Metal Dynamic Transformation And Destruction Of Molecular Dynamics Simulations

In this thesis, the dynamic structural transition in single crystal iron and dynamic failure in single crystal aluminum have been studied with molecular dynamics simulations, where the embedded-atom method was used to describe the interatomic forces. The main research contents are as following:(1) We successfully simulated the bcc to hcp structural transition in single crystal iron under isothermal compression along the [001] direction. It is revealed that the above transition is conducted by the adjacent (110) or (110) faces shuffling relatively along the [110] or [110] directions. The softening of C33and hardening of C3](or C32) prior to the transition, as well as the over-relaxation of the stress after transition, were discovered. The hcp growth can be divided into four stages:hcp homogeneously nucleated, columnar grains formed, nuclei competed and merged, and a laminar structure formed along{110} planes. In the mixed phases, hcp phase has negative shear stress and its potential is higher than that of the bcc phase.(2) The shock-induced bcc to hcp transition in iron was simulated. We obtained the pressure intervals of double wave structure and the mixed phase. The simulation results reveal that the compression process can be divided into five stages:elastic compression, softening of elastic constant, structural transition, over-relaxation and elastic compression of high-pressure phase. The stress change was analyzed by the corresponding slip amplitude.(3) Further, the reversibility of bcc to hcp structural transition under [001] uniaxial strain loading and unloading were analyzed. The stress history indicates a super-elastic deformation in the sample. The phase boundaries for the bcc to hcp structural transition and its inverse were found along the same (101) plane, implying the memory effect of morphology.(4) In addition, the heterogeneous nucleation and the orientation dependence of structural transition were investigated. We simulated the shock-induced transition in nanovoid-concluded iron. The results demonstrated the transition time reduces exponentially with increasing shock pressure. The transformed atoms do cross a shear pressure barrier and then show an over-relaxation of pressure, while their potential increases to a much higher value than bcc atoms. Meantime, the orientation dependence of the structural transition simulated. It is found that the transition pressures are less dependent on the crystal orientations. However, the pressure interval of mixed phases for [011] loading is much shorter than other orientations, and the temperature increase for [001] loading is evidently lower. For loading along [011] and [111] directions, both hcp and fcc nucleations were observed.(5) We simulated the microjet from a grooved aluminum surface under shock loading. The effect of release melting was discussed in detail. It was found that the microjet mass keeps a linear increase with the post-shock particle velocity prior to release melting, and the release melting can evidently enhance the microjet. However, while the release melting speed is fast, the microjet mass shows a linear increase again, because the material strength can already be neglected. Also, our simulations suggest that the head speed of microjet always keeps a linear increase with the post-shock particle velocity, nearly independent of melting.(6) The failure modes of single crystal aluminum under decaying shock loading were also simulated. The results explain the failure morphology and mechanical properties under dynamic tension, and especially the difference between solid and melted states. The change of destroyed depth form solid to melted states was obtained. Besides, the fracture strength of aluminum was analyzed from surface velocity and virial theorem respectively.(7) Finally, we simulated the effects of preset voids or He bubbles on the failure in single crystal aluminum. The initial spall in the voids or He bubbles-included samples was captured, where we observed the disordered atoms generation from the preset voids or He bubbles and confluence. The homogeneous void nucleation can be reduced by the preset voids and He bubbles growth, while the impact velocity is high enough, a wide voids-included amorphous zone generates, where the homogeneous void nucleation is dominant and all the samples have the same spall strength.