Atomistic Simulation Of The Phase Transition In Singal-craystal Al Induced By High-pressure

Aluminum (Al) is the most abundant metallic element in the earth’s crust, and one of the common materials of industrial materials. The study about the basic characteristics of Al can help us to understand the knowledge of the material science and geophysics.In this thesis, By molecular dynamics simulations employing an embedded atom method potential, we have investigated fcc-hcp structural transformations in single crystal Al caused by uniaxial strain loading along the [001],[011] and [111] directions. The results show that the structural transition is strongly dependent on the crystal orientations. The entire fcc-hcp structural transformation only occurs when loading along [001] direction, and the transition pressure is about90GPa. While for the [011] and [111] loadings, the fcc-hcp structural transition pressures are14GPa and17GPa respectively.The dynamics structural transformation mechanism and the structural transformation characteristics of Al under uniaxial strain loading along [001],[011] and [111] directions have been discussed. For [001] loading, after the structural transformation, about80%atoms exist in twins in the hcp structure and the twin boundaries exist in the (110)-plane. It is found in the process of the structural transformation that the (010) fcc or (100) fcc planes transit into (0001) hcp planes, and the twins of the hcp phase along the (112)-plane appear, whose boundaries finally become along the (110)-plane. The structural transformation mechanism can be realized by a two-step process. Firstly, the fcc (010)-plane and the (100)-plane transit to the hcp (0001) plane; secondly, the atoms in the area where the fcc (010)-plane transits to the hcp (0001)-plane slip along the [100]-and [100]-directions, and atoms in the area where the fcc (100)-plane transits to the hcp (0001)-plane slip along the [010]-and [010]-directions, and that finally results in the hcp phase.The unloading process for [001] loading is analyzed in our work. The pressure of reverse transition is about71GPa, hysteresis with the fcc-hcp structural transition. The mechanism and morphology evolution of backward transitions are analyzed in detail. We find the twinning (along the fcc (110)-plane) in the hcp structure prior to the back transition (hcp-fcc).For [011] and [111] loadings, we find that the structural transformation mechanisms are completely different from the [001] loading. The results indicate that only20%atoms transit to hep phase and the appearance of the hcp phase owes to the partial dislocation moving forward on fcc{111}-family. For [011] loading, the hcp phase grows to form laminar morphology in four planes which belong to the fcc{111}-family; while for [111] loading, the hcp phase grows into laminar structure in three planes which belong to the fcc {111}-family except the fcc (111)-plane. Our simulations show the coexistence of fcc and hcp phases over a wide range of pressures. The coexistence between the fcc-Al and hcp-Al phases over a relatively wide pressure region could be attributed to the small total-energy difference between both phases.The fcc-bcc structure transformation in single crystal Al caused by uniform compression is investigated in this thesis too. Results show that the transition pressure is about250GPa, in reasonable agreement with the calculated value through density functional. The transition time increases as an exponential function with the decreasing of the pressure. Our simulation indicates that the structural transformation mechanism can be realized through four motions of the lattice:compression, shear, slid, and rotation. The distance of atoms in the fcc [211]direction in (111) plane and in the fcc [211] direction in (111) plane are compressed, and the distance of atoms in [011] direction are elongated accompanying a shear course along the [010] and [001] direction respectively. In the mean time, the atoms in the (111) and (111) planes slid along [211] and [211] directions respectively. A rotation with about6.5°of fcc (111) and (111) planes follow on the heels of the above three motions of the lattice. After the four motions, the fcc (111) and (111) planes transit to bcc (011) plane, and the fcc (011) planes, which are the interfaces of the two nucleating planes, grow to form the boundaries after the phase transition.The structural transformations induced by the uniaxial strain loading along [001],[011],[111] directions and uniform compression in single crystal Al has been investigated at atomic scale in this thesis. The results of the study about the structural transformations and mechanisms of Al provide important basis for the micro-research on the phase transitions.