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Modeling solidification microstructure and microsegregation of U-6%Nb binary alloy during solidification process

LI Qiang,MA Yingche,LIU Kui,KANG Xiuhong,LI Dianzhong,

金属学报 , 2007,
Abstract: An improved model of calculating solid fraction is presented in this paper that the solid fraction of solid/liquid interface zone can be expressed as an second order equation of one variable based on calculating phase diagram, instead of calculating the solid fraction based on the assumptions of the position and the shape of the solid/liquid interface, which make the physical meanings of the model more clearly. Using the model the solidification microstructure and solute microsegregation of U-6%Nb binary alloy are simulated. The simulated results show that Nb concentration in the initial nucleated position is high. As the solidification proceeds, Nb concentration of the nucleus decreases continuously due to the back diffusion in the solid phase. At the end of solidification, there are poor Nb concentration zones formed at the grains boundary where Nb concentration can be lower than 3%.

金属学报(英文版) , 2004,
Abstract: Microstructure evolution of wrought aluminum alloy extruded rods and the mechanism of liquid phase formation during reheating were investigated. And the relation between the volume fraction of liquid phase and the recrystallization microstructure was proposed. The results show that increase in reheating temperature and time can augment the volume fraction of liquid phase and accelerate the grain spheroidization, as a result of which the requirement of semi-solid forming can be satisfied. Due to the higher aberration energy of grain boundary, the melting point is lowered as a result of the easy diffusion of atoms. At higher reheating temperature the grain boundary melts, the growth of the recrystallized grain is inhibited and the grain is refined. The composition of the low melt-point phase along the recrystallized grains was determined using EDS. It can be seen from the experimental results that when the extrusion rod of the wrought aluminum alloy is reheated at 610℃ for 20min, perfect fine equiaxial grains can be obtained, the average grain size is about 66.34μm and the volume fraction of solid phase is about 68%.
Microsegregation and Improved Methods of Squeeze Casting 2024 Aluminium Alloy
Yong ZHONG,Guoyue SU,Ke YANG,

材料科学技术学报 , 2003,
Abstract: The squeeze casting of a 2024 Al alloy was carried out to investigate the effect on microsegregation in the alloy of the application of pressure followed by diffusion annealing. The experimental results indicate that an optimum applied pressure followed by an optimum diffusion annealing process can markedly reduce the degree of microsegregation and improve the mechanical properties to a degree that can approach the level of forged 2024 Al alloy.


物理学报 , 2001,
Abstract: The microstructure and crystallization process of an amorphous Cu56Zr44 alloy at isothermal annealing were studied by using the X-ray diffraction, differential scanning calorimetry and transmission electron micrograph techniques. The experimental results seem to show that the short-range order of amorphous Cu56Zr44 alloy at room temperature is of hard sphere random dense packing. It is found that the crystallization products of amorphous Cu56Zr44 alloy annealed at 703K within its supercooled liquid range is mainly Cu8Zr3 phase in an annealing time of 3min, and when the time is 6min, Cu8Zr3 phase disappears completely and Cu10Zr7 phase precipitates a lot, while if the time is longer than 30min, several kinds of phases such as Cu10Zr7, CuZr2 and Cu8Zr3 will form. Based on the above the diffusion mechanism of amorphous Cu56Zr44 alloy at isothermal annealing is presented.
Microstructure Evolution of AZ91D Magnesium Alloy During Semi-Solid Isothermal Heat Treatment Under Direct Current

ZHOU Quan,YANG Yuansheng,TANG Junli,HU Zhuangqi,

金属学报 , 2006,
Abstract: The influences of direct current density, heating temperature, and holding time on the microstructure evolution of magnesium alloy AZ91D during semi-solid isothermal heat treatment were studied, and the related mechanism was also discussed. When heating temperature and holding time are constant, with the increment of current density the primary phase becomes small and smooth. But if current density is too high, the primary grains become coarser. Direct current can accelerate the course of semi-solid isothermal heat treatment and improve globular effect of dendrites obviously. It is proposed that the activation energy is increased by direct current, so the diffusivity of solute atoms is increased, which accelerates the microstructure evolution. The effect of electromigration, Joule heat and Peltier heat caused by direct current will promote remelting and globuling of dendrites.
Microstructure Characteristics of Ni-Nb Near Eutectic Alloy during EBFZM Directional Solidification

Jinjiang YU,Jun ZHANG,Hengzhi FU,Jinchen ZHAO,

材料科学技术学报 , 2001,
Abstract: Microstructure Characteristic of Ni-Nb near eutectic alloy is systematically investigated during directional solidification with electron beam floating zone melting (EBFZM). The effect of the Zone melting rate on the microstructure has also been studied.
Phase field simulation of the columnar dendritic growth and microsegregation in a binary alloy

Li Jun-Jie,Wang Jin-Cheng,Yang Gen-Cang,

中国物理 B , 2008,
Abstract: This paper applies a phase field model for polycrystalline solidification in binary alloys to simulate the formation and growth of the columnar dendritic array under the isothermal and constant cooling conditions. The solidification process and microsegregation in the mushy zone are analysed in detail. It is shown that under the isothermal condition solidification will stop after the formation of the mushy zone, but dendritic coarsening will progress continuously, which results in the decrease of the total interface area. Under the constant cooling condition the mushy zone will solidify and coarsen simultaneously. For the constant cooling solidification, microsegregation predicted by a modified Brody-Flemings model is compared with the simulation results. It is found that the Fourier number which characterizes microsegregation is different for regions with different microstructures. Dendritic coarsening and the larger area of interface should account for the enhanced Fourier number in the region with well developed second dendritic arms.
Numerical simulation of dendrite growth and microsegregation formation of binary alloys during solidification process
Li Qiang,Guo Qiao-Yi,Li Rong-De,
李 强

中国物理 B , 2006,
Abstract: The dendrite growth and solute microsegregation of Fe-C binary alloy are simulated during solidification process by using cellular automaton method. In the model the solid fraction is deduced from the relationship among the temperature, solute concentration and curvature of the solid/liquid interface unit, which can be expressed as a quadric equation, instead of assuming the interface position and calculating the solid fraction from the interface velocity. Then by using this model a dendrite with 0 and 45 degree of preferential growth direction are simulated respectively. Furthermore, a solidification microstructure and solute microsegregation are simulated by this method. Finally, different Gibbs-Thomson coefficient and liquid solute diffusing coefficient are adopted to investigate their influences on the morphology of dendrite.
Microsegregation and dendritic growth mode of Al-5wt%Cu alloy based on non-equilibrium mush zone model  [PDF]
Xiaoping Ma
Physics , 2013,
Abstract: The microsegregation and dendritic growth mode of Al-5wt%Cu alloy was investigated. In the early solidification stage, the crystal growth mode of interrupted growth and periodic boundary trapping will happen, which results in the segregationless dendritic grains. Microsegregation only exists at the final solidification stage with extremely tiny residual melt fraction. In the tiny residual melt zone, the diffusion of solute from the enriched boundary layer to the residual melt and the convergence of enriched boundaries produce the final microsegregation.

LI Wen,JIN Tao,HU Zhuangqi,

金属学报 , 2008,
Abstract: The Ni-based single crystal superalloy was bonded by the transient liquid phase (TLP) bonding using Ni-15Cr-3.5B amorphous ribbon as an insert alloy. The microstructure characteristics in the joint during TLP process were evaluated by OM, SEM and TEM. The electron back scattering diffraction (EBSD) method was applied to determine the crystallographic orientation between the bonded interlayer and substrates. The results indicated that the joint consists of a bond, diffusion and base zones, in the meantime M23B6 +γ and MB+γ eutectics formed in the bond center and the fine M3B2 phases occurred in the diffusion zone. The γ' phases both in the bonded interlayer and in the superalloy substrate are almost identical after homogenization treatment. Due to the epitaxial growth of the isothermal solidification fronts from each mating surface, the crystallographic orientation between the bonded interlayer and the bonded substrate has a good match.
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