Materials Science Forum Vols. 745-746

Abstract: HGM-Ni_0.5Co_0.5Fe₂O₄ core-shell particles were prepared by plating Ni_0.5Co_0.5Fe₂O₄ magnetic film on hollow glass microsphere (HGM) from the aqueous solution containing NiCl₂·6H₂O, FeCl₂·4H₂O, CoCl₂·6H₂O and HGMs without sintering. Urea was used as precipitator, and air was used as oxidizer in homogeneous coprecipitation process. The morphologies, phase structures, shell thickness, chemical compositions and magnetic performances of the core-shell particles were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS) and vibrating sample magnetometer (VSM), respectively. The results showed that a compact and continuous film with thickness at about 250 nm was coated on the HGM by the homogeneous coprecipitation process. The film was spinel ferrite phase, and was determined as the Ni_0.5Co_0.5Fe₂O₄. The saturation magnetization (Ms) and the coercivity (Hc) of as-synthesized HGM-Ni_0.5Co_0.5Fe₂O₄ core-shell particles were 20.886emu/g and 97.174G, respectively.

Abstract: The effect of severe plastic deformation at ambient temperature on microstructures and mechanical properties of aluminum alloy 2519 was investigated by means of tensile test, micro-hardness test, optical microscopy and scanning electron microscopy. The results showed that tensile strength of as-queched 2519 alloy was greatly enhanced to nearly 550MPa (ultimate tensile strength, UTS) and 520MPa (yield strength, YS) by severe cold rolling or equal channel angular processing (ECAP) while the elongation decreased to 5%. The 2519 alloy could obtain quite well mechanical properties as much as 80 % and 12 passes cold rolling deformation. This indicated that pre-deformation by ECAP is effective in improving the mechanical properties of 2519 alloy by grain refinement, strain aging and high density dislocations.

Abstract: In the present work, a peak-aged 6061 Al-Mg-Si aluminum alloy was subjected to equal channel angular pressing (ECAP) at 110 °C. The microstructure of the sample was characterized by high-resolution transmission electron microscope and weak-beam dark-field method. It was shown that the dislocation density in some local areas is much lower than the average dislocation density expected in the usual alloys processed by severe plastic deformation. High-resolution transmission electron microscope observations indicated that many full dislocations were dissociated into partial dislocations connected by stacking faults. In addition, a Z-shaped defect (i.e., a type of dislocation locks) probably formed by the reactions of the partials in different {111} planes was first observed in the ECAPed alloy. Furthermore, the precipitation behavior and sequence in the present ECAPed sample were identified by high-resolution transmission electron microscopy.

Abstract: Perovskite lead titanate crystals with various morphologies were successfully synthesized via a hydrothermal reaction route with different lead sources. X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were employed to characterize the phase composition and the morphology of the synthesized products. In order to investigate the effect of the lead source on the phase formation and morphology evolution of the synthesized pervoskite PbTiO₃ crystals, PbO, PbF₂, PbSO₄ and Pb (CH₃COO)₃ ·3H₂O, were used as starting precursor lead source introduced into the hydrothermal reaction system, respectively. Accordingly, perovskite PbTiO₃ brken cubes, irregular particles, cubic particles, and microplates were obtained, respectively. Based on the experimental results, the effect of lead source was simply discussed.

Abstract: Using molecular dynamics simulations, the influence of transverse tensile stresses on the plastic deformation behaviour of nanocrystalline (NC) Ni under tension has been investigated. The sample with an average grain size of 20 nm was created using a Voronoi construction, and two different tensile tests of the sample were performed at a constant strain rate. The simulation results revealed that more partials were emitted from the grain boundaries and propagate into the grain interiors after adding the transverse tensile stress, enhancing the dislocation density in the grain interiors. This increased dislocation density can cause additional strain hardening observed in the stress strain curve. Meanwhile, it was observed from microstructures that nanovoids are easier to form and coalesce into cracks under the biaxial stress state, causing strain softening. The two competing effects of the transverse tensile stress on the plastic deformation behaviour of NC Ni resulted in the flow stresses from 4% to 10% strain in the biaxial stress state slightly larger than those in the uniaxial stress state.

Abstract: The composite filament structure was produced in the Cu-5.7%Cr and Cu-12.4%Cr as-cast alloy ingots by using equal channel angular pressing (ECAP) at room temperature. Optical and TEM microstructure, micro-hardness, tensile strength and electrical conductivity of ECAPed samples were investigated. The rotation and spreading of Cr particles took place during ECAP, and resulted in long thin in-situ filaments. The tensile strength increased with the number of the ECAP passes. A strengthening model was recommended to predict the enhancement of the tensile strength in Cu-Cr in situ fibrous composites.

Abstract: Nanosized tungsten carbide (nanoWC) has been widely studied and applied in many industries as hard materials since it has good combination of high hardness and strength. Thermal mechanical alloying method consisting of high-energy ball milling and subsequent carbonization is a common synthetic approach to prepare nanoWC. In this paper, a special milling process has been reviewed, in which the dielectric barrier discharge plasma (DBDP) is introduced, for the preparation of nanoWC. The DBDP milling renders the W+C powders with specific flake-like structure possible owing to the cooperative effect of DBDP and mechanical milling. The obtained W+C powders are activated significantly within shortened milling period. Furthermore, pure nanoWC powders can be synthesized after subsequent carbonization at a significantly lowered temperature. DBDP milling is demonstrated to be an efficient way to synthesize nanoWC.

Abstract: Nanostructured Al alloy powder was prepared by ball milling of a mixture of Fe-based metallic glass (FMG) powder and 2024-Al alloy powder. Microstructural evolution and mechanical properties of the milled composite powder were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and micro-hardness test. It is revealed that after 24h milling, the grain size of the Al alloy powder reduced to about 30nm, and the FMG particles were uniformly distributed throughout the Al matrix. The mechanical test indicated that the micro-hardness of the powder was significantly improved.

Abstract: Polyether imides (PEI) and nanoattapulgite particles organically synthesized by the active silane containing epoxide groups (OAT) were used to modify the epoxy resin (ER). The effect of the concentration of OAT on the morphology, thermal and mechanical properties of ER/PEI/OAT nanocomposites was explored in the present work. The microscopic structure of nanocomposites was investigated by Scanning electric microscopy (SEM). The results showed that a homogeneous nanoattapulgite organically-modified by silane in the polymeric matrix was obtained. SEM images showed that the morphology transformation increased with the increase of OAT content, which would lead the improvement of the impact strengths of nanocomposites. The results of mechanical and thermal measurements indicated that the introduction of PEI and OAT into the epoxy resulted in the great improvement of the impact strength and storage module. With the OAT concentration increasing, the glass translation temperature (T_g) value of nanocomposites was gradually increased. Additionally, the results of thermo-gravimetric analysis (TGA) revealed that thermal stability of nanocomposites was apparently improved in comparison to the epoxy resin systems.

Abstract: The stress relaxation behavior of as-deposited and rolling nanocrystalline NiFe alloy was studied by nanoindentation tests. The results indicated that both the hardness and activation volume of rolling NiFe are larger than that of as-deposited samples. Furthermore, the hardness decreases with increasing indentation depth. The reduction of indentation stress during holding becomes much faster with decreasing the indentation depth. Dislocation density is remarkably enhanced by rolling deformation, leading to the hardening behavior. Dislocation multiplication and accumulation mediated process is believed to the dominant plastic deformation mechanism.