An Atomistic Study On Shock Induced Plasiticity And Phase Transtion Of Iron

Since α(?)ε phase transition of iron, as the first example of high pressure phase transitions, had been inferred from shock experiments at 1950s, it has been attracting lots of efforts devoted to uncover the transformation processes and its mechanisms. Due to its transient and reversible, analyses of the free surface velocity profile have long been a main mean to access this problem because of the restrictions of experimental techniques. It is not until recent decade, experimental detection techques have enabled a direct observation of the transition of iron under shock compressions. Still, it is hard to capture the whole processes of the phase transformation under shock compressions at present. Therefore, some theoretical methods should be employed to understand experimental data. To date, great achievements have been made in studying the phase transition of iron via combinations of non-equilibrium molecular dynamic (NEMD) simulations and experiments, for example, experiments have successfully verified the phase transition mechanism of iron firstly observed in NEMD simulations. However, some severe problems are encountered by these NEMD simulations, such as the emergence of quantites of fake FCC phases and the lackness of proceding plastic processes. The underlying reason of these problems is that the interatomic potential of iron employed in these NEMD simulations is constructed under ambious pressure conditions without condering high pressure applications.In this dissertation, we construct a new interatomic potential of iron by firstly generating lots of potential at ambient conditions and then choosing the optimal one based on whether it satisify the cretiria for high pressure applications, which involve phase transition pressure and equation of states at high pressure here. After varication statically and dynamically, we find that the new potential of iron could well overcome the shortcoming of previous potentials (lacking of preceding plasticity and emerging large fractions of fack FCC phases). Thus, a faithful potential of iron for the studies of shock induced phase transition and plasticity has been constructed here.Effects of phase transition on the quasi-isentropy have been studied by ramp compressions upon single crystalline iron with the new potential of iron. The results suggest that the rising in temperature is mainly contributed by the phase transition which will destroy the "quasi-isentropy" of ramp compressions. The rising in temperature decreases with the growing ramp rising time and, increases with the growing final particle velocity. However, these two factors do not affect the quasi-isentropy in terms of ramp slope or vp/rrising. It is explained by deriving an analytic relation between the "quasi-isentropy" and the two parameters of ramp compressions. Our results show that the max power of vp, working on the quasi-isentropy, is n/vpn-1, while the quasi-isentropy is linearly dependent on rrising according to Swegle-Grady law. Besides, we find that formation of the first shock wave results from the compressing induced lattice instabilities. But the shock wave does not cause a big rising in temperature until it is overdriven by the behind plastic wave or phase transition wave.In order to study microscopic coupling mechanism between plasticity and phase transition of iron under shock compressions, a new c axis analysis method, lattice analyses and some related crystallographic methods have been proposed herein. With these proposed methods, the nature of the coupling between plasticity and phase transition of iron is uncovered via the interaction between plasitic mechanism and phase transition mechanism. The results indicate that only stress assisted phase transition (SAT) mode happens in the shock upon single crystalline iron, while both stress assisted and strain induced phase transition (SIT) mode happen in the shock upon polycrystalline iron. By comparing these two phase transition modes kinetically and dynamically, we show that they are distinctly different in the paths of transformation, which acts as an important indicator of different physical processes. In the SAT mode, the phase transition begins with a compression along<001>BCC direction, and finishes with shuffling among alternative{110}BCC planes. The compression in the first step contributes to the driven force of phase transition via accumulating strain energy, which eventually leads to the phase transformation. In the SIT mode, the phase transformation could be interpreted by two basic processes:firstly compress along [110]BCC and [110]BCC to form a FCC lattice, and then slip along [101]BCC among every two (101)BCC planes. However, the two processes are happened nearly simultaneously so that FCC phase does not emerge during the phase transition. Plastic slippage, due to large local stress concentrations, contributes to the driven force of the phase transition via the strain field of relevant lattice defects. According to characters of each phase transition modes, it could be inferred that the martentic variant selection rules of these two transition modes satisfy strain energy criterion and Schmid factor criterion, respectively, which shows a good consistency with our simulation results.