Materials Science Forum Vols. 745-746

Abstract: The morphology, microstructure and magnetic properties in 1K107 alloy annealed by nitrogen and transversal magnetic (N₂) are studied. The results show that coercivity (H_c), remanence ratio (B_r/B_s), core loss (P_c), and effective permeability (μ_e) are efficiently optimized by transversal magnetic annealing (N₂) according to the methods of digital electric bridge, rectifier bridge, X-ray diffraction (XRD), and field emission scanning electron microscopy (FESEM), etc.

Abstract: The effect of annealing treatment on the mechanical properties and microstructure of cold-rolled Cu-20% Zn alloys was investigated in this work. Mechanical properties changed dramatically with the increase of temperature. According to the microhardness test, it can roughly concluded that 150 is the optimal annealing temperature for deformation, at which a uniform elongation increased from 1.4658% before annealing to about 6.89%, and the elongation to failure increased from 7.426% to 16.81% with the same strength almost retained. The changes of microstructure during the annealing process are mainly distributed to the improvement of mechanical properties.

Abstract: Carbon nanospheres of uniform diameter around 40 nm have been achieved in the absence of catalyst by microwave pyrolysis chemical vapor deposition. The products have been characterized by field emission scanning electron microscopy, high resolution transmission electron microscopy, X-ray diffraction, Raman spectroscopy and thermogravimetric analysis. The results show that the nanospheres with high purity, which are solid by their cross-sections, consisting of concentric incompletely closed graphitic layers. Their thermal stability in dynamic air atmosphere is also discussed.

Abstract: Gemini amphiphiles have been widely investigated in many aspects due to the unique structure. In this study, two gemini-type Schiff base amphiphiles containing aromatic spacers were synthesized and characterized in Langmuir-Blodgett films. Both compounds formed stable multilayer films and showed distinct phase behaviours on water surface and ion subphase. When the compounds spread on Cu (II) ions subphase, an in-situ coordination occurred for both ligands. While spectral measurements confirmed the coordination with Cu (II) ions and aggregation mode, the morphological investigations showed the distinct nanostructures and molecular rearrangement depended on the molecular spacers and aggregation modes. For both process, the aromatic spacers in gemini amphiphiles have predominant effect in regulating the aggregation modes and spectral changes in organized molecular films.

Abstract: Pre-perovskite PbTiO₃ nanofibres were synthesized by hydrothermal method with four butyl titanate ((C₄H₉O)₄Ti) and lead nitrate (Pb (NO₃)) as the main precursors and Tetramethylammonium Hydroxide (TMAH) as mineralizer. X-ray diffraction (XRD), Raman, Scanning Electronic Microscopy (SEM), Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and the corresponding selected area electron diffraction (SEAD) were used to characterize the crystal structure and morphology of the obtained products. The results showed that the nanofibers are of a few to tens micrometer at length size, and 50~300 nm at diameter size. Based on the experimental results, the effect of Tetramethylammonium Hydroxide (TMAH) was simply discussed.

Abstract: In the present study, surface mechanical grinding treatment method was developed to induce a gradient nanograined structures in commercial pure iron bar. The gradient structures and hardness were characterized by optical microscope and micro-indentation. Fatigue behaviors of samples in gradient nanograined structures and in as-annealed state were examined at room temperature, and the fatigue fracture behavior was characterized with scanning electron microscope. The result showed that the thickness of deformation layer was about 300 μm, and the fatigue strength of the treated case was 252 MPa based on fatigue life of 10⁷ cycles, but the fatigue strength of the as-annealed sample was 186 MPa. Fatigue cracks source of samples in gradient nanograined structures gradually transformed into the internal of gradient structures layer, but cracks sources of as annealed samples were observed to be initiated at surface.

Abstract: Rhombohedral bismuth ferrite (BiFeO₃) powders were hydrothermally synthesized by using C₆H₈BiNO₇ and Fe (NO₃)·5H₂O as precursors, KOH as a mineralizer and poly sodium-p-styrenesulfonate (PSS) as an additive. The phase formation and morphology of the as-prepared powders were characterized by X-ray diffraction,raman spectroscopy, and field-emission scanning electron microscopy. In order to investigate the effects of PSS, a series of experiments were performed by changing the introduced amount of PSS and the hydrothermal reaction time. The results showed that the introduction of the PSS leads to the synthesized bismuth ferrite changes from the mixture of Bi₂Fe₄O₉ and BiFeO₃ to pure rhombohedral BiFeO₃. Corresponding to the phase composition evolution, the particle morphology alters from nanosheets to microparticles. The magnetic properties of single-crystalline BiFeO₃ powders were also characterized. Based on the experimental results, the phase formation mechanism of the rhombohedral BiFeO₃ hydrothermally synthesized assisted with PSS was discussed.

Abstract: Nanostructured ferritic alloys (NFA) of Fe-9Cr-0.2Ti-0.3Y₂O₃ (in mass) incorporating nanoscale oxide particles, was produced by mechanical milling (MM) followed by hot pressing (HP). The formation process of the nanoscale oxide particles were structurally characterized at each step of the elaboration processes by means of transmission electron microscope (TEM). The observations of structure of the mixed powders and the nanoscale oxide particles in the NFA after MM indicated that the initial powders, coupled with the original yttria powders, get fractured by severe plastic deformation and ultrafine bcc grains (~20 nm) of the matrix, and Y2O3 nanocrystals with irregular edges are formed during MM. The addition of titanium (Ti) promotes the formation of amorphous phase of [YM_amorphous during MM. Microstructure of the NFA observed by TEM exhibited a very fine structure of nanostructured grains in which large number of nanoscale oxide particles were distributed after HP process. HRTEM observations of partially and fully crystallized oxide nanoparticles in the matrix of NFA gain an insight into the formation mechanism of nanoscale oxide particles in HP process. The formation process of nanoscale oxide particles consist of the reaction between Y2O3 fragments and titanium (Ti) and crystallization of [YM_amorphous.Threshold stress which can quantitatively shows the effect of dispersion strengthening was carefully evaluated on the basis of higher magnified images of the nanoscale oxide particles. Different values of threshold stress were obtained due to the various dispersions of the nanoscale oxide particles within different areas. That may be the reason why the threshold stress cannot be clearly obtained by the results of creep tests.

Abstract: The falling process of melt electrospinning jet is different from those of solution electrospinning in which there is apparent solvent volatilization. In order to study the factors influencing on the forming process of fibers in melt electrospinning, dropping process of fibers is recorded and analyzed via high speed video camera in the article. Results showed that there was an optimal spinning temperature for melt electrospinning of the polymer; the greater the voltage was, the more obvious stretching action on jet was. However, the voltage did not exceed a certain value, because there was a spinnable voltage limit corresponding to every receiving distance. When the spinning distance was generally short, the jet swinging radius decreased with increasing spinning distance; when the spinning distance was long, the jet was subject to the influence of the environment temperature easily. The changes of viscosity had dominant influence on the motion of jet.

Abstract: Microstructures of a maraging steel subjected to equal-channel angular pressing (ECAP) up to 16 passes at room temperature were investigated. Electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) examinations showed that the microstructures of the as-processed samples were characterized by a well-defined lamellar structure. With increasing the number of ECAP passes from 1 to 16, the lamellar spacing was significantly decreased from ~323 nm to ~54 nm, while the volume fraction of high-angle lamellar boundaries (HALBs, having the misorientation larger than 15^o) was obviously increased form ~46% to ~80%. The real lamellar-type nanostructured microstructures were completely accomplished in the tested steel after 8 or more ECAP passes. In addition, the grain refinement mechanism of maraging steel during the processing was discussed in detail.