D. Akhtar

Effect of processing parameters on the microstructure and soft magnetic properties of Fe88Zr7B4Cu1 alloy ribbons

Structure and soft magnetic properties of melt spun Fe88Zr7B4Cu1 alloy have been investigated using x-ray diffraction, differential scanning calorimetry, transmission electron microscopy and vibrating sample magnetometer. On decreasing the cooling rate the structure of as spun ribbon changes from completely amorphous to a cellular structure of bcc solid solution along with the amorphous phase at intercellular regions. Annealing leads to the precipitation of nanocrystalline bcc-Fe phase from both amorphous phase and already existing bcc solid solution. The saturation magnetization increases sharply for all samples on annealing at 500 °C due to the precipitation of nanocrystalline bcc phase and then remains almost constant at higher annealing temperatures. The coercivity decreases initially with annealing temperature, attains a minimum value and then increases at higher temperature.

Glass formation in the NiCrNb system

The development of metallic glasses which can be produced by rapid-quenching techniques has led to a new class of materials with extraordinary properties [1]. Depending upon the alloy composition, metallic glasses have been found to exhibit any of several attractive properties, such as good corrosion resistance, stability under irradiation, high strength and hardness and interesting magnetic and electrical properties. Many of the metallic glasses studied so far crystallize at relatively low temperatures and structural relaxation of the glass may occur as much as 200 K below the crystallization temperature leading to further deterioration in thermal stability. It is therefore desirable to identify glasses of higher crystallization temperature. Some effort has been made in this direction to develop metal-metalloid glasses [2], but little attention has been paid to glasses containing only metallic elements. Of the known metal-metal glass-forming alloys, the Ni60Nb40 (subscripts refer to at %) composition readily forms a glass of high crystallization temperature (Tx~923K) [3-6]. Since the addition of chromium has been found to be most effective in improving corrosion resistance in metalmetalloid glasses [1], we have investigated the effect of its addition on the glass-forming ability of Ni60Nb40 alloys. Alloy buttons of compositions Ni60CrxNb40 x and Ni60 yCryNb40 (x, y = 0, 5, 10, 13 and 15) were made by arc melting high-purity components in a pure argon atmosphere over a water-cooled copper crucible. The buttons were remelted at least six times to ensure homogeneity. Weight losses during melting were negligible and the nominal compositions were accepted. Rapidly quenched ribbons were prepared in s i tu by the melt-spinning technique. After induction melting in a quartz nozzle of around 1 mm orifice diameter, the molten metal was ejected onto a rotating polished copper wheel (~ 200mm diameter) using an ejection pressure of pure argon. Before melting, the nozzle containing the alloy pieces was repeatedly flushed with argon and an additional argon flow around the nozzle was maintained throughout the experiment by incorporating a concentric quartz tube of larger diameter to prevent oxidation of the melt pool. The ejection pressure and wheel velocity were adjusted to provide around 20 to 25 #m thick and 2 to 3 mm wide ribbons. X-ray-diffraction analyses of the asquenched ribbons were made using a Philips diffractometer with CoK~ radiation at a scanning rate of 1 ° (20)min -j . X-ray-diffractometer traces of the asquenched alloy ribbons are shown in Fig. 1. In the melt-spun Ni60CrxNb40 x alloy ribbons, X-ray diffraction revealed only diffuse intensity maxima characteristics of an amorphous

Isothermal crystallization kinetics of Ni60Nb4o-xCrx, glasses

Isothermal crystallization kinetics of Ni60Nb40-xCrx (x = 0, 5, and 13 at %) glasses was investigated by differential scanning calorimetry. It was possible to separate out the kinetics of formation of various phases which are obtained on crystallization of these glasses. The results are compared with the earlier investigations [1] in which the data were obtained by constant heating rate experiments. The crystallization of M-phase, Ni3Nb and NbCr2 phases could be described by Johnson-Mehl-Avrami kinetics. The activation energies for the formation of these phases were found to decrease in the same order. A decreasing activation energy with increasing transformed fraction (time) was also observed. The results are interpreted in light of the values obtained for Avrami exponents.

Structure and magnetic properties of Nd4.5Fe77−xSnxB18.5 nanocomposite alloys

The effect of partial substitution of Sn(Fe) on the crystallization behaviour, magnetic properties and microstructure of rapidly solidified Nd4.5Fe77?xSnxB18.5 (x = 0.0, 0.5, 1.0 and 1.5) nanocomposite alloys has been studied. Sn decreases the crystallization temperature and suppresses the formation of the undesired Nd2Fe23B3 phase. Remanence (Mr), remanence ratio (Mr/Ms) and maximum energy product (BH)max increase from 10.1?kG, 0.70 and 9.1?MG?Oe to 12.6?kG, 0.82 and 12.8?MG?Oe, respectively, on addition of 1.5% Sn. The coercivity (iHc) decreases slightly from 2.6 to 2.2?kOe with an increase in Sn from x = 0.0 to x = 1.5. TC of the Nd2Fe14B increases from 588 to 602?K. The results are discussed and correlated with crystallization behaviour, microstructure and M?ssbauer studies.

Crystallization of amorphous Ni60 Nb40−x Cr x alloys

Crystallization behaviour of amorphous Ni60 Nb40-x Crx (x = 0, 5, 10 and 13 at%) alloys was studied by differential Scanning calorimetry and X-ray diffraction measurements. It is shown that the addition of chromium reduces the crystallization temperature, stages of crystallization and activation energies associated with the crystallization sages of the Ni60Nb40 glass. Crystallization of the Ni60Nb40 glass occurred in three stages; in the initial stage a metastable M-phase formed in the amorphous matrix as reported earlier [1] . However, contrary to earlier observation [1], M -phase was not very stable and transformed together with some amorphous phase to the equilibrium Ni3Nb phase in the second stage of crystallization. In the third stage, the remaining amorphous matrix transformed to the equilibrium NiNb phase. On addition of chromium the formation/stability of the M-phase was found to be suppressed and equilibrium NbCr2 phase precipitated preferentially in the first stage. The second stage, corresponding to the formation of Ni3Nb phase, remained almost unaltered. The third stage corresponding to the crystallization of NiNb phase disappeared completely at 13 at% Cr. In the fully crystallized samples the proportion of the NiNb phase decreased and that of NbCr2 phase increased continuously with chromium concentration.

The effect of substitution of Nd by Pr on the magnetic properties of melt-spun Nd?Fe?B nanocomposite alloys

The effect of partial substitution of Nd by Pr on the magnetic properties of exchange-coupled Fe3B/Nd2Fe14B nanocomposites with compositions (Nd1−yPry)4.5Fe77B18.5 (y = 0, 0.22, 0.45 and 0.66) has been investigated. Microstructural studies were carried out and correlated with the hard magnetic properties. The substitution of Nd by Pr improves iHc and (BH)max from 0.264 T and 73 kJ m−3 in the Pr-free alloy to 0.363 T and 106 kJ m−3, respectively, in the y = 0.45 alloy. Remanence magnetization decreases marginally on substitution. An increase in the Pr content beyond y = 0.45 leads to a deterioration of the hard magnetic properties, possibly due to a weakening of the exchange coupling interactions between the hard and soft magnetic phases.

Magnetic Properties of Melt Spun Fe3B/Nd2Fe14B Nanocomposites Containing Mn

The magnetic properties of Nd4.5Fe77−xMnxB18.5 (x = 0, 1 and 2) nanocomposites prepared by crystallization of amorphous precursors was investigated. Coercivity (iHc) and remanence ratio (mr = Mr/Ms) has been improved for 1 at% Mn addition for Fe. The temperature dependence of coercivity is analyzed and the best linear fit Hc(T)/Ms(T) vs. HNmin(T)/Ms(T) indicates that Hc controlled by nucleation process. The microstructural parameters αkαex and Neff were determined for these composites.

Structural relaxation and embrittlement of Fe74Co10B16 and Fe74Co5Cr5B16 metallic glasses

Structural relaxation and annealing embrittlement behaviour of Fe74Co10B16 and Fe74Co5Cr5B16 metallic glasses has been studied by differential scanning calorimetry (DSC) and bend ductility measurements. A pre-anneal technique was employed with DSC to determine activation energies of relaxation at various temperatures. Activation energies of embrittlement were derived from measurements of the embrittlement kinetics. The results obtained for both the alloys are compared to ascertain the effects of chromium addition. A spectrum of activation energies is obtained corresponding to structural relaxation and embrittlement. The activation energies of structural relaxation are found to be slightly higher for the chromium-containing alloy than for the ternary alloy. This observation is consistent with the higher crystallization temperature of the chromium-containing alloy as reported earlier. The ductile-brittle transition temperature of the Fe74Co10B16 glass, however, decreases by ≈ 50 K (for 15 min anneal) on addition of 5 at% chromium. Activation energies for embrittlement of the chromium-containing alloy are also considerably smaller than for the ternary alloy. It is concluded that despite increasing the thermal stability, chromium reduces the mechanical stability of Fe74Co10B16 glass.

Mössbauer studies of a-Fe74Co10-xCrxB16 alloys

Fe57 Mössbauer spectroscopy has been employed to study hyperfine interactions in melt-spun a-Fe74Co10−xCrxB16 (x = 0, 2.5, 5, 7.5 and 10) ribbons. The Curie temperature and the effective hyperfine fields observed for the spectra at 80 K show a systematic fall with increasing chromium content. Increasing chromium content also brings about a change in the orientation of the magnetization axis in that it comes closer to the direction of γ-ray propagation. The temperature dependence of hyperfine fields in a-Fe74Co5Cr5B16 alloy is accounted for using the existing theoretical models. The field distribution curves show a temperature dependence, while the e.f.g. distribution evaluated from the paramagnetic doublet spectra aboveTc are temperature independent. The spin-wave coefficients B (B3/2) and C (C5/2) and the critical exponent, β, were determined and their significances explained.

Transport properties of some transition-metal-based amorphous alloys

Abstract The electrical resistivity and thermoelectric power of amorphous Ni 60 Nb 40− x Al x alloys have been studied over a temperature range of 300 to 600 K. A negative temperature coefficient of resistivity (TCR) is observed for the compositions possessing higher resistivity. The thermoelectric power of the amorphous alloys is small but positive and it increases linearly with temperature. The temperature dependence of both the properties is explained on the basis of Zimans theory.