Grain Boundary Energy And Migration In Al And Ni Investigated By A Synthetic Driving Force-molecular Dynamics Method

Grain boundaries, as a significant part of material microstructure, directly affect material properties, such as mechanical and chemical properties. Grain boundary energy (GBE) and grain boundary mobility (GBM) are two parameters characterizing GB properties, both are anisotropically distributed in the microstructure, and depict strong dependences on GB orientation and temperature. Quantitative measurements and calculations of GBE and GBM can provide important information in exploring the mechanisms of microstructure evolution during recrystallization and grain growth.In this work, molecular dynamics (MD) was used to construct a bicrystal model, which can completely characterize orientations of various GBs. With this model, we calculated energies of GBs in Al and Ni, and discussed the correlation between GBE and GB orientation. A modified synthetic driving force MD (SDF-MD) method was proposed to study GB migration. GBM values of various GBs were calculated, and the effects of orientation, driving force and temperature on boundary migration have been explored. Finally, the present results were compared with existing experimental data and modeling results from other methods. The main results and conclusions from this work are as follows:(1) GBE results of<100>,<110>,<111>STGB and<111>TWGB in Al and Ni indicate that GBE depends on all the five parameters of GB orientation, and shows no specific relation with misorientation angle and coincidence factor. For any of the four GB types, the GBE in Ni, which is of higher potential energy than Al, is larger than that in Al, but GBE in both metals shows same dependence on the misorientation angle.(2) Compared to the original SDF-MD method, the modified method introduces a subtracted order parameter to characterize the deviation of atom arrangement in the whole bicrystal from the original state. Grain and boundary atoms are distinguished based on an order parameter threshold value and a definition of trans-GB region. The threshold value depends on the atom arrangement and temperature, and the trans-GB region characterizes the GB width. These modifications overcome successfully the problem of the original method in making the bicrystal unnormally suffering forces.(3) GB migration results calculated by the modified method reveal that, the migration velocity does not change with the migration direction and increases linearly with the driving force; mobility, showing no specific relation with misorientation angle and coincidence factor, still depends on the GB plane for GBs with the same misorientation. For GBs with the same misorientation axis, mobility in both metals almost shows the same dependence on the misorientation angle, but the mobility of each GB in Al does not show obvious differencs with that in Ni.(4) The mobility results of Al<111>STGB calculated by the modified method show same orientation and temperature dependences with other experimental and simulation results, in which GB is driven under the interface energy. However, the GBM values of Ni<110> and<111> STGB by the original and modified methods differ not only in specific values but also in their variation with orientations. Mobility extracted by the modified method shows a clear dependence on GB orientations and temperature.