摘要 :
Adopting glass fluxing and cyclic superheating, high undercooling up to ～ 550 K was achieved in bulk eutectic Ni_(78.6)Si_(21.4) alloy melt. With increasing undercooling, the as-solidified microstructure shows an interesting evol...
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Adopting glass fluxing and cyclic superheating, high undercooling up to ～ 550 K was achieved in bulk eutectic Ni_(78.6)Si_(21.4) alloy melt. With increasing undercooling, the as-solidified microstructure shows an interesting evolution, i.e., regular lamellar eutectic, coarse directional dendrite, quasi-spherical dendritic colony, fine directional dendrite, fine quasi-spherical dendritic colony, and superfine anomalous eutectic. In combination with different theories for nucleation and growth, the microstructure evolution was analyzed and described using competitions incorporated in rapid solidification of the bulk undercooled eutectic Ni_(78.6)Si_(21.4) alloy. For undercooling below and above 180 K, Ni_3Si, and α-Ni are primarily solidified, respectively. This phase selection can be ascribed to competitive nucleation. As undercooling increases, a transition of the prevalent nucleation mode from site saturation to continuous nucleation was interpreted in terms of competition of nucleation mode. Accordingly, the superfine anomalous eutectic is obtained, due to the substantially increased continuous nucleation rate, i.e., grain refinement occurring at high undercooling (e.g., ～ 550 K).
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Effects of secondary flow of the melt on the separation of inclusions from molten aluminum flowing in a square channel by a solenoid were investigated theoretically and experimentally. Numerical methods were used to calculate the ...
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Effects of secondary flow of the melt on the separation of inclusions from molten aluminum flowing in a square channel by a solenoid were investigated theoretically and experimentally. Numerical methods were used to calculate the secondary flow, the separation efficiency of inclusions, and the particle motion. It is found that there appear two recirculating loop flows in a quarter cross section of the channel and the separation efficiency of inclusions is significantly improved by the secondary flow mainly owing to the mixing effect. The separation efficiency increases with the increase of the effective magnetic flux density and the frequency of magnetic field, and decreases significantly with the increase of the size of the separator channel for a constant value of a/δ. However, it is possible to achieve high separation efficiency by using large-sized square channels and high frequency magnetic field with the help of the mixing effect of secondary flow. The computed results of particle trajectories show that the secondary flow accelerates the transportation of the particles from the inner region to the vicinity of the wall and greatly shortens the separation time of those particles. The effects of secondary flow on the separation efficiency were confirmed by comparing the measured separation efficiency with the computed results.
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Stability analysis of a growing solid/liquid interface is the fundamental concept of modern solidification theory. Here, serial laser rapid solidification experiments wee performed on a hypoperitectic Ti_47Al_53 alloy to explore t...
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Stability analysis of a growing solid/liquid interface is the fundamental concept of modern solidification theory. Here, serial laser rapid solidification experiments wee performed on a hypoperitectic Ti_47Al_53 alloy to explore the dendritic growth behavior near the limit of high-velocity absolute stability. SEM and TEM techniques were carried out to investigate the microstructure and identify the phase composition. By adopting an improved sampling method of TEM, the growth morphology evolution of the laser-resolidified layer was observed directly and high-velocity banding structure was firstly detected in Ti-Al peritectic alloys.
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High undercooling (about 392 K) was achieved in bulk eutectic Ni_(70.2)Si_(29.8) alloy melt using glass fluxing combined with cyclic superheating. Surprisingly, the microstructure of eutectic Ni_(70.2)Si_(29.8) transits from irreg...
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High undercooling (about 392 K) was achieved in bulk eutectic Ni_(70.2)Si_(29.8) alloy melt using glass fluxing combined with cyclic superheating. Surprisingly, the microstructure of eutectic Ni_(70.2)Si_(29.8) transits from irregular to quasiregular when the undercooling exceeds 340 K. The morphology transition at large undercooling is analyzed based on the solution entropy of eutectic phases. The forming process of quasiregular microstructure is ascribed to a transition from faceted-faceted to nonfaceted-nonfaceted eutectic concurring with increased undercooling.
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Laser rapid solidification experiments were performed on a Ti-53at%Al hypoperitectic alloy to investigate the dendritic growth behavior near the limit of high velocity absolute stability. By adopting an improved sampling method of...
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Laser rapid solidification experiments were performed on a Ti-53at%Al hypoperitectic alloy to investigate the dendritic growth behavior near the limit of high velocity absolute stability. By adopting an improved sampling method of TEM, the growth morphology evolution of the laser-resolidified layer was observed directly. High velocity banding structures was firstly detected in Ti-Al peritectic alloys. The high velocity banding structures are parallel to the solid-liquid interface and made of the oscillation structures grown alternatively in modes of cell and plane morphologies. In the light bands with cellular growth mode, all dislocation assembles parallel to the growth direction and forms cell boundaries; while all dislocation distributes randomly in the dark bands. The determined growth velocity range for the appearance of banding structures is 0.5-1.1 m/s and the origin of the banding structure agrees well with the prediction of the CGZK phenomenological model.
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An Fe-30 at/100 Co hyperperitectic alloy is undercooled in B_2O_3 glass slag to reveal dirctly the fragments of the primary dendrite. Different morphologies of the fragments have been observed by TEM technique. The fragments of th...
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An Fe-30 at/100 Co hyperperitectic alloy is undercooled in B_2O_3 glass slag to reveal dirctly the fragments of the primary dendrite. Different morphologies of the fragments have been observed by TEM technique. The fragments of the primary dendrite are isolated in the remaining liuqid after recalescence. The mismatch of the crystal structures of the primary phase and the subsequently solidifed equilibrium phase makes it possible to have the fragments recognizable after solidifcation. The relation between the radius of primary trunk and the theoretical tip radius is discussed on the basis of Rayleigh instability.
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The directional dendrite growth velocity in the Cu_70 Ni_30 alloy melt undercooled by 90～185 K was measured by high speed cinematography, and compared with the calculated free dendrite growth velocity. It was found that at lower ...
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The directional dendrite growth velocity in the Cu_70 Ni_30 alloy melt undercooled by 90～185 K was measured by high speed cinematography, and compared with the calculated free dendrite growth velocity. It was found that at lower undercoolings the interaction between the adjacent directionally growing dendrites is weak, and there is not marked difference between the velocity of the directionally growing dendrite and that of free dendrite. But with undercooling increasing, the interaction is enhanced due to the decrease of the primary arm spacing, which makes the velocity difference enlarges quickly.
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