mainly distributes in Nd-rich phases, sometimes combining with Co which replaces Fe of matrix mainly. Dy203 that is added during milling improves the coercivity and prevents the grain growth.
良好的烧结NdFeB的铸锭组织是细小的板条状Nd2Fe_(14)B主相被富钕相均匀的分开，不存在块状的富钕相、α-Fe及其他非磁性夹杂相。α-Fe呈树枝状和颗粒状析出，成分中稀土总量越低、冷却速度越慢、浇铸温度越高，α-Fe越容易析出。甩带法制备的NdFeB铸锭的显微组织细而均匀。
NdFeB的烧结组织中存在成分和形貌不同的富钕相；主相晶粒均匀性越好，颗粒形状越规则，其磁性能越好。晶粒的异常长大严重地恶化烧结体的磁性能，粉末颗粒的尺寸差异越大、烧结温度越高、时间越长、磁粉含氧量越低，晶粒越容易出现异常长大。晶粒度对磁体性能的影响很大，晶粒越细，磁体的磁能积和矫顽力都越高。
粉末形状的不规则性和不均匀性对烧结组织和磁性能影响很大。气流磨制得的粉末形貌规则性、颗粒的尺寸分布较差：球磨制得的粉末形状更差，存在长条状的大颗粒，甚至出现了没有完全破碎的大块。得出了一种取向度计算方法，该方法得出的取向度值与X射线衍射谱所反映的取向情况比较相符。氧含量与工艺过程密切相关，一般来讲，气流磨+防氧化剂制粉可显著降低氧含量，提高磁性能。
加入一些合金元素可以改善烧结NdFeB的磁性能。Dy的加入对提高矫顽力非常显著，加入3.8％Dy后，矫顽力可达18kOe。Nb加入到合金中形成了NbFe和Nb_2Fe_3两种类型的化合物，在一定程度上阻碍了α-Fe的析出，细化了烧结体的晶粒。Cu主要分布在富钕相中，有时与Co伴生，而Co主要进入了主相。在制粉时加入Dy_2O_3，可以改善烧结体的矫顽力，阻碍了颗粒的长大。
The microstructure of ingots and sintered magnets of NdFeB and the effective factors on the magnetic properties have been investigated by means of optical metallurgical analysis, SEM, EDAX, X-ray diffraction analysis, determination of oxygen by infrared analysis and the measurement of density and magnetic properties. The sintered NdFeB magnets with high coercivity have been prepared by traditional powder metallurgy, and the magnets with Br=l. 229T, H.j=12. 87kOe, (BH)粁=35. 2MGOe and Br=l. 162T, Hcj=17. 24kOe, (BH)MI=32MGOe have been gained.
The results are as following:
The ideal microstructure of NdFeB ingots is composed of fine Nd2Fe,4B lath uniformly divided by Nd-rich phase, without block Nd-rich phase, a -Fe and other nonmagnetic phases. The lower the total rare earth is, the slower the cooling rate is and the higher the casting temperature is, the easier a -Fe with dendritic and granular character separates out. The microstructure of NdFeB ingots prepared by strip cast is fine and uniform.
Nd-rich phase with difference composition and form exits in NdFeB sintered magnets. The better the matrix distributes and the more regular the grains are, the higher the magnetic properties. The abnormal grain growth deteriorates the magnetic properties seriously, which has close relation with the size distribution of powder, the sintering temperature and time and the oxygen content, etc. The grain size has important effects on the magnetic properties. The finer the grain is, the higher the magnetic properties.
The regularity and size distribution of powders have great effects on the magnetic properties. The powder produced by jet milling has bad regularity and size distribution, which are worse for the powder produced by ball milling. A method calculating alignment degree has been gained, the results achieved by which agrees with the alignment reflected by X-ray diffraction patterns. The oxygen content has close relations with the process. Generally speaking, the oxygen content can be reduced notably by jet milling with additions against oxide.
The additions of some elements can improve the magnetic properties of sintered
NdFeB. When 3. 8%wt Dy is added, the coercivity can reach 18kOe. The addition of Nb forms the compounds: NbFe and Nb2Fe3, prevents the formation of a -Fe and refines the grains. Cu mainly distributes in Nd-rich phases, sometimes combining with Co which replaces Fe of matrix mainly. Dy203 that is added during milling improves the coercivity and prevents the grain growth.
引文
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