Showing papers on "Directional solidification published in 2016"

Defect characterization of β-Ga2O3 single crystals grown by vertical Bridgman method

Abstract: The characteristics of structural defects observed on (100) wafers in β-Ga2O3 single crystals grown by directional solidification in a vertical Bridgman furnace were studied in terms of crystal growth conditions. No high-dislocation-density regions near the wafer periphery were observed owing to the lack of adhesion between the as-grown crystal ingot surface and the crucible inner wall, and directional solidification growth in a crucible with a very low temperature gradient resulted in β-Ga2O3 single crystals with a low mean dislocation density of 2.3 × 103 cm−2. Line-shaped defects up to 150 µm long in the [010] direction were detected at a mean density of 0.5 × 102 cm−2, which decreased with decreasing growth rate. The line-shaped defect structure and formation mechanism were discussed.

Fabrication of lamellar porous alumina with axisymmetric structure by directional solidification with applied electric and magnetic fields

Abstract: A novel directional solidification method was used to fabricate lamellar porous alumina with an axisymmetrical structure by combining multiple cold sources, electrostatic field, and magnetic field during the freezing of aqueous alumina slurries. Multiple cold sources with mutually perpendicular directions formed by the bottom and parallel sides of rectangular copper molds induced a specific growth direction for ice crystals. The pore channels of porous ceramic are arranged parallel to the direction of electric field lines, and the axisymmetrical structure was obtained after applying the electrostatic field. The applied magnetic field can reduce pore channel size; however, the field has no effect on pore channel direction and distribution, and leads to cracking in the central region of porous ceramics. The compressive strength along the electrostatic field direction is higher than that achieved by conventional freeze casting.

Degenerate seaweed to tilted dendrite transition and their growth dynamics in directional solidification of non-axially oriented crystals: a phase-field study

Abstract: We report the results of a phase-field study of degenerate seaweed to tilted dendrite transition and their growth dynamics during directional solidification of a binary alloy. Morphological selection maps in the planes of (G, Vp) and (e4, Vp) show that lower pulling velocity, weaker anisotropic strength and higher thermal gradient can enhance the formation of the degenerate seaweed. The tip undercooling shows oscillations in seaweed growth, but it keeps at a constant value in dendritic growth. The M-S instability on the tips and the surface tension anisotropy of the solid-liquid interface are responsible for the formation of the degenerate seaweed. It is evidenced that the place where the interfacial instability occurs determines the morphological transition. The transient transition from degenerate seaweed to tilted dendrite shows that dendrites are dynamically preferred over seaweed. For the tilted dendritic arrays with a large tilted angle, primary spacing is investigated by comparing predicted results with the classical scaling power law, and the growth direction is found to be less sensitive to the pulling velocity and the primary spacing. Furthermore, the effect of the initial interface wavelength on the morphological transition is investigated to perform the history dependence of morphological selection.

Dendritic segregation and arm spacing in directionally solidified CMSX-4 superalloy

Abstract: Dendritic segregation of directionally solidified columnar and single-crystal CMSX-4 was investigated. The partition coefficients derived from EDS measurements on as-cast samples in both transversal and longitudinal sections have shown strong segregation tendency of elements either to dendrite cores (Re, W) or to the interdendritic regions (Al, Ti, Ta). The dendritic segregation as a function of various secondary dendrite arm spacings (SDAS) has highlighted that Ti segregation is more sensitive to the SDAS than that of the other elements. It was also found that Cr has a tendency to accumulate at the coarse gamma prime–matrix interface. The solidification partition coefficients calculated with Pandat™ and the PanNi thermodynamic database showed a good agreement with the experimental data. Pandat™ calculations using the local chemical composition of segregated areas were also used for the optimisation of the solutioning heat treatment window. Primary (λ1) and secondary (λ2) arm spacings as important paramet...

Large-scale Phase-field Studies of Three-dimensional Dendrite Competitive Growth at the Converging Grain Boundary during Directional Solidification of a Bicrystal Binary Alloy

Abstract: 1) Faculty of Mechanical Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto, 606-8585 Japan. 2) Graduate School of Science and Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto, 606-8585 Japan. 3) Division of Materials Science and Engineering, Faculty of Engineering, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo, 060-8628 Japan. 4) Department of Materials Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656 Japan. 5) Global Scientific Information and Computing Center, Tokyo Institute of Technology, 2-12-1-i7-3, Ohokayama, Meguro-ku, Tokyo, 152-8550 Japan.

Effect of B, Zr, and C on Hot Tearing of a Directionally Solidified Nickel-Based Superalloy

Abstract: The effect of the minor elements B, Zr, and C on the castability of a Nickel-based γ′-strengthened superalloy has been investigated. Tube-like specimens were prepared by directional solidification where the rigid ceramic core leads to hoop stresses and grain boundary cracking. It was found that an important improvement in castability can be achieved by adjusting the minor elemental composition. Too low C (≤0.15 pct) and too high B and Zr contents (≥0.05 pct) lead to material that is very prone to solidification cracking and should be avoided. The results cannot be rationalized on the basis of the current models for solidification cracking. Instead, pronounced hot tearing is observed to occur at high amounts of γ/γ′-eutectic and high Zr contents. The critical film stage where dendrites at the end of solidification do not touch and are separated by thin liquid films must be avoided. How Zr promotes the film stage will be discussed in the paper.

Microstructural Control of Colloidal-Based Ceramics by Directional Solidification Under Weak Magnetic Fields

Abstract: The use of weak magnetic fields to control the microstructural evolution of colloidal-based systems in conjunction with directional solidification is demonstrated as a convenient processing route to fabricate anisotropic ceramic scaffolds with complex microarchitectures. A variety of graded and aligned microstructures were formed by applying external static magnetic fields oriented radially, axially, and transversely with respect to the solidification direction of freezing slurries containing micro/nanoparticles of ZrO2 and Fe3O4. The graded structures, formed by the radial and axial fields, resemble core–shell architectures composed of dense outer perimeters surrounding porous inner cores. The aligned structures, formed by transverse fields, exhibit two modes of microstructural alignment: lamellar walls aligned by the growing ice crystals and mineral bridges aligned by the magnetic fields. The alignment of mineral bridges that connect adjacent lamellae, provide these scaffolds enhanced strength and stiffness when compressed parallel to their orientation (parallel to the direction of the magnetic field).

Refinement and growth enhancement of Al2Cu phase during magnetic field assisting directional solidification of hypereutectic Al-Cu alloy

Abstract: Understanding how the magnetic fields affect the formation of reinforced phase during solidification is crucial to tailor the structure and therefor the performance of metal matrix in situ composites. In this study, a hypereutectic Al-40 wt.%Cu alloy has been directionally solidified under various axial magnetic fields and the morphology of Al2Cu phase was quantified in 3D by means of high resolution synchrotron X-ray tomography. With rising magnetic fields, both increase of Al2Cu phase’s total volume and decrease of each column’s transverse section area were found. These results respectively indicate the growth enhancement and refinement of the primary Al2Cu phase in the magnetic field assisting directional solidification. The thermoelectric magnetic forces (TEMF) causing torque and dislocation multiplication in the faceted primary phases were thought dedicate to respectively the refinement and growth enhancement. To verify this, a real structure based 3D simulation of TEMF in Al2Cu column was carried out, and the dislocations in the Al2Cu phase obtained without and with a 10T high magnetic field were analysed by the transmission electron microscope.

In-situ fabrication of amorphous/eutectic Al2O3–YAG ceramic composite coating via atmospheric plasma spraying

Abstract: The microstructure and property of plasma-sprayed Al2O3–Y3Al5O12 (YAG) composite coatings were investigated. The sprayable feedstock was composed of Al2O3/Y2O3 granules obtained from the spray drying. As-sprayed Al2O3–YAG composite coating showed obvious amorphous characteristics. Post-heat treatment facilitated the complete crystallization of the Al2O3–YAG composite coating. The heat-treated Al2O3–YAG ceramic coating possessed the deposition microstructure comprising three-dimensional interpenetrating network of α-Al2O3/YAG eutectic phases, which is very similar to the directional solidification microstructure of Al2O3/YAG eutectic bulk ceramics. This eutectic coating microstructure does not correspond to a traditional eutectic microstructure. Al2O3–YAG ceramic coating with eutectic composition is obtained from the crystallization of an amorphous solid ceramic instead of the solidification of the liquid. Furthermore, the plasma-sprayed Al2O3–YAG eutectic ceramic coating with the heat treatment exhibits excellent microstructure and performance stabilities under high temperature.

Microstructure, fracture toughness and compressive property of as-cast and directionally solidified NiAl-based eutectic composite

Abstract: Microstructure and room temperature mechanical property of as-cast and directionally solidified NiAl-Cr(Mo) eutectic composite were investigated by SEM, EDS, three point bending test and compression test. The microstructure of as-cast alloy consists of Cr(Mo) primary dendrites and equiaxed eutectic cells. For the directional solidification of liquid metal cooling technique (LMC), only the small quantity of Cr(Mo) primary dendrites appear in the beginning of solidification, and the fully and well-aligned lamellar structure parallel to the growth direction is obtained in the steady-state zone. However, the Cr(Mo) primary phase is always observed in the whole directional solidification of zone melted liquid metal cooling technique (ZMLMC), and the eutectic lamellas are disordered and not parallel to the growth direction in the steady-state zone. Compared to the ZMLMC alloys, the LMC alloy has the better room temperature fracture toughness and compressive property due to the well-aligned lamellar structure. Crack propagation and fracture surface are also observed to understand the fracture behavior.

Crystallization Behaviors of Blast Furnace (BF) Slag in a Phase-Change Cooling Process

Abstract: Blast furnace (BF) slag is the principal byproduct formed during the iron-making process, which generates a considerable amount of thermal energy. However, to simultaneously recover the maximum amount of residual heat and obtain vitreous slag using a low conduction coefficient of air is a primary challenge of the dry granulation technique, which has not been solved by either fundamental research or engineering applications. In the current study, the evolution of temperature inside molten BF slag was experimentally investigated using the directional solidification technique to calculate the local heat-transfer coefficient and the average cooling rates. Furthermore, the effects of the average cooling rate on the vitreous phase content of the BF slag during solidification were investigated based on X-ray diffraction (XRD) technology. The results indicate that the average cooling rate from the cooling side to the adiabatic side gradually decreased due to the increase in heat resistance and latent heat release...