Materials Science Forum Vols. 561-565

Abstract: Compared to conventional single-phase fluids, the latent functionally thermal fluid (LFTF), with phase change particles of μm magnitude in size dispersed in it, shows much greater apparent specific heat and heat transfer rate between the fluid and the duct wall. Therefore, for given heat transportation quantity, the mass flow rate and the pump consumption can be reduced greatly. Due to these, LFTF is a promising material in the fields of heat exchanging. In this paper, phase change material (n-tetradecane) was encapsulated by polymethyl mathacrylate (PMMA), polystyrene (PS) and blend of them by internal phase separation method to form microcapsule of 1~2μm in size. The chemical structures were demonstrated by using Fourier transform infrared spectroscopy (FTIR). The core-shell structure was observed using phase contrast microscope. Differential scanning calometry (DSC) results indicated that the phase change enthalpy of the containing 75wt% n-tetradecane as core material reaches 150.7 J/g.

Abstract: The precipitation processes from G.P. zones to γ’ in a Cu−0.9wt%Be alloy single crystal containing only the G.P. zones parallel to the matrix (001)α plane are investigated by high-resolution electron microscopy. The precipitate phases follow a G.P. zone → γ” → γI + γ’ sequence. The G.P. zone to γI phase transformation occurs successively via γ” during aging, while the γ’ phase heterogeneously precipitates on the γI phase. From length-change measurements during aging, the misfit strains of γ’ precipitates in directions perpendicular and parallel to [001]α are estimated as ε11 =ε22 = −0.03 and ε33 = −0.09, respectively. The observation that the estimated absolute value of ε33 is much smaller than that of ε33 = −0.25 calculated using lattice parameters of the γ’ phase and Cu matrix is understood in terms of the relaxation of ε33 by interfacial misfit dislocations.

Abstract: Studying austenitisation in steel, so far, was either limited to observations at the surface of the material or to the determination of the average grain growth behavior in the bulk. The development of the three-dimensional X-ray diffraction (3DXRD) microscope at beam line ID11 of the European Synchrotron Radiation Facility in Grenoble, France, made it possible to study the transformation kinetics in-situ and at the level of individual grains in the bulk of the material. Unique in-situ observations of austenite growth kinetics during continuous heating experiments were made for two commercial low-alloy steels (C22 and C35). The observed growth behavior of individual austenite grains gives a valuable contribution to understanding the phase transformations on heating, i.e. austenite formation from ferrite and pearlite.

Abstract: There have been many reports about the bainitic phase decomposition of annealed Cu-Zn alloy. During annealing at 523 K, the hardness of this alloy increased at the early stage of annealing, although the β-phase couldn't be observed in the matrix of β'-phase (CsCl). The crystal structure of β' phase, thus, was investigated by a high-resolution transmission electron microscopy (HRTEM) to find its structural change. As the result, the striations were observed in the HRTEM image of matrix at the early stage of annealing. The selected area diffraction pattern was also obtained from this area, and it showed the extra streaks which couldn't be explained by the normal CsCl structure of the β’-phase.

Abstract: The free energy function of martensitic transformation is established using Landau polynomial. According to the free energy function, the interfacial structural modification of austenite-martensite with the chemical driving force of martensitic transformation and elastic constants of materials is discussed. Some characteristics of martensitic transformation, such as the difference between thermoelastic and nonthermoelastic martensitic transformation, martensitic growth, are explained.

Abstract: The initial coarse grains of Ti-50.9at%Ni were obviously refined after eight ECAE processes. Effects of pressing number on the transformation behavior have been investigated in detail. Comparing with the initial solution-treated Ti-50.9at%Ni, the R-phase transformation was stimulated and the martensitic temperature Ms decreased with ECAE processes at 773K. The shape memory properties of Ti-50.9at%Ni alloy before and after ECAE processes at 773K have been measured. Good one way shape memory and superelasticity were obtained for the submicron-grained Ti-50.9at%Ni alloy.

Abstract: The microstructural evolution and associated strengthening during isothermal ageing of an Al-3Cu-0.05Sn reinforced by non-spherical θ' (Al2Cu) precipitates is studied. Emphasis is placed on the plate shape of the θ'. It is found that the microstructure parameters governing the magnitude of the precipitation strengthening effect are completely different from the classical case of spherical precipitates. New possibilities for microstructure design are discussed.

Abstract: The effects of P and Cr addition and two-step aging on the microstructure and mechanical properties of a Cu-4wt%Ni-0.95wt%Si alloy have been examined. The addition of 0.02wt%P improves both strength and elongation because it suppresses discontinuous precipitation reaction. The Cr addition to the Cu-Ni-Si-P alloy decreases greatly the grain size, resulting in an increase in elongation. Two-step aging, pre-aging at 300 or 350oC and subsequent second-step aging at 450oC, causes an increase in strength without reducing elongation. The increase in strength is attributable to the decrease in inter-precipitate spacing by the two-step aging. The two-step aged Cu-4wt%Ni- 0.95wt%Si-0.02wt%P-0.02wt%Cr alloy attains a tensile strength of 830MPa, an elongation of 13% and an electrical conductivity of 35%IACS.

Abstract: Small spherical specimens of ZrO2-1.5mol%Y2O3 with diameters ranging from 150 to 900μm were prepared and the burst temperature was measured on an individual specimen under optical microscope with cooling stage. The range of scatter of the burst temperature increased with decreasing specimen’s size. While the upper limit of the scatter remained nearly the same, the lower limit of the scatter markedly decreased with decreasing specimen’s size. The distribution of potential embryos was deduced by fitting the simulated distribution of burst temperatures to the observed result by adjusting the assumed distribution of potential embryos used for the simulation.

Abstract: Tantalum (Ta) addition is one of the promising method for increasing the strength of Ni-based wrought alloys such as Inconel 718, because Ta is an element having a high melting temperature. For wrought alloys, it is necessary to make clear the phase relationships at 700~1000°C, but there is a few report on phase diagrams of Ni-Ta and Ni-Ta-Al systems at those temperatures. In this study, the phase diagram in Ni-rich region of Ni-Ta-Al system at 800°C, which is the important temperature for the practical use of the wrought alloys, was investigated. The equilibrium relations of each phase were examined by a conventional XRD, SEM/EDX and TEM observations. It was found that the γ-phase region expanded considerably towards Ta-rich compositional region in Ni-Ta-Al system at 800°C. Also, it was observed that the γ phase precipitated secondarily in the primary precipitated γ’ phase in Ni-10.5mol%Ta-5.5mol%Al alloy. Ni8Ta phase was not detected even in Ni-Ta binary system in this study, although this phase was reported previously.