Atomic Simulations On Interface Evolution Related To Atomic Shuffling Mechanism In Metals

Grain boundary and phase boundary are the most common two-dimensional defects in metals.Their evolution is closely related to plastic deformation and phase transformation mechanism.Generally speaking,the migration of grain boundaries or phase boundaries can be achieved by the thermal activation of atoms from one side of the interface to the other,or by the movement of dislocations.Meanwhile,the migration and movement of some defects on the interface(such as steps and disconnections)are related to the interface evolution process as well.In recent years,twinning mechanism in magnesium,zirconium and titanium has been investigated.Results reveal that due to the complex crystal structure,homogeneous shear can not ensure all lattice points form a symmetric lattice structure with the matrix after deformation.Therefore,atomic shuffling becomes an important mechanism of twinning in hcp metals and the atomic displacement in deformation twinning can be divided into two parts: homogeneous shear and atomic shuffling.The concept of SHUFFLE was first brought up in the research of martensite transformation.The formation and migration of twin boundary and phase boundary in Ni Ti alloy are also closely related to the mechanism of atomic shuffling.For further understanding the interfacial migration behavior and plastic mechanism of metals,it is of great significance to analyze and compare the atomic shuffling mechanism in the plastic deformation of hcp metals and Ni Ti alloy.Since displacement vectors of atoms in atomic shuffling mechanism have different magnitudes and directions,atomic simulation has become an effective method to deeply explore the mechanism.Therefore in this work,the interface structures together with interface migration characteristics related to atomic shuffling in hcp magnesium and Ni Ti alloy are studied through atomic simulations such as molecular dynamics(MD),with emphasis on temperature effect of interface structure evolution and the influence of interface defects on interface evolution.The specific research work and results are listed as follows:(1)Through atomic simulations on tensile and shear deformation in hcp magnesium,the plastic deformation behavior related to atomic shuffling is studied.The tensile deformation simulation is for the research on single crystal nanowires with different cross-sectional shapes,while the shear deformation simulation is aimed at the research on coherent twin boundary(CTB)in {10(?)2} extension twinning.In the c-axis tensile deformation simulation of magnesium single crystal nanowires,the initial plastic deformation mechanism is different under different temperature and strain rate conditions.Pyramidal partial dislocation dominates the initial plastic deformation under relatively low and relatively high temperature ranges,while lattice reorientation is responsible for the initial plastice deformation in medium temperature range.Lattice reorientation is the embryo of {10(?)2} extension twin and it is related to the formation and migration of basal-prismatic interface,which is mainly achieved by atomic shuffling.When the strain rate is increased,the lattice reorientation mechanism needs higher temperature to be activated,meanwhile,the temperature range in which lattice reorientation dominates the initial deformation also expands.When the temperature increases,the migration rate of basal-prismatic interface increases.These temperature and strain rate effects of lattice reorientation mechanism above reflect the thermal activation properties of the atomic shuffling mechanism.In the simulation on {10(?)2} extension twin,it is found that <10(?)1 disconnection dipole and < 1(?)10 disconnetion dipole in twinning loop have different structural and energetic characteristics.It is clarified that a <1(?)10 dipole has lower formation energy and growth energy,which leads to the fact that the {10(?)2} extension twinning loop can expand more easily in <1(?)10 direction than in <10(?)1 direction.Moreover,the growth energy of disconnection dipoles also changes due to the change of external stress and dipole size.(2)The nucleation and growth mechanism of <011 type II twinning in Ni Ti B19’ phase are studied by atomic simulation method.The reasons for the formation of stepped structure on twin boundary(TB)together with its effect on TB migration are discussed.Through molecular static simulations,it is found that type II twinning can nucleation in Ni Ti B19’ single crystal system under applied stress in certain direction.The corresponding initial B19’ phase type II twin is 6-atomic-layer thick,which is about 13.2 (?).The nucleation and growth of type II twinning need the combination of shear and atomic shuffling.There is no disconnection found in TB migration.Through constructing initial stepped TB structure together with initial stepless TB structure for comparison,it can be deduced that the stepped structure in B19’ phase type II TB has little effect on TB migration mechanism but plays a role in coordinating the lattice mismatch on both sides of TB.Moreover,through MD simulations,temperature is found to have effect on type II TB migration rate.Within a certain temperature range,the TB migration rate increases with the increase of temperature.(3)The angular and planar B2/B19’ phase interface models of Ni Ti are constructed to systematically study the effects of temperature,applied stress and interface steps on the phase interface migration and phase transformation process.The results gained by MD simulations show that the transformation between B2 and B19’ phase involves the combination of shear and atomic shuffling,which is,in detail,the combination of a-axis lattice shear of B19’ phase and atomic shuffling required to meet the lattice change.There is no disconnection observed in the process of phase interface migration.It can also be found that the applied stress can promote austenite phase transformation.When the applied shear direction is consistent with the shear direction of lattice phase transformation,the promoting effect is the most obvious.In the process of temperature-induced austenite transformation,the angular phase interface can emit dislocations,resulting in local lattice distortion and change of free energy,so that the austenite transformation tends to have an “interface priority” characteristic,while the planar phase interface maintains homogeneous transformation feature without an “interface priority” characteristic.Besides,under applied stress,the main deformation mechanism of planar interface model is relative sliding of grains on both sides of the interface,while for angular interface model,there is anisotropic deformation behavior due to its stepped structure.