Molecular dynamics simulations of effects of pressure and void size on melting of aluminum

Molecular dynamics simulations are performed using isobaric-isoenthalpic (NPH) ensembles to implement the concept of defect nucleated melting. The effect of void size on the melting temperature is studied for bulk as well as particulate aluminum. The interatomic interactions are simulated using the Glue potential and a combination of structural and thermodynamic parameters such as the potential energy, Lindemann index, translational-order parameter, and radial-distribution functions are employed to characterize the melting process. The mechanism of the whole melting process is examined using snapshots of time evolution of the atomic positions and density contours. Molecular dynamics studies have been used as a tool to provide detailed information about the time evolution of system at the atomic level, providing unique insight into the atomic mechanisms involved during melting. For the bulk studies, crystals composed of 864, 2048 atoms are considered. The phenomenon of defect nucleated melting is investigated using voids of different shapes and sizes and results are compared with pure crystals without any voids. Voids with different shapes but same effective void size are considered for bulk as well as particulate phase to explore the effect of void shape. For the particulate phase aluminum, spherical nanoparticles up to 8.5 nm are considered. For comparative analysis results for 5.5 nm and 8.5 nm particles, composed of 5072 and 20736 atoms respectively, are investigated. The bulk is characterized by a sharp increase in structural and thermodynamic properties, whereas the particulate phase involves surface pre-melting. A perfect crystal simulating bulk with periodic boundary conditions is associated with structural melting, predicting melting point greater than thermodynamic melting point. From the study of bulk crystals with 864 and 2048 atoms, it is concluded that the ratio between structural and thermodynamic melting points (f = T_s/T_m) for aluminum is 1.32. The effect of defect nucleated melting is negligible in case of nanoparticles because of the presence of surface which acts as a nucleation site. Increasing the void size beyond a critical value results in the collapse of the crystal and the limiting value depends on the number of atoms considered in pure crystal. The effect of pressure on the defect nucleated melting of aluminum has also been investigated and is found to be insignificant.