Papers by Keyword: Phase Transformation

Abstract: Zirconia (ZrO2) films were deposited by metal-organic chemical vapor deposition (MOCVD) on {1 0 0} Si single crystal using Zr(thd)4 precursors. The thickness of obtained films is typically of 3.5 μm. The samples have been characterized by Field-Emission-Gun Scanning Electron Microscopy (FEG-SEM) for morphologic and microstructure study, and by X-ray Diffraction (XRD) for crystalline structure. The microstructure analysis showed that unexpected stable single tetragonal phase preferentially grew in low temperature area. According to the literature, the tetragonal phase stabilization is related to the crystalline size and the internal compressive stress. To analyze the effect of grain size and internal stress on the phase transformation, the thermal annealing were carried out in different temperatures and internal stress was measured by XRD method.

Abstract: The Thermo-Calc and DICTRA software/database/programming-interface packages, through many successful applications in the fields of Computational Thermodynamics and Kinetics, have tremendously contributed to quantitative conceptual design and processing of various advanced materials. Materials scientists and engineers can efficiently apply such unique and comprehensive tools in calculating material properties, predicting material structures and simulating material processes, which are of wide-ranging industrial and academic importance.

Abstract: Some dry and wet grinding experiments have been respectively conducted on titanium dioxide which is a noble photocatalyst material in a mortar, a tumbling mill and a planetary mill. Anatase is apt to transform to rutile via a metastable phase brookite in every kind of mills in the case of dry grinding. And it hardly takes place for phase transformation from rutile to other forms. It is shown that the kind of mill has not decisive effect on the mechanochemical polymorphic transformation of titanium dioxide, which merely influences the rate of phase transformation. On the other hand, the addition of other liquid media, such as water and acetone, is helpless for phase transformation of anatase. Only anatase can transform to metastable phase brookite by wet grinding. When ground titanium dioxide is heated, the amorphous phase is easier to transform to rutile than metastable phase brookite at lower temperature.

Abstract: The massive transformation, austenite (γ) → ferrite (α), in Fe-4Cr alloy subjected to cooling from the high-temperature austenite down to room temperature at cooling rates of 75, 150, 330 K min^-1 were experimentally investigated by means of high-resolution differential dilatometry. It is found that the onset transformation temperature and the overall transformation time decrease with increasing the cooling rate applied. Furthermore, by fitting the calculated transformed fraction according to developed phase-transformation model to the experimental data obtained by differential dilatometry, the kinetic parameters are obtained which show the activation energy for growth and the activation energy for nucleation are considerably different, this may be due to considerably different interfacial structure with the matrix for minute embryos and for the much larger growing particles.

Abstract: Porous reaction-bonded silicon nitride ceramics with different values of α/β ratio were obtained by setting the nitriding temperature and time. The influence of α/β phase transformation on the dielectric properties of porous silicon nitride ceramics has been investigated. The results show that the α/β transformation occurs when the nitriding temperature is higher than 1400°C . The values of α/β ratio decrease with increase of nitriding temperature and time. The dielectric constant ε′ and the dielectric loss tan δ of the samples increase because of the α/β transformation, and the change of the dielectric loss is more obvious. The increase of concentrations of point defects due to the α/β transformation leads to the significant increase of the dielectric loss.

Abstract: The microstructural evolutions of 7055 aluminum alloy after single and two-step homogenization with or without applying 12T high magnetic field were investigated. The as-cast microstructures of 7055 alloy consist of α+AlZnMgCu eutectics, Al7Cu2Fe phase and AlTiCuFe phase. During homogenization, coarse α+AlZnMgCu eutectics located at interdendritic region become smaller, discontinuous and spheroidized, some of them transform into Al2CuMg (S) having a higher melting point. High magnetic field significantly accelerates the dissolution of α+AlZnMgCu eutectics and S phase. The least amount of α+AlZnMgCu eutectics and S phase is obtained when the alloy homogenized at 4650C/10h+4850C/8h under 12T high magnetic field.

Abstract: In the present work, ultra-fine Al₂O₃/ZrO₂-MgO-CeO₂ powders were used as the starting materials to produce Al₂O₃-ZrO₂ composite coatings on 45# carbon steel by flame spray. The ultra-fine Al₂O₃/ZrO₂-MgO-CeO₂ powders were prepared via the co-precipitation method accompanied a crushing sintering process. The microstructure, the phase compositions and the bond strength of the crushed sintered powders and the corresponding composite coatings were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and a tensile bond strength tests respectively. It was found that the phase composition, the surface topography and the coating adhesion were different from the difference kinds of coatings. Results indicated that the phase composition of the flame sprayed ceramic coatings consisted of tetragonal-ZrO₂, cubic-ZrO₂, ZTA (zirconia toughened alumina) and γ-Al₂O₃, with a small amount of α-Al₂O₃, and that bond strength values attained 45.1 MPa for 42.6wt.% Al₂O₃/57.4wt.% Mg_0.13Ce_0.1Zr_0.77O_1.87 coating.

Abstract: The purpose of our research is to generate the ultrahigh pressure by using high explosive and to transform a phase of a material. The extremely high impulsive pressure generator that has been developed by us uses the head-on collision between metal jets. Because the velocity of the metal jet is very high, the ultrahigh pressure will generate. If a powdered material is mixed to metal jets, it is expected that the material is transformed to a high pressure phase by this ultrahigh pressure. A graphite powder was used to synthesize a diamond. The existence of the diamond was confirmed by X-ray diffraction (XRD). In this paper, the mechanism of the generation of the ultrahigh pressure is explained and the results of the observation of the powder by using scanning transmission electron microscope (STEM) are reported.

Abstract: The microstructure characterization of Al73Mn23Pd4 and Al73Mn21Pd6 alloys was done after annealing at 900°C for 312 h and subsequent water quenching, as well as after thermal cycling. DTA and EDX/WDX/SEM techniques were used in the investigation. It was found out that the alloys consist of the single ternary T-phase after annealing and water quenching. The DTA experiment confirmed the stability of this phase also at lower temperatures. After DTA, the alloys exhibited double-phase microstructure consisting of the ternary T-phase and probably the icosahedral I-phase. It was proved an incongruent transformation of the ternary T-phase into the liquid and vice versa.

Abstract: Through phrase transformation analysis on ceramics samples after applied laser shock wave, study was made on micro-structure and mechanical performance transformation and transformation laws. Thus conclusions were drawn through analysis as follows: tensile stress was caused on back surface of TZP ceramics sample by laser shock wave, and further transformation was generated under tensile stress in a way that about 48% tetragonal phrases t-ZrO2 transformed into monoclinic phrases m-ZrO2, which expanded volume of and gave rises to micro-cracks, and relief stress and micro-cracks caused will absorb the energy of main cracks, which can impede expansion of main cracks and realize the aim of phrase transformation toughening.