Study On {10-12} Twinning Behavior Of Magnesium Alloys Based On Molecular Dynamics And In-situ EBSD Methods

Magnesium alloy,as the lightest metal structural material,holds broad application prospects in automotive,aerospace and other fields.However,magnesium alloy lacks sufficient slip systems at room temperature due to the close-packed hexagonal structure.As a result,deformation twinning plays an important role in the plastic deformation and processing of the alloy,and also affects the strength,plasticity,and plastic anisotropy.Therefore,a deep and comprehensive understanding of twinning characteristics,including twin growth mechanism,the structure and evolution mechanism of twin interface,the interaction mechanism between twins and crystal defects,and the nucleation and growth characteristics of twins will provide theoretical basis and basic data support for optimizing alloy structure,designing and developing high-performance magnesium alloy.{10-12}twinning is the most common twinning mode in magnesium alloy,and thus the exploration of{10-12}twinning features has attracted wide attention.However,the current researches still exhibit the following deficiencies:In terms of twin growth mechanism.It is known that the actual twin is a three-dimensional(3D)structure.However,the current exploration of twin growth mechanism is aways based on a two-dimensional(2D)model whether it is atomic scale simulation or crystallographic analysis,ignoring the extension of a twin along the[11-20].There is no doubt that there is a quite difference between the 2D twin model that extends along the[11-20]normal plane and the true twin structure that extends in the 3D domains,as a result,a controversial growth mechanism of{10-12}twins emerges.Obviously,investigating the 3D growth features of{10-12}twin nucleus is necessary to fully understand twin growth mechanism.On the other hand,the state of stress in the matrix is critical to the expansion of twins.However,at present,there is a lack of systematic analysis of how the elastic stress in the matrix affects the evolution of the twin interface.Apparently,such analysis is the key to understanding twin growth mechanism.In terms of the structure and evolution of the twin interface.It has been known that there are periodically arranged lattice sites on the{10-12}twin boundary({10-12}TB),and these periodic lattice sites play a vital role in the annealing and strengthening of magnesium alloys.However,the formation mechanism of these lattice sites is still unclear.Exploring the periodic lattice sites will be delightful for a deep understanding of the evolution mechanism of the twin interface.In addition,a full understanding of the lattice sites can provide theoretical guidance for optimizing the twin interface structure and thereby improving the mechanical properties of the alloy from the perspective of engineering applications.In terms of the interactions between twins and crystal defects(plannar defects).It has been known that grain boundaries and twin interfaces can obviously hinder the migration of{10-12}twins.In recent years,the interaction between stacking faults and{10-12}TB has attracted more and more attention.Stacking faults can improve alloys strength by hindering the movement of{10-12}TB.However,to study the interaction between stacking faults and{10-12}TB is currently limited to the use of molecular dynamic simulations,based on 2D interface models.Notably,the actual{10-12}twin is a 3D structure.If one attempts to fully understand the influence of stacking faults on the evolution of twins,it is necessary to understand the interaction between the 3D twins and stacking faults,or how the twins evolve when different twin interfaces interact with stacking faults.However,for now,there are no reports of interactions between{10-12}twins and stacking faults.In terms of the nucleation and growth characteristics of twins.It has been commonly accepted that pre-strain can change the initial microstructure,thereby regulating the nucleation and growth of twins.In addition to different numbers and sizes of twins obtained under different pre-strain paths and pre-strain levels,the dislocation density in the microstructure will also be significantly different.Although numerous researches have focused on how different pre-strain paths and pre-strain levels affect twin nucleation and evolution,it remains to be clarified how the differences in initial microstructures caused by different loading conditions affect subsequent twin nucleation and growth.If the nucleation and growth rules of twins in the structure are clarified,the characteristic parameters(including number and size)of the twins will be regulated,so as to control the mechanical properties and deformation behavior of the alloy.Regarding to such problems,firstly,molecular dynamics simulation combined with stress field analysis was used to systematically study the 3D evolution characteristics of{10-12}twins in magnesium and the migration mechanism of the twin interfaces.Next,the formation mechanism of the periodic arrangement lattice sites on the{10-12}TB was explained,by using the 3D{10-12}twin model.Then,based on molecular dynamics simulation,the interaction between the 3D{10-12}twin and basal stacking faults(I₁,I₂)was analyzed.Further,combined with the analysis of topological model,the evolution mechanism of the twin interfaces on stacking faults was discussed in depth.On the other hand,the in-situ EBSD characterization technique was used to analyze how the difference in initial microstructure under different pre-strain loading conditions(direct compression,compression-tension)affected subsequent twinning behavior,and attempted to find the initial microstructure features corresponding to the twin nucleus and growth.The results are promising to provide theoretical guidance and basic data support for designing high-performance magnesium alloys.The main research results obtained in the thesis are as follows:(1)The 3D morphological characteristics of{10-12}twins were obtained for the first time,and it was proved that the growth of{10-12}twins was not dominated by a single mechanism.The stabilized 3D twin nucleus consists of BP/PB interfaces and{10-11}interfaces.Later,a{10-12}TB occurs at the junctions of the BP and PB interfaces with the growth of the twin nucleus.Two twinning mechanisms are involved in twin growth:a pure-shuffle mechanism,in which the movement of the{10-11}interface along the[11-20]is dominated by atomic shuffle and rearrangement,and a glide-shuffle mechanism,in which BP/PB and{10-12}TB movements are mediated by disconnections migration along the relevant interfaces.Interfaces migration allows stress release in matrix regions,which triggers quick propagation of the twin nucleus.(2)The formation mechanism of periodically arranged lattice sites on{10-12}TB was clarified.There are two nucleation modes of interface disconnections activated alternatively on the BP/PB interface:I:b_1/-1b,b _1/1a;II:b _1/-1a,b _1/1b.In addition,the alternate nucleated disconnections can be transformed into alternate lattice sites after migrating to the junction position.Finally,the periodic arrangement lattice is formed by the continuous transformation from disconnections to lattice sites.(3)The strengthening potential of the stacking faults to{10-12}twins was clarified,and a novel method of alloy strengthening was proposed.Both I₁ and I₂stacking faults can obviously hinder the migration of twin interfaces.The boundary of the I₂ stacking fault is more resistant to the twin interfaces than the I₁ fault boundary.Moreover,the BP interface displaies a larger displacement after entering the I₂ stacking fault with respect to entering the I₁ fault.Therefore,the I₂stacking fault shows a stronger hindering ability for{10-12}twins than the I₁ stacking fault.Moreover,the stacking faults exhibit different strengthening effects on different twin interfaces.The barrier effect of stacking faults on the BP/PB interface is better than that on the{10-11}interface.(4)The influence of the microstructure of initial twins and dislocations on the nucleation and growth of twins in the subsequent deformation process was discovered.The stress field around the initial twin and dislocation density can significantly affect the evolution of subsequent twins.When the grain contains a small amount of dislocations,the twins are easy to expand and interact with the grain boundary.As a result,the twin stress field promote subsequent twins to nucleate at the grain boundary.However,when the grain contains high-density dislocation dislocations,the twins and dislocations can obviously interact with each other,and the growth of the initial twins is significantly inhibited.At this time,the twin stress field excite the subsequent twin nucleation around the existing twins.In the end,the initial grains containing fine twins and high-density dislocations benefit to obtain high-density fine twins during subsequent deformation.