Atomistic Studies On Effect Of Cylindrical Nanopores On Shock-induced Phase Transformation In Single Crystalline Iron

The study of the mechanical and structural response behavior of solids under extreme high-pressure is important to understand geophysical, astronomical processes and technological application. Since iron is the main component of inner core of Earth and has been widely used in modern industry and defense, the high-pressure phase transition of iron has been investigated extensively. It is noteworthy that, in real engineering materials, there are different kinds of defect inevitably or artificially, the pre-existing defects can affect phase transformation property of material under shock loading. However, up until now the micro-mechanism that how the cylindrical nanopores influences the phase transformation of iron is yet to be systematically clarified. So, it is meaningful to investigate the effects of cylindrical nanopores on the phase transformation of iron.In the present work, non-equilibrium molecular-dynamic simulations with our modified analytic embedded-atom model potential were carried out on single crystalline iron of idealized cylindrical nanopores to study shock-induced phase transformation. Our work concludes as followings:(1) The cylindrical nanopore influences the shock response. Reflection wave can be observed when shock wave swept over the cylindrical nanopore, and the temperature of the region around the cylindrical pore was much higher than the other regions. In addition, the nucleation and growth of dislocations can be clearly observed under [110] and [111] loading directions, and the surfaces of cylindrical nanopore act as effective sources for dislocations. The existence of cylindrical pore decreases the critical stress of phase transformation, and the cylindrical pore is the favorite nucleation site of the phase transformation of iron for different loading directions; (2) The cylindrical nanopore influences the martensitic variants. The existence of cylindrical nanopore increased a martensitic variants for different loading direction. For the shock along [001] direction, the increased martensitic variants is (101) variants, for the shock along the [110] and [111] directions, the increased martensitic variants is (1-10) variants; (3) Stress assisted transformation mechanisms and strain induced transformation mechanisms were discussed. For the shock along [001] direction, (110), (1-10) and (101) variants were generated through the strain induced transformation mechanisms. For the shock along [110] direction, (110) variants was generated through the strain induced transformation mechanisms and (1-10) variants was generated through the stress assisted mechanisms; (4) The number of variants' atoms were investigated. Cylindrical pores can slow down the increase rate of the old variants while promote the nucleation of the new variants. In addition, there is a critical pore size which determines whether the nucleus of the new variants could grow up.The effects of cylindrical nanopores on the phase transformation of iron were investigated. To draw these conclusions, I hope to provide a theoretical basis and guidance for future experimental studies of the phase transformation of iron with defect.