Simulation Study On Solidification Processes And Microstructure Evolution Characteristics Of Liquid Metal Lead

Some basic areas are firstly reviewed briefly, including the characteristic of mi-crostructures in liquid and glass metal, the basic theories on solidification, amorphous-crystallizing and solid-solid phase transition, and the development of the computer sim-ulation for the liquid as well. Then the simulation methods are validated by comparing the computational results with the corresponding experimental results, and several micro-structural quantifications adopted in the present works are introduced in detail. Based on a series of effective approaches (such as the pair distribution function, the cluster index method (CTIM-2), the mean square displacement (MSD), and the non-gaussian parame-ter (NGP)), the mechanism of the microstructure evolution during the solidification under different conditions are studied systematically from different viewpoints.A simulation study for the influence of cooling rate on the evolutions of micro-clusters structures has performed. And the results show that there is a critical cooling rate in the range of5x1012and1x1012K·s-1separating glass forming and crystal form-ing. When the cooling rate is higher than the critical cooling rate, amorphous structures are formed mainly with the1551,1541and1431bond-types. When the cooling rate is lower than the critical cooling rate, the crystal structure mainly with the1421and1422bond-types (the basic clusters of (1200066) and (12000120)of hcp and fcc coexisted) are finally formed. At the same time, it has been found that there are obvious effects of the cooling rate on the relative proportion of the fcc basic cluster to the hcp basic clusters, the smaller the cooling rate is, the bigger relative proportion of the fcc basic cluster, and the system tends to form highly perfect fcc crystal structure.The rapid solidification process at the cooling rate of5x1011K·s-1for a system containing10,000Pb atoms is simulated to reveal the microstructure evolution focusing on the bcc-hcp, bcc-fcc transformation mechanisms. It is demonstrated that with temper-ature decreasing, the system undergoes twice phase transitions:liquid stateâ†'metastable bcc phaseâ†'more stable phase-the hcp and fcc coexisted crystal structure. The hcp and fcc crystal structures are also competing to each other during the transformation process. The detailed trace to the concrete transformation processes illustrates that the bcc-hcp and bcc-fcc transformation are corresponding to the revised Burgers mechanism and Bain mechanism respectively, which provides valuable complement to theory and experiment. It is also found that the hcp and fcc coexistence will transformed back to bcc phase at high temperature during the rapid heating process, in other words, the martensitic transforma-tion in metal Pb is revisable.Going further, the isothermal relaxation process of supercooled liquid and glassy Pb have been investigated by molecular dynamics simulation. The formation and evolution characteristics of bcc phase during isothermal crystallization are analyzed with the bond-type index method, cluster-type index method and tracing method. It is found that the metastable bcc phase plays an important role as precursor for crystallization. And the stability of the bcc phase strongly depends on the initial temperature and structure:at the higher temperature with the a supercooled liquid initial state, the bcc phases can form and stably maintain in the relaxation processes; while at the lower temperature with a glassy initial state, the bcc phase form at first and then partially transform into hcp phase, just metastable. When the initial temperature is lower than153K, the hcp and fcc phase in-cline to directly form in the glass structure without undergoing a metastable bcc phase. This result can be well explained from the view point of the free energy barrier:at rel-ative high temperatures, the bcc phase has the lowest free energy barrier, so it is easy to form and keep stable; as temperature decreases, the free energy barrier for hcp and fcc phases are lowered, and both the two phases are easy to form and without undergoing the metastable bcc phase. However, take the structure fluctuation and kinetic influence into consideration, the lowest free energy barrier rule is the most probable case, but does not always work, such as the case at233K in our simulation.Based on the above work, the correlation of dynamic character with the microstruc-ture evolution is studied, and the physical interpretation has also been provided. Re-sults suggest that, the supercooled liquid and glassy Pb exhibit the dynamic heterogene-ity during the relaxations; and the lower the temperature, the more apparent the dy-namic heterogeneity. According to the time dependence of thermodynamic, dynamic and structural functions in supercooled liquid and glassy Pb, we plot the temperature-time-transformation (TTT) diagram for metal Pb. Furthermore, the simulation study is also found that the relaxation, nucleation and crystallization can be consistently correlated: the β—relaxation regime is corresponding to minor structural rearrangement because of the "cage effect", while the a—relaxation regime relates to a more diffusive movement favoring the microscopic configuration rearrangement, where the nucleation and growth take place; and the appearance of the second plateau of MSD and the non-zero plateau of NGP is corresponding to the completion of crystallization. Therefore, the nucleation and growth is a diffusive atom rearrangement process, which exhibits three distinct stages of nucleation, increase of nuclei and the coarsening of crystalline grain.