| The research of strength and ductility of metals is an important topicat present. It has a positive effect in the field of industrial applications formetals. Above all, the hcp structure magnesium and its alloys have aunique high specific strength property compared to other materialbecause of its special lattice structure, and present excellent applicationperformance and broad prospects in aerospace, automotive and othermanufacturing industries. However, their poor plasticity limits the furtherwidely applications. The obdurability of magnesium is related todislocation glides, twins, stacking fault and other corresponding microdeformation mechanisms. To enhance the plastic deformation ability ofmagnesium, on the one hand, we should understand the mechanisms andinfluence factors of non-basal slips, on the other hand, we have to explorethe reasons and methods to motivate the non-basal slips. As a result, inthis work, we focus on the research of non-basal slips such as twinnucleation and twin boundary migration, and also combine the firstprinciple calculations of the alloying element’s influence on basal planestacking fault. In this way, we can study and discuss the improvement of the strength and plasticity of magnesium and its alloys further.We use the molecular dynamics to build a special model which have a crack tip on the top and a symmetry tilt grain boundary in the middle, and get two type twins as {1121} primary twin,{1122} secondary twins. In this way we can analysis the twin nucleation induced by the dislocation-grain boundary interactions and their twin boundary migrations. Also, tensile deformation of a bi-crystalline thin film of hcp Mg containing a {10-12} coherent twin boundary (CTB) has been studied with molecular dynamics (MD) simulations. Based on these work, we can understand the non-basal slips better. Moreover, using the DFT and CINEB methods to calculate the influence of Zn element on the basal plane stacking fault energy of Mg help us research the alloying effect on the cross slips and the influence on the non-basal slips as twin boundary migrations.The research results show that:(1) under the effect of dislocation-grain boundary interactions, two types of primary-secondary twins as {1121}{1122} are produced respectively under10K. This is related to their own twin boundary migration processes. As for {1121} twin boundary migration, it is accomplished by pure-shuffle mechanism and only need relatively low energy. So it appears as the first at both two temperatures.{1122} twin boundary migration is formed by pyramidal slip. These twin boundaries reveal the complexity of non-basal slips and its contributions to the overall ability of plastic deformation of Mg, so wecan further study how to motivate more non-basal slips.(2) Impingedbasal slip at the CTB from a dislocation source (crack tip) at one grainmay activate complex dislocation reactions at the CTB. To transfer theslip to the opposite grain, different mechanical events, such as basal slip,non-basal slip as well as deformation twinning, can be observed in theopposite grain.(3) According to the first-principle calculations we canconclude that in the Mg-Zn alloying system, the addition of Zn atomsincreases the system steady-state stacking fault energy. This means thatalloying element Zn can help motivate dislocation slip mechanism of Mgand prompt the dislocation slips from basal planes to non-basal planes,namely, the process of cross slips. This is mainly because after theaddition of Zn elements, the bonding mode of Mg is changed in a way,thus fundamentally affect the choice of the deformation mechanism ofMg and motivate the cross slips to start more non-basal slips. As a result,the magnesium material can better adjust and adapt to further plasticdeformation process and have better strengthening and tougheningperformance. |
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