{332}<113> detwinning in a multilayered bcc-Ti–10Mo–Fe alloy

We have studied {332}<113> detwinning mechanism in a bcc-Ti–10Mo–xFe (x = 1–3 wt%) multilayered alloy upon annealing treatment at 900 °C by electron backscatter diffraction and electron channeling contrast imaging. Our analysis reveals that in the present material the detwinning process consists of two independent detwinning events that occur at two different microstructural regions, namely twin tips located at grain interiors and grain boundaries. Boundary dissociation reactions and mobilities were analyzed by tracking the evolution of the twin structure upon thermal annealing. We find that the first detwinning process is characterized by the evolution of incoherent twin boundaries into a Σ3 boundary and its subsequent migration. The second detwinning event is characterized by the detachment of the twin crystal from a grain boundary by the formation and migration of Σ11 {113} incoherent twin boundaries. Both detwinning modes can be explained from a thermodynamic standpoint where the boundary dissociation processes minimize the boundary free energy.     

Structural characterization of the { 11-22 } 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 {11-2 2} TB of a deformed pure titanium by transmission electron microscope.The { 11-2 2} 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 face-centered cubic structure transformation.The zonal dislocation was produced by the dissociation of the 1/3 < 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 {11-2 2} twin boundary.     

Annealing twins in a multifunctional beta Ti-Nb-Ta-Zr-O alloy

The grain boundary character distribution and annealing twins in a multifunctional β-type Ti-23Nb-0·7Ta-2Zr-O alloy having a stable bcc phase structure are investigated by electron backscattering diffraction (EBSD). The results show that the coincident site lattice (CSL) boundaries, including Σ3 and Σ11 twin boundaries mainly form at the early stage of recrystallization. {112}〈111〉 and {332} 〈113〉 annealing twins are observed to occur in the completely recrystallized Ti-23Nb-0·7Ta-2Zr-O alloy.     

Interaction of {112¯2} twin variants in hexagonal close-packed titanium

As multiple {11$\overline{2}$ 2} twin variants are often formed during deformation in hexagonal close-packed (hcp) titanium, the twin-twin interaction structure has a profound influence on mechanical properties. In this paper, the twin-twin interaction structures of the {11$\overline{2}$2} contraction twin in cold-rolled commercial purity titanium were studied by using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). Formation of the {11$\overline{2}$2} twin variants was found to deviate the rank of Schmid factor, and the non-Schmid behavior was explained by the high-angle grain boundary nucleation mechanism. All the observed twin-twin pairs manifested a quilted-looking structure, which consists of the incoming twins being arrested by the obstacle twins. Furthermore, the quilted-looking {11$\overline{2}$2} twin-twin boundary was revealed by TEM and high resolution TEM observations. De-twinning, lattice rotation and curved twin boundary were observed in the obstacle twin due to the twin-twin reaction with the impinging twin. A twin-twin interaction mechanism for the {11$\overline{2}$2} twin variants was proposed in terms of the dislocation dissociation, which will enrich the understanding for the propagation of twins and twinning-induced hardening in hcp metals and alloys.     

Effect of downscaling nano-copper interconnects on the microstructure revealed by high resolution TEM-orientation-mapping

In this work, a recently developed electron diffraction technique called diffraction scanning transmission electron microscopy (D-STEM) is coupled with precession electron microscopy to obtain quantitative local texture information in damascene copper interconnects (1.8 μm-70 nm in width) with a spatial resolution of less than 5 nm. Misorientation and trace analysis is performed to investigate the grain boundary distribution in these lines. The results reveal strong variations in texture and grain boundary distribution of the copper lines upon downscaling. Lines of width 1.8 μm exhibit a strong <111> normal texture and comprise large micron-size grains. Upon downscaling to 180 nm, a {111}<110> bi-axial texture has been observed. In contrast, narrower lines of widths 120 and 70 nm reveal sidewall growth of {111} grains and a dominant <110> normal texture. The microstructure in these lines comprises clusters of small grains separated by high angle boundaries in the vicinity of large grains. The fraction of coherent twin boundaries also reduces with decreasing line width.     

The assisting and stabilizing role played by ω phase during the {112} <111>β twinning in Ti-Mo alloys:A first-principles insight

The ω phase is commonly observed in β-Ti alloys and plays a significant role on various properties of β-Ti alloys.Although many results about the role ofω phase on mechanical properties of β-Ti alloys have been derived from theoretical and experimental studies,the role ofω phase on deformation mechanism hitherto remains elusive and deserves to be further studied.In this work,the role played by ω phase during the {112 } <111>β twinning in Ti-Mo alloys were investigated by first-principles calculations at atomic scale.In the energy favorable interface of(112)β/(10(1)0)ω,we found that partial dislocations slipping on the successive (10(1)0)ω planes ofω phase can lead to the formation of { 112} <111>β twin nucleus.And the twin nucleus grows inwards ω grain interior through atomic shuffle.Thus,a new twinning mechanism of {112 } <111>β assisted by ω phase was proposed.Furthermore,our calculations indicated that the Pearance of ITB (interfacial twin boundary) ω phase can improve the stability of the symmetrical 12 } <111 >β twin boundary (TB),which can well explain the experimental phenomenon that the ITB ω phase always accompanies the formation of {112 } <111>β twin.Finally,a probable microstructure evolution sequence was suggested,namely β matrix → β matrix + athermal ω phase → (112)[11(1)]twin → (112)[11(1)]β twin + ITB ω phase.Our calculations provide new insights on the role played by ω phase during the twinning process of {112} <111>β,which can deepen the understanding on the deformation behaviors of β-Ti alloys.     

Atomic-resolution studies on reactions between basal dislocations and {1012} coherent twin boundaries in a Mg alloy

Reactions between basal 〈a60〉 dislocations and {1012} coherent twin boundaries(CTBs) in a Mg alloy were studied with atomic resolution. Individual dislocation-CTB reactions produced steps with residual dislocations and multiple t winnin g dislocations(TDs) gliding away, consequently resulting in TB migration. Reactions between {1012} CTBs and low-angle grain boundaries composed of basal 〈a60〉 dislocations created either basal-prismatic/prismatic-basal interfaces or asymmetric tilt grain boundaries, depending on whether TDs gliding away or not. Not only the emission of TDs by dislocation-TB reactions may drive TB migration, but also the resultant steps or facets along TBs can act as TD sources to facilitate TB migration. Our results indicate that roughness or severe loss of local coherency induced by dislocation-TB reactions does not intrinsically impede TB migration in Mg alloys. Dislocation-TB reactions may provide another feasible strategy to improve the ductility of Mg alloys, in addition to other techniques like grain refinement and texture modification.     

Diffraction contrast of fold domain boundaries in polyethylene single crystals

The dark-field diffraction contrast images of several types of fold domain boundaries in polyethylene single crystals have been characterized. Macrosector, microsector, {110} and {310} twin boundary contrast are shown to be due to the image shift which occurs when two diffracted beams contribute to the dark-field image.     

Evolution of a martensitic structure in a Cu–Al alloy during processing by high-pressure torsion

A Cu-11.8 wt% Al alloy was quenched in iced water from a high temperature (850 °C) to introduce a martensitic phase and then the alloy was processed using quasi-constrained high-pressure torsion (HPT). The micro-hardness and the microstructures of the unprocessed and severely deformed materials were investigated using a wide range of experimental techniques (X-ray diffraction, optical microscopy, scanning electron microscopy, transmission electron microscopy, and high- resolution TEM). During HPT, a stress-induced martensite–martensite transformation occurs and an $ \alpha^{\prime}_{1} $ martensite phase is formed. In the deformed material, there are nanoscale deformation bands having high densities of defects and twins in the $ \alpha^{\prime}_{1} $ martensite. It was observed that a high density of dislocations became pinned and accumulated in the vicinity of twin boundaries, thereby demonstrating a strong interaction between twin boundaries and dislocations during the HPT process.     

Annealing twin boundary cracking in the low-temperature brittle fracture of a high-nitrogen bearing austenitic steel

Crack propagation paths in the low temperature brittle fracture of 18Cr-18Mn-0.7N austenitic steel were investigated by means of scanning electron microscopy. Corresponding relationships of the fracture facets with microstructures were established by the simultaneous observation of the fracture surface and the microstructure of the adjacent side surface. It was shown that the annealing twin boundary cracking occurred during fracture. A great deal of twins formed during solution treatment of the steel, with steps several microns high on the twin boundaries, and a considerable amount of planar deformation structures developed on {111} planes in the fracture process. The fracture facets of the annealing twin boundary are fairly flat and smooth, with bent steps of micron-scale height, and a pattern of three sets of parallel straight-lines intersecting at 60°. But there is no river pattern on the facets. The bent steps result from partial propagation of crack along steps that are developed on annealing twin boundaries during solution treatment, while the line-pattern is the intersection traces of the planar deformation structures with the fracture facet. It is believed that the annealing twin boundary cracking is attributed to the stress concentration arising at the intersection of planar deformation structures and the annealing twin boundaries.     

HRTEM study of [001] low-angle tilt grain boundaries in fiber-textured BaTiO3 thin films

Low-angle tilt grain boundaries in [001] fiber-textured BaTiO₃ thin films were investigated by high-resolution transmission electron microscopy. Extensive observation revealed a very high density of low-angle tilt grain boundaries in the film. The low-angle tilt grain boundaries can be described as periodical arrays of dislocations on {100} and {110} boundary planes. The boundaries with (100) plane on {100} planes are composed of perfect dislocations with Burgers vectors b = a < 100 > (a = lattice constant of BaTiO₃: 0.3992 nm), while the boundaries with (110) plane on {110} planes are composed of the dissociated dislocations with Burgers vectors a/2 < 110 >. It was thus found that the difference in the boundary plane leads to different dislocation structures along the low-angle grain boundaries.     

Detwining in Mg alloy with a high density of twin boundaries

To investigate the role of preexisting twin boundaries in magnesium alloys during the deformation process, a large number of {10-12} tensile twins were introduced by a radial compression at room temperature before hot compressive tests with both low and high strain rates. Unlike the stable twins in Cu-based alloys with low stacking fault energies, {10-12} twins in Mg alloy are extremely unstable or easy to detwin through {10-12}-{10-12} re-twinning. As a result, non-lenticular residual twins and twin traces with misorientation of 5°–7° were present, as confirmed by electron backscatter diffraction. The extreme instability of the twins during compression indicates that both twin and detwinning require extremely low resolved shear stresses under our experimental conditions.     

Role of slip and {10-12} twin on the crystal plasticity in Mg-RE alloy during deformation process at room temperature

The deformation mechanism of slips and twins has a considerable influence on the plasticity of magne-sium alloys.However,the roles of slips and twins in the room-temperature deformation of Mg-rare earth(Mg-RE) alloys with high contents of rare earth elements is rarely investigated.Here,the microstruc-tural evolution and deformation mechanism of an aged Mg-SY-2Nd-3Sm-0.SZr alloy during uniaxial compression at room temperature were systematically investigated using in-situ electron-backscattered diffraction and transmission electron microscopy.The results indicated that in the early stage of defor-mation,the Mg-RE alloy was mainly controlled by the slip of dislocations in the basal plane and the coordinated c-axis strain of the {10-12} twin.With an increase in the strain,the grain orientation became more suitable for the initiation of pyramidal Ⅱ dislocations in the later stage of deforma-tion;these dominated the deformation mechanism.In the twin evolution of the Mg-RE alloy,there were three types of twin-twin interaction behaviors:(i) single twin variant 'parallel'structure,(ii) single twin variant 'cross' structure,and (iii) multi twin variant 'cross' structure.In addition,three types of twin-grain boundary interaction behaviors were summarized:(i) twin 'refracting through'grain boundary,(ii) twin'parallel through'grain boundary,and (iii) twin 'fusing through'grain boundary,which are expected to act as new means and solutions for the twin strengthening of magnesium alloys.     

An analytical approach and experimental confirmation of dislocation–twin boundary interactions in titanium

The morphology of $ \{ 10\overline{1} 2\} \left\langle {\overline{1} 011} \right\rangle $ { 10 1 ¯ 2 } 〈 1 ¯ 011 〉 deformation twins formed in commercial purity titanium during an initial pass of equal-channel angular pressing was studied by transmission electron microscopy (TEM). The corresponding diffraction patterns show a symmetry line splitting of $ (10\overline{1} 2) $ ( 10 1 ¯ 2 ) twin boundaries (TB) which is related to the presence of interfacial defects. A simple modeling for the interaction between non-screw a-slip lattice dislocations (Burgers vector b =  $ \frac{1}{3}[\overline{1} \overline{1} 20] $ 1 3 [ 1 ¯ 1 ¯ 20 ] ) and the $ (10\overline{1} 2) $ ( 10 1 ¯ 2 ) twin plane is used according to crystallographic geometry and vector conservation. The results show that dislocation dissociation into different Frank partial dislocations on the interfacial plane is more favorable than its transmission to the other side of the interface. The formation of the Frank partials at the TB can produce a small change in the TB misorientation angle and this is consistent with the symmetry line splitting of the $ (10\overline{1} 2) $ ( 10 1 ¯ 2 ) twin boundaries observed by TEM.     

Reversible motion of twin boundaries in AZ31 alloy and new design of magnesium alloys as smart materials

Attractiveness of magnesium alloys for structural applications is caused by their intrinsic properties i.e. low density and high specific strength. The main challenge in development of magnesium alloys is connected with requirement to fulfill the main function of structural materials i.e. to bear load. Wrought magnesium alloys possess strong basal texture which causes anisotropy of mechanical properties. It would be interesting to find out the way how to benefit from this anisotropic behavior of magnesium alloys. One way is to take into account strong basal texture and {1 0 –1 2} twinning in magnesium alloys under compressive deformation. Parallelepiped samples of AZ31 magnesium alloy were successively deformed in compression with 3.5% strain along two perpendicular directions. During first compression the sample contracts along the RD direction parallel to compression axis, elongates only in one perpendicular ND direction and no deformation is observed in third perpendicular TD direction. Subsequent compression along the ND direction recovers the initial shape of the sample. Microstructure analyses shows that the {1 0 –1 2} twinning is the main deformation mode during compression along the RD direction and twin variants which gives 0% strain to TD direction are predominant in microstructure. Twin-free microstructure is observed after subsequent compression along the ND direction. Crystallographic analyzes and calculations explain why reversible motion of twin boundaries is more favorable than nucleation of other twin variants in matrix grains during compression along the ND direction. The experiment presented in this article profile wrought magnesium alloy as smart material and emphasize the importance of strong {0 0 0 1} <1 0 –1 0> texture and {1 0 –1 2} twinning in obtaining the properties characteristic for smart materials. In the presented case, it is the ability to produce and recover significant strains in a controlled manner under compressive stress.     

The role of vicinal Σ3 boundaries and Σ9 boundaries in grain boundary engineering

Σ 3 grain boundary planes and triple junctions containing Σ 3 boundaries have been investigated in grain boundary engineered brass using an adaptation of electron backscatter diffraction (EBSD) data combined with a single surface trace analysis methodology. The data have been analysed from the standpoint of whether or not the interface planes in the Σ 3 boundaries are close to {111} (i.e. coherent annealing twins), vicinal to {111} (i.e. a small deviation from the {111} reference structure) or not on {111}. At triple junctions composed of Σ 3/Σ 3/Σ 9 it was shown that a combination of one {111} and one vicinal-to-{111} Σ 3 was more likely to occur than two {111} Σ 3s or two not-{111} Σ 3s. The explanation for the preferred {111}/vicinal-to-{111} combination is that mobile Σ 9 boundaries with high deviations encounter {111} Σ 3s, and in consequence regenerate a Σ 3 at the triple junction which must be vicinal-to-{111} according to the crystallographic constraints at the junction. Analysis of the not-{111} Σ 3s indicated that more than half the boundaries in this category could not have {211}{211}, {774}{855}, {111}{511}, {001}{221} or {110}{411} planes. The possible distribution of these planes types, based on information from the single-surface trace analysis, had a high rank correlation coefficient, 0.925, with previous data from nickel which was based on a two-surface, full boundary planes analysis.     

Mg-0.4Zn镁合金挤压板拉伸变形组织的演变

用电子背散射衍射(EBSD)技术结合原位拉伸,研究了在0%~20%应变条件下,Mg-0.4%Zn二元镁合金晶界、织构和裂纹的变化。结果表明,在拉伸应变从0%增加到20%的过程中,随着应变量的增大材料微观组织中的孪晶逐渐增加。孪晶的类型以{10-12}拉伸孪晶为主;这种孪生使材料的组织织构类型发生了显著的变化,随着应变量的增大(0001)//TD面的新织构组分的强度提高;微观裂纹优先在原始晶界和孪晶尖处萌生并在部分晶粒出现穿晶裂纹,随着应力的增大微裂纹进一步扩展并相互连接最终使材料断裂。     

Effect of initial texture on dynamic recrystallization of AZ31 Mg alloy during hot rolling

Wedge-shaped AZ31 plates with two kinds of initial textures were rolled at 573 K to investigate the effect of initial texture on dynamic recrystallization (DRX). The results indicated that the initiation and nucleation of DRX were closely related to the initial texture. The initiation and completion of DRX in the TD-plate were significantly retarded compared with that in the ND-plate. Twin related DRX nucleation was mainly observed in the ND-plate samples; while gain boundary related DRX nucleation was mainly observed in the TD-plate samples. The different DRX behavior between the TD- and ND-plates was attributed to the different deformation mechanism occurring before DRX initiation. For the ND-plate, dislocation glide was considered as the main deformation mechanism accompanied with {1 0 −1 1}-{1 0 −1 2} double twin, which led to the increment of a faster increasing stored energy within the grains. And {1 0 −1 1}-{1 0 −1 2} double twin was mainly found to be DRX nucleation site for the ND-plate. For the TD-plate, {1 0 −1 2} extension twin was the dominant deformation mechanism which resulted in a basal texture with the c-axis nearly parallel to ND. The stored energy caused by dislocation motion was relatively small in the TD-plate before a basal texture was formed, which was considered as the main reason of that DRX was retarded in the TD-plate compared with that in the ND-plate. Based on the difference in deformation mechanism and DRX mechanism caused by the different initial texture, the variation in grain size, micro-texture and misorientation angle distribution in the ND and TD plates were discussed.     

De-twinning and Texture Change in an Extruded AM30 Magnesium Alloy during Compression along Normal Direction

Twinning and de-twinning are the salient deformation mechanisms in hexagonal close-packed(hcp)metals. The aim of this study was to examine and quantify the de-twinning process involving a reversible motion of twin boundaries in an extruded AM30 magnesium alloy after re-compression along the normal direction(ND) of pre-compressed samples along the extrusion direction(ED). {1012} extension twins were first introduced at a compressive strain of 3.7% along the ED. The subsequent compressive deformation along the ND induced a gradual shrinkage of twins with increasing cumulative true strain,and the complete de-twinning occurred at a strain of ~7.7%. The twin width decreased linearly with increasing true strain. Texture measurements verified the rotation of c-axes of hcp unit cells towards the anti-compression direction due to {1012} extension twinning after compression along the ED, and a gradual return of c-axes to the initial orientation due to twin shrinking or de-twinning during the following compression along the ND. The {1012} twinning corresponded to the formation of new texture components C{1210}<0001> and D{0110}<0001> and a decrease in the initial texture components A{0001}<2110> and B{0001}<1010>, while the twin shrinking or de-twinning was characterized by a gradual vanishing of components C{1210}<0001> and D{0110}<0001> and an increase in the components A{0001}<2110> and B{0001}<1010>.     

Crystal growth of the primary silicon in an Al-16 wt % Si alloy

Studies on the crystallographic growth habit of primary silicon crystals in an Al-16 wt% Si alloy were carried out by X-ray micro focus Laue analysis and ECP (electron channelling pattern) analysis. The plate-like primary silicon crystals grow by the same mechanism as that for germanium dendrites, i.e. the TPRE (twin plane re-entrant edges) mechanism. The spherical primary silicon crystal in sodium treated melts is composed of several pyramidal grains with tops at the centre of the sphere. Many of these grains have a twin relation to each other. The sodium enriched regions are found at the boundaries of these pyramidal silicon grains. The external surfaces of the spherical primary crystals exhibit regular crystal facets. The surface facets are most frequently parallel to {111} plane but there are also some facets parallel to other less densely packed planes such as {100}, {211} and so on.