Papers by Keyword: Dendrite

Abstract: Surface quality as well as internal quality of cast products of aluminum alloys are strongly affected by the process of initial solidification. Control of solidified structure in this region is therefore quite important. In order to understand the growth of solidified grain, crystallographic characterization has been performed using EBSD (Electron Backscattered Diffraction) in this study. Al-6 mass%Si alloy was cast at 750°C on the chill plate. Longitudinal cross section of solidified shell was analyzed. In the region of initial solidification, many small crystals nucleated on the mold surface. The crystallographic orientations of these grains were random. It is normally found that an unfavorable grain was eliminated by a favorable grain. However, occasionally, we have found that an unfavorable grain enlarged its size. In this case, dendrite, the growth direction of which was far from the heat flow direction, gradually changed its crystallographic orientation from unfavorable one to favorable one. The grain enlarged its size by multiplication of dendrite arms. Crystallographic orientation of dendrite changed little by little when it branched. This kind of phenomena may take place in unsteady condition, such as initial solidification region.

Abstract: This study was designed to determine the effect of the solidification rate on the acoustic properties of the bronze alloy of 20% wt. Sn. Copper and commercially pure tin is melted in a furnace to a temperature 1000, 1100 and 12000C. The melted metal is poured into molds variation temperature of 200, 300 and 4000C. Materials castings were cut and machined for specimen damping capacity test. The results showed that the reduction in mold temperature leads to an increase solidification rate, which causes the shortness of the solidification time. The variation of the solidification rate affects on the morphology of the microstructure and acoustical properties of the material. By increasing the solidification rate influence on the secondary dendrite arm spacing (SDAS) decreases. It causes the material hardness increases and the damping capacity of material decreases. There is a significant correlation between the material hardness and the damping capacity of materials.

Abstract: In this research, heat treatment is the final finishing process applied on nanocrystalline CoNiFe to improve microstructure for good hardness property. Nanocrystalline CoNiFe has been synthesized using the electrodeposition method. This study investigated the effect of heat treatment at 500°C, 600°C, 700°C and 800°C on electrodeposited nanocrystalline CoNiFe. The heat treatment process was performed in the tube furnace with flowing Argon gas. By changing the heat treatment temperature, physical properties such as phase and crystallographic structure, surface morphology, grain size and hardness of nanocrystalline CoNiFe was studied. The nanocrystalline CoNiFe phase revealed the Face Centered Cubic (FCC) and Body Centered Cubic (BCC) crystal structure. FESEM micrographs showed that the grain sizes of the coatings were in the range of 78.76 nm to 132 nm. Dendrite shape was found in the microstructure of nanocrystalline CoNiFe. The nanocrystalline CoNiFe prepared in heat treatment temperature of 700°C, achieved the highest hardness of 449 HVN. The surface roughness of nanocrystalline CoNiFe heated at 700°C was found to be smaller than other temperatures.

Abstract: In this study, we present a numerical technique for the improvement of computational efficiency for computation of microstructural evolution in alloy during solidification process. The goal of this technique is for the computational domain to grow around the microstructure and fixed the grid spacing, while solidification advances into the liquid region. The growth around the microstructure is controlled according with the solute diffusivity for binary alloy in the liquid region. The computation showed that the microstructure with well-developed secondary arms can be obtained with low computation time and moderate memory demand.

Abstract: The electrochemical behavior of zinc electrode with indium addition in 35%KOH(or saturated with ZnO) solutions has been investigated systematically by electrochemical methods including linear polarization, potentiostatic polarization, potentiodynamic anodic polarization, potential-time measurements at a constant current density, combining the observations of scanning electron microscopy (SEM). It is indicated that the indium addition makes the corrosion potential of Zn shifted positively and its corrosion current increased. Galvanostatic results showed that the addition of indium shortened the passivation time, indicating indium is an active element to the electrochemical performance of zinc alloy electrode. The potentiostatic polarization curves of the pure zinc plate and zinc-indium alloy electrodes in a 35%KOH solution saturated with ZnO indicated that the addition of indium improved the cathodic polarization of alloy electrodes and the deposition overpotential,mean while it depressed the deposition morphology of zinc on the electrode and accelerated the dendrite growth. Scanning electron microscopy images showed that the addition of indium aggravated the corrosion of zinc electrode which may be responsible for the increased tendency to passivation at high current densities. It has been found that at low current densities the reaction kinetics may be increased by indium addition , which is agreement with the discharging test of actual alkaline batteries.

Abstract: A modified cellular automaton model for describing the dendritic solidification of pure substance was developed. Instead of using the high mesh-induced anisotropy capture rules, such as Von Neumann’s and Moore’s method, a new capture rule---random zigzag method was developed, which greatly reduced the mesh-induced anisotropy in crystallographic orientation. The calculation method for the solid/liquid interface curvature was also improved. The effect of interfacial energy anisotropy on the dendritic growth behavior was analyzed.

Abstract: Bulk metallic glasses (BMGs) and high entropy alloys (HEAs) have unique structures at the length scales of micro- and nano-metre, and exhibit unique properties, which make them potential materials for structural applications. The tensile ductility of dendrite/BMG composites can be greatly improved by Bridgman solidification. The BCC structured HEA with a composition of Ti0.5AlCoCrFeNi exhibits ultrahigh fracture strength which is competitive to most of the BMGs, moreover, the strength can be sustained at high temperatures. The phase changes of HEAs are closely related to the atomic packing efficiency (APE).

Abstract: We have developed the refinement process of the microstructure of metallic materials by imposition of electromagnetic vibration force during solidification. This process is effective for both wrought magnesium alloys and cast ones. By simultaneous imposition of a static magnetic field of 10 Tesla under an alternative electric current of 60A, the average grain sizes of the AZ31B wrought alloy and the AZ91D cast alloy were obtained about 50 micron in both alloys. The grain size was affected by electric current frequency and had the minimum value at the special electric current frequncy of 500 to 2000 Hz and 900 Hz for wrought alloy and cast alloy, respectively. From experimental results, we suggested the mechanism of refinement of microstructure during solidification by imposition of electromagnetic vibration force. The cavitation phenomenon in liquid phase during electromagnetic vibration was effective to break down th esolid phase. And also the difference of electric conductivity between the solid phase and the liquid one brought vigorous vibration of the solid phase. Then the solid phase was suppressed its growth.

Abstract: Dendrite needles grow from an undercooled melt and their shapes depend on the temperature distribution on the solidification front, which are specified by some parameters such as undercooling, capillary length, diffusivity, convection and kinetic effects. Neglecting the convection and kinetic effects, this study numerically computes the quasi-steady-state integral-differential equation to obtain the shape of a dendrite using solvability condition and investigates the effect of parameters changing the temperature field on the shape of a dendrite. The results reveal that the tip shape enlarges with the decreasing undercooling and increasing capillary length. On the other hand, the increase of thermal diffusivity only slightly reduces the tip radius and shape of a dendrite.

Abstract: Rather than designated directly as solid if the micromesh (or cell) larger than a nucleus is chosen as the nucleation site, the growth of a nucleus in the cell is considered in the application of the modified cellular automaton model to simulate the evolution of dendritic microstructures in the solidification of Al-Cu alloy. The growth velocity of a nucleus or a dendrite tip is calculated according to the KGT (Kurz-Giovanola-Trivedi) model, which is the function of the undercooling. In this study, the dendritic microstructures, such as the free dendritic growth in an undercooled melt and the dendritic growth in the directional solidification, are simulated with the modified growth algorithm in the nucleation cell. The simulated results for the temporal and final morphologies are shown and are in agreement with the experimental ones.