Twin Nucleation Research Articles

Atomic insights into interface-mediated plasticity and engineering principles for heterogeneous serrated interfaces

Metallic nanolayered composites typically experience substantially enhanced resistance to irreversible deformation as the portion of interfaces increases. Three-dimensional (3D) serrated interfaces possess considerably higher resistance to interface-facilitated plasticity than two-dimensional (2D) planar interfaces; however, the atomistic mechanisms underlying this phenomenon are little explored, while the engineering principles of tailoring atomic serrations are nearly unknown. In this study, two known representative serrated interfaces, i.e., Cu{991}//{112}Nb and Cu{112}//{112}Nb interfaces, are analyzed using atomic-scale simulations and interfacial dislocation theory and comprehensively compared with their planar counterparts. The Cu{991}//{112}Nb and Cu{112}//{112}Nb serrated interfaces exhibit the novel interface-facilitated deformation behaviors of deformation twinning and near-interface dislocation nucleation, respectively. The stress inhomogeneity arising from the geometrical mismatch between Cu and Nb serrations contributes to deformation twinning rather than dislocation nucleation, while the improved symmetry of highly distorted atomic hexagons on the extended Cu{111}//{110}Nb facets dominates the near-interface dislocation nucleation. Both deformation twinning and dislocation nucleation are closely related to the geometry and characteristics of atomic serrations at the interfaces, which differ from those observed in planar interfaces. Further systematic investigations of fourteen serrated interfaces derived from Cu{991}//{112}Nb and Cu{112}//{112}Nb suggest that the screened facet planes, free volume, and Poisson's ratio mismatch may be used as critical descriptors to tailor the mechanical properties and responses, which presents a convenient solution for interface engineering. These findings provide not only novel atomistic mechanisms that explain the localized interface-facilitated plasticity, but also general principles for engineering atomically serrated interfaces.

Advanced in-situ experimental techniques for characterization of deformation mechanisms in magnesium alloys

A combination of advanced in-situ experimental techniques, comprised of neutron diffraction, acoustic emission (AE), and in-situ electron backscattered diffraction (EBSD) was selected to provide extensive insight into the deformation behavior of magnesium alloys based on their mutual complementarity. The potential and limitations of these techniques were shown and discussed in the scope of the study of the influence of the crystallographic texture influence on the activity of individual deformation mechanisms in a hot-rolled sheet of the AZ31 magnesium alloy. The neutron diffraction experiments coupled with a recording of the AE signal allowed monitoring of the twinning activity and the evolution of its dynamics from nucleation toward twin growth. The AE suggested microplastic behavior, which was confirmed by the lattice strain evolution analysis for the sample compressed in the sheet normal direction. The in-situ EBSD experiments provided direct observation of the deformed microstructure, including the formation of twinning bands and the transition from twin nucleation to growth with progressing strain, being in very good agreement with the indirect neutron diffraction and AE measurements. Furthermore, the subsequent analysis of the EBSD maps enabled the quantification of the twinned volume fraction and its qualitative comparison with the evolution measured by neutron diffraction. The EBSD-based slip trace analysis revealed a notable non-Schmid basal activity and offered insight into the mechanisms accommodating strain once the twinning was exhausted.

In-situ observation of twinning and detwinning in AZ31 alloy

Twinning and detwinning behavior of a commercial AZ31 magnesium alloy during cyclic compression–tension deformation with a total strain amplitude of 4% (±2%) was evaluated using the complementary techniques of in-situ neutron diffraction, identical area electron backscatter diffraction, and transmission electron microscopy. In-situ neutron diffraction demonstrates that the compressive deformation was dominated by twin nucleation, twin growth, and basal slip, while detwinning dominated the unloading of compressive stresses and subsequent tension stage. With increasing number of cycles from one to eight: the volume fraction of twins at -2% strain gradually increased from 26.3% to 43.5%; the residual twins were present after 2% tension stage and their volume fraction increased from zero to 3.7% as well as a significant increase in their number; and the twinning spread from coarse grains to fine grains involving more grains for twinning. The increase in volume fraction and number of residual twins led to a transition from twin nucleation to twin growth, resulting in a decrease in yield strength of compression deformation with increasing cycles. A large number of < c >-component dislocations observed in twins and the detwinned regions were attributed to the dislocation transmutation during the twinning and detwinning. The accumulation of barriers including twin boundaries and various types of dislocations enhanced the interactions of migrating twin boundary with these barriers during twinning and detwinning, which is considered to be the origin for increasing the work hardening rate in cyclic deformation of the AZ31 alloy.

Rolling speed dependence of cryogenic rollability of bimodal non-basal textured AZ31 Mg alloy sheet

The rolling speed dependence of cryogenic rollability of bimodal non-basal textured AZ31 Mg alloy sheet has been investigated through multi-pass rolling experiments with a constant 5% thickness reduction per pass and characterization experiments via X-ray diffractometer (XRD) and electron backscatter diffraction (EBSD) techniques. The chosen rolling speeds include 30 mm/s (low-speed), 300 mm/s (mediate-speed) and 600 mm/s (high-speed). Rolling results show that the accumulated thickness reduction (ATE) of low-speed rolled sheet reaches up to about 22.6%, which is 58.0% bigger than that (14.3%) of high-speed rolled sheet. This means that the rollability of studied sheet under high-speed rolling is obviously worse than that under low-speed rolling. Characterization results confirm that high-speed rolled sheet possesses the smallest tilted angle (about 16.3°) between the maximum intensity point in (0002) pole figure and normal direction (ND) by comparison with other two sheets, leading to the enhancement of difficulty to accommodate the thickness plastic strain during rolling at liquid-nitrogen temperature. Moreover, high-speed rolling can not only realize the saturation of {10-12} extension twin (ET) nucleation at the beginning of cryogenic rolling, but also promote the growth of {10-12} ET nuclei. This issue benefits in improving the frequency of occurrence of Ortho-position (OP) -typed {10-12} ET variant interaction, which would block the motion of dislocation slip effectively. The deteriorated rollability of high-speed rolled sheet is mainly attributed to these two issues.

Effect of twin nucleation and twin on the recrystallization of a nickel-based superalloy during hot compression

The nucleation mechanism of deformation twin (DT) and annealing twin (AT), and the effect of twins on recrystallization were studied comprehensively for Nickel-based superalloys during dynamic recrystallization (DRX). The aim is to improve the understanding of microstructure evolution during hot deformation. Hot compression tests were carried out at the temperature of 900-1150 °C with the strain rate of 0.01-1 s -1 for a Nickel-base superalloy. The results show that the DT and the Ʃ3 boundary exist simultaneously in the DRX stage. DTs are nucleated by the rotation of Franck partial dislocations and the decomposition of perfect dislocations. The DTs hinder the movement of dislocations in deformed grains and promote the nucleation of DRX grains. Ʃ3 boundaries associated with stacking faults are formed at recrystallized grain boundaries. In the condition of continuous DRX (CDRX), the Ʃ3 boundary changes the grain orientation and restrain the CDRX nucleation. While for the discontinuous DRX (DDRX), the Ʃ3 boundary provides an additional nucleation site and increase the nucleation rate. In addition, the comprehensive effect of DTs and ATs can promote the DRX nucleation, for which increase at first and then decrease with nucleation mechanical transform from CDRX to DDRX. • Deformation twins are nucleated by the rotation of Franck partial dislocations and the decomposition of perfect dislocations. • The quantity of deformation twins decreasing with the increase of DRX volume fraction. • Deformation twins hinder the movement of dislocations and promote the nucleation of CDRX. • The Ʃ3 boundaries hinder the CDRX nucleation and accumulate the DDRX nucleation process.

Multi-scale simulation of deformation and recrystallization of austenitic stainless steels with solidified columnar crystal structures

Columnar grain structures (CCS) often distinct in steel ingots, which have to be refined and homogenized during forging. In this investigation, simulation of deformation and dynamic recrystallization of austenitic stainless steels with CCSs were carried out by macroscopic, mesoscopic (microscopic) and nanoscale simulation techniques. (1) Using molecular dynamics simulation method, the nano-CGSs model with different loading directions was simulated. The results show that the deformation stresses are anisotropic with variation of angles between the loading direction and the columnar crystal growth direction. The higher stresses present at the 0° and 90° angles due to higher dislocation density; However, the lower stresses present at the angles from 30° to 60° due to higher stacking faults and twins. (2) The cellular automata (CA) fractal rules were proposed to simulate nucleation and grain growth of dynamic recrystallization by introducing weighted variables considering Σ3 twin nucleation rate for the twinning-promoted recrystallization. The CA method of nucleation at primary columnar crystal boundaries, secondary dendrites and deformation bands was proposed to simulate the joint nucleation. (3) The coupling simulation of macroscopic thermal parameters by finite element method, solidification of columnar structures and hot deformation dynamic recrystallization by CA was realized.

Activation of second-order pyramidal slip and other secondary mechanisms in solid solution Mg-Zn alloys and their effect on tensile ductility

This work investigates the strain-dependent activation of second-order <c+a> pyramidal slip and other non-basal systems with increasing tensile strain in a Mg-Zn alloy. With that purpose, a polycrystalline solid solution Mg-Zn alloy with ∼2 wt.% Zn (∼0.75 at. %) and with an average grain size of ∼36 μm was processed by casting, hot rolling and subsequent annealing. The slip activity with tensile strain was analyzed by electron backscattered diffraction (EBSD)-assisted slip trace analysis. Basal slip was found to be the dominant deformation mechanism at low strains (∼5-10%). However, at higher strains the activity of pyramidal slip increases significantly due to the hardening of basal slip by Zn solutes. Other non-basal deformation mechanisms, such as compression twinning, cross-slip and slip transfer between pyramidal planes, come into play also at higher strains and are shown to be induced by pyramidal slip. Molecular dynamics (MD) simulations have been used, in particular, to demonstrate the influence of pyramidal dislocations on the nucleation of compression twins. The large availability of deformation mechanisms at high strains contributes to improve the RT ductility of the Mg-Zn alloy.

Generation of fine extension twins in rolled magnesium alloy AZ31B at room temperature by simple shear

A textured AZ31B plate was subject to simple shear along 45° to its normal direction at room temperature, and extension twinning under biaxial stress was investigated by statistical analyses of electron backscatter diffraction maps. Extension twinning is the primary deformation mechanism up to a plastic shear strain of 15% (8.66% equivalent plastic strain), and the most frequent twin thickness remains to be 1.5 μm. The simple shear suppresses twin growth, yet brings sustained twin nucleation. The critical resolved shear stress of extension twinning is determined as 25 MPa and remains virtually constant up to a plastic shear strain of 10%. The present study suggests that by using biaxial stress state that promotes multi-twin nucleation, a microstructure with dense and fine twins can be produced at room temperature in a conventional Mg alloy, without jeopardizing the contribution of twinning to plastic straining.

Structure and glide of Lomer and Lomer-Cottrell dislocations: Atomistic simulations for model concentrated alloy solid solutions

Lomer (L) and Lomer-Cottrell (LC) dislocations have long been considered to be central to work hardening in face-centered cubic (FCC) metals and alloys. These dislocations act as barriers of motion for other dislocations, and can serve as sites for twin nucleation. Recent focus on multicomponent concentrated FCC solid solution alloys has resulted in many reported observations of LC dislocations. While these and L dislocations are expected to have a role in the mechanical behavior of these alloys, little is understood about how variations in composition and associated fault energies change the response of these dislocations under stress. We present atomistic simulations of L and LC dislocations in a model Cu-Ni system and find that changes in composition and applied stress conditions result in a wide variety of responses, including changes in core configuration and (100) glide. The results are compared to and extend previous literature related to the nature of L/LC core structures and how they vary with respect to intrinsic materials properties and stress states. This study also provides insights into mechanisms such as twin nucleation that could have important implications for work hardening in FCC solid-solution alloys.

Improved strength and plasticity of magnesium matrix nanocomposites reinforced by carbonaceous nanoplatelets and micro-clusters

The uniform distribution of carbonaceous nanomaterials in a magnesium matrix composite can dramatically improve the strength without sacrificing ductility. In this work, graphene oxide (GO) reinforced AZ31 magnesium alloy matrix composites were fabricated using friction stir processing to achieve higher strength, toughness, and plasticity compared with the unreinforced condition. The composite microstructure consisted of individual GO nanoplatelets and GO micro-clusters, having different effects on the deformation behavior of the matrix. Texture analysis revealed that adding GO led to the activation of different twin types and variants during tensile deformation. In contrast, only one extension twin variant was dominant in the case of the unreinforced condition. In spite of the GO micro-clusters, the individual GO nanoplatelets cannot hinder the growth of twins. The GO micro-clusters inhibit twin growth, change the stress state in the parent grains, and provide a driving force for twin nucleation. The extensive twinning occurrence was followed by twin intersections and significantly enhanced the plasticity of the alloy by providing easy orientations for further basal slip.

Swap motion-directed twinning of nanocrystals.

Twinning frequently occurs in nanocrystals during various thermal, chemical, or mechanical processes. However, the nucleation and propagation mechanisms of twinning in nanocrystals remain poorly understood. Through in situ atomic resolution transmission electron microscopy observation at millisecond temporal resolution, we show the twinning in Pb individual nanocrystals via a double-layer swap motion where two adjacent atomic layers shift relative to one another. The swap motion results in twin nucleation, and it also serves as a basic unit of movement for twin propagation. Our calculations reveal that the swap motion is a phonon eigenmode of the face-centered cubic crystal structure of Pb, and it is enhanced by the quantum size effect of nanocrystals.

Influencing Mechanisms of Prior Cold Deformation on Mixed Grain Boundary Network in the Thermal Deformation of Ni80A Superalloy.

Within the grain boundary engineering (GBE) of alloys, a mixed grain boundary network with random grain boundaries interrupted by twin boundaries, contributes to enhancing the overall grain boundary-related properties. The higher density of twin boundaries is pursued herein. Furthermore, a two-stage deformation method, i.e., prior cold deformation followed by thermal deformation, was proposed for improving the mixed grain boundary network in the thermal deformation of Ni80A superalloy. The influence of prior cold deformation on the mixed grain boundary network was investigated through a series of two-stage deformation experiments. The analysis of the stress–strain curves shows that the critical strain for dynamic recrystallization (DRX) and peak strains decrease significantly under the effect of prior cold deformation. In comparison to the necklace-like microstructures that occur after a single thermal deformation, the microstructures apparent after a two-stage deformation are characterized by finer DRX grains with abundant Σ3n twin boundaries, with a significantly improved density of the Σ3n twin boundaries (BLDΣ3n) by a factor of around nine. With increasing prior cold strain, the grain size, after a two-stage deformation, decreases continuously, while the BLDΣ3n increases firstly and then decreases. The mechanisms for improving the mixed grain boundary network via two-stage deformation were uncovered. The sub-grain boundaries formed in prior cold deformation stimulate the nucleation of DRX grains and twins; meanwhile, the driving force for grain boundary migration is enhanced due to prior stored energy. Then, DRX is activated in advance and occurs more completely, thereby promoting the formation of Σ3n twin boundaries.

Microstructure evolution and constitutive modeling for mesoscaled tension of pure titanium thin sheet

The design and fabrication of micro components of pure titanium thin sheets face a challenging problem with size effects. Size effects will affect the plastic deformation mechanism of thin sheets in addition to causing unpredictable flow stresses. Although the impact of size on strength is obvious, further research is still needed, particularly for metal thin sheet with HCP lattice, to thoroughly understand the impact on deformation mechanisms. To assess the size effects on flow stress, uniaxial tensile experiments were carried out on specimens with varying thicknesses and grain sizes in this work. Additionally, Electron Backscattered Diffraction (EBSD) technology was used to analyze the dislocation evolution and twinning behavior in surface and inner grains for specimens with various thicknesses and grain sizes employing electrolytically polished specimens under different strain conditions. The results demonstrate that the feature size (t/d) has a substantial impact on the flow stress of pure titanium sheet, as seen by a rapid reduction in flow stress when t/d < 7. Microstructure analysis showed that the feature size effect has a significant impact on the deformation mechanism. Inhomogeneous strain distribution and geometrically necessary dislocation (GND) density distribution are produced by the reduction in thickness and the increase in grain size. There is a change from slip dominated to slip-twin deformation mechanism as the t/d decrease. Decreased twin nucleation stress and worsened inhomogeneous deformation are the inherent causes of twin activation as t/d decreases. When the t/d is less than 1, the dislocation starving mechanism causes a paradoxical increase in the flow stress. Furthermore, there are similarities and variances between the surface and the inner grains. Regarding GND density, there is not much of a difference between the two. However, there is a significant difference between the two in terms of twin volume fraction. The twinning volume fraction in the surface grains is much lower than that in the inner grains. To evaluate the contribution of size effect to flow stress, the variation Equation of twinning volume fraction and dislocations in surface and inner grains are quantified. These results were incorporated into a novel mesoscaled constitutive model that includes lattice friction and size-dependent grain boundaries, dislocations and twinning strengthening. The reduction of flow stress in relation to t/d is correctly predicted by the novel model, which also agrees with the experimental results. This study deepens the understanding of deformation mechanism of pure titanium thin sheets in microforming and provides numerical model for accurate forming.

RETRACTED: Shuffling pathway of anti-twinning in body-centered-cubic metals

In this study, anti-twinning and deformation twinning of body-centered-cubic (BCC) metals under shear loading in the anti-twin direction were examined. Molecular dynamics simulations of BCC Ta showed both the existence of anti-twinning and a novel two-layer-by-two-layer anti-twinning mechanism consisting of a staggered shuffle sliding of { 112 } atomic planes in the [ 11 1 ¯ ] and [ 1 ¯ 1 ¯ 1 ] directions, generating finite shear displacement along the anti-twin direction. The density functional theory multi-dimensional interlayer slipping energy landscape of the newly observed anti-twinning process has a lower energy barrier for both twin nucleation and growth than the recently proposed layer-by-layer anti-twinning process.

Retainable short-range order effects on the strength and toughness of NbMoTaW refractory high-entropy alloys

Short range order (SRO) can affect the dislocation glide and twin nucleation in high-entropy alloys (HEAs), leading to varying mechanical performance. However, the mechanical implications of SRO in refractory HEAs (RHEAs) remain largely unknown due to the lack of understanding of the nanoscale deformation. Here, the role of SRO on the tensile mechanical properties of single crystal equiatomic NbMoTaW RHEAs is investigated using the hybrid Monte Carlo method and molecular dynamics simulation. We find that the presence of SRO can simultaneously enhance the strength and toughness over a wide range of temperatures under very fast strain rate. The enhancement is larger at lower temperature due to higher degree of SRO. The effects of SRO strengthening and toughening are retainable after unloading from the plastic stage, due to the unexpected twinning-assisted pseudo-elastic deformation. The fundamental principles of SRO are provided to facilitate the rational design and applications of RHEAs.

The influence of initial texture, microstructure and deformation behavior on the low mechanical anisotropy of extruded Mg alloy tube

In this study, the extruded Mg–Zn–Mn–Ce–Ca alloy tube with a weak basal fiber texture, i.e. the {0001} plane is distributed along the TD with 0°∼30° deflection towards ED, was fabricated. The alloy tube exhibits a low mechanical anisotropy in the tensile process at room temperature. To clarify the relationship between the weak texture and the low mechanical anisotropy, the microstructure and deformation mechanism of the alloy tube in the tensile process along the ED, 45ED and TD were studied. The results revealed that during yielding, the deformation in ED sample was governed by predominant prismatic slip, and a portion of additional basal slip and lowest pyramidal <c+a> slip, while predominant basal slip and accompanied by a high fraction of prismatic slip and lowest pyramidal <c+a> slip for both 45ED and TD samples. These relatively high additional slips result from the weak texture, which attenuates the deformation differences among the three samples. After yielding, the three samples underwent similar prismatic slip transition and twin nucleation quantity. Additionally, the {0001} plane displayed to converge towards the RD after tensile fracture, which may be related to the twins and intragranular slip. Especially for 45ED and TD samples, relatively high basal slip may cause their {0001} plane to tend to be parallel to the loading direction. In summary, the similar deformation mechanisms governed by weak texture along the three directions resulted in the low mechanical anisotropic of the alloy tube.

Abnormal Twinning Behavior Induced by Local Stress in Magnesium

This study investigated the twinning behavior with increasing compressive strain in rolled AZ31 alloy. With that purpose, a polycrystalline structure with an average grain size of 30 μm was utilized to perform the uniaxial compression tests. Microstructure evolution was traced by in situ electron backscattered diffraction (EBSD). Multiple primary twin variants and extension double twins were observed in the same grain. A comprehensive analysis of kernel average misorientation (KAM) and Schmid factor (SF) revealed that the nucleation of twins in one special grain is not only based on the SF criterion, but that it is also strongly influenced by surrounding grains. Moreover, the existing primary twins modified the inner and outer strain distribution close to the twin boundaries. With continued compression, the strain inside the primary twins stimulated the nucleation of double twins, while the strain in the matrixes facilitated twin growth. Therefore, the primary twin growth and the new nucleation of secondary twins could take place simultaneously in the same twinning system to meet the requirements of strain accommodation. Twinning behaviors are controlled by the combined effect of the Schmid factor, strain accommodation between surrounding grains, and variation in the local stress state. The local stress exceeded the critical resolved shear stress (CRSS), implying that twin nucleation is possible. Hence, the twinning process tends to be a response of the local stress rather than the applied stress.

On the plasticity of a hot-rolled Zircolay-4 via a combination of in-situ acoustic emission and ex-situ EBSD techniques

The plastic anisotropy of a hot-rolled Zircaloy-4 (Zr-1.56Sn-0.22Fe-0.11Cr) occurring during compressive loading in three orthogonal directions was analysed using the acoustic emission (AE) technique and scanning electron microscopy (SEM). In particular, AE was used for in-situ monitoring of the activity of individual deformation mechanisms. The microstructure was characterized by the electron backscatter diffraction (EBSD) technique in the initial state and after deformation of 2% and 12%, respectively. The grain reference orientation deviation (GROD) maps estimated from the EBSD data reveal the evolution of slip system activities in individual grains. During compressive loading along the rolling and transverse directions (RD and TD, respectively), dislocation glide and twinning are dominant deformation mechanisms. Moreover, the AE technique in combination with EBSD analysis indicates that more intensive twinning in RD, reflected by a high twin volume fraction, is rather given by twin thickening than massive twin nucleation. With respect to the initial texture, the twinning is almost suppressed during loading along the normal direction (ND) and plastic deformation is realized by several slip systems, including multiple slip.

Formation of contact and multiple cyclic cassiterite twins in SnO2-based ceramics co-doped with cobalt and niobium oxides.

Contact and multiple cyclic twins of cassiterite commonly form in SnO2-based ceramics when SnO2 is sintered with small additions of cobalt and niobium oxides (dual doping). In this work, it is shown that the formation of twins is a two-stage process that starts with epitaxial growth of SnO2 on CoNb2O6 and Co4Nb2O9 seeds (twin nucleation stage) and continues with the fast growth of (101) twin contacts (twin growth stage). Both secondary phases form below the temperature of enhanced densification and SnO2 grain growth; CoNb2O6 forms at ∼700°C and Co4Nb2O9 at ∼900°C. They are structurally related to the rutile-type cassiterite and can thus trigger oriented (epitaxial) growth (local recrystallization) of SnO2 domains in different orientations on a single seed particle. While oriented growth of cassiterite on columbite-type CoNb2O6 grains can only result in the formation of contact twins, the Co4Nb2O9 grains with a structure comparable with that of corundum represent suitable sites for the nucleation of contact and multiple cyclic twins with coplanar or alternating morphology. The twin nucleation stage is followed by fast densification accompanied by significant SnO2 grain growth above 1300°C. The twin nuclei coarsen to large twinned grains as a result of the preferential and fast growth of the low-energy (101) twin contacts. The solid-state diffusion processes during densification and SnO2 grain growth are controlled by the formation of point defects and result in the dissolution of the twin nuclei and the incorporation of Nb5+ and Co2+ ions into the SnO2 matrix in the form of a solid solution. In this process, the twin nuclei are erased and their role in the formation of twins is shown only by irregular segregation of Co and Nb to the twin boundaries and inside the cassiterite grains, and Co,Nb-enrichment in the cyclic twin cores.

Atomistic migration mechanisms of [formula omitted] symmetric tilt grain boundaries in magnesium

Grain boundary (GB) is an important microstructure and plays a vital role in the mechanical properties of polycrystalline materials by GB migration and sliding. In this work, molecular dynamic (MD) simulations were performed to investigate the migration mechanisms of [12¯10] symmetric tilt grain boundaries (STGBs) in magnesium. A total of 15 STGBs with the rotation angle θ from 0° to 90° were studied under a pure shear loading. The results show that the GB migration mechanisms are significantly influenced by the GB structure. For small angle STGBs (θ<28°), the GB migration is mediated by twin nucleation from GB and subsequent twin growth. For large angle STGBs (θ>83°), the GB migration is achieved by the glide of GB dislocations. The medium angle STGBs (28°<θ<83°), which are the majority of studied STGBs, were observed to be transformed into twin boundary (TB) by emitting lattice dislocations/stacking faults (SFs) during migration. The migration mechanisms for medium angle STGBs can be explained by two rules: GB decomposition and emission of lattice dislocations/SFs. This work provides atomic mechanisms on the GB migration, which are important for understanding the GB behaviors and mechanical properties in magnesium.