4.5 Influence of Boron Content in NdFeB Alloys on \(Nd_{2}Fe_{14}B\) Phase Formation and Magnetic Properties
Investigated Ce - Fe - B phase diagram in 1972. Afterwards Вилонжко worked out Y - Co - B, Ce - Co - B and Sm - Co - B ternary phase diagrams one after the other, in 1976 he worked out Co - La - B phase diagram, and in 1979 worked out Fe - Nd - B, Fe - Sm - B and Fe - Gd - B phase diagrams.
In 1980 Croat, Koon, Beckor, and C. C. Hadjipanayis investigated Fe - Pr and Fe - Nd series microcrystal permanent magnets. It found the coercivity of 7.5 kOe (596.8 kA/m), \((BH)_{max}=3 - 4\) MGs·Oe \((23.9 - 31.8\ kJ/m^{3})\) in \(Nd_{0.4}Fe_{0.6}\) and \(Pr_{0.4}Fe_{0.6}\). Alloys prepared by J. J. Croat. He considered that the produce of coercivity is related to formation of metastable microcrystal structure. In 1981, N.C. Koon, C. M. William, B.N. Das reported that the coercivity reached 9 kOe (716.2 kA/m) after crystallizing amorphous \(Tb_{0.05}La_{0.05}(Fe_{0.82}B_{0.18})_{0.9}\) at 930K. In 1983, C.C. Hadjipanayis and R. Hazelton prepared US Army worked out \(Pr_{16}Fe_{78}B_{6}\) with the intrinsic coercivity of m\(H_{c}=15\) kOe (1193.7 kA/m), \((BH)_{max}=13\) MGs·Oe \((103.4\ kJ/m^{3})\), and the specimen only containing boron \((Pr_{16}Fe_{78}B_{6})\) also reached the similar level. They pointed out that the crystallized specimen became preferable permanent magnetic phase because \(Fe_{20}RE_{3}B_{6}\) with the square phase has a great of magnetic anisotropy. In June of 1983, Sumitomo Special Metal Corporation announced that successfully developed the \(Nd_{15}Fe_{77}B_{8}\) perma - nent magnet with a high magnetic energy product.
This section is to report that study the function on formation of phase texture by addition of boron and study the relation between variation of alloy microstruc - ture and the coercivity by using electronic microscope, 1000 kV HVEM, X - ray microscope analysis instrument cooperating with magnetic measurement instru - ment.
Specimen Preparation Process and Experimental Methods for Boron-Modified NdFeB Alloys
NdFeB alloy for experiment was made by using normal powder metallurgy method. At first the high purity neodymium, high purity iron and high purity bo - ron was melted in non - consumable arc furnace. In the melting process the furnace was pumped to vacuum in advance and filled argon later. In order to ensure component homogenization the alloy was melted three continually. The melted alloy was cracked roughly at protection of argon atmosphere and pulverized in a ball grinder about 3μm. The powder was formed under 1.5 kGs magnetic field and sintered at 1100°C. After aging at 600°C it was cooled to room tempera - ture as sample for electronic microscope.
The above sample was sliced into lamellae of 0.25 mm in direction perpendicu - lar to \(c\) axis, and thinned to 0.025 mm mechanically, and then the lamella were electrolyzed to open a hole basically in an electrolyte of 20% perchloric acid and 80% glacial acetic acid and finally be thinned and cleaned by ionic thinning appa - ratus. Then the specimen was ready for observation.
Observation of the filmy specimen was carried out in JEM - 1000 HVEM. The operating voltage was 1000 kV, output voltage was 185V, electric current was 6.6 A, vacuum was \(0.7\times10^{-7}\) Torr \((9.3\times10^{-6}\ Pa)\) and the ion beam was 10 μA. The specimen was inserted at first into the side inserting type heating dais of JEM - 1000 in the observation and observed by electronic microscope in the condition of room temperature and heat condition.
Effects of Boron Content on Magnetic Properties and Phase Structure of NdFeB Alloyse
This section investigated the relationship between the B content and the magnetic property. The addition of B (6% - 8%(at.)) facilitates the formation of hard - magnetic phase resulting in high performance magnetic materials.
Variation of magnetism with change of boron content
It can be seen from Table 4.9 and Fig.4.22 that boron content has an important influence on the property of the magnet of NdFeB permanent magnetic alloy. It is seen from the condition of Table 4.9 that the magnetism will be optimal when the boron content is 7% (at.). In fact, preferable magnetic performance can be obtained in the boron content range of 6% - 8% (at.). The addition of boron enables NdFeB alloy to form a square phase.
Using Mössbauer effect to \(Nd_{15}Fe_{85 - x}B_{x}\) (\(x = 0\), 4, 8, 11) we conclude as follows: as the boron (B) content increases, the magnetism of \(Nd_{2}Fe_{14}B\) gradually increases in the alloy. As \(x<4\), there is no appearance of boron - rich phase, and the proportion of \(Nd_{2}Fe_{14}B\) decreases by 24%, and the magnetic performance also decreases. If \(x = 11\), the proportion of \(Nd_{2}Fe_{14}B\) decreases by 5%. That does not affect the intrinsic coercivity. To enhance the quantity of \(Nd_{2}Fe_{14}B\) so as to enhance the saturation magnetization intensity, one should properly control the boron (B) content and alloy ingredients, and properly increasing Fe is an effective means to enhance the proportion of \(Nd_{2}Fe_{14}B\) (Pan, Zhao, Li and Ma, 2011).
Influence of boron content on phase structure
In order to study the formation of strong single - axis anisotropy the following work has been conducted:
- Confeccted specimen of \(Nd_{16}B_{x}Fe_{84 - x}\), while \(x = 0\), 3, 5, 7, 11;
- Carried out phase analysis by X - ray diffraction for above specimen (Fig. 4.23).
The result of component analysis of micro area being carried out by electronic probe is shown in Table 4.10 (the boron element here is unable to be analyzed by this instrument).
It can be seen from above experiment result that without boron the alloy was composed of \(\alpha - Fe\), \(Nd_{2}Fe_{17}\) base phase and Nd - rich phase. The composition of Nd - rich phase, \(\alpha - Fe\) and \(Nd_{2}Fe_{17}\) base phase was changed when boron content of 3% (at.) was added into the alloy. And a new peak, being analyzed as \(Nd_{2}Fe_{14}B\), appeared besides the original \(\alpha - Fe\) and \(Nd_{2}Fe_{17}\). While the alloy with a boron content of 5% (at.) the base phase composition change to be Fe 65.62%, Nd 34.38%, being consistent with \(Nd_{2}Fe_{14}B\) phase, besides the change in percentage of iron and neodymium. Thus increase of boron content promotes formation of quadrangle crystal system and strong magnetic phase of \(Nd_{2}Fe_{14}B\). Also because the neodymium and ferromagnetic moment of ferromagnetic - coupling is along \(c\) axis all of crystalline structures are square and so that an anisotropy structure is conducted for the high coercivity. The main part of intrinsic anisotropy inside of \(Nd_{2}Fe_{14}B\) phase originated from the split of crystal field with \(4f\) energy level of rare earth materials. Thus it can be seen from Fig. 4.22 that the coercivity heightened rapidly because of increase of boron content, and neodymium element enriched degree in crystal boundary increased largely from planar distribution phase of neodymium. When boron content was 7% the \(\alpha - Fe\) and \(Nd_{2}Fe_{17}\) would disappeared referring to X - ray diffraction spectrum, which was also verified by micro - area analysis experiment using electronic probe. There are no \(\alpha - Fe\) and \(Nd_{2}Fe_{17}\) in \(Nd_{16}B_{7}Fe_{77}\) (i.e., 7% (at.) boron content) in Table 4.10, and that neodymium con - tent raised to 82.45% in Nd - rich phase and the neodymium enrichment becomes more obvious in crystal boundary. Then the main phase is the base phase, i.e., \(Nd_{2}Fe_{14}B\). It can be seen from Fig.4.22 and Table 4.9 that this boron content en - ables the magnetic performance to reach the optimum status. J. F. Herbst, J. J. Croat, et al conducted studies on cells in crystal of \(Nd_{2}Fe_{14}B\) and found: there are four \(Nd_{2}Fe_{14}B\) and total 68 atoms in each crystal cell, among them iron atoms are 56, neodymium atoms are 2 and boron atoms are 4; and the boron atoms occupied triangle prism constituted of 3 nearest iron atoms in upside and downside of base plane. All moments of neodymium and iron paralleled with \(c\) axis of square crys - tal cells. The more perfectible configuration is the reason to improve \(T_{c}\) (for \(Nd_{2}Fe_{14}B\) is 627 K, for \(Nd_{2}Fe_{17}\) is 330 K). That only praseodymium and neodym - ium can provide a higher product of magnetic energy. M. Sagawa believed that boron in NdFeB square phase acted as enlarging atomic space of Fe - Fe and reduced effect of the nearest neighbor atomic number of iron, and that addition of boron was the reason to induce rising of Curie temperature. Therefore, controlling ap - propriate boron became a key to obtain excellent magnetic performance. It can be seen from Fig.4.22 and Table 4.9 that when boron content is higher than 7% (at.) i.e., increases to 11% (at.) \(B_{r}\) will be declined and \(_{m}H_{c}\) will be enhanced somewhat, that indicated decline in the product of magnetic energy which indi - cates integrated magnetic performance. And that enriched degree of neodymium in boundary of prismatic crystals (casting status) was decreased referring to pla - nar distribution of neodymium by study using scanning microscope. \(Nd_{2}Fe_{14}B\) peak was weakened by referring to X - ray diffraction spectrum of \(Nd_{16}Fe_{73}B_{11}\) alloy and the phase composition in micro - area was changed, as shown in Table 4.10.
The saturated magnetization intensity of square phase was 15.7 kGs at room temperature (Blonizhko, Kuzma, IVZ, et al, 1974), and that the remanence \(B_{r}\) can be estimated in expression as below as per the square phase in alloy and orienta - tion angle of magnetic moment:
\(B_{r}=M_{s}(V'/V)\cos\theta\) (4.5)
where \(V'\) and \(V\) represent the volume of square phase and volume of the whole specimen, respectively; \(M_{s}\) represent saturation magnetic intensity. The rema - nence of alloy estimated for \(x = 3\), 5, 7, and 11 by the Eq. 4.5 is well consistent with the result in actual measurement and the difference is within 7%.
Conclusions: Understanding the Role of Boron in Tuning NdFeB Magnetic Performance
Conclusion was derived through above studies:
- Boron content in NdFeB permanent magnetic alloy has an important influence on the magnetic performance of the alloy. Appropriate boron content may result in a good magnetic performance.
- When \(B = 0\%\) (at.) the NdFeB alloy is mainly constituted of Nd - rich phase, \(\alpha - Fe\) and \(Nd_{2}Fe_{17}\). When boron content increased from 0% - 7% (at.) the \(\alpha - Fe\) and \(Nd_{2}Fe_{17}\) disappeared and \(Nd_{2}Fe_{14}B\) phase formed gradually. While \(B = 11\%\) (at.) the strength of base phase weakened but there still is \(\alpha - Fe\) and \(Nd_{2}Fe_{17}\). That resulted in degradation in magnetic induction intensity and magnetic performance (but \(_{m}H_{c}\) was raised). Thus controlling boron content to obtain NdFeB permanent magnetic alloys with different brand, different performance in practicality.
- Addition of boron into NdFeB alloy may promote formation of strong magnetic performance phase.
- Observation of NdFeB alloy specimen using 1000 kV HVEM found a filmy belt between crystal boundaries of two hard magnetic square phases. This filmy belt was widened regularly in proportion with rising of temperature ranged from 25 - 600°C.
