Papers by Keyword: Rapid Quenching

Abstract: In this work, an experimental measurement, contour method, is implemented for an after quenching IN718 forging specimen to obtain the distribution of residual stress field. A sequentially coupled thermal mechanical finite element model is developed with the similar 3D geometry of the experimental specimen and implemented the same heat transfer boundary of the rapid quenching with the experimental condition. A thermal mechanical rate dependent continuum plasticity model for IN718 alloy, with the dynamic strain ageing (DSA) effect incorporated, is developed to study the impact of DAS effect on the evolution of residual stress during rapid quenching. The modelling predictions of residual stress are in good agreement with the contour method measurements. The impact of DSA effect is further quantified, indicating that an annular high plastic strain rate region in the core part of the disc is captured during the simulation of the quenching process.

Abstract: The ingot of Sm-Fe alloy was prepared by vacuum melting. After a process of coarse crushing, it was made into Sm-Fe ribbons by melt-spinning. By analysis of XRD and SEM, it was confirmed that the ribbons composed of fine Sm₂Fe₁₇ columnar grains with almost the same orientation can be obtained under the condition of 5~7m/s surface rotating velocity of Cu wheel, suitable nozzle size, injection pressure, temperature and composition of the Sm-Fe melt to regulate cooling rate and crystallization. The achievement of the ribbons lays a foundation for preparing anisotropic Sm₂Fe₁₇N_x magnetic powders by rapid quenching.

Abstract: Fe-6.5wt.%Si alloy is one excellent soft magnetic material. In the present study, rapid quenching was applied to fabricate ultra-thin ribbons of Fe-6.5wt.%Si alloy by melt spinning. Continuous ribbons of 20 mm wide, 35-45 μm thick were fabricated with optimized experimental parameters. The ribbons exhibited metallic luster appearance, and can be wound without cracks. Microstructure, texture and mechanical properties of the ribbons were investigated. It was found that the rapidly quenched ribbons had intense {100} plane texture, and exhibited some extent of bending capability. Finally the ribbons were cold rolled and the surface smoothness was improved for stacking purpose.

Abstract: Methods of X-ray diffraction, transmission and scanning electron microscopy, and electron diffraction have been used to study phase and chemical compositions and structure of Ti₂NiCu alloys. The alloys of the quasi-binary section TiNi–TiCu, which exhibit in the initial as-cast state thermoelastic martensitic transformations B2↔B19 and related shape memory effects, have been produced by rapid quenching of the melt (melt spinning technique). The chemical composition of the Ti₂NiCu alloys was varied with respect to titanium and nickel within x ≤ ±1 at. %. The mechanical properties of the alloys have been measured in the initial state and after subsequent heat treatment. The kinetics of the crystallization from the amorphous state, devitrification processes and the forward and reverse thermoelastic martensitic transformations were investigated. Their characteristic temperatures have been determined by measuring temperature dependences of the electrical resistivity of the alloys. The diagram of the dependence of the critical temperatures on the chemical composition has been constructed.

Abstract: Direct magnetic field was imposed during the process of rapid quenching by melt spinning of MlNi_3.6Co_0.7Mn_0.4Al_0.3 and MlNi_3.6Co_0.35Mn_0.5Al_0.3Cu_0.25. The effect of rapid quenching in magnetic field on the microstructures and electrochemical performances was investigated in detail. The results show that rapid quenching decreases the grain size of both alloys, and magnetic field results in an oriented growth especially for MlNi_3.6Co_0.35Mn_0.5Al_0.3Cu_0.25 alloys, but they have little influence on the typical CaCu₅ structure of AB₅-type alloys except for the increase of cell parameters (a₀, c₀) and cell volume. Electrochemical studies indicate that rapid quenching enhances the cycle stability significantly but inevitably decreases the electrochemical capacities of the alloys unless magnetic field was imposed. Additionally, the diffusion of hydrogen and rate properties are improved with the emergence of magnetic field compared with the alloys processed by rapid quenching only.

Abstract: The influences of rapid quenching on the structures and electrochemical behaviors of La-Mg-Ni system (PuNi₃-type) La_0.7Mg_0.3Co_0.45Ni_2.55-xFe_x (x = 0, 0.1, 0.2, 0.3, 0.4) electrode alloys were systemically investigated. The results show that the rapid quenching, instead of changing the phase structures of the alloys, leads to c axis enlarged and the a axis and cell volumes of the LaNi₅ and (La, Mg)Ni₃ major phases decreased slightly. The morphologies of the as-quenched alloys exhibit a massive structure, which differ from that of the as-quenched AB₅-type alloy. The rapid quenching clearly impairs some electrochemical performances of the alloys, involving discharge capacity, high rate discharge ability (HRD) and discharge potential, but it significantly prolongs the cycle life of the alloy. With an increase in the quenching rate from 0 (As-cast was defined as quenching rate of 0 m/s) to 30 m/s, the discharge capacity of the alloy (x = 0.4) decreases from 351.06 to 313.36 mAh/s, the HRD from 69.36 to 50.54%, whereas its cycle life increases from 106 to 166 cycles at a charging discharging current density of 600 mA/g.

Abstract: The performance of SiC MOSFET devices to date is below theoretically expected performance levels. This is widely considered to be attributed to defect at the SiO2/SiC interface that degrade the electrical performance of the device. To analyze the relationship between defect structures near the interface and electrical performances, advanced computer simulations were performed. A slab model using 444 atoms for an amorphous oxide layer on a 4H-SiC (0001) substrate was made by using first-principles molecular dynamic simulation code optimized for the Earth-Simulator. Simulated heating and rapid quenching was performed for the slab model in order to obtain a more realistic structure and electronic geometry of a-SiO2/4H-SiC interface. The heating temperature, the heating time and the speed of rapid quenching were 4000 K, 3.0 ps and -1000 K/ps, respectively. The interatomic distance and the bond angles of SiO2 layers after the calculation are agree well with the most probable values of bulk a-SiO2 layers, and no coordination defects were observed in the neighborhood of SiC substrate.

Abstract: In order to meet the requirements of various applications, considerable efforts have been focused on the new approaches for synthesizing carbon nanotubes. Recently, simple thermal chemical vapor deposition growth technique by pyrolyzing ethyl alcohol has been developed. However, this method needs metallic catalyst. Here, we demonstrate that it is possible to synthesize multi-walled carbon nanotubes without the aid of any catalyst. The as-synthesized products were analyzed by high-resolution transmission electron microscopy (HRTEM). The HRTEM images show that the products are straight multi-walled carbon nanotubes. The synthesis technique of carbon nanotubes is cost-effective because of the catalyst-free process.

Abstract: Structural states of Al-Sc, Al-Hf and Al-Zr alloys rapidly quenched from the liquid state have been studied. Features of structures in dependence on the nature of alloying elements, on the quenching temperature and the cooling rate by the crystallization of ribbons have been investigated. Fractal structures are formed by the quenching from the certain temperature-concentration region. The change of the morphology of structures has been researched during the aging process.

Abstract: The Sm(Co0.71Fe0.1Cu0.12Zr0.04B0.03)7.5 (2:17 type magnet) melt spun ribbons have been produced from bulk as cast samples at low (5 m/sec) to medium (40 m/sec) wheel speed by the melt spinning technique. The crystallographic texture on wheel side, the microstructural characteristics and magnetic properties have been investigated. The soft magnetic fcc-Co forms a very high degree of texture especially at low velocities but for the first time a degree of texturing has been remarked on fcc-Co grains. Diffraction patterns have been traced by x-ray scattering using Cu-Kα radiation on the wheel and free side of the ribbons. In the pattern of ribbons which have been produced at 5 m/sec the (002) plane of fcc-Co is almost the dominant peak while at 40 m/sec this peak diminishes in parallel to the appearance of the structure type TbCu7 and (111) plane of fcc-Co structure. Scanning electron microscopy on the wheel side of the ribbons has been used to observe microstructural characteristics and showed that the formation of texture is attributed to the appearance of dendrites, with their long axis parallel to the longitudinal direction of the ribbons. Dendrites’ density depends on the wheel speed of the roller and boron content. It decreases as the velocity increases while for constant velocity of about 40 m/sec, higher boron content stabilizes higher degree of texturing. Magnetic properties are also examined from low to medium wheel speed by also using the magnetooptical Kerr microscopy. Therefore coercive field as high as 3.4 kOe and reduced remanence (mr) of ~0.76 has been detected from the hysteresis curve for as spun Sm Co0.71Fe0.1Cu0.12Zr0.04B0.03)7.5 ribbons at 5 m/sec.