Gang WANG
,
Dechang ZENG
,
Zhongwu LIU
金属学报(英文版)
The phase-field model of a liquid-to-solid transition was constructed where the model parameters were linked quantitatively to the interfacial properties, and the variation of nucleation barrier height in undercooled metallic melts with respect to undercooling was studied respectively based on two kinds of forms of local free energy density. The calculation results show that, with the increase of undercooling, the critical nucleus does not show bulk properties, and the nucleation barrier height decreases gradually and deviates more and more from that predicted by the classical nucleation theory in both cases. The physical spinodal occurs for a specific form of the local free energy density, where the nucleation barrier height vanishes when the undercooling reaches a critical value and the reduced nucleation barrier height can be expressed by a function of the ratio of undercooling to critical undercooling.
关键词:
Nucleation
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null
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Renguo GUAN
,
Zhanyong ZHAO
,
Runze CHAO
,
Hongqian HUANG
,
Chunming LIU
金属学报(英文版)
Heat transfer of flow melt and grain refining mechanism during melt treatment by the cooling sloping plate were investigated. The results show that the cooling sloping plate can refine not only grains of alloys but also can obviously refine pure metal. Cooling ability of the plate is the key factor that induces grain refining, the plate material and the flow amount can affect cooling rate of the melt and thus affect refining effectiveness. The cooling rate of the melt on the cooling sloping plate is much faster than that of the conventional casting process, which can reach 1000 K/s and belongs to meta-rapid solidification scope. The thickness of the temperature boundary layer is much larger than that of the velocity boundary layer on the sloping plate, but the temperature gradient is small in the temperature boundary layer. Under strong cooling action by the cooling plate, most parts of the melt on the plate surface can form undercooling, which causes continuous eruptive nucleation, this is the main grain refining mechanism, and the heterogeneous nucleation on the plate surface is a helpful supplement for the nucleation.
关键词:
Grain refining
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杜桂香
,
康志荣
,
宋金玲
,
赵江红
,
宋昌
,
朱珍平
新型炭材料
从理论和实验角度研究了金属-碳协同催化的化学气相沉积反应中碳纳米管的成核和生长过程.结果表明:多壁碳纳米管的成核和生长不仅受金属的催化作用,碳核一旦形成也会促进碳纳米管向轴向和径向的生长.金属催化剂颗粒仅仅在促进最内层碳核的形成及生长,碳原子向有序的石墨结构转化有催化作用.多壁碳纳米管和单壁碳纳米管形成的本质区别在于是否存在碳的自催化作用.
关键词:
碳纳米管
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协同催化
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化学气相沉积
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成核
,
生长
,
自作用
Qingyan XU
,
Weiming FENG
,
Baicheng LIU
材料科学技术（英文）
A 3D stochastic modeling was carried out to simulate the dendritic grains during solidification of aluminum alloys, including time-dependent calculations for temperature field, solute redistribution in liquid, curvature of the dendritic tip, and growth anisotropy. The nucleation process was treated by continuous nucleation. A 3D simplified grain shape model was established to represent the equiaxed dendritic grain. Based on the Cellular Automaton method, a grain growth model was proposed to capture the neighbor cells of the nucleated cell. During growing, each grain continues to capture the nearest neighbor cells to form the final shape. When a neighbor cell was captured by other grains, the grain growth along this direction would be stopped. Three-dimensional calculations were performed to simulate the evolution of dendritic grain. In order to verify the modeling results, the predictions were compared with the observation on samples cast in the sand mold and the metal mold.
关键词:
3D stochastic modeling
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null
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