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The melting and solidification of Ag was simulated by molecular dynamics method. The structural transfor- mation of Ag during the metal solidification and amorphous crystallization was analyzed based on the variations of the internal energy, radial distribution function (RDF), common neighbor analysis (CNA), and atomic visualization technique. The simulation results showed that the embryonic crystals similar to body-centered cubic (bcc-like) structure (about 11%) already exist in the liquid metal, the content of bcc increases with cooling and it is up to 31% near the solidifying. About 24% bcc structure has in the amorphous structure. The volume per atom of bcc is larger than that of fcc, which is beneficial in the cooling liquid metal and amorphous structure. In the nucleation process, bcc-like embryonic crystals can be used as the nucleus with disorder atoms near the bcc to transform fcc crystal. There is no fcc and hcp structure in the cooling melt and amorphous structure. They are formed in the solidification directly.
本文采用分子动力学方法研究了Ag-Cu共晶合金的结晶过程。依靠体系内能变化、公共近邻分析和原子可视化技术对Ag-Cu共晶合金的结构演变进行了分析。模拟结果表明，在10%~40%Cu范围内，随着Cu含量的增加，合金形成非晶结构的临界冷却速度减小，玻璃化转变温度增加。公共近邻分析的结果表明，在非晶晶化前的阶段，表征非晶结构的1551、1541和1431键对占据主要部分，但也存在少量的bcc和fcc晶核团簇。其中，表征正二十面体团簇的1551键对，随着Cu含量的增加而增加，表征缺陷二十面体团簇的1541和1431键对则相应减少，这表明，随着Cu含量的增加非晶体系的稳定性增加。与此同时，表征晶体相结构的键对随Cu含量的增加都有所减少，其中，bcc晶核的数量减少较小，但fcc晶核的数量却有较大的下降，从而使非晶晶化的形核核心减少，这也从结构上解释了非晶合金的玻璃化转变温度随成分增加而增加的现象。在非晶晶化以后，虽然体系以fcc结构为主，但仍存在一些无序的非晶态结构，这些结构存在于共晶两相界面，随Cu含量的增加而增加。利用可视化技术，我们可以发现，随着Cu含量的增加，Ag-Cu合金的共晶组织存在从固溶体形、网状共晶到层状共晶的演变。The crystallization of Ag-Cu alloys was studied by molecular dynamics method (MD) with embedded atom potential (EAM). The structural developments of Ag-Cu alloys were analyzed based on the variations of internal energy, common neighbor analysis (CNA), and atomic visualization technique. The simulation results showed that with the increase in Cu composition, the critical cooling rate of amorphous formation decreased and the glass-transition temperature of amorphous increased under the same heating rate. The results of CNA showed that the amorphous structure is main, and a few crystal clusters (bcc and fcc) are in it. Among the amorphous bond pairs, the pairs 1551 increases with the increase in Cu composition, which is the character of regular icosahedrons cluster. Meanwhile, the pairs 1541 and 1431, which are the character of defect icosahedrons clusters, reduce correspondingly. These show that the stability of amorphous increases. With the increase in Cu composition, the pairs 1441, 1661 and 1421 are all reduced, the 1441 and 1661 decrease little and the 1421 decreases great, which implies that the nuclei reduce during the crystallization of amorphous and the glass-transition temperature increases. After crystallization, the fcc structure is dominant but there are a few defect icosahedrons clusters between the eutectic boundaries. Moreover, the eutectic structure of Ag-Cu alloys can be transformed from solid solution, net-like into the lamellar morphologies with composition during the solidification and crystallization.