Rajat K Roy

Crystallization behavior and high temperature magnetic phase transitions of Nb-substituted FeCoSiBCu nanocomposites

The effect of Nb substitution on the nanocrystallization process and high temperature magnetic properties of FeCoSiBCu nanocomposites is reported. Magnetization changes accompany the primary crystallization of α-Fe(Co) nanocrystals and the secondary crystallization of (FeCo)23B6 and (FeCo)2B phases. With increasing the Nb fraction in the alloys, the diffusion barrier to growth of α-Fe(Co) nanocrystals increases, resulting in an increase of thermal stability and a delay of Fe(Co) dissolution required for (FeCo)23B6 phase formation. The transmission electron microscopy images reveal finer grains with increasing Nb content.

Heat treatment induced martensitic accommodation and adaptive anisotropy in melt spun Ni55Mn22Ga23 (at. %) ribbons

The effect of annealing on melt spun Ni55Mn22Ga23 (at. %) ribbon has been addressed in terms of structural, thermal, thermomagnetic, and magnetic field induced strain (MFIS) behaviour. In comparison to as-spun ribbons, the samples annealed within the L21 domain at 1073 K for 30 h with subsequent furnace cooling, showed a rise in martensitic transformation temperature. This was endorsed from electron microscopy studies revealing change in morphology of martensite plate and stacking of dislocations in a preferential orientation. The mechanism is associated to martensitic accommodation which enhanced ferro-elastic magnetic coupling and also lowered magnetic coercivity. Such accommodations and adaptive anisotropy improved the MFIS behavior in the annealed sample.

Effect of P addition on nanocrystallization and high temperature magnetic properties of low B and Nb containing FeCo nanocomposites

The P content dependencies of the nanocrystallization behaviors and high temperature magnetic properties of the Fe54.6Co29.4Si2.8(B0.8−YPY)14Nb1Cu1 (Y = 0, 0.2, and 0.3) alloys have been investigated. Alloys were prepared by melt spinning and subsequently annealed in an argon atmosphere to induce nanocrystallization. P addition increases primary crystallization temperature (Tx1), thermal stability (ΔTx), and activation energy (QJMA) for secondary crystallization in as-cast alloys. The saturation induction (Bs) of 1.68 T for as-cast P free alloy decreases continuously with the addition of P. However, the soft magnetic properties are enhanced for P added alloys. The XRD pattern reveals that grain refinement increases with increasing P contents. Alloys annealed at 430 °C confirm primary nanocrystallization of α-FeCo in the amorphous matrix, while annealing at 550 °C causes secondary crystallization of other non-magnetic phases as well. The magnetic moment of as-cast and annealed alloys, measured by vibrating...

Temperature-Dependent Giant Magnetoimpedance Effect in Amorphous Soft Magnets

Giant magnetoimpedance (GMI)-based devices offer potential as next-generation low-cost, flexible, ultrasensitive sensors. They can be used in applications that include current sensors, field sensors, stress sensors, and others. Challenging applications involve operation at high temperatures, and therefore studies of GMI temperature dependence and performance of soft magnetic materials are needed. We present a high-temperature GMI study on an amorphous soft magnetic microwire from room temperature to 560°C. The GMI ratio was observed to be nearly constant at ∼86% at low temperatures and to decrease rapidly at ∼290°C, finally reaching a near-zero value at 500°C. The rapid drop in GMI ratio at 290°C is associated with a reduction in the long-range ferromagnetic order as measured by the spontaneous magnetization (M) at the Curie temperature (Tc). We also correlated the impedance with the magnetic properties of the material. From room temperature to 290°C, the impedance was found to be proportional to the square root of the magnetization to magnetic anisotropy ratio. Lastly, M(T) has been fit using a Handrich–Kobe model, which describes the system with a modified Brillouin function and an asymmetrical distribution of exchange interactions. We infer that the structural fluctuations of the amorphous phase result in a relatively small asymmetry in the fluctuation parameters.

Interplay of stress, temperature, and giant magnetoimpedance in amorphous soft magnets

Giant Magnetoimpedance (GMI)-based sensing devices have attracted attention from both academia and industry due to their low cost, flexibility, and excellent sensitivity. Potential applications range widely from current and stress sensors, navigation systems, magnetic recording, to more demanding ones such as field sensors for deep drilling and oil fracking at elevated temperature. To realize the latter, the temperature dependence of GMI effect must be well understood. Herein, we report a study on the GMI effect in a Cobalt-based amorphous microwire under temperature cycles between 20 °C–560 °C. The GMI ratio was observed to decrease from 126.1% at 20 °C to 68.5% at 230 °C, rapidly drop at ∼290 °C and reach a near zero value above 320 °C in the first half of the measurement where the temperature was increased. Upon cooling down from 560 °C to 20 °C, the GMI ratio exhibits little variation at ∼95% in the 260 °C–20 °C regime. Similarly, the anisotropy-temperature profile was also observed to change irrevers...

Development of high strength stainless steel brazed joints using rapidly solidified filler alloys

Abstract Two types of rapidly solidified filler alloys of nominal composition Cu–40Mn–10Ni (C50) and Ni–7Cr–3·2B–4·5Si–3Fe (N82) were used for stainless steel (SS304) brazing joints. The C50 foil is crystalline in nature, whereas N82 foil shows amorphous structure. The SS304/C50/SS304 joint shows solid solution phases at interfacial area, with maximum bond strength of 500 MPa, which qualifies to 80% of base metal strength. Conversely, the SS304/N82/SS304 joint develops brittle CrxBy intermetallic phases, which lowers bond strength to 330 MPa.

Failure Analysis of SA213-T22 Re-heater Rear Tube of Thermal Power Plant

SA213-T22 steel had been widely used in petrochemical and power generation plants. But due to prolonged service exposure in various conditions, these tubes have been ageing continuously and deteriorations of the materials cannot be avoided. This paper presents a case study of SA213 type-T22 alloy steel reheater tube that failed in a boiler of a power plant. The failed tube was investigated through visual examination, mechanical properties evaluation, and microstructural analysis to find the root cause of failure. The tube was found to fail by long term over-heating due to formation of thick oxide scale formation leading to creep.

Functional gradation through preferential crystallisation and interfacial activity in rapidly quenched Fe/Co-based bilayered ribbons for bend sensors

Rapidly quenched bilayered ribbons comprising one layer as Fe73.5Nb3Cu1Si13.5B9 (FM) and the other layer either as Fe74.5Nb3Si13.5B9 (FNb) or Co72.5Si12.5B15 (CSB) alloy designated as BLFM/FNb and BLFM/CSB, respectively, have been addressed. Phase transformation, thermomagnetic transitions, saturation magnetization, and soft magnetic properties of individual layers were revealed in the functional properties of bilayered ribbons. Properties of bilayer could be endorsed with respect to the single layered Fe73.5Nb3Cu1Si13.5B9, Fe74.5Nb3Si13.5B9, and Co72.5Si12.5B15 alloy ribbons. Selective devitrification of the layers in the bilayers could be induced through optimum heat treatment. Enhanced bend sensitivity was revealed in BLFM/CSB through generation of desirable phases in the bilayers of the bilayered ribbon. Synergistic diffusivity of Fe and Co with its consequent effect on interfacial zone of BLFM/CSB bilayer was observed.

Generation and detection of guided waves in a defective pipe using rapidly quenched magnetostrictive ribbons

In this investigation, rapidly quenched ribbons prepared by a melt spinning technique have been used as a magnetostrictive sensor (MsS) element for the generation and detection of guided waves traversing through a pipe. The materials we have used in the present study are the rapidly quenched Fe80Si8B12 and Fe40Ni40B20 which have a high magnetostriction constant. The experiments were conducted on a bare Al tube after generating proper dispersion curves. The experiments were carried out using as-spun and annealed ribbons. After optimizing the AC magnetizing field, DC bias field and frequency (from software), longitudinal guided waves were generated in the tubes using MsS ribbons. Measurements were carried out to test the sensitivity of this sensor to both circumferential discontinuity (saw cut) and defect free bare aluminum tubes at a frequency of 76?kHz. The effect of defect size on the sensor signal was also investigated. Using the present technique, the defects were not only identified but their location in the pipe was also estimated.

Assessment of recovery and recrystallisation behaviours of cold rolled IF steel through non-destructive electromagnetic characterisation

Abstract The recovery and recrystallisation behaviours of cold rolled IF steel have been investigated by destructive (optical microscopy and hardness) and non-destructive electromagnetic sensor, (which allows direct measurement of strip samples with no surface preparation) techniques. The onset and completion of recrystallisation are clearly monitored through destructive techniques of optical microscopy and hardness measurements. The nucleation of new recrystallised grains is observed in the sample annealed at 600 °C/15 min, while completion of recrystallisation takes place at 700 °C/15 min. The destructive techniques are not very accurate in monitoring recovery, for example, changes in hardness of <20% are seen. In contrast, the magnetic properties of annealed steel show the onsets of both recovery and recrystallisation, with recovery accounting for ≈60% change in the coercivity value. Therefore, the measurement of magnetic softening through an electromagnetic sensor acts a crucial role for understanding recovery and recrystallisation behaviours of steels during industrial processing. The present investigation is aimed not only for controlling product quality but also saving characterisation time through off line monitoring during steel processing at industry.