T.S. Chin

Electrochemical behavior of rare-earth magnet alloys in various solutions

Electrochemical behavior of rare earth magnet alloys SmCo/sub 5/, Sm/sub 2/TM/sub 17/, and Nd-Fe-B in various aqueous solutions was investigated. Although they are very reactive both in air and aqueous solutions, passivation is still possible in orthophosphoric acid. The ability to passivate, the passivated current density, and breakdown voltage of the passivated films were found to be related to the content of rare earth element, single- or multiple-phase state, and iron content of the alloys, respectively. Galvanic current density of the coupled pairs between one of the magnet alloys and one of the biomedical alloys (amalgam, Co-Cr, and Ni-Cr alloys) was found to be within 3-15 mu A/cm/sup 2/ in synthetic saliva. However a large value of 25 mu A/cm/sup 2/ between a Nd-Fe-B magnet and a Ni-Cr alloy was observed. >

Spinodal decomposition and magnetic properties of Fe–Cr–12Co permanent magnet alloys

This report is concerned with the spinodal decomposition and magnetic properties of Fe–21∼30 Cr–12Co ternary alloys (by weight) as studied by transmission electron microscopy and magnetic measurements. Wavelength growth of the isothermally aged spinodal structure is found to be proportional to time1/3 for the later stage of aging at all aging temperatures, which suggests a coarsening process. Spinodal temperatures of these alloys are found to be higher than each of their Curie temperatures; nevertheless magnetic aging is effective in greatly improving the magnetic properties. Hence, a prerequisite of a higher Curie temperature in the magnetic aging theory is concluded to be unnecessary. Moreover, as the extrapolated onset spinodal wavelength, of 80–100 A, is superparamagnetic in nature, this suggests a prerequisite for the magnetic aging theory. Before magnetic aging can be operative, the particle should grow to a certain critical size, determined by the aging temperature and the anisotropy energy at that...

The anomalous phase instability near the Curie temperature of the Fe‐Cu system

Taking into account the effect of the magnetic free energy, the phase stability (i.e., the onset of Cu‐rich precipitation) at the Fe‐rich end of the Fe‐Cu binary system near the magnetic ordering (Curie) temperature has been evaluated with a regular solution model and experimental verification. The calculated results show that the stability of alpha iron is significantly lowered near the Curie temperature, and the solvus of copper in alpha iron is bent toward the Curie temperature line, hence a protrusion should be added to the known phase diagram. Microhardness indentation tests, lattice constant determination by the careful x‐ray diffraction (XRD), and transmission electron microscopy (TEM) were carried out on the solution heat treated and then aged Fe‐Cu alloys containing 0.25‐, 0.95‐, and 1.5‐at. % Cu. All the three experimental results confirm the calculated ones. The Cu‐rich precipitate is identified by TEM on an alloy quenched from the protruded region.

Magnetic properties and microstructures of Fe-Cr-10 wt% Co-M (M= Si/Ti/Ni/Mo/Ge/Ta) permanent magnet alloys

Abstract Effect of alloying elements and heat treatments on magnetic properties and microstructures of Fe-Cr-10 wt% Co alloys was studied by magnetic measurements and transmission electron microscopy (TEM). Among the alloying elements tried, it was found that only Si and Ti can greatly improve the hard magnetic properties of the Fe-Cr-Co alloys. An Fe-10Co-26Cr alloy added with 0.5 wt% Si-0.5 wt% Ti showed the best combination of magnetic properties of H c = 43 kA/m. B r = 1.42 T, ( BH ) m = 41 kJ/m 3 . The variation in magnetic properties was correlated to quantitative microstructural features, particle size and aspect ration. It was found that for alloys of the same composition, H c is determined by shape anisotropy induced by magnetic aging.

Effect of alloying on magnetic properties of Fe-Cr-12 wt% Co permanent magnet alloys☆

Abstract Effect of alloying additional elements (Al, Si, Ti, V, Ni, Cu, Zr, Nb and Mo) with Fe-22/30 wt% Cr-12 wt% Co permanent magnet alloys manufactured by magnetic aging has been systematically studied with magnetic measurements and transmission electron microscopy (TEM), on a laboratory scale. It is found that the alloying shifts the optimum Cr content to a lower lever. Ti and Si show the greatest enhancement on magnetic properties yet narrow the optimum magnetic aging temperature range. Mo, Nb and Cu, when used together with Si, promote magnetic properties and widen the optimum magnetic aging temperature range. Ni and Zr are simply magnetic diluents. V is good for the magnetic properties only at a large amount of addition. The origin of alloying effects explored by TEM microscopy is attributed to the degree of particle entanglement as well as elongation.

Deteriorating effect of manganese on magnetic properties of Fe-Cr-Co permanent magnet alloys

Effect of manganese on magnetic properties of an Fe-24Cr-12Co-1.5Si alloy (weight fraction) has been systematically studied by electron microscopy and magnetic measurements, in order to estimate the availability of certain grades of ferro-chromium as raw material for manufacturing Fe-Cr-Co magnet alloys. It has been found that manganese deteriorates magnetic properties of the Fe-Cr-Co alloys. As the Mn content of the alloy exceeds 0.2%, the degraded magnetic properties become very difficult to recover by heat treatments. The reasons are attributed to, as evidenced from transmission electron microscopy, (1) much inferior enlongation and alignment of the spinodal phase in Mn containing alloys, (2) the possible existence between the spinodally decomposed α 1 and α 2 phases a third phase which degrades the squareness of the B-H loops, (3) the gamma stabilizing effect of Mn. Trials to compensate for the Mn effect by adding sulfur are not pronounced. It is concluded that ferro-chromium or any other raw materials for manufacturing Fe-Cr-Co permanent magnets should be free from manganese.

The phases and magnetic properties of (Ti, Co), and Cr doped Zn2Y-type hexagonal ferrite

Abstract The phases and magnetic properties of Y-type hexagonal ferrite, Ba 2 Zn 2 (Ti, Co) y Fe 12−2 y O 22 doped with two sets of ions, (Ti, Co) and Cr were studied. In (Ti, Co) - doped ferrites the second phase appears at y ⩾ 0.6, which is a spinel type with the formula of (Zn 1−η Co η )(Fe 2−δ Co δ )O 4 . Two resonant peaks are observed in ESR studies at the fields of 1020 and 2430 Oe, respectively, at a frequency of 9.684 GHz. The linewidth increases with the addition of the dopants. In chromium doped ferrite, two phases are identified as the amount of chromium is up to 0.2: spinel type of Zn(Fe 2−ϵ Cr ϵ )O 4 and orthorhombic BaCr 2 O 4 . Although the amount of Cr used does not influence the resonant field of the unique peak of the derivative curves from ESR, it eventually enlarges the linewidth.

Study of SmTM7•3 microstructure by a simplified method and its correlation to properties

By using a newly developed potentiostatic etching technique, the microstructural evolution of an SmTM7.3 (TM=Co/Fe/Cu/Zr) alloy during different stages of heat treatment has been easily and successfully observed under a conventional scanning electron microscope (SEM). Specimens up to several centimeters were examined, with their features being statistically measured. It is found that the cell structure has been well developed with a median size of 110 nm after aging 2 h at 1123 K, while the intrinsic coercivity (iHc) was as low as 2 kOe. The cell size grows slowly during subsequent heat treatment to a median size of 220 nm after the whole treatment. For aging at 1123 K for 4 h, iHc value is around 4 kOe, and the platelet phase appears. After aging at 1123 K for 8 h, a large zonal phase and grain boundary phase are visible. It is thus believed that the zonal phase appears after aging for a time period between 4 and 8 h, corresponding to the iHc of 600 kA/m (7.5 kOe). No new microconstituent appears after t...

The effect of carbon on magnetic properties of an Fe-Cr-Co permanent magnet alloy

The effect of carbon on magnetic properties of an Fe- 24Cr-12Co-1Si-1Ti alloy with carbon content from 0.012 to 0.291 percent (by weight) was studied. It has been found that for a carbon content larger than 0.05 percent there is an abrupt degradation in magnetic properties for alloys treated according to a normal procedure to develop the magnetically optimum state. The reasons have been thoroughly explored by optical metallography, X-ray diffractometry, transmission electron microscopy, carbide extraction, and analysis and hardness measurements. Carbon stabilization of the γ phase, the amount of which increases with carbon content, is the main deteriorating factor found in this alloy. Abnormal spinodal decomposition between α phase and non-α-phase regions and Cr-rich carbide precipitated in or near α-phase impair magnetic properties further. It has been shown that prolonged solution treatment at temperatures near the solidus can compensate part of the carbon effect at the expense of (B-H)m value. Hence, for practical purposes, the 0.05 percent C is set to be the upper limit for ensuring (B-H)m value greater than 5.5 MGOe for this alloy, and the 0.05 percent C limitation is suggested to be common to all Fe-Cr-Co permanent magnet alloys.

Novel method for studying the complex microstructure of a Sm(Co/Fe/Cu/Zr)7.3 permanent magnet alloy

Abstract A novel and simple method is proposed for investigating the complex microstructure of a Sm(Co/Fe/Cu/Zr)7.3 permanent magnet alloy. It makes use of potentiostatic etching in concentrated orthophosphoric acid at specific potentials to reveal the platelet, zonal, and/or grain boundary phases so that they can be viewed by conventional scanning electron microscopy. The whole procedure can be accomplished within a short period of time, no sophisticated techniques are involved, and bulk specimens up to several cm in diameter can be examined. Details of the etching technique are described and scanning electron micrographs are compared with transmission electron micrographs from the same specimen.