Papers by Keyword: Nucleation Mechanism

Abstract: The foaming and nucleation and growth mechanism of soybean oil-based polyurethane (SPU) were determined by the degree of hydrogen bonding, and isocyanate groups. New types of SPU were prepared by the different NCO/OH molar ratio (isocyanate index) from 1.0 to 2.0 in a soy polyol/polyether polyol (MDI) system. Foaming and nucleation and growth mechanisms of SPU were studied by fluorescence microscope (FM), scanning electron microscope (SEM), energy disperse spectroscopy (EDS) and Fourier transform infrared spectroscopy (FT-IR). It indicated that the isocyanate index affected remarkably the velocity of foaming and the critical nucleation radius of SPU and the ester functional group increased with the increase of isocyanate index. The nucleation and growth phase transition were dominated by the diffusion controlled nucleation and isocyanate content was the key factor of foam formation.

Abstract: The Al-Cu-Mg-Ag-Ce-Er alloy wires with different extrusion ratio (λ=12, 25, 50, 100) were produced by hot extrusion at 450oC. The effects of extrusion ratio on the microstructures, tensile strength and elongation of Al-Cu-Mg-Ag-Ce-Er alloy wires were researched by means of OM, SEM, TEM and mechanical test.The results show that with the extrusion ratio increasing, the average grain size decreased from 83μm to 42μm, the Al₂Cu, Al₈Cu₄Ce and Al₈Cu₄Er phases was broken gradually, and the homogeneousdistribution about these second-phases in the alloy wires increased. The tensile strength increased from 366MPa to 459MPa with extrusion ratio, and the elongation initially decreased and then increased with the increase of extrusion ratio. Dynamic recrystallization for Al-Cu-Mg-Ag-Ce-Er alloy occurred at different extrusion ratios. Withincreasing of extrusion ratio, the main nucleation mechanism of dynamic recrystallization changed from the sub-grain coalescence at lower extrusion ratioto the acceleration of second-phases for nucleation at higher extrusion ratio.

Abstract: The weldability of the steel can be improved by formation of intra-granular ferrite (IGF) in heat affected zones (HAZs) on the edge of weld bead. The nucleation mechanism of IGF of Ti-killed high strength low alloyed (HSLA) steel has already been investigated with the aid of transmission electron microscope. Titanium oxides (Ti₂O₃) particles with the diameter of 0.4μm and Si-rich complex inclusions (Ti₃O₅+MnS) with that of 0.5μm can serve as the nuclei of IGF. The nucleation mechanism of IGF is proposed as follows: (1) inclusions are inert substrate. (2) The depletion of the austenite former Mn local to the inclusion increases the thermodynamic driving force of γα for transformation. (3) Lattice matching between inclusion and ferrite reduces the interfacial energy of opposing nucleation.

Abstract: The powder metallurgy (PM) nickel-base superalloy GH720 was extruded at 1100°C and the as-extruded samples were treated at 1110°C during 10min, 20min and 30min apart. The microstructure and recrystallization behavior of as-extruded alloy GH720 were discussed. The results indicate that there are two different types of γ' phases: primary γ' phase is mainly distributed on the grain boundary with an average size ranging from 0.3 to 0.5μm and dissolves partially at 1110°C during 30min, whereas the secondary γ' phase is distributed inside the grains with an average size of 50nm and dissolves largely at 1110°C during 30min. Recrystallization mechanism is subgrain nucleation when as-extruded GH720 is treated at 1110°C at which a lot of γ' phase exist. γ' phase inhibits growth of recrystallized grain availably and the uniform, fine and equiaxed grains in GH720 were formed during the extrusion and following heat treatment process.

Abstract: The formation of 111-112 and 111-110 recrystallization textures during annealing of cold rolled low carbon steels at low heating rate was investigated by electron backscatter diffraction (EBSD) and X-ray diffraction techniques. The orientation characteristics during recrystallization of this steel were determined, results show that there is a strong competition between the 111-112 component and the 111-110 component along the γ-fiber. The former was developed from the deformed matrix with the same orientation by means of subgrain coalescence at early stage of recrystallization, while the latter nucleated at the grain boundary areas of deformed grains with 111-112 or 112-110 orientations by means of preferred nucleation and evolved into stable recrystallization texture at later stage of recrystallization.

Abstract: In the present work, the oriented nucleation origin of the recrystallized {h11}<1/h,1,2> fibre is characterized. Aiming to investigate the substructural evolution of <110>//RD fibre grains and {001}<110> grains in particular, a detailed microstructure and texture analysis is performed by high resolution orientation scanning microscopy on a cross-rolled sample. The reason to work with cross-rolled material is the increased incidence of rotated cube orientations after cross rolling. The present data have revealed the presence in the deformed substructure of a crystallite volume that has rotated towards the {311}<136> component in the interior of <110>//RD fibre grains as a result of a grain fragmentation process. Preliminary simulations of the deformation texture suggested that the observed orientation fragmentation might be produced by strain localizations of a shear band nature.

Abstract: The nucleation mechanism during the epitaxial graphene films on Si-terminated SiC (0001) surfaces was investigated by atomic force microscopy (AFM) and Raman scattering spectrum. By imaging the change of Si-terminated SiC substrate surfaces, we observed the process of the initial nucleation and the wrinkle formation of graphene. The nucleation of epitaxial graphene phase initiates at 1450°C and the wrinkle formation depends on the thermal decomposition time.

Abstract: The structural and electronic properties of bulk and (001) surface of TiAl₃ have been examined by the first-principles total-energy pseudopotential method based on density functional theory. The lattice constants and heat of formation of bulk TiAl₃ we obtained are in good agreement with the experimental and other theoretical values. The calculated bulk properties indicates that bonding nature in TiAl₃ is a combination of metallic and ionic, in which the metallic bonding become the predominate one. the strongest hybridization exist in the DO₂₂ structure, the Al-3p and Ti-3d bonding of TiAl₃ play the dominant role in hybridization. The structural relaxation and surface energy for (001) slab have been simulated to make sure the stability of slabs with different atomic layers. Compared to TiB₂ (0001) slab, TiAl₃ surfaces shows smaller structural relaxation and lower surface energy, furthermore, the charge redistribution of (001) slab shows almost the same characteristics as bulk TiAl₃, which confirms structural stability of TiAl₃ with (001) slab. This present work makes a beneficial attempt at exploring TiAl₃ surface as an ab initio method for studying possible nucleation mechanism of Aluminum on it.

Abstract: The effects of Zr addition on the microstructure and magnetic properties of the sintered NdFeB magnets were investigated by X-ray diffraction (XRD) and BH magnetic tester. The XRD analysis showed that both the Zr-free and Zr-doped samples are composed of main amount of Tetragonal phase Nd2Fe14B (P42/mnm) and trace amount of Nd-rich phase, but adding Zr reduces significantly the values of pole density factor of (004), (006) and (008) crystal faces for the sintered Nd(Fe,Zr)B magnets estimated by Horta-formula. Accordingly, the coercivity of the sample with Zr addition is reduced from 2038 kA/m down to 1951 kA/m. The study of the Kronmüller-plot shows that the nucleation is the dominating mechanism for the magnetization reversal in these two kinds of magnets, and two microstructural parameters of αk and Neff are obtained also.

Abstract: Microstructure evolution during dynamic recrystallization (DRX) of hot deformed GH625 superalloy was investigated by optical microscope (OP) and transmission electron microscope (TEM). Hot compression tests were conducted using Gleeble-1500 simulator. It was found that the nucleation mechanism of DRX for the alloy deformed at 1150°C is composed of discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) in the vicinity of the serrated grain boundaries. With the increasing strain, the fraction of the DRX grains increases, while the size of the DRX grains almost remains in the same range. As the deformation temperature increasing, the size and fraction of the DRX grains increase, and no precipitation of intergranular carbides are found when the deformation temperature increases to 1150°C. At lower strain rate, the size and volume fraction of DRX grains decrease with the increasing strain rates. However, the size and volume fraction of DRX grains increase at higher strain rates due to the deformation thermal effect.