Correspondence between low-energy twin boundary density and thermal-plastic deformation parameters in nickel-based superalloy

To deeply understand and even describe the evolutions of the low-energy twin boundary density(BLDΣ3n)in a thermal-plastic deformation process,an improved twin density model as a function of average grain size and stored energy is developed.For Nimonic 80A superalloy,the model is solved based on the EBSD statistical results of grain size and BLDΣ3n in the specimens compressed at temperatures of 1273−1423 K and strain rates of 0.001−10 s−1.The corresponding relationships of BLDΣ3n with stored energy and grain size varying with temperature and strain rate are clarified by the superimposed contour plot maps.It is summarized that BLDΣ3n increases with increasing stored energy and decreasing grain size,and higher BLDΣ3n with finer grains corresponds with lower temperatures and higher strain rates.Such relationships are described by the improved twin density model,and the prediction tolerance of the solved model is limited in 2.8%.

Basal stacking fault induced twin boundary gliding,twinning disconnection and twin growth in hcp Ti from the first-principles

First-principles calculations were performed to investigate the structures and energetics of {101n} coherent twin boundaries(CTBs) and glide twin boundaries(GTBs) in hexagonal close-packed(hcp) Ti. The formation mechanism of GTBs and their correlation with twin growth were fundamentally explored. Results suggested that GTBs can form from the gliding of CTBs, through their interaction with basal stacking fault. The gliding eventually restored the CTB structures by forming a pair of single-layer twinning disconnections. The pile-up of twinning disconnections should be responsible for the wide steps at twin boundaries as observed in high-resolution transmission electron microscopy, which can further promote twin growth. Possible effects of various alloying elements on pinning twin boundaries were also evaluated, to guide the strengthening design of Ti alloys.

Effect of twin boundary on nanoimprint process of bicrystal Al thin film studied by molecular dynamics simulation

The effects of a twin boundary（TB） on the mechanical properties of two types of bicrystal Al thin films during the nanoimprint process are investigated by using molecular dynamics simulations.The results indicate that for the TB direction parallel to the imprinting direction,the yield stress reaches the maximum for the initial dislocation nucleation when the mould directly imprints to the TB,and the yield stress first decreases with the increase of the marker interval and then increases.However,for the TB direction perpendicular to the imprinting direction,the effect of the TB location to the imprinting forces is very small,and the yield stress is greater than that with the TB direction parallel to the imprinting direction.The results also demonstrate that the direction of the slip dislocations and the deformation of the thin film caused by spring-back are different due to various positions and directions of the TB.

Twin boundary dominated electric field distribution in CdZnTe detectors

The performance of CdZnTe X/γ-ray detectors is strongly affected by the electric field distribution in terms of charge transport and charge collection.Factors which determine the electric field distribution are not only electric contact,but also intrinsic defects,especially grown-in twin boundaries.Here,the electric field distribution around twin boundaries is investigated in a CdZnTe bicrystal detector with a{111}–{111}twin plane using the Pockels electro-optic effect.The results of laser beam induced current pulses are also obtained by the transient current technique,and we discuss the influence of the twin boundary on the electric field evolution.These studies reveal a significant distortion of the electric field,which is attributed to the buildup of space charges at twin boundaries.Also,the position of these space charge regions depends on the polarity of the detector bias.An energy band model based on the formation of an n–n+–n junction across the twin boundary has been established to explain the observed results.

In situ observation of the pseudoelasticity of twin boundary

Twin boundary(TB)is a special and fundamental internal interface that plays a key role in altering the mechanical and physical properties of materials.However,the atomistic deformation mechanism of TB re-mains under debate,of which the most concerned aspect is how TB would affect the mechanical strength and plasticity of a material.Herein,we introduce our new discovery that the pseudoelastic strain of a TB can recover with decomposition and escape of pile-up dislocations,demonstrated by imposing a sponta-neous pseudoelastic deformation with recoverable plastic bending strain up to 5.1%on a TB.We found that the steps on the curved TB gradually annihilated during the migration of the TB,which was in-duced by the slip of decomposition dislocations on the TB.The TB not only provides local strain harden-ing through interaction with dislocations during the loading stage but also acts as a channel for the fast movement of decomposition dislocations during the recovery stage.Beside,the TB can maintain excellent pseudoelasticity under a multicycle bending test,which may play an important role in improving the fa-tigue resistance of materials.These findings could open up a new avenue for optimizing the mechanical properties of materials by manipulating their twin boundaries at the nanoscale.

Effects of twin orientation and twin boundary spacing on the plastic deformation behaviors in Ni nanowires

Spreading twins throughout nano metals has been proved to effectively mediate the mechanical behaviors in face-centered-cubic(fcc)metals.However,the experimental investigation concerning the roles of twin boundary(TB)during deformation is rarely reported.Here,with the joint efforts of in-situ nanomechani-cal testing and theoretical studies,we provide a systematic investigation regarding the effects of TB orien-tation(θ,the angle between tensile loading direction and the normal of TB)and spacing on deformation mechanisms in Ni nanowires(NWs).As compared with single-crystalline counterparts,it is found that nano-twinned(nt)NWs withθ∼0°exhibit limited ductility,whereas TB can serve as an effective block-age to the dislocation propagation.In contrast,in nt NWs withθ∼20°and 55°,TB migration/detwinning induced by TB-dislocation reaction or partial dislocation movement dominates the plasticity,which con-tributes to enhanced NW ductility.Regarding nt NWs withθ∼90°,dislocations are found to be able to transmit through the TBs,suggesting the limited effect of TB on the NW stretchability.Furthermore,de-creasing TB spacing(λ)can facilitate the detwinning process and thus greatly enhance the ductility of NW withθ∼55°.This study uncovers the distinct roles that TB can play during mechanical deforma-tions in fcc NWs and provides an atomistic view into the direct linkage between macroscopic mechanical properties and microscopic deformation modes.

Solute-solute interactions and their impacts on solute co-segregation and interfacial cohesion of{1012}twin boundary in zinc

Interactions of solute atoms in biodegradable zinc alloys and their effect on alloy mechanical properties have been less investigated.In this work,the interactions between the common solutes(Li,Mg,Mn,Cu,and Ag)used in the biodegradable Zn alloys,including a solute-solute pair with the same element or with two different elements,are investigated based on first-principles calculations.It is found that the energetically favorable configuration is the third nearest-neighboring for most solute-solute pairs in the bulk lattice because of the relatively strong electronic interaction between solute and Zn atoms or the relatively small local elastic deformation associated with the configuration.Considering that interfacial cleavage is a key fracture mode of zinc,the segregation ability of these solutes and their effect on the{1012}twin boundary cohesion are also examined.The result shows that Li tends to fully occupy its preferred site in the twin boundary,while Mg,Mn,Cu,or Ag has a concentration limitation in the twin boundary.The twin boundary cohesion can be significantly enhanced by the segregation of Mn,followed by Cu and Ag,because of the contribution of their d states close to the Fermi level.Furthermore,the co-segregation ability of two solute atoms in the twin boundary increases with increasing the binding tendency of these two solute atoms in the boundary.Mn and Li or Mg show a relatively strong co-segregation ability in the twin boundary.Adding Mn to Zn-Li or Zn-Mg alloys can significantly enhance the resistance to fracture of twin boundaries.

Temperature effect on nanotwinned Ni under nanoindentation using molecular dynamic simulation

Temperature effect on atomic deformation of nanotwinned Ni (nt-Ni) under localized nanoindentation is investigated in comparison with nanocrystalline Ni (nc-Ni) through molecular simulation.The nt-Ni exhibits enhanced critical load and hardness compared to nc-Ni,where perfect,stair-rod and Shockley dislocations are activated at (111),(111) and (111) slip planes in nt-Ni compared to only SSockley dislocation nucleation at (111) and (111) slip planes of nc-Ni.The nt-Ni exhibits a less significant indentation size effect in comparison with nc-Ni due to the dislocation slips hindrance of the twin boundary.The atomic deformation associated with the indentation size effect is investigated during dislocation transmission.Different from the decreasing partial slips parallel to the indenter surface in nc-Ni with increasing temperature,the temperaturedependent atomic deformation of nt-Ni is closely related to the twin boundary:from the partial slips parallel to the twin boundary (~10 K),to increased confined layer slips and decreased twin migration(300 K–600 K),to decreased confined layer slips and increased dislocation interaction of dislocation pinning and dissociation (900 K–1200 K).Dislocation density and atomic structure types through quantitative analysis are implemented to further reveal the above-mentioned dislocation motion and atomic structure alteration.Our study is helpful for understanding the temperature-dependent plasticity of twin boundary in nanotwinned materials.

Deformation behaviors of Cu bicrystals with an inclined twin boundary at multiple scales

Cu bicrystals of different sizes with a sole twin boundary（TB） inclined at 45？with respect to the loading direction were deformed under unidirectional and cyclic loading, respectively. It is found that the slip bands（SBs） parallel to the TB can be activated near the TB at all scales without obeying the Schmid＇s law.It is concerned with the local stress enhancement in the macroscale while it is more closely related to the scarce dislocation sources in the microscale. Moreover, a wedge-shaped zone formed near the TB in the microscale ascribed to the limited specimen size.

Twin boundary： Controllable interface to fatigue cracking

Twin boundaries（TBs） are key factors influencing the mechanical properties of crystalline materials. We have investigated the intrinsic fatigue cracking mechanisms of TBs during the past decade. The effects of TB orientations on the fatigue cracking mechanisms were revealed via cyclic deformation of a series of grown Cu bicrystals with a sole TB. Furthermore, the combined effects of crystallographic orientation and stacking fault energy（SFE） on the fatigue cracking mechanisms were clarified through cyclic deformation of polycrystalline Cu and Cu alloys. Both developments were reviewed in this report which will provide implications to optimize the interfacial design for the improvement of fatigue performance of metallic materials.

Structural characterization of the{1122}twin boundary and the corresponding stress accommodation mechanisms in pure titanium

{11-22}compression twin plays an important role in accommodating deformation along the c axis of HCP metals.However,the studies on the interface structure of{11-22}twin boundary(TB),especially the twin tip,and the corresponding local stress release mechanism are still limited.This work studied the interface characters of{11-22}TB of a deformed pure titanium by transmission electron microscope.The{11-22}TB presented serrated character,consisting of coherent twin boundary(CTB)and(0002)_(T)//(11-2-2)_(M)or(11-2-2)_(T)//(0002)_(M)steps,and the twin tip was fully composed of the basal-pyramidal(BPy)and pyramidal-basal(PyB)steps.The twin tip presents asymmetric morphology and the step height at the twin tip is much larger than that away from the twin tip.Two types of local stress accommodation mechanisms were observed:zonal dislocation emission and hexagonal close packed structure to facecentered cubic structure transformation.The zonal dislocation was produced by the dissociation of the1/3<11-2-3>dislocation that was nucleated at the twin tip and the phase transformation was introduced by emission of Shockley partial dislocations from the{11-22}twin boundary.

Effect of Rare Earths on Electronic Structure of (110) Twin Martensite Boundary for Ni_2MnGa Alloy

The doping effect of rare earth elements (Tb and Sm) on the electronic structure of (110) martensitic twin boundary in Ni2MnGa alloys was investigated by using ab initio method within the DFT and the supercell implementation. The calculated results show that the atomic relaxation lowers the boundary energy and the segregation energy. Sm seems easier to segregate to the boundary and has a greater doping effect compared with Tb due to its lower segregation energy and bigger bonder order with neighboring atoms. Tb makes a greater contribution to the magnetic properties of the twin boundary than Sm.

A realistic topological p-n junction at the Bi2Se3 （0001） surface based on planar twin boundary defects

We propose a realistic topological p-n junction （TPNJ） by matching two Bi2Se3 （0001） slabs with opposite arrangements of planar twin boundary defects. The atomistic modeling of such a device leads to dislocation defects in the hexagonal lattice in several quintuple layers. Nevertheless, total energy calculations reveal that the interface relaxes, yielding a smooth geometrical transition that preserves the nearest-neighbors fcc-type geometry throughout these defect layers. The electronic, magnetic, and transport properties of the junction have then been calculated at the ab initio level under open boundary conditions, i.e., employing a thin-film geometry that is infinite along the electron transport direction. Indeed, a p-n junction is obtained with a built-in potential as large as 350 meV. The calculations further reveal the spin texture across the interface with unprecedented detail. As the main result, we obtain non-negligible transmission probabilities around the F point, which involve an electron spin-flip process while crossing the interface.

Primary and secondary modes of deformation twinning in HCP Mg based on atomistic simulations

Deformation twinning, i.e., twin nucleation and twin growth （or twin boundary migration, TBM） activated by impinged basal slip at a symmetrical tilt grain boundary in HCP Mg, was examined with molecular dynamics （MD） simulations. The results show that the {1^-1^-21}-type twinning acts as the most preferential mode of twinning. Once such twins are formed, they are almost ready to grow. The TBM of such twins is led by pure atomic shuffling events. A secondary mode of twinning can also occur in our simulations. The {112^-2} twinning is observed at 10 K as the secondary twin. This secondary mode of twinning shows different energy barriers for nucleation as well as for growth compared with the {1^-1^-21}-type twining. In particular, TBMs in this case is triggered intrinsically by pyramidal slip at its twin boundary.

EFFECT OF NANOCRYSTALLINE AND TWIN BOUNDARIES ON CORROSION BEHAVIOR OF 316L STAINLESS STEEL USING SMAT

By means of surface mechanical attrition treatment （ SMAT）, the groin size with a diameter of aboat 60hm formed at about 20μm depth and numerous mechanical twins at about 50μm depth from the treated surface were synthesized in 316L stainless steel because of the different distributions of strain and strain rate along depth orientation. For instance the maximum strain rate reached 10^3-10^4s^-1 on the top surface. The relationship between the microsturcture and the corrosion property was studied in 0.05M H2SO4＋ 0.25M Na2SO4 aqueous solution, and the results show an extreme improvement of corrosion resistance owing to the appearance of twin boundaries and the obvious reduction in corrosion resistance attributed to the presence of nanocrystaline boundaries.

Atomistic simulations of interactions between screw dislocation and twin boundaries in zirconium

Molecular statics was employed to simulate interaction between screw dislocation and twin boundaries（TB） in hexagonal close-packed zirconium. In the moving TB model, the interaction of a moving {10ˉ12} TB with a static 1/311ˉ20{10ˉ10} screw dislocation was investigated. Twinning dislocation（TD） nucleation and movement play an important role in the interaction. The screw dislocation passes through the moving TB and changes to a basal one with a wide core. In the moving dislocation model, a moving 1/31120{1010} dislocation passes through the TB, converting into a basal one containing two partial dislocations and an extremely short stacking fault. If the TB changes to the {1011} one, the moving1/31120{1010} prismatic screw dislocation can be absorbed by the static TB and dissociated into two TDs on the TB. Along with the stress-strain relationship, results reveal the complicated mechanisms of interactions between the dislocation and TBs.

Current Progress of Mechanical Properties of Metals with Nano-scale Twins

Focus on face-centered cubic （fcc） metals with nano-scale twins lamellar structure, this paper presents a brief overview of the recent progress made in improving mechanical properties, including strength, ductility, work hardening, strain rate sensitivities, and in mechanistically understanding the underling deformation mechanisms. Significant developments have been achieved in nano-twinned fcc metals with a combination of high strength and considerable ductility at the same time, enhanced work hardening ability and enhanced rate sensitivity. The findings elucidate the role of interactions between dislocations and twin boundaries （TBs） and their contribution to the origin of outstanding properties. The computer simulation analysis accounts for high plastic anisotropy and rate sensitivity anisotropy by treating TBs as internal interfaces and allowing special slip geometry arrangements that involve soft and hard modes of deformation. Parallel to the novel mechanical behaviors of the nano-twinned materials, the investigation and developments of nanocrystalline materials are also discussed in this overview for comparing the contribution of grain boundaries/TBs and grain size/twin lamellar spacing to the properties. The recent advances in the experimental and computational studies of plastic deformation of the fcc metals with nano-scale twin lamellar structures provide insights into the possible means of optimizing comprehensive mechanical properties through interfacial engineering.

Direct characterization of boron segregation at random and twin grain boundaries

Boron distribution at grain boundaries in hot-deformed nickel is directly characterized by the time-of-flight secondary ion mass spectrometry. The segregations of boron are observed at both the random and twin grain boundaries. Two types of segregations at random grain boundaries are observed. The first type of segregation has a high intensity and small width. Its formation is attributed to the incorporating of dislocations into the moving grain boundaries. The second type of segregation arises from the cooling induced segregation at the dislocations associated with the grain boundaries. The segregation at twin boundary is similar to the second type of segregation at random grain boundaries.

Intrinsic Strengthening of Coherent Twin Boundaries in Copper

Molecular dynamics （MD） simulations were applied to simulate the deformation process of copper with different density of parallel coherent twin boundaries （TBs）. It is shown that the strength of perfect copper crystal enhances with increasing coherent TB density. Based on the local hydrostatic pressure analysis, we found that stress concentrations are more likely to form in the interior of the crystal rather than around the TBs. Since the dislocation nucleation is suppressed in the vicinity of the coherent TBs and each TB plane hinders dislocations from propagating, the coherent TBs can be regarded as an intrinsic strengthening phase relative to perfect crystal.

Atomic scale structural characterization of {10 12} twin boundaries in zinc

The structure and the migration mechanisms of {10 12} twin boundaries（TBs） of pure zinc deformed by rolling were studied using high-resolution transmission electron microscopy（HRTEM） at atomic scale. We found the presence of basal/prismatic（BP/PB） planes serrations on {10 12} TBs and the coexistence of two kinds of TBs with different structures in the same {10 12} twin： TBs composed of {10 12} coherent twin boundaries（CTBs） plus short BP/PB serrations, and TBs composed of successive BP/PB segments without {10 12} CTBs. The formation of BP/PB serrations has no relation to the c/a ratio of hexagonal-closepacked（HCP） metals because the BP/PB serrations are energetically preferred and geometrically favored. Based on dislocation theory, we proposed the migration mechanisms of the TBs to be the glide of twinning dislocations（TDs） on the CTBs and the climb of interface dislocations（IDs） on the BP/PB segments.