Molecular Dynamics Study On The Orientation And Temperature Effects Of Plastic Deformation Mechanisms In Hexagonal Closed-packed Metals

Due to many special properties,hcp metals have been used more and more widely in industry in recent decades,such as aerospace,automobile manufacturing,communication and electronics industry,etc.Because of the low symmetry of the crystal structure of hcp metals,there are few silp systems that can be activated at the same time.And the ductility of hcp metals is poor compared with that of fcc and bcc metals.When hcp metals are used as structural material,their strength and toughness should be considered emphatically.Therefore,a deep understanding of the plastic deformation mechanisms is of great significance for improving the mechanical properties of hcp metals.In this thesis,the plastic deformation mechanisms of hcp metals have been systematically studied by molecular dynamics simulations.And the effects of temperature and orientation on the plastic deformation mechanisms of Mg,Zr and Ti single crystals have been discussed.Based on these,the nucleation and growth mechanisms of{10(?)1} twin are analyzed.The interaction between point defect and twin boundary is studied to investigate the fatigue mechanical properties of Mg and Mg alloy.The main research work and results are listed as follows:1.In order to investigate the orientation effect of the plastic deformation in hcp metlas,the models of Mg,Zr and Ti single crystals with different orientations are established,and the deformation mechanisms of Mg,Zr and Ti single crystals under uniaxial tension are systematically studied.The results show that slips and twinning are main plastic deformation mechanisms in hcp metals.The activation of slip system is determined by the combined effect of the Schmid factor and the critical resolved shear stresses(CRSS).The simulation results are consistent with the related experimental results.Twinning plays a crucial role during plastic deformation since basal and prismatic slips are limited.The {10(?)2} twinning is popularly observed in Mg,Zr and Ti due to its low CRSS.Moreover,the stress-induced hcp-fcc phase transformation occurs in Ti.Due to the lower CRSS for slips and twining in Mg,the strenghth of Mg is much lower than that of Ti or Zr.For Zr,the CRSS for twinning is higher,thus dislocation slips dominate the plasticity.For Ti.more types of plastic deformation mechanisms(including the cross-slip,double twins,and hcp-fcc phase transformation)are activated.Multiple deformation mechanisms coordinate with each other,resulting in the higher strength and good ductility of Ti.2.In order to investigate the temperature effect of the plastic deformation mechanism of hcp metals,the intial plastic deformation mechanisms of Mg,Zr and Ti single crystals at different temperatures are systematically studied,combining the analysis of CRSSs for deformations.The results show that the temperature effecs on plastic deformation mechanisms of Mg,Zr and Ti single crystals are different.For Mg,prism slips and {10(?)1} twinning are more easierly activated at higher temperatures.For Zr,the CRSSs of pyramidal dislocations decreases the fastest with temperature increasing,thus the density of the pyramidal dislocations increases significantly at higher temperatures.For Ti,the CRSSs of basal<a>dislocation decreases with temperature increasing,thus the basal<a>dislocation is favored at higher temperatures.Besides,the {10(?)2} twinning in Mg,Zr and Ti single crystals is not sensitive for the the change of temperature..3.The nucleation and growth of {10(?)1} twins are further discussed.The decomposition of<a>dislocations cause the nucleate of {10(?)1} twin in Ti single crystal.In the nucleation of a twin embryo,one-layer,two-layers and four-layers twin dislocations are found.The {10(?)1} twin grows through the glide of two-layers twin dislocations.Two-layers twin dislocations always nucleate doe to the stress concentration.Based on the topology analysis,the minimum energy of the coherent twin boundary can be achieved when the one-layer twin dislocations are connected with the basal stacking faults.Thus the basal stacking faults are polularly observed in{10(?)1} twin.Moreover,it is revealed that the shear of {10(?)1} twinning is consistent with the that of the two-layers twin dislocation,which is contradictory with the shear of tranditional {10(?)1} twinning.4.The molecular dynamics simulation is employed to study the {10(?)2} detwinning under shear loads in Mg,focusing on the interaction between the twin boundary and Self-interstitial atoms(SIAs)in the detwinning process.The simulation results show that the SIAs are absorbed by the cohenrent twinning boundaries(CTB s)and migrate along with them.The absorbed SIAs can be released with the disappearance of the CTBs during the detwinning process.By the SIA adsorption and release,the detwinning process will result in a more concentrated distribution of SIAs.The simulations reveal that SIAs will be adsorbed by CTB if the distance between the CTB and SIA is less than 0.752 nm at 0 K and 3.59 nm at 273 K.The energy barrier for the adsorption is also obtained using the nudged elastic band(NEB)method.The structure of SIA will be changed after the SIA interacts with the CTB in the detwinning process.Given that the crystal defects such as dislocation loops can be induced by the dense distribution of SIAs at a long timescale,this study helps to understand the fatigue mechanical properties of magnesium and magnesium alloys.