S. Pandian

Effect of Al, Cu, Ga, and Nb additions on the magnetic properties and microstructural features of sintered NdFeB

This study describes the relative effect on the permanent magnet characteristics viz. remanence (Br), intrinsic coercivity (Hci), Curie temperature (TC), and rectangularity of the intrinsic demagnetization curve, when Al, Cu, Ga, and Nb are added individually to NdFeB. Each elemental addition causes significant improvement in Hci but the level of improvement differs from one additive element to the other. The addition of Nb is favored over other elements for realizing maximum enhancement in Hci and rectangularity of the demagnetization curve. The microstructural features of the sintered samples of NdFeB with elemental addition show the formation of a new phase, in addition to the phases (φ,η, and Nd-rich) generally found in the ternary sample. The factors influencing the permanent magnet characteristics of sintered samples are the distribution of the Nd-rich phase in the intergranular region, the size and distribution of the minor phases at the grain junctions, the formation and distribution of new phases...

Microstructure and magnetostriction of Tb0.3Dy0.7Fe1.95 prepared under different solidification conditions by zoning and modified Bridgman techniques

An intermetallic compound of nominal composition Tb0.3Dy0.7Fe1.95 was conventionally cast in the form of cylindrical rods of 8mm diameter and directionally solidified by zoning at three different growth rates. Compounds of the same nominal composition were also directionally solidified in the form of cylindrical rods of 20mm diameter by modified Bridgman technique, at two different growth rates. The microstructure of the directionally solidified compounds has been investigated as a function of solidification rate and compared with that of conventionally cast compound. The observed microstructural features of these samples have been correlated with the magnetostriction measured on the corresponding samples. Further, by examining the longitudinal sections cut along its cylindrical axis, a correlation of microstructure with magnetostriction has been brought out for each directionally solidified sample as a function of distance from the initially solidified end to the other. It has been observed that a large ...

Effect of Co, Dy and Ga on the magnetic properties and the microstructure of powder metallurgically processed Nd–Fe–B magnets

Abstract The elemental additions to Nd–Fe–B for the simultaneous enhancement of the Curie temperature (TC), intrinsic coercivity (Hci), remanence (Br) appear to be the viable option for improving the temperature stability of the magnetic properties. In this work, an attempt has been made to understand the effect of individual and successive additions of Co, Dy and Ga on the degree of variation in these magnetic properties and in the microstructure of anisotropic Nd–Fe–B magnets. Co additions increase the Curie temperature in proportion to its concentration in the alloy but reduces the coercivity, Hci. Formation of new phases such as Nd(FeCo)2 and Nd(FeCo)3 in Co added alloy has been proved by X-ray diffraction and SEM-EDAX/EPMA studies. From the thermo-magnetic plots, the new phases appear to be ferromagnetic, which may be responsible for easy demagnetization and reduction in Hci. The increase in TC is attributed to the solubility of Co in the Nd2Fe14B (φ phase). With the additions of Dy and Ga, solubility of Co in φ phase increases. In 36Nd–1.2B–7Co–Fe, with successive additions of 4% Dy and 1% Ga, a significant improvement in Hci (from 300 to 970 kA/m) along with a marginal improvement in TC (from 695 to 715 K) has been observed. The changes in the composition of the φ phase as well as the improvement in the microstructural features are attributed to the enhancement in TC and Hci in Co, Dy and Ga substituted Nd–Fe–B magnets.

Pressure-magnetic field induced phase transformation in Ni46Mn41In13 Heusler alloy

The effect of hydrostatic pressure and magnetic field on the magnetic properties and phase transformation in Ni46Mn41In13 Heusler alloy was investigated. Pressure (P)-magnetic field (H)-temperature (T) phase diagram has been constructed from experimental results. In the P–T contour of the phase diagram, the slope of the austenite-martensite phase boundary line appears positive (dT/dP > 0), while it appears negative (dT/dH < 0) in the H–T contour. The results revealed that pressure and magnetic field have opposite effect on phase stabilization. The combined effect of pressure and magnetic field on martensitic transition has led to two important findings: (i) pressure dependent shift of austenite start temperature (As) is higher when larger field is applied, and (ii) field dependent shift of As is lowered when a higher pressure is applied. The pressure and magnetic field dependent shift observed in the martensitic transformation has been explained on the basis of thermodynamic calculations. Curie temperatur...

Microstructure and magnetostriction of melt-spun Fe73Ga27 ribbon

Melt spun ribbon of Fe73Ga27 was prepared and characterized for microstructural features and magnetostriction. The phase equilibria observed from structural investigations have been correlated with Mossbauer studies. The magnetostriction of the melt spun ribbon has been found to be significantly large compared to the bulk sample. The large magnetostriction is attributed to the absence of ordered fcc (L12) phase in the melt spun ribbon, which is otherwise seen in slow cooled bulk Fe73Ga27 alloy.

Effect of B on the microstructure and magnetostriction of zoned Dy0.7Tb0.3Fe1.95

It had been reported that B has beneficial effects on the magnetostriction of anisotropy compensated Dy0.7Tb0.3Fe2 alloys. In the present work, Dy0.7Tb0.3Fe1.95 and Dy0.7Tb0.3Fe1.95Bx (x=0.1, 0.15, and 0.2) alloys, in the form of 8mm diameter cylindrical rods, were investigated both in the as-cast and zoned conditions. At 5kOe dc magnetic field, magnetostriction of ∼1200 and 900 microstrains was observed for zoned samples with x=0 and 0.1, respectively. For zoned samples with x>0.1 and for the as-cast sample of the parent alloy, magnetostriction of ∼500 microstrains was measured. X-ray diffraction of the zoned x=0 and x=0.1 samples showed a strong prevalence of the ⟨112⟩ and the ⟨110⟩ grain orientations; in contrast, the other samples appeared to possess a random grain orientation. Optical and scanning electron microscopy studies revealed a significant increase in the volume fraction of the RFe3 phase with increasing boron content. This increase in the volume fraction of the (Tb,Dy)Fe3 phase and the reduc...

Investigation on the Microstructure, Texture and Magnetostriction of Directionally Solidified Alloys

Effect of V addition on the microstructure and magnetostriction of directionally solidified Tb0.3Dy0.7Fe1.95 has been investigated. The microstructure of V added alloys (Tb0.3Dy0.7 with , 0.025, 0.05, and 0.075) indicate that Fe-50 at.% V is formed as primary phase, which subsequently undergoes spinodal decomposition. The spinodially decomposed Fe-rich phase reacts with the liquid and forms the matrix phase, (Tb,Dy)Fe2. The V-rich spinodally decomposed product, on the other hand, exists as remnant phase without undergoing any metallurgical transformation. Texture studies indicate that the grains of (Tb,Dy)Fe2 show /rotated and orientations for all compositions investigated in the directionally solidified condition. An improvement in magnetostriction has been noticed for small addition of V and with further addition the magnetostrictive property decreases. The formation of additional phases containing vanadium is attributed to be the reason when V is added in higher concentration levels.

Favorable Effect of Ho Addition on the Microstructure and Magnetostriction of Textured Tb0.3Dy0.7Fe1.95

This paper reports the effect of Ho on the microstructure, texture and magnetostrictive properties of Tb0.3Dy0.7-xHoxFe1.95 (with x = 0, 0.05, 0.1, 0.15 and 0.2) alloys. The alloys were vacuum induction melted and directionally solidified by modified Bridgman technique. Ho addition has been found to improve the magnetostriction at lower concentration due to absence of deleterious pro-peritectic (Tb,Dy)Fe3 phase. The static strain co-efficient is found to be better in Ho added alloys and the prevalence of strong and texture is attributed to be the reason.

Evolution of Texture during Directional Solidification of Giant Magnetostrictive Tb0.3Dy0.7Fe1.95 Alloy

The development of preferred grain orientation has been investigated in the directionally solidified samples of Tb0.3Dy0.7Fe1.95 as a function of pulling speed viz. 10, 40, 70 and 100 cm/h. The study indicates that at lower solidification rate (10 cm/h) growth of and texture components are preferred, whereas, texture component becomes dominant at higher pulling rate (100 cm/h). However, as the solidification progresses, growth of texture component is observed subduing the other components. Consequently, the magnetostriction improves from 1100 to 1350 micro-strains with higher pulling speed.

Structural And Magnetic Properties Of Ho1−xTbxFe1.95[x = 0.0,0.25, 0.5, 0.75 And 1.0]

Alloys of Ho1−xTbxFe1.95 [x = 0, 0.25, 0.5, 0.75 and 1] were prepared and investigated for the structural and magnetic properties. A three phase microstructure consisting of Laves phase (Ho,Tb)Fe2 as matrix, and (Ho,Tb)Fe3 and (Ho,Tb)‐rich as minor phases has been observed for alloys x>0.5. For alloys x⩽0.5, the (Ho,Tb)Fe3 phase is non‐existent. The lattice parameter and Curie temperature of the alloys are found to increase with Tb addition. At ambient condition, the easy magnetization direction lies along 〈100〉 for HoFe1.95 (x = 0) and changes to 〈111〉 for Tb substituted alloys [x = 0.25–1].