Papers by Keyword: Directional Solidification

Abstract: Recently, high strength at high temperature can be achieved by inducing kink bands in alloys having aligned lamellar microstructure. However, the kink-bands formation has been confirmed only in alloys with lamellar microstructures, where slip plane is limited to the plane parallel to the lamellar interface, and not confirmed in alloys with rod-like or Chinese script microstructures. In this study, we clarified the contribution of rod-like Si phases in Al-Si alloy on the mechanical properties and focused on the feasibility of introduction of kink bands in the alloys without lamellar structure. The results showed that in Al-Si eutectic alloys, the non-lamellar second phase, i.e., the Si phase, is aligned by directional solidification, and refined by rolling. The directionally-solidified sample showed high yield strength with long and aligned Si phase, while the rolled samples showed high ductility with refined microstructure. The rolled samples were uniformly deformed in all the samples with variety of reduction ratios, and wedge-shaped deformation bands were observed after the compression test, especially in the 5-10% rolled specimens. Crystallographic orientation analysis indicated that these deformation bands were not kink bands but were localized slip bands.

Abstract: Microstructures and mechanical properties of directionally solidified Mg-xGd (5.21, 7.96 and 9.58 wt.%) alloys were investigated at a wide range of growth rates (V = 10-200 μm/s) under the constant temperature gradient (G = 30 K/mm). The results showed that when the growth rate was 10 μm/s, different interface morphologies were observed in three tested alloys: cellular morphology for Mg-5.21Gd alloy, a mixed morphology of cellular structure and dendritic structure for Mg-7.96Gd alloy and dendrite morphology for Mg-9.58Gd alloy, respectively. Upon further increasing the growth rate, only dendrite morphology was exhibited in all experimental alloys. The microstructural parameters (λ₁, λ₂) decreased with increasing the growth rate for all the experimental alloy, and the measured λ₁ and λ₂ values were in good agreement with Trivedi model and Kattamis-Flemings model, respectively. Vickers hardness and the ultimate tensile strength increased with the increase of the growth rate and Gd content, while the elongation decreased gradually. Furthermore, the relationships between the hardness, ultimate tensile strength, the growth rate and the microstructural parameters were discussed and compared with the previous experimental results.

Abstract: The effect of Ca on the microstructure and mechanical properties of directionally solidified (DSed) Mg-3Zn-xCa alloys (x=0.2,0.5,0.8wt.%) was investigated in the present work. The results showed that the DSed samples with the growth rate of 120 μm/s had columnar dendritic structures and the primary dendritic arm spacing (PDAS) decreased with the content of Ca increase. The TEM result indicated that the growth orientation of the DSed Mg-Zn-xCa alloys was , which was independent of the content of Ca. The tensile tests at room temperature showed that the mechanical properties of the DSed Mg-Zn-xCa alloys were strongly affected by the content of Ca. The addition of Ca remarkably improved the ultimate tensile strength (UTS) and the yield strength (YS), while dramatically reduced the elongation (El). Prismatic slip and twinning were the main deformation mechanisms in tensile tests.

Abstract: The Nb-24Ti-12Si-14Cr-2Al-2Hf-0.1Y (at.%) alloys were fabricated by directional solidification with selected withdrawal rate 1.2 and 18 mm/min, followed by a heat treatment at 1375 °C for 10 h. The microstructure of directional solidified samples were composed of Nb_SS, Cr₂Nb and eutectics (Nb_SS+Nb₅Si₃), aligning with the growth direction. After heat treatment, the Nb_SS in the eutectic structures and Nb_SS dendrites were connected to form the matrix, and the silicide and Cr₂Nb tended to spheroidize. The sample prepared by higher withdrawal rate plus heat treament shows higher average K_Q values. The results suggested that the Nb-Si based alloy showed higher room-temperature fracture toughness when the microstructure consists of continuous Nb_SS distributed with finer Nb₅Si₃ and Cr₂Nb.

Abstract: The directional solidification process of Sn–10 wt% Bi alloy with low melting point was observed by synchrotron X-ray imaging technology. The mold temperature was controlled, and the dynamic images of a series of alloy solidification behavior were obtained. The results show that columnar crystal grows in dendrite morphology. It is also found that dendrite morphology changes at different mold temperature. With the decrease of the mold temperature, the dendrite morphology begins to change from irregular to regular, and finally, the primary dendrites and the secondary dendrites are perpendicular to each other.

Abstract: Solidification of alloys in a thermal gradient usually involves the generation of flows by thermal or thermosolutal convection. We experimentally study their effects on the dynamics of a solidification interface by inducing a controlled sweeping flow in a directional solidification device. Flow is induced in the sample from an external thermosiphon. Downstream inclination of microstructures and downstream sidebranch development are observed. However, the major outcome is the evidence of large scale travelling waves on the solidification interface. They are induced by the coupling between solidification and flow and yield repetitive striations of the solid phase. Two waves are observed and characterized.

Abstract: In this paper, we review our results from phase field simulations of tilted dendritic growth dynamics and dendrite to seaweed transition in directional solidification of a dilute alloy. We focus on growth direction selection, stability range and primary spacing selection, and degenerate seaweed-to-tilted dendrite transition in directional solidification of non-axially orientated crystals. For growth direction selection, the DGP law (Phys. Rev. E, 78 (2008) 011605) was modified through take the anisotropic strength and pulling velocity into account. We confirm that the DGP law is only validated in lower pulling velocity. For the stability range and primary spacing selection, we found that the lower limit of primary spacing is irrelative to the misorientation angle but the upper limit is nonlinear with respect to the misorientation angle. Moreover, predicted results confirm that the power law relationship with the orientation correction by Gandin et al. (Metall. Mater. Trans. A. 27A (1996) 2727-2739) should be a universal scaling law for primary spacing selection. For the seaweed-to-dendrite transition, we found that the tip-splitting instability in degenerate seaweed growth dynamics is related to the M-S instability dynamics, and this transition originates from the compromise in competition between two dominant mechanisms, i.e., the macroscopic thermal field and the microscopic interfacial energy anisotropy.

Abstract: Directional solidification is a paradigm process to gain the desired microstructure via certain applied solidification parameters. A thorough understanding of the diffusion-limited solid-liquid interface morphology evolution from initial transient to steady state is of uppermost importance to optimize the solidification processes. The rapid development of quantitative phase-field model provides a feasible computational tool to explore the underlying physics of the morphological transition at different stages. On basis of the diffusion-limited quantitative phase-field simulations using adaptive finite element method, the directional solidification of Al-4wt.%Cu alloy is characterized and both the solid interface propagation speed and solute profile are analyzed. The simulations are then compared with the in situ and real-time observation by means of synchrotron radiation x-ray radiography image. Good agreements are obtained between simulations and experimental data. Detailed mechanism that controls the morphological instability and transition are then addressed.

Abstract: Large temperature gradient was introduced to improve the removal rate of metal impurity in silicon ingot during direction solidification. The concentration of metal impurities in the silicon ingot with a large temperature gradient is 0.96 ppmw. The solidification time is reduced by 20% due to the fast speed of crystal growth improved; meanwhile the purity is increased by 64%.

Abstract: The paper investigates the novelty of application of end chills in fabricating Aluminum alloy metal matrix composites. An effort has also been made to evaluate the effect of chill material on the soundness of the castings obtained. The required composites were prepared using LM-25 Aluminum alloy as matrix material in which different weight percent of Borosilicate glass particles were added ranging from 3 wt.% to 12 wt.%. The variation in weight percent was brought about in steps of 3%. The fabrication of the composites was carried out in sand molds by incorporating two metallic (copper and Steel) and two non-metallic (Graphite and Silicon carbide) end chills. The specimens for strength and hardness tests were prepared as per ASTM standards and the specimens were drawn from near chill-end as well as from farther away from chill end. The microstructure of the specimens reveal a refined grain structure proving the sound quality of the castings. The result analysis also leads to the conclusion that metallic chills are more beneficial as compared to non-metallic chills for obtaining a good quality composites. Copper chill with a high volumetric heat capacity proved to be the best chill material amongst the others.