Simulation Study On The Microstructure Evolution During Phase Transition Under Rapid Cooling Of Copper And Copper-zircoium Alloy

After briefly reviewing the development of basic solidification theory, the industrial application of rapid solidification technology and the importance of computer simulation in scientific research and industry, the origin, development and application of Molecular dynamics(MD) simulation were summarized, from general principles to specific implements including interatomic potentials, simulation ensembles and computer algorithms. Various microstructural analysis methods including pair distribution function, HA bond-types method and the Cluster Type Index Method-3(CTIM-3) are also clarified. Then the microstructure evolution during the phase transition of rapid cooling MD simulations for liquid metal Copper(Cu) and Copper-Zirconium(Cu-Zr) alloy are extensively investigated, and the results are summarized.A MD simulation of rapid cooling at 1×1012 K/s is conducted. The twice changes on the energy curve reveals that crystallization takes places within 707 K and 622 K. With more than 80% of the atoms being involved, the details of structural evolution during crystallization are extensively examined by CTIM-3. Marked by the critical changes in number percentages of different basic clusters, the phase transition experiences several sequential stages(partly overlapped). Particularly, there is an ico-saturated stage existing in T ∈(792K, 745K) where the number of icosahedrons-like structures keeps stable, which is believed to play an important role for stabilizing the super-cooled liquid and breeding the precursor of crystal--the metastable body-centered cubic(bcc) structures. At the end of such saturation stage, the bcc clusters begin to rapidly increase. At the beginning of phase transition of 707 K, the ico-bcc-like clusters start to decrease abruptly, and the most stable clusters of face-centered cubic(fcc) increase rapidly. At 622 K, the end of this phase transition, only fcc clusters still keep a rather high increase rate, resulting in a final solid at 300 K mainly composed of fcc structures. Therefore the phase transition of melt Cu follows the Ostwald’s rule of stages under rapid solidification.Focusing on icosahedrons, the formation and evolution mechanism of nano-clusters in a 106-atoms Cu64.5Zr35.5 alloy was investigated. Linked by 1551 HA-pairs, icosahedrons comprise the so-called IS-ICO nano-clusters, and their sizes(the total number of atoms in the IS-ICO nano-clusters) form a serial of magic numbers of 19, 23, 25, 27, 29, 31, 33, 35, 37, 41, 43, … and so on. The morphology formed by the central atoms of icosahedrons can be chain-like, triangle-tailed and quadrilateral-tailed. With the highly local symmetry of icosahedrons, the IS-ICO nano-clusters have higher structural stability, resulting in the stronger mechanical stability and higher loading capability of Cu-Zr amorphous solid that contain a abundant of IS-ICO nano-clusters.Based on 5 solidification simulations from different temperatures, the effect of initial melt temperatures on microstructure and mechanical properties of amorphous Cu46Zr54 alloy have been studied. It is found that the prominent HA bond-types are(1551、1541 and 1431) that are closely related to amorphous, and the icosahedral(12 0 12 0 0 0 0 0 0) and defective icosahedral(12 0 8 0 0 0 2 2 0) play a key role during the microstructural evolution. And the effect of initial melt temperatures does not present until T< Tg. For the 5 amorphous solids at 300 K, the main amorphous bond-types, the major basic clusters, the elastic constants, and the extent of structural softening are non-linearly related to the initial melt temperature, varying in a certain range. While there exists a best initial melt temperature of 2000 K, from which the final amorphous solid obtained holds the highest packing density, the most optimised “strength”, and the smaller extent of structural softening under deforming.