Twin Lamellae Research Articles

Grain and twin boundaries dependent mechanical behavior of FeCoCrNiCu high-entropy alloy

The nanoindentation response of FeCoCrNiCu high-entropy alloy is revealed through molecular dynamics simulation. The influences of various grain sizes and twin lamellae thicknesses on the mechanical characteristics and plastic deformation are carefully studied. The result indicates that the indentation force and substrate hardness increase as the grain size increases from 55.16 Å to 114.46 Å and the twin layer thickness rises from 6.74 Å to 30.04 Å, which reveals the inverse Hall-Petch effect in the relationship between the material strength versus grain size and the twin lamellae thickness. The shear strain and residual stress are converged around the indentation regions and then enhanced deep into the substrate along the grain boundary and twin boundary. The presence of grain boundaries significantly affects the movement of the atoms, resulting in an asymmetric dispersion of the atomic displacement vector during the deformation. Moreover, the surface morphology reveals that the pile-up height generally increases with increasing the grain size and decreasing the twin lamellae spacing. The microstructural evolution suggests that the rotation of the grain related to grain boundary sliding and the migration of the grain boundary are the dominant mechanism in the deformation with grain size reduction. Moreover, the dislocation nucleation at the intersections of the twin and grain boundaries is also a remarkable characteristic, which significantly affects the plastic deformation of the material. The result also shows that the dislocation density tends to increase with increasing grain size and twin lamellae thickness. The figure shows the simulation model of FeCoCrNiCu high-entropy alloy (a), atomic displacement vectors of samples with different grain sizes (b), atomic configuration of the specimens with various twin lamellar thicknesses (c). ● The inverse Hall-Petch effect is recognized with the variation of grain size and twin lamellae thickness. ● The deformation mechanism of FeCoCrNiCu HEA is significantly dependent on the grain/twin boundaries. ● The rotation of grain related to grain boundary sliding and the grain boundary migration are dominant mechanism. ● The dislocation nucleation at the intersections of the twin and grain boundaries is also a remarkable characteristic.

Twin suppression by atomic scale engineering of precipitate-matrix interfaces

This work aims to investigate the influence of solute segregation at particle-matrix interfaces on the twin activity of aged Mg alloys. With that purpose a binary Mg-8Al alloy (wt%) and two ternary Mg-8Al-1 Zn and Mg-8Al-1Ag alloys (wt%) were first homogenized and then aged in order to obtain comparable precipitate distributions. Particle-matrix interfaces in the binary alloy were observed to be clean, while Zn and Ag atomic segregation was clearly present at the interfaces of the ternary alloys. Room temperature micropillar compression tests were carried out in grains with favorable orientations for tensile twinning in the homogenized and the aged conditions for the three materials under investigation. The activation of twinning and slip was then characterized by electron microscopy. This study shows that while in the binary alloy precipitation did not lead to any major changes in the twinning activity and, in particular, it did not prevent the formation of thick twin lamellae encompassing the entire width of the tested micropillars, a drastic reduction of the twin activity took place in the aged ternary alloys. In the latter, straining led to the formation of narrow twin lamellae belonging to a single variant, and occasionally arrested at the interior of the micropillars. We propose that the drastic reduction of the twin volume fraction in the aged ternary alloys may be attributed to the decrease of the particle-matrix interface energy due to Zn and Ag segregation, which would hinder re-nucleation at such interfaces, thus suppressing twin propagation.

Thermally induced planar defect formation in sputtered V1-xMoxB2-Δ films

This work reports on various thermally induced planar defects formed in ternary V1-xMoxB2-Δ coatings with different influence on mechanical properties. V0.58Mo0.42B1.9 and V0.37Mo0.63B1.2 films were prepared by magnetron sputtering and annealed in vacuum up to 1300 °C. Depending on the stoichiometry, different types of planar defects were identified by scanning transmission electron microscopy (STEM). In the slightly understoichiometric V0.58Mo0.42B1.9 system, anti-phase boundary defects (APB) were observed, which was accompanied by chemical separation of V and Mo. Accumulation of these defects led to formation of MoB-Cmcm orthorhombic phase in otherwise hexagonal VB2-P6/mmm structure, with no apparent effect on mechanical properties. On the other hand, in case of the highly understoichiometric V0.37Mo0.63B1.2 coating, no chemical separation was observed. Here, annealing resulted in crystallization from amorphous state into orthorhombic Cmcm phase with high density of twin lamellae, which was accompanied by a significant hardening.

High-mobility 4 μm MOVPE-grown (100) β-Ga2O3 film by parasitic particles suppression

AbsrtractIn this work, we comprehensively investigate the development of unwanted parasitic particles in the MOVPE chamber while growing μm level films. The density of the parasitic particles is found to be pronounced at film thicknesses starting from >1.5 to 2 μm. These particles seem to induce structural defects such as twin lamellae, thereby harming the electrical properties of the grown film. The origin of the parasitic particle is attributed to the parasitic reactions within the chamber triggered by the promoted gas-phase reactions during the growth process, which can be largely reduced by increasing the total gas flow and decreasing the showerhead distance to the susceptor. A film thickness of up to 4 μm has been achieved after minimizing the density of parasitic particles. Thereby, RT Hall measurements reveal carrier mobilities of 160 cm2V−1s−1 at carrier concentrations of 5.7 × 1016 cm−3.

Improvement of severe plastic deformation realized by several passes rotary swaging in the microstructure and properties of Mg-0.6Mn-0.5Al-0.5Zn-0.4Ca alloy

The industrial applications of Mg alloys are greatly limited by their trade-off of high strength and ductility. In this work, conventional extrusion and rotary swaging (RS) were applied to Mg-0.6Mn-0.5Al-0.5Zn-0.4Ca (wt.%) alloy to obtain high strength-ductility synergy Mg alloys. The microstructure evolution and mechanical properties of as-extruded and as-swaged alloys at different (2, 6 and 10) RS passes were investigated. The results show that large quantity of dislocations occur in twin lamellae and original dynamic recrystallized (DRXed) grains after RS. Those dislocations promote the formation of low-angle grain boundaries (LAGBs) and subgrains. As the RS process continue, these subgrains transform into fine grains with high-angle grain boundaries (HAGBs), which result in grain refinement. Compared to the as-extruded alloys, the strength of as-swaged alloys increases significantly. The edge region of the as-swaged Mg-0.6Mn-0.5Al-0.5Zn-0.4Ca alloy after 10 passes exhibits comprehensive excellent mechanical performance with a yield strength (YS) of 403 MPa, an ultimate tensile strength (UTS) of 427 MPa and a fracture elongation (FE) of 9.1%. The high strength of as-swaged alloys is the results of the combination of dislocation strengthening, fine grain strengthening and dispersion strengthening. The deformed grains deflected to ED and the tensile twins produced during RS enhance the basal texture and reduce the plasticity of the alloy.

The morphologies of twins in tetragonal ferroelectrics

Twins, as a special structure, have been observed in a small number of experiments on ferroelectrics. A good understanding of the morphologies of twins is very important for twin engineering applications in ferroelectric materials. In this work, the morphologies of twins in tetragonal ferroelectrics are investigated using the compatibility analysis of the transformation strains and spontaneous polarization and the energetic analysis. The tetragonal BaTiO3 single crystal is chosen as an example for the material system. The results show that the lamellar twin structures with 111 as the twin plane have been identified by both compatibility analysis and energetic analysis, which is consistent with the experimental observations. In addition to 111 twin structures, 12¯1 and 21¯5 twin structures can also appear in tetragonal ferroelectrics. Furthermore, stable state uncharged twin boundaries and metastable charged twin boundaries are distinguished by energetic analysis, where the spontaneous polarization is compatible across the uncharged twin boundaries, while it is not compatible across the charged twin boundaries. The results predicted by the compatibility analysis and energetic analysis are consistent, thus providing a pathway for understanding the morphologies of twins.

Microstructure of rapidly‐quenched ZrO2‐SiO2 glass‐ceramics fabricated by container‐less aerodynamic levitation technology

AbstractIn this work, an aerodynamic levitation technology (ALT) was utilized to prepare ZrO2‐SiO2 glass‐ceramics with two different ZrO2 contents, that is, 35 mol% and 50 mol%. The glass‐ceramics were partially melted at ∼2000°C or fully melted at ∼3000°C by ALT, followed by rapid quenching to obtain spherical glass‐ceramic beads. The phase compositions and microstructures of the glass‐ceramics were characterized. Crystallization of ZrO2 occurred during the solidification process and ZrO2 content, processing temperature, and the addition of yttrium (3 mol%) affected the crystalline phase of ZrO2. No ZrSiO4 or crystalline SiO2 were formed during the solidification process and the glass‐ceramics were away from thermodynamic equilibrium due to rapid quenching. The glass‐ceramics showed a microstructure of irregular‐shaped ZrO2 micro‐aggregates embedded in an amorphous SiO2 matrix, with lamellar twins and lattice defects formed within ZrO2 crystals. For samples prepared at ∼3000°C, a liquid‐liquid phase separation occurred in the melt, which eventually resulted in the formation of large and irregular‐shaped ZrO2 aggregates. In comparison, for samples prepared at ∼2000°C, pre‐existed ZrO2 crystals formed during heating acted as nucleation sites during the cooling process, followed by grain growth to form large ZrO2 aggregates. Solidification and microstructure formation mechanisms were proposed to elucidate the solidification process during rapid cooling and the microstructure of the glass‐ceramics obtained.

Metalorganic chemical vapor deposition of (100) β-Ga2O3 on on-axis Ga2O3 substrates

Metalorganic chemical vapor deposition (MOCVD) growths of β-Ga2O3 on on-axis (100) Ga2O3 substrates are comprehensively investigated. Key MOCVD growth parameters including growth temperature, pressure, group VI/III molar flow rate ratio, and carrier gas flow rate are mapped. The dependence of the growth conditions is correlated with surface morphology, growth rate, and electron transport properties of the MOCVD grown (100) β-Ga2O3 thin films. Lower shroud gas (argon) flow is found to enhance the surface smoothness with higher room temperature (RT) electron Hall mobility. The growth rate of the films decreases but with an increase of electron mobility as the VI/III molar flow rate ratio increases. Although no significant variation on the surface morphologies is observed at different growth temperatures, the general trend of electron Hall mobilities are found to increase with increasing growth temperature. The growth rates reduce significantly with uniform surface morphologies as the chamber pressure increases. By tuning the silane flow rate, the controllable carrier concentration of (100) β-Ga2O3 thin films between low-1017 cm−3 and low-1018 cm−3 was achieved. Under optimized growth condition, an (100) β-Ga2O3 thin film with RMS roughness value of 1.64 nm and a RT mobility of 24 cm2/Vs at a carrier concentration of 7.0 × 1017 cm−3 are demonstrated. The mobilities are primarily limited by the twin lamellae and stacking faults defects generated from the growth interface. Atomic resolution scanning transmission electron microscopy reveals the formation of twin boundary defects in the films, resulting in the degradation of crystalline quality. Results from this work provide fundamental understanding of the MOCVD epitaxy of (100) β-Ga2O3 on on-axis Ga2O3 substrates and the dependence of the material properties on growth conditions. The limitation of electron transport properties of the (100) β-Ga2O3 thin films below 25 cm2/Vs is attributed to the formation of incoherent boundaries (twin lamellae) and stacking faults grown along the on-axis (100) crystal orientation.

The Effect of Initial Grain Size on the Nanocrystallization of AZ31 Mg Alloy during Rotary Swaging.

Nanograins were obtained in the AZ31 Mg alloy bars with different initial grain sizes via cold rotary swaging. Microstructure evolution during deformation was investigated through electron backscatter diffraction analysis and transmission electron microscopy studies. The results indicate that initial grain size had little effect on the mechanism of grain refinement during swaging. The nanocrystallization process of the alloys with different initial grain sizes included extensive twinning followed by the further refinement of the twin lamellae through the formation of massive dislocation arrays. However, as the initial grain size decreased, the formation rate of nanograins increased, resulting in a higher degree of nanocrystallization after the same swaging pass. The mean grain size and yield strength of the sample with the smallest initial grain size were about 91 nm and 489 MPa, respectively. The slower rate and lower degree of nanocrystallization in the alloy with a larger initial grain size were mainly attributed to the less grain boundary areas and higher activity of twinning.

Tunable band gap of diamond twin boundaries by strain engineering

Diamond thin films are promising wide band gap semiconductor materials for applications in electronic and microwave devices. Revealing the mechanism of how twin boundaries impact the band gap of diamond is of critical importance since they are the most common defects in polycrystalline diamond films. Here, nanocrystalline diamond films are synthesized by microwave plasma-enhanced chemical vapor deposition. The atomic and electronic structures of diamond lamellar and fivefold twins, and their evolution behaviors with the increase of axial tensile strain, are investigated by combining aberration-corrected transmission electron microscopy with first-principles calculations. There is no intrinsic stress concentration at the lamellar twin boundary, and its band gap equals to that of the bulk diamond ( i.e. , 5.3 eV). An intrinsic in-plane compressive stress field is formed and the band gap is increased evidently ( i.e. , 0.6 eV) in the center of fivefold twins. When applying an axial tensile strain up to 15%, the band gaps of the bulk diamond, lamellar and fivefold twins reduce significantly to 2.2 eV, 2.1 eV and 2.4 eV, respectively, which are mainly due to the decrease of p z orbital energy caused by the increase of axial bond length during the tensile process. Under the same axial tensile strain, the band gap of the fivefold twin is always larger than those of the lamellar twin and the bulk diamond due to the formation of in-plane five-membered carbon rings in the center. The findings of the tunable band gap by strain engineering will benefit for the innovation and design of advanced diamond functional devices. • Atomic and electronic structures of diamond lamellar and fivefold twins under tensile strains have been investigated. • The band gap of fivefold twins is evidently larger than that of the bulk diamond. • Under axial tensile strains, the band gap of bulk diamond, lamellar and fivefold twins reduces significantly. • The reduction of band gap is due to the decrease of p z orbital energy caused by the increase of axial bond length.

Analysis of strain hardening behavior of a high-Mn TWIP steel using electron microscopy and cyclic stress relaxation

Electron microscopy and cyclic stress relaxation experiments were employed to study the dislocation-obstacle interactions and activated hardening mechanisms, during tensile straining of a high-Mn TWIP steel specimen, and to estimate the contributions of hardening mechanisms to the strain hardening behavior. Electron contrast channeling and high-resolution electron backscattered diffraction images showed that deformation twins and their twin boundaries affect the dislocation evolutions inside both the matrix and twin lamellae. High-resolution transmission electron microscopy suggested that twin boundaries (TBs) can be passed by dislocations through the dislocation multiplication mechanism resulting in the nucleation of sessile dislocations and alternated stacking faults at TBs. Consequently, high dislocation density was developed behind the TBs resulting in enhancement of the strain hardening rate through the composite hardening effect. Various thermally activated parameters determined through cyclic stress relaxation tests indicated that dislocation evolution, developed by TBs in terms of the composite effect, was the prevalent rate-controlling mechanism, imparting higher strengthening than TBs in terms of the dynamic Hall–Petch effect. Finally, the high nucleation tendency of Shockley partial dislocations at TBs was interpreted from the variation of the stress exponent and independent plastic strain rate, and also from the general stacking fault energy point of view.

Enhanced Toughness of High‐Temperature Carburizing Gear Steel via Refining Twin Martensite and Retained Austenite by Nb Microalloying

High‐temperature carburizing technology is replacing traditional carburizing technology due to energy savings and emission reductions. However, further effort is required to understand the mechanical properties for practical high‐temperature carburizing. Herein, the microstructures and mechanical properties of the 20MnCr5 gear steel with different Nb contents after carburizing at the temperature from 900 to 950 °C and the corresponding holding time from 8 to 5 h are investigated. For a given steel, the toughness slightly decreases with the carburizing temperature because of the increase of prior austenite grain size. For high‐temperature carburizing, the impact energy increases from 45.9 to 84 J after Nb microalloying. The toughening mechanism is analyzed through theoretical discussion and various characterizations of fracture morphology and microstructure. It is indicated that the refinement of prior austenite grain caused by the solute drag effect of Nb leads to the refinement of acicular martensite and retained austenite. The combined effect of microstructure refinement results in the significant enhancement of impact toughness and accompanies by coordinated plastic deformation. The fracture spreads along the martensite blocks due to the decrease in the thickness of twin lamellas in acicular martensite, which also contributes to the enhancement of toughness.

Effect of aging treatment and equal channel angular pressing on microstructure evolution and properties of a CuCrZrSc alloy

Precipitation and severe plastic deformation have been proven to be effective methods for separately enhancing the properties of copper alloys. This work studied microstructure evolution and properties of a CuCrZrSc alloy undergoing aging treatment and equal channel angular pressed (ECAP). After 12 passes of combined thermomechanical treatment, the high-angle grain boundary spacing of the alloy was reduced to 300 nm, and many low-angle grain boundaries appeared. When the ECAP deformation reached 8 passes, stacking faults occurred in the grains due to severe shear deformation. After 12 passes, lamellar twins were detected in the sample. The pre-aging treatment before ECAP can refine the microstructure of CuCrZrSc alloy and significantly improve the mechanical properties and electrical conductivities of the alloy. After 12 passes of deformation and aged at 400 °C for 15 min, the studied CuCrZrSc alloy had a UTS of 664 MPa, elongation of 17.5%, and electrical conductivity of 80.1% IACS. Electron back-scattered diffraction and transmission Kikuchi diffraction techniques were employed to analyze the microstructure evolution of samples with different ECAP passes.

Observing Dislocations Transported by Twin Boundaries in Al Thin Film: Unusual Pathways for Dislocation-Twin Boundary Interactions.

Twins are generally regarded as obstacles to dislocations in face-centered cubic metals and can modify individual dislocations by locking them in twin boundaries or obliging them to dissociate. Through in situ tensile experiments on Al thin film in a transmission electron microscope, we report a dynamic process of dislocations being transported by twin lamella via periodic twinning and detwinning at the atomic scale. Following this process, a 60° dislocation first transforms into a sessile step of the twin boundary, then migrates under stress as a step and finally reverts back into a 60° dislocation. Our results reveal a novel evolution route of dislocations by a dislocation-twin interaction where the twins act as transport vehicles rather than as obstacles. The potential implications of this mechanism on toughening are also discussed.

Manipulating the microstructure of Cu from direct current electrodeposition without additives to overcome the strength-ductility trade-off

The attainment of both high strength and ductility has been a long-standing quest for most metallic materials in engineering. However, these properties were generally mutually exclusive. In this study, columnar nanotwinned Cu (col-nt-Cu) with a hexagonal pyramid surface morphology was fabricated using direct current (DC) electrodeposition in an additive-free electrolyte. The formation mechanism of col-nt-Cu was studied from the perspective of surface morphology, and the results indicated that both nanotwins and hexagonal pyramids were the result of screw dislocation-driven growth. This unique microstructure is at the basis of excellent mechanical properties, including a high ultimate tensile strength up to 443 MPa and superior elongation to failure (∼21%), thereby overcoming the strength-ductility trade-off. This unusual strengthening is caused by the synergy of multiple deformation mechanisms, including dislocation transfer across twin boundaries (TBs) and threading dislocation gliding in the confined twin lamella. In particular, the interaction between dislocations and TBs exhibited variability in the spatial distribution of col-nt-Cu. The TBs in the top grains dominate the strength of the sample, whereas the grain boundaries (GBs) in the core grains dominate the ductility of the sample. These results not only provide new insight to overcome the strength-ductility trade-off, but also steer the way for optimization of mechanical properties through the design of nanotwinned structures in spatial distribution.

Investigation of microscale and nanoscale twin lamellae in monocrystalline silicon grown by Czochralski method

In this paper, twin lamellae with microscale and nanoscale in 300 mm (001) Czochralski monocrystalline silicon were characterized by nanotopography (NT), X-ray topography (XRT), scanning infrared depolarization (SIRD), and localized light scattering (LLS). The structure of twin lamellae was investigated via scanning electron microscopy (SEM) and transmission electron microscopy (TEM). According to the characterization results, the geometry of micro-scale and nano-scale twin lamellae are reconstructed and the starting points of twinning were located. It is found that the two types of twin lamellae are both on {111} plane and identified as Σ3 twin. Furthermore, the formation mechanism and origins of twin lamellae in this work were discussed.

Achieving maximum strength-ductility combination in fine-grained Cu-Zn alloy via detwinning and twinning deformation mechanisms

• The FG Cu-30%Zn was prepared by ECAP and subsequent annealing. • The optimal strength and ductility combination was achieved. • Microstructural evolutions during tensile deformation were revealed by EBSD. • Deformation-induced detwinning of pre-existing ultrafine twin lamellae occurred. • Grain orientation with< 111 > // TD facilitates deformation twinning. Although deformation twinning has been demonstrated to improve the strain hardening and ductility of metals and alloys with low stacking fault energies (SFEs), the optimum grain size for maximum strength-ductility combination still needs to explore. In this work, we selected Cu- 30 wt% Zn alloy with extremely low SFE (7 mJm -2 ) acting as a model material. Specifically, Cu- 30 wt% Zn samples with different grain sizes ranging from 628 nm to 30.6 µm were prepared by equal-channel-angular pressing (ECAP) and subsequent annealing. Tensile test revealed that the maximum strength-ductility combination (ultimate tensile strength of 565 MPa and ductility of 20%) corresponds to a mean grain size of 3.8 µm. Electron backscatter diffraction (EBSD) indicated that pre-existing annealing twins in fine-grained Cu- 30 wt% Zn alloy annihilated at the initial deformation stage (<10% tensile strain) via detwinning of thin twin lamellae (<1 µm) and conversion of twin boundaries of thick twin lamellae (>1 µm) into conventional high-angle grain boundaries. In the later stage of deformation (>10% strain), deformation twinning occurred in grains with<111>orientation parallel to tensile direction, suggesting the combination of both detwinning and twinning deformations caused the maximum strength and ductility synergy. Our findings provide insights into optimization of strength and ductility of metals with low SFEs and detwinning-twinning deformation mechanisms.

In-Situ Evolution of Calcite Twinning during Uniaxial Compression of Carrara Marble at Room Temperature

Calcite twinning is a dominant deformation mechanism at low temperatures. It is often used to reconstruct paleostresses: orientations of the principal stress axes, stress ratios and differential stress. Despite numerous studies, on single crystals and aggregates, questions remain about the initiation and evolution of the twinning. In particular, the existence of a critical value for the activation of twin planes is debated. In this study, Carrara marble samples were uniaxially deformed at low temperature. The experiments were monitored in situ in an SEM (Scanning Electron Microscope) and a deformation analysis was performed at regular intervals using image correlation. Image correlation analysis shows the link between the overconcentration of strains and the appearance of the first twinned planes. This is followed by a densification and a gradual thickening of the twin lamellae. Fracturing only appears in a third stage as a precursor to the collapse of the sample. The inversion, using the CSIT-2 technique, showed that the twinned planes are globally related to the applied macroscopic stress. The inversion allows one to retrieve the macroscopic stress tensor. Schmid factors were extracted from this analysis and correlated to the loading curves. For crystals of about 200 µm diameter, the threshold value is in between 6.75 and 8.25 MPa.

Nanotwin-induced ductile mechanism in thermoelectric semiconductor PbTe

•Pb-CTBs weaken ionic Pb-Te bonds to generate an easy dislocation source at CTBs •Twinning partial dislocations glide on (111) twin planes, leading to CTB migration •“Catching bond” induces high ductility and maintains integrity for nanotwinned PbTe Coherent twin boundaries (CTBs) with the lowest interfacial energy provide a strong phonon-CTB scattering source to suppress the lattice thermal conductivity needed for thermoelectric properties, but the impact on mechanical properties of PbTe remains unexplored. We construct nanotwinned structures with Pb- or Te-terminated CTB (Pb- or Te-CTB) along (111) plane and employ molecular dynamics simulations to examine structural evolution. We find that Pb-CTBs weaken ionic Pb-Te bonds to generate an easy dislocation source at CTBs. Due to nucleation and motion of partial dislocations on each Pb-CTB plane driven by shear load, Pb-CTBs gradually migrate to Te-CTBs, which is accompanied by breaking and re-forming of Pb-Te bonds. This “catching bond” maintains structural integrity while dramatically enhancing deformability of nanotwinned PbTe. Dislocations move from Te-CTBs toward twin lamellae, resulting in the structural slippage and fracture. These findings provide a theoretical strategy to improve the ductility of PbTe-based semiconductors through TB engineering. Coherent twin boundaries (CTBs) with the lowest interfacial energy provide a strong phonon-CTB scattering source to suppress the lattice thermal conductivity needed for thermoelectric properties, but the impact on mechanical properties of PbTe remains unexplored. We construct nanotwinned structures with Pb- or Te-terminated CTB (Pb- or Te-CTB) along (111) plane and employ molecular dynamics simulations to examine structural evolution. We find that Pb-CTBs weaken ionic Pb-Te bonds to generate an easy dislocation source at CTBs. Due to nucleation and motion of partial dislocations on each Pb-CTB plane driven by shear load, Pb-CTBs gradually migrate to Te-CTBs, which is accompanied by breaking and re-forming of Pb-Te bonds. This “catching bond” maintains structural integrity while dramatically enhancing deformability of nanotwinned PbTe. Dislocations move from Te-CTBs toward twin lamellae, resulting in the structural slippage and fracture. These findings provide a theoretical strategy to improve the ductility of PbTe-based semiconductors through TB engineering.

Effect of yttrium on nanocrystallization of magnesium alloys during cold rotary swaging

In this study, binary Mg-2Y alloy, pure Mg, and AZ31 alloy were prepared via cold rotary swaging. Microstructure evolution during the grain refinement process was explored using transmission electron microscopy. The results showed that it was impossible to achieve nanocrystallization in pure Mg via cold rotary swaging, which was attributed to the difficulty of forming dislocation arrays. In contrast, after swaging, nanograins were obtained in an alloy formed by adding 2 wt% Y to Mg. In the early stage of swaging, the initial coarse grains of the Mg-2Y alloy were subdivided by the formation of twin lamellae, dislocation arrays (in the matrix) and deformation bands. With increasing strain, more dislocation arrays were formed within the twin lamellae and deformation bands, which resulted in further refinement of the twin lamellae and deformation bands and promoted the formation of nanoscale subgrains. As the swaging process continued, these nanoscale subgrains transformed into nanograins with high-angle grain boundaries. Nanograins were also developed in the swaged AZ31 alloy. The formation rate of nanograins during swaging was slower, and the grain refinement effect after swaging was weaker, in the Mg-2Y alloy than the AZ31 alloy. • It cannot form nanograins in pure Mg due to the absence of dislocation arrays. • Deformation bands formed in Mg-2Y alloy but not in pure Mg and AZ31 alloy. • Accelerated nanocrystallization in AZ31 alloy due to enhanced twinning activity. • A weaker grain refinement effect obtained in Mg-2Y alloy than in AZ31 alloy.