The Study Of Twinning Behaviors In Magnesium By Molecular Dynamics Simulation

Due to the hexagonal close-packed(HCP)lattice,magnesium(Mg)alloys have few independent slip systems during plastic deformation at room temperature,resulting in its low plasticity and difficulty to be processed,which has become an obstacle to the large-scale application of Mg alloys.Since the independent slip system is inadequate,twinning plays a vital role in plastic deformation of HCP metals.The research on the twinning mechanism of Mg alloy can not only enrich the basic theory of metal plastic deformation,but also make a significant contribution to optimize plastic processing technology and develop new materials with high forming properties.The dynamic evolution of defects in materials is hard to be observed through traditional experimental method which is usually limited by sample size.Molecular dynamics(MD)simulation has its unique advantages in studying the intrinsic plastic deformation mechanism of small-sized samples especially for nanocrystalline(NC)materials.Due to the lack of experimental and theoretical research on Mg alloys in nano scale,MD simulation was applied to study the twinning behavior in Mg.In this paper,MD simulation of twin boundary(TB)motion and twin evolution in Mg was carried out.First of all,deformation behavior in Mg bicrystals with[2110]symmetric tilt grain boundary(STGB)under uniaxial tension and compression is investigated.Based on the lowest grain boundary(GB)energies,stable STGBs with different misorientation angle are divided into six sets consisting of different structures containing specific base plane and an array of grain boundary dislocations(GBDs).The yielding behavior and nucleation mechanism at GB is determined by the atomic structure of STGB varying with the misorientation angle.The deformation mode of the model changes with the misorientation angle as well.Tension-compression asymmetry of hcp metals is revealed obviously by different deformation modes under tension and compression,which is explained partly by Schmid factor.Dislocations prefer to nucleate from the GBD parts where intrinsic stacking fault facets may nucleate prior to dislocation emission.And the TB parts which usually perform as twin-like structure are invariable during deformation.The effects of TB spacing and temperature have been studied for polycrystalline Mg with lamellar {1101} twins constructed by the Voronoi method.It was found that the introduction of lamellar twins can decrease the strength that behaves as a softening mechanism.This softening mechanism is a result of twinning behaviors including initial TB migration controlled by TD nucleation and newly formed twins.The strength decreases monotonically with the reduction of TB spacing due to the consequently increasing TD nucleation sites.The polycrystalline Mg models with lamellar twins show obvious temperature sensitivity on plasticity.The migration mechanism of basal-prismatic(BP)boundary as a special {1012}TB is investigated in bicrystalline models.By introducing a cylindrical void,the interaction between void and GB was observed,and the effect of different locations and sizes of voids on BP boundary migration was also studied.The BP boundary migration starts with the glide of disconnection.Then new disconnections nucleated from the boundary to form the {1012} facets.The glide of TDs along the {1012}facets and the BP facets jointly drive the BP boundary migrate forward.The location of the void will affect the start position of BP boundary migration,and the void will behave as an obstacle during migration and parts of GB will be pinned by void surface.In addition,the effect of intergranular and intragranular void on the BP boundary migration were observed and explained respectively.The effect of Mg17Al12 precipitate against {1012} twinning in Mg alloy has been studied.Three sets of simulation have been performed to investigate the influence of precipitate on twin nucleation,propagation and growth respectively.It is found that the precipitate has little effect on twin nucleation.While precipitate can block the twin propagation.When the precipitate is not long enough,part of twin tip can bypass the precipitate and continue its propagation.When the length of precipitate is much larger than the twin thickness,the precipitate will completely block the twin tip and elastic bending will occur at the junction.Precipitate behaves as both a source and an obstacle for TDs during TB migration.The TDs can be nucleated from the precipitate in a manner of dislocation loop,and can also be blocked by the precipitate that leads to pile-up to form a BP facet.The TDs pile-up leads to stress increase and strengthening of model.The plate-like precipitate shows a size effect of strengthening on twinning mediated plasticity.