A.C. Neiva

Chemical composition and coercivity of SmCo5 magnets

The effects of alloy composition and heat treatment on the intrinsic coercivity iHc of SmCo5 magnets were studied. Alloys having six chemical compositions near that of stoichiometric SmCo5 were used to produce magnets via the usual powder metallurgy techniques. Magnets were either as sintered (1150 °C) or sintered (1150 °C) and treated at 850 °C. The substantial increase in iHc due to the 850 °C heat treatment occurs reversibly and with a negligible change in lattice parameters, Curie temperature Tc and anisotropy field HA. Quantitative metallography and thermomagnetic measurements showed that the microstructural constituent present in addition to the SmCo5 phase in hyperstoichiometric alloys is composed of both the Sm2Co7 phase and the Sm5Co19 phase. Measurements of magnetization in dc magnetic fields up to 33 T suggest that HA is around 53 T, considerably higher than previously reported values. The coercivity of the magnets is discussed in terms of thermal equilibrium populations of lattice defects.

Phase diagram of the PrFeB system

Abstract The PrFeB phase diagram was determined in the low boron regions of the ternary system. It is presented and discussed in terms of four vertical sections, a polythermic projection and a Scheil diagram. Six ternary invariant reactions are found in the region investigated: a peritectic maximum, a eutectic maximum, three transition reactions, and a degenerate reaction. The final solidification sequence, with the degenerate reaction, can also be applied to NdFeB.

A model relating remanence and microstructure of SmCo5 magnets

Abstract The question of remanence in SmCo5 sintered magnets is reviewed, giving special attention to the influence of microstructure. In SmCo5 magnets, the microstructural constituents are the matrix phase SmCo5, another ferromagnetic phase (Sm2Co7), oxides (Sm2O3), a recently reported carbide SmCoC2 and pores. A method to estimate alignment via the ratio Jr/Js from remanence measurements and microstructural analysis is proposed. The results agree very well with the degree of alignment calculated from Schulz pole figures. During sintering it was observed that the nucleation of SmCo5 on the Sm2Co7 phase is epitaxial. Hexagonal Sm2Co7 is present in our sintered magnets but rhombohedral Sm2Co7 may also be present. The effect of chemical composition (Sm content and oxygen content) on remanence and on the ratio Jr/Js was evaluated. In the sintering step, the densification occurs more slowly for compositions with higher samarium content (or higher Sm2Co7 volume fraction). A model able to calculate the best chemical composition to maximize coercivity and remanence is presented and discussed.

Comparison of metastable NdFe phases and the NdFeAl μ phase

Abstract Metastable ferromagnetic phases with Tc = 245°C, but different morphologies, are found to occur in arc-melted (A1) and annealed (A′1) samples of Nd-rich binary NdFe alloys. The coercivity Hc≈5 kOe encountered in as-cast Nd-15 at% Fe-5 at% B has been attributed to the same A1 + Nd eutectic that is encountered in binary alloys. Both μ of the NdFeAl system and A′1 possess a platelet microstructure, both exhibit magnetic domains and EDS analysis shows that both of these phases contain about 34 at% Nd. Mossbauer spectra of A1, A′1, and μ, obtained at room temperature, can be fitted with very similar sets of hyperfine parameters, indicating an intimate relation between these phases.

Magnetic and microstructural properties of NdFeAl alloys

Abstract Arc-melted samples of Nd-(20-χ)at.%Fe-xat.%Al ( x =1−10) were studied by optical metallography, magnetic measurements and Mossbauer spectroscopy. The coercivity H c , and the Curie temperature T c were determined for the as-cast samples as well as for materials annealed at 600 †C. In order to understand the annealed materials, samples with compositions Nd-58at.%Fe-5at.%Al (600 °C for 20 days) (μ phase) and Nd-47.5at.%Fe-20at.%Al (850 °C for 30 days) (δ phase) were produced and studied. It is found that annealing of arc-melted samples with χ =1−3 produces μ and other phases while, for χ >5, non-magnetic δ is obtained. The implications for permanent magnet production are discussed.

The (SmZr)Co

Detailed microstructural characterization of magnets and homogenized as-cast alloys, which included X-ray diffraction Rietveld analysis, has indicated that the so-called platelet or lamellae phase is (SmZr)₁(CoFeCu)₃ with the PuNi₃ structure and lattice parameters a~0.5 nm and c~2.4 nm. The structural and magnetic properties of the (SmZr)Co₃ phase were investigated. The microstructure shows two phases differing in their Zr/Sm ratio. Magnetization curves for the samples (Sm_0.33Zr _0.67)Co₃, (Sm_0.33Zr_0.67)Co _2.97Fe_0.03, and (Sm_0.67Zr_0.33)Co₃ are consistent with the two-phase microstructure observed. Room temperature coercivity values of these samples are low (ap1 kOe.)

_3

SmCo5 samples were investigated under the following conditions: ‘as-sintered’ at 1150 ◦ C (low coercivity—0.1–0.5 T), heat-treated at 850–880 ◦ C (coercivity ∼2.5–3.0 T) and after heat treatment at 750 ◦ C during 25 days (low coercivity—0.1–0.5 T). Transmission electron microscopy investigation showed that the increase of coercivity at 850–880 ◦ C is not due to second phases precipitated at grain boundaries in the samples nor to the elimination of stacking faults or dislocations. After the heat treatment during 25 days at 750 ◦ C, no clear evidence of eutectoid decomposition of SmCo5 phase was found, but a slight change of lattice parameters was detected.

Phase in Sm(CoFeCuZr)

Abstract Ferromagnetic phases in Pr—Fe and Pr—Nd—Fe were investigated by energy-dispersive X-ray analysis, optical metallography and magnetic measurements. The Pr—Fe phase analogous to Nd 5 Fe 17 was not obtained. In Pr—Nd—Fe alloys, samples with higher Pr:Nd ratios have slower formation rates for (Pr,Nd) 5 Fe 17 .

_rm z

Sm(CoFeCuZr)z commercial magnets are manufactured by powder metallurgy techniques. Microstructural investigations of Sm(CoFeCuZr)z magnets have shown that, increasing the Zr content, some impurity phases may appear. An alloy with composition (at%): 60.5% Co – 15.5% Fe – 11.5% Zr - 8.5% Sm - 4% Cu, homogenized at 1050oC, was investigated. Three main phases were identified: rhombohedral 1:3 (ZrSm)1(CoFeCu)3, hexagonal 1:7 (SmZr)1(CoFeCu)7 and cubic 6:23 (Zr)6(CoFe)23. Knowledge of possible phases present in 2:17-type magnets allows a better understanding of the nanocrystalline microstructure responsible for high coercivity of these magnets.

Magnets

The achievement of high coercivity in 2:17 magnets depends upon the presence of Zr, although there is some disagreement as to its role. The important role of Zr in stabilizing the high temperature 1:7 phase has been emphasized. It has been proposed that the function of the Zr might be to improve the imperfect shape of the cell walls and thereby increase coercivity. A Rietveld refinement of an X-ray diffraction spectrum was performed using Topas software. This analysis revealed that the sample consisted mainly (~70%) of a rhombhohedral 1:3 phase with lattice parameters a = 0.5 nm and c = 2.4 nm.