| The energy has always been attracting much attention in global world.Developing the low energy comsuption materials is of strategic significance for energy suistainability.As the lightest structural metals,magnesium(Mg)alloy,a hexagonal close-packed(HCP)structural metal,has great potential usefulness for improving energy efficiency across the automobile,aircraft,and aerospace industries due to its high specific strength and specific stiffness.Its plasticity and ductility are poor at room temperature due to less activated slip systems compared with the face-centered cubic(FCC)and body-centered cubic(BCC)structural metals.Hence the further investigation for the microstructural plastic deformation of Mg and its alloys contributes to the designed and optimized alloys.The plastic deformation mechanisms of Mg and its alloys involve the dislocation and its slipping,grain-boundary movement,and twining.In addition,the HCP to BCC phase transformation occurs under high pressure for Mg and its many alloys.Due to the transient and reversible process during the phase transition,the present experimental probing techques are difficult in tracing and recording directly the whole shock.The nonequilibrium molecular dynamic(NEMD)method is a powerful tool for understanding and exoplaining the plastic deformation,phase transition,and the coupling between plasticity and transition in Mg under shock compression.The accuracy of the NEMD simulation is intensively depend on the interatomic potentials to describe the interactions between atoms.Due to the lack of appropriate interatomic potentials describing the plasticity and phase transition under high pressure,large-scrale computational simulations can hardly be peformed for Mg and its alloys.In this dissertation,the potentials for Mg and Zn,and the alloy potential for MgZn systems are constructed and used in high-pressure simulation.The developed potentials are based on the Finnis-Sinclair formalism,which has high computational efficiency.The accuracy of these potentials has been tested.For the pure elements,the potentials reproduce their physical parameters,such as the lattice constant,cohesive energy,elastic constants,formation energies of vacancy,and the most stable interstitial atom.In addition to the basis physical properties,the potentials accurately predict some high-pressure properties,such as equation of states and bulk moduls at high pressure,and stress-strain relation.And that,the developed potential reproducing the phase transformation between HCP and BCC comes true for Mg under uniaxial compression,and accurately predicts the plastic deformation before the phase transition.For alloy potential,some major properties are well produced,such as the formation energies,lattice constants,and elastic constants of artificial ordered alloys.Based on the present tests,the developed potentials in this dissertation are suitable for high-pressure studies,and also provide a basis for fitting interatomic potentials for Mg-based ternary alloys.By using the constructed potentials and molecular dynamic method,the shock compression simulations in pure Mg and Mg-Zn systems are performed.The shocked samples in simulations are designed to be perfect single crystal,single crystal with prismatic nanopore,nano-polycrystals with hexagonal columnar grains,and Mg-Zn alloy.Using analysis techiniques,such as adaptive common neighbor analysis(ACNA)and dislocation extraction algorithm(DXA),the shock-wave propagation and microstructure evolution during shock are analyzed in detail.Results illustrate that single crystal Mg under low velocity shock compression exhibits strong anisotropy in shock-wave propagation,plastic deformation and phase-transformation mechanism,and the anisotropy weakens with increase of shock strength.The plastic deformation for Mg under shock compression is as follows: for shock compression in the [0001]direction,the amorphization dominates the plastic deformation;for shock compression in the [10-10] and [-12-10] directions,the reorientation contributes to the plastic defomation,and the incoherent {10-12} twin forms between intial and reorientated crystalline grains.Because nanopore in single crystal Mg,grain boundary in nanocrystalline,and adulteration in Mg-Zn alloy lead to local stress concentration during shock compression,the basal,prismatic dislocation slippings,and twin slipping are activated to accommodate plastic deformation of Mg under high pressure.The nanopore in single crystal Mg collapses during shock loading,and its collapse mechanism is the plasticity mechanism,leading to different collapse modes.For the nano-polycrystal,the {10-12} twin grain boundary nucleates around the grain boundary due to local stress concentration during shock,and then migrates due to nucleation and dissociation of twin grain dislocation,leading to the {10-12} twin.The phase transformation between HCP and BCC in Mg induced by shock compression occurs.The analysis of the crystallographic relations between structures is an intuitive way for elucidating the phase-transition mechanism.And,the phasetransformation mechanism between HCP and BCC for Mg involves a homogeneous deformation and a shear in every second plane along the [10-10] direction.The homogeneous deformation includes contraction along [0001] and 〈10-10〉orientations,and tension along〈-12-10〉orientation.The shear deformation involves the slipping in every second(0001)plane along 〈 10-10 〉 orientation.The coupling between plasticity and phase transformation also relys on the shock compression direction.For shock compression in the [0001] direction,a small part of HCP phase directly tranforms into BCC phase leading to the phase-transformation nucleation.The amorphous atoms recrystallize into BCC phase based on the nucleation as the shock compression continues,resulting in the larger phase-transformation region.For shock compression in the [10-10] and [-12-10] directions,Reorientation leads to the compressed HCP structure(RHCP),and then the phase transformation occurs between RHCP and BCC.These results provide a basis for understanding the plastic deformation,phase transformation,and their coupling relation in Mg under shock compression,and for optimizing the design of Mg alloys. |
