A Mitra

Effect of quenching rate on the properties of melt-spun FeNbCuSIB ribbons

Abstract Amorphous ribbons of composition Fe 73.5 Nb 3 Cu 1 Si 13.5 B 9 and Fe 72 Nb 4.5 Cu 1 Si 13.5 B 9 were prepared by rapid solidification technique. A convergent nozzle of suitable dimensions was chosen so as to minimise the turbulent flow of the fluid. The separation between the nozzle and the quenching wheel surface was kept constant at 0.5xa0mm. The quenching rate was directly proportional to the wheel velocity as the dimension of the crucible and the position of crucible with respect to wheel surface were kept fixed in all the trials. The thickness of ribbon was found to decrease while the ductility increased with the increase of wheel surface velocity, i.e. with the increase of quenching rate. The amorphous to nanocrystalline transformation temperature was found to remain constant when the wheel surface velocity was more than 32xa0m/s and also the soft magnetic properties were found to be superior.

Structural and soft magnetic properties of Fe73.5Nb3M1Si13.5B9 (M = Cu, Mn, Pt)

The magnetic and the structural properties have been investigated for Fe73.5Nb3M1Si13.5B9 (M=Cu, Mn, Pt) alloys in their as-received and heat-treated state. Differential scanning calorimeter and scanning electrical resistivity were used to study crystallization behaviour and to examine the influence of the nucleation elements Cu, Mn and Pt on the structural and magnetic properties of Fe-based nanocrystalline materials. The temperature coefficient of resistivity above Curie temperature was found to be very low (0.2×10−4 K−1) in the Cu containing alloy as compared to the alloys which contained Mn or Pt. A broad exothermic DSC peak for the formation of Fe3Si nanoparticles was absent when Cu was replaced by Mn or Pt. Instead, a sharp exothermic peak for the formation of Fe3(SiB) phase was observed with an onset at 875 K. The magnetic properties of the three alloys measured after stress relaxed annealing were similar. However, after first-stage crystallization, the soft magnetic properties were further enhanced only in Cu containing alloys due to the formation of Fe3Si nanoparticles. Such nanoparticles together with different intermetallic phases like FeMn3 or FePt3 were formed in Mn or Pt containing alloys. Moreover, the NbFeSi phase was also formed in the latter alloys after higher temperature annealing which reduced not only the amount of the Fe3Si phase but also the grain refining effect due to the lesser amount of Nb available at the grain boundary. When Cu was replaced by Mn or Pt, the formation of different intermetallics reduced the volume fraction of Fe3Si nanoparticles and the lesser amount of grain refining Nb made the materials magnetically inferior after annealing above 775 K.

Crystallization study of amorphous Fe40Ni40B20 by electrical resistivity measurement

The crystallization in amorphous Fe40Ni40B20 alloy has been studied by electrical resistivity measurement. It shows a three stage compared to the more conventional two-stage crystallization behaviour in many metallic glasses. The Johnson-Mehl-Avrami equation was found to be operative only for 40% transformed crystallized volume for the highest measured isotherm (740 K). The Avrami exponent and activation energy were found to be 1.75 and 53 kcal mol−1, respectively. The low activation energy in the amorphous alloy has been explained by the structural relaxation model.

Crystallization and magnetic properties of rapidly solidified Fe-Nb-M-Si-B (M=Cu, Mn, Pt)

Abstract Crystallization behavior and magnetic properties of rapidly solidified Fe 73.5 Nb 3 Si 13.5 B 9 alloys were studied with 1xa0at.% Cu or Mn or Pt as the fifth alloying element. Crystallization behavior was investigated by electrical resistivity and differential scanning calorimetry techniques. Addition of Mn or Pt enhanced the temperature coefficient of resistivity. A two stage crystallization process was observed in Cu-containing alloys whereas the Mn- and Pt-containing alloys exhibited three-stage processes. A broad exothermic maximum with the onset at 808xa0K, which is characteristic of the formation of Fe 3 Si nanoparticles was observed only in Cu-containing samples. The formation of various intermetallic phases along with the Fe 3 Si nanoparticles in annealed Mn and Pt systems made these two alloys magnetically inferior compared to the Cu-containing alloy.

Magnetic characterization of 5Cr-0.5Mo steel used in process heater tubes

The magnetic properties of a process heater tube were determined to evaluate the structural integrity of its use in the oil industry. Carbon diffusion inside the tube during processing of petroleum is a cause of concern in the oil industry because it leads to brittle fracture. Both magnetic hysteresis and magneto-acoustic emission (MAE) parameters were determined for three specimens made from virgin, laboratory-aged and service-exposed materials. It was expected that the coercivity would be a good damage parameter by which to assess the service-exposed material because it increased with carbon concentration. However, the observed insignificant difference in coercivity between service-exposed material and the virgin one ruled out the possibility of using coercivity as a suitable damage parameter. The present study indicated that saturation induction could be used to find the extent of carbon deposition inside the material. The RMS voltage (Vrms) of the MAE signal signal at low field was found to be higher in the laboratory-aged specimen due to greater interparticle distances which allowed domain walls to move longer distances before encountering a pinning centre. At high field, Vrms in the virgin specimen was found to be higher than that in laboratory-aged and service-exposed materials. That was attributed to the presence of higher anisotropy in the virgin sample, originating from the metallurgical processing.

Phase transformation studies on Mn-Al-C alloy, by DSC and electrical resistivity measurements

Phase transformation studies carried out on Mn−Al−C permanent magnet alloy employing DSC and electrical resistivity measurements, are reported and discussed. The transformation of the hexagonal Mn−Al phase (disordered and non-magnetic) to the ferromagnetic fct phase proceeds via the formation, in at least two stages, of the ordered orthorhombic phase. The activation energy for the formation of the fct phase is ≈34.65 kcal/mol. Microstructural changes occurring at various stages of the transformation are also studied.ZusammenfassungBeschrieben und diskutiert werden vorliegend die Phasenumwandlungsuntersuchungen an der permanentmagnetischen Legierung Mn−Al−C unter Anwendung von DSC- und elektrischen Widerstandsmessungen. Die Umwandlung der hexagonalen Mn−Al-Phase (ungeordnet und nicht magnetisch) in die ferromagnetische fct-Phase erfolgt—in mindestens zwei Schritten—über die Bildung einer geordneten orthorhombischen Phase. Die Aktivierungsenergie der fct-Phase beträgt ca 34.65 kcal/mol. Veränderungen der Mikrostruktur, die in verschiedenen Schritten der Umwandlung erfolgen, wurden ebenfalls untersucht.

Enhancing end-user bandwidth using content sharing over optical access networks

A steady increase in the consumption of bandwidth-hungry Internet services (e.g., on-demand content) has resulted in a simultaneous increase of the end-user bandwidth demand. Moreover, the growing bandwidth demand is foreseen to immensely pressurize content delivery networks (CDNs) in the near future. CDNs stream these services through the core network on a per-user basis. Hence, the bandwidth consumed by a CDN increases when content is requested by multiple users. In this paper, we propose optical access network (OAN) architectures and associated media access control protocols that enhance the end-user bandwidth by facilitating supervised sharing of content among end users with security. This is an attractive solution for OANs, which otherwise require technology upgrade and redeployment of fiber infrastructure for enhancing the end-user bandwidth. The proposed schemes provide bandwidth benefits in the access network which are simultaneously observed at the metro and core network levels. In comparison, competent technologies like caching and P2P solutions provide bandwidth benefits at the metro and core network levels only, and do not enhance the enduser bandwidth. Analytical models are derived for the schemes proposed in this paper, as well as the caching and CDN solutions, in order to compare bandwidth benefits. Validation of the analytical models is performed using simulations.

The generation of magnetic bistability in annealed Fe-Si-B fibre by hydrogenation

The magnetically bistable nature of as-quenched amorphous fibre disappeared when the material underwent stress-relief annealing at for 30 min. A reduction of internal stress leading to a lowering of the magnetoelastic anisotropy was the cause of such a change from bistable to normal ferromagnetic behaviour. Cathodic hydrogen impregnation caused the re-appearance of magnetic bistability, the magnitude of which was dependent on the duration of hydrogen charging (and hence on the hydrogen concentration). Perfect bistability was restored when the annealed fibre was charged for 120 min at the charging current density of . Owing to the positive molar volume of hydrogen, internal stresses were generated by hydrogenation, which increased the size of the longitudinal anisotropy region. When this longitudinal anisotropy region was big enough to act as a monodomain at the core of the fibre, bistability with a large switching field was observed.