Twin Grain Research Articles

Temperature-induced twinning mechanism transformation and strain hardening behavior of pure rhenium in compression

This work investigates the activation twinning mechanism at different temperatures and its role on the strain hardening in rhenium. The microstructure of compression at 200 °C to 1000 °C was analyzed using electron back scattered diffraction (EBSD). Only {112¯1} twins were activated during compression at 200 °C. {101¯2} twins were observed during compression at 400 °C, and its number increased with increasing temperature. The strain hardening rate (SHR) could be classified into multiple plastic deformation stages governed by twin-induced geometrical hardening and softening, and grain boundary strengthening. The preferentially activated {112¯1} twin variants usually conform to Schmidt law and contribute to grain refinement and grain boundary strengthening, inducing localized geometrical hardening. In contrast, the subsequently activated {101¯2} twin variants released the local high stresses and decreased SHR, in contrast to the regularity of other HCP metals. Whether the selective behavior of the {101¯2} twin variants conform to Schmidt law were dependent on the local stress. A large number of {101¯2} twins with negative Schmidt factors were activated to accommodate stress compatibility at grain boundaries and {112¯1} twin boundaries. As the deformation temperature increases, it was more likely to activate {102¯2} twins with negative Schmidt factor at grain boundaries and {112¯1} twin grain boundaries to accommodate local stress compatibility.

Influence of cooling rate on microstructure and defect evolution in GaAs during solidification

The fabrication of high-quality GaAs crystals is essential to approach optimal performance in optoelectronic and microelectronic devices. In this study, a molecular dynamics simulation study was conducted for the solidification of liquid GaAs at three cooling rates (1010 K s−1, 1011 K s−1, and 1012 K s−1) at 300 K. The structural evolution in terms of crystal structure and defect formation in GaAs was thoroughly investigated using pair distribution function, average atomic energy, the largest standard cluster analysis, and visualization techniques. The results showed that the cooling rate of 1010 K s−1 led to the development of the best crystal quality with ease of eutectic twin grain boundary coherent twin boundary formation. Increasing the cooling rates to 1011 K s−1 and 1012 K s−1 resulted in the amorphous structure. Both high and low cooling rates profoundly affected the formation of As8 structure, but a maximum amount of 2.2% of As8 crystal structure was formed at a cooling rate of 1011 K s−1. The reduction in cooling rate to 1010 K s−1 induced the formation of numerous Schottky and Frenkel types of partial dislocations in the GaAs system. Results of this study can serve as potential guidelines to the theory of crystal growth and may be implemented in the fabrication of high-quality GaAs crystals for optimal device performance.

The mechanical properties of TCP phase of rapidly cooled molybdenum

The rapidly-cooling of pure molybdenum (Mo) at 1010 Ks−1 and the uniaxial tensile of the solid at a strain rate of 2 × 10 s−1 were studied by molecular dynamics simulation; then the structure evolution was investigated in terms of pair distribution function and the largest standard cluster analysis. It is found that Mo melt was cooled into a complex crystal with multiple characteristic lengths. Further analysis revealed that it is a mixture topological close packing (TCP) crystal composed of the dominated A15 phase (Mo-A15) and the less Z phase, with H phase as twin grain boundary. The Phonon spectrum, potential energy, and cohesion energy revealed that Mo-A15 is stable slightly inferior to Mo-bcc. Compared to the prevailing bcc Mo, Mo-A15 holds much higher Young’s modulus, ultimate tensile strength, and Yield strength, being a representative material for barrier layer with high hardness.

Growth of Germanium Thin Films on Sapphire Using Molecular Beam Epitaxy

Germanium films were grown on c-plane sapphire with a 10 nm AlAs buffer layer using molecular beam epitaxy. The effects of Ge film thickness on the surface morphology and crystal structure were investigated using ex situ characterization techniques. The nucleation of Ge proceeds by forming (111) oriented three-dimensional islands with two rotational twin domains about the growth axis. The boundaries between the twin grains are the origin of the 0.2% strain and tilt grains. The transition to a single-grain orientation reduces the strain and results in a better-quality Ge buffer. Understanding the role of thickness on material quality during the Ge(111)/Al2O3(0001) epitaxy is vital for achieving device quality when using group IV material on the sapphire platform.

Apparent symmetry rising induced by crystallization inhibition in ternary co-crystallization-driven self-assembly

The concept of apparent symmetry rising, opposite to symmetry breaking, was proposed to illustrate the unusual phenomenon that the symmetry of the apparent morphology of the multiply twinned particle is higher than that of its crystal structure. We developed a unique strategy of co-crystallization-driven self-assembly of amphiphilic block copolymers PEO-b-PS and the inorganic cluster silicotungstic acid to achieve apparent symmetry rising of nanoparticles under mild conditions. The triangular nanoplates triply twinned by orthogonal crystals (low symmetry) have an additional triple symmetry (high symmetry). The appropriate crystallization inhibition of short solvophilic segments of the block copolymers favors the oriented attachment of homogeneous domains of hybrid nanoribbons, and consequently forms kinetic-controlled triangular nanoplates with twin grain boundaries.

A re-evaluation of stannopalladinite using modern analytical techniques

AbstractAn investigation of sample 41647 from the Platinum Placer of Ugol'nyi Ruchei, Norilsk Cu–Ni deposit, Russia, stored in the systematic collection of the Fersman Mineralogical Museum, revealed that it contained a mineral that can be identified as the original stannopalladinite described in 1947. As the literature information on the latter is controversial, the mineral was re-investigated using modern analytical techniques. Stannopalladinite occurs as spherical and ovoid-shaped grains up to 0.08 mm, closely intergrown with tetraferroplatinum as part of flattened, prismatic crystals up to 6 mm long. The micro-indentation hardness of stannopalladinite (VHN, 30 g load) is 244 kg/mm2 (range 233–266, n = 4), corresponding to a Mohs hardness of 4. The calculated density is 9.781 g/cm3. In reflected light, stannopalladinite is pale pink. The bireflectance is noticeable only in oil immersion. In crossed nicols the mineral exhibits strong anisotropy with colour effects changing from reddish purple to greyish blue and polysynthetic twinning of grains. The reflectance curves show distinct anomalous dispersion. The chemical composition (wt.%, electron microprobe data, mean of 11 analyses) is: Cu 8.48, Pd 61.21, Pt 0.89, Sn 25.87, Pb 3.70, total 100.15. The empirical formula calculated on the basis of 4 atoms per formula unit is (Pd2.42Cu0.56Pt0.02)Σ3.00(Sn0.92Pb0.08)Σ1.00. The ideal chemical formula of stannopalladinite is therefore proposed as (Pd,Cu)3Sn instead of Pd3Sn2 used in the official IMA List of Minerals. The strongest powder X-ray diffraction lines are [dobs, Å (I, %) (hkl) ]: 2.292 (42) (231), 2.166 (100) (331), 2.034 (10) (710), 1.916 (15) (141) and 1.851 (15) (630). The powder X-ray data are indexed in the orthorhombic unit cell with a = 14.634(2), b = 8.5253(6), c = 4.5946(3) Å and V = 573.24(7) Å3 (Z = 8). Stannopalladinite can be reliably identified among all other minerals belonging to the binary Pd–Sn and ternary Pd–Cu–Sn systems by a combination of chemical, optical and X-ray data.

Quasi-in-situ observation and analysis of grain boundary evolution of GH4169 nickel-based superalloy during the micro-strain stage of thermal deformation

A systematic study was conducted to analyze the grain boundary microstructure evolution of GH4169 nickel-based superalloy during the micro-strain stage of thermal deformation. A quasi-in-situ EBSD (electron backscatter diffraction) approach was used for observation. Moreover, at a temperature of 1100 °C and a strain rate of 0.1 s−1, a continuous strain uniaxial compression experiment was carried out. We observed the development of the grain boundary structure and the grain boundary deformation. The experimental findings revealed that GH4169 underwent locally inhomogeneous thermal deformation during the micro-strain stage, and the simultaneous occurrences of grain boundary slip, grain rotation, and grain boundary migration, exhibiting microstructural diversity. The grain boundary structure changed continuously as a result of the random movement of high-angle grain boundaries. Twin grains displayed a stable structure during the migration of grain boundaries, and their content indicated a considerable decrease as the strain increased. This provides a basis for directing the design of grain boundary engineering since it suggests that twin boundaries are easier to migrate during thermal deformation. In addition, no evidence of dynamic recrystallization was found throughout the entire micro-strain stage and a <101>/<111> micro-texture was observed, which is compatible with common dislocation slips in face-centered cubic crystals. Then, on the basis of the transmission electron microscope images of the microstructure, the numerous forms of grain boundary deformation were rationally explained from the standpoint of the relation mechanisms among grain boundaries and dislocations. Finally, the synergistic mechanism of grain boundary slip and migration is established as the primary mechanism of thermal deformation in the micro-strain stage.

Creep deformation and damage characteristics of nickel-based superalloy GH4169

To further understand the creep behavior of nickel-based superalloys, creep deformation and damage characteristics of nickel-based superalloy GH4169 were investigated systematically by using SEM, EBSD and TEM characterizations in this study. Creep tests were conducted at 650 °C under 820 MPa.The results indicated that the fractions of low-, high-angle, and twin grain boundaries changed significantly after creep. Due to the grain rotation mechanism, the overall microstructure changed from random to preferred crystallographic orientation. <110>//loading direction (LD) oriented grains were rotated to <100>//LD and <111>//LD fibers, resulting in the decrease of <110> fiber, and the increase of <100> and <111> fibers. Cases of <110> oriented grains rotating into <100> and <111> orientations were also provided. Deformation twin and stacking faults were determined as the main deformation mechanisms. The failure of the specimen was induced by intergranular brittle fracture. The grain boundaries with special characteristics, namely high Schmid factor m and low Schmid factor mismatch Δm of neighbouring grains, and low geometric compatibility factor m’, exhibited high susceptibility to cracking.

Insights into factors that affect non-Arrhenius migration of a simulated incoherent Σ3 grain boundary

Non-Arrhenius grain boundary migration, sometimes referred to as antithermal migration where temperature and GB velocity values are inversely related to each other, is examined in an incoherent twin Σ3 [111] 60° (11 8 5) nickel grain boundary. Molecular dynamics is used to simulate migration and examine the effect of various factors on the migration, including interatomic potential, system size, driving force, and variation of atomic grain boundary structure. A classical model for grain boundary migration, in its unsimplified form, is used to analyze the results. The grain boundaries exhibit migration mechanisms with very low apparent barrier heights to migration. As a result, the boundaries migrate quickly but exhibit a velocity saturation similar to that of dislocations. The various interatomic potentials exhibit different migration velocities, but their similarities suggest they all predict similar overall behaviors of migration. The variation of atomic structure in the same incoherent twin grain boundary leads to diverse behaviors with barrier heights that vary from non-Arrhenius to Arrhenius migration. Facet nucleation is confirmed not to be a requirement for this boundary based on an examination of simulation cell size; however, the presence and/or interaction between numerous facets does suggest a slowing and increased barrier height to migration for larger boundaries.

Synergistic effect of multi-element co-segregation on mechanical properties of Mg twin grain boundary

Properties of grain boundaries (GBs) are pronouncedly influenced by alien elemental segregation. Synergistic effect of co-segregation behavior on mechanical properties of Mg twin GB was systematically investigated with first-principles calculations. Ten elements (Ni, Mn, Zn, Al, Ag, Ti, Li, Zr, Y, Ca) for single segregation and four solute combinations (Al–Ca, Al–Y, Zn–Ca, Zn–Y) for co-segregation were chosen respectively, the segregation energies and solubility energies with different elemental concentrations were calculated to evaluate the thermodynamic stability of corresponding segregated GBs. These solute atoms have unique effects on the stability of the system, and the interaction between solute atoms also has a synergistic effect on the stability of the system. It is found that the segregation tendency is enhanced and the GB structure tends to be stable with the increase of the concentration of solute atoms at the appropriate sites, regardless of single-element segregation or co-segregation. The redistribution of GB charges due to co-segregation significantly affects the binding and mechanical properties of GBs. With the increase of the co-segregation concentration, the lamellar charge is transferred between the matrix and the twins, which further enhances the electron hybridization and leads to a notable enhancement of interface binding. This shows that the appropriate combination of solute elements can effectively improve the mechanical properties of the interface. The manuscript reveals the theoretical understanding of the effect of elemental segregation on the mechanical properties of GB, and provides design ideas for the tuning of mechanical properties of magnesium alloy by GB engineering.

Mixing enthalpy near grain boundaries in Fe-Cr alloy: The results of atomistic simulation

Energy characteristics of symmetric tilt (twin) grain boundaries (GBs) Σ5(210) and Σ5(310) with the tilt axis [001] and boundaries Σ3(111) and Σ3(112) with the tilt axis [11¯0] in a bcc random binary FeCr alloy have been studied using atomistic simulation. The simulation has been carried out based on DFT calculations and two-band semi-empirical potential. We have proposed the formula for calculating the mixing enthalpy near GBs. The relationship between the mixing enthalpy and the energy of the boundary in the alloy has been revealed. Both simulation methods have shown a decrease in the mixing enthalpy near a GB compared to the ideal crystal. Moreover, DFT has predicted a stronger decrease in the mixing enthalpy compared to estimates based on semi-empirical interaction. Based on the obtained dependences of the mixing enthalpy on the alloy composition, phase diagrams for the considered GB regions have been constructed. It has been shown that the GB region is characterized by a higher equilibrium concentration of chromium, and the precipitates of the α'-phase have a lower concentration of chromium compared to the bulk. The smallest difference in equilibrium compositions has been observed for the Σ5(310) and Σ3(112) boundaries, and the largest one for the Σ3(111) boundary, which has the highest energy among considered GBs.

The Microstructure Evolution and Formation Mechanism of Gradient Nanostructure Prepared on CrCoNi Medium-Entropy Alloy.

An equiatomic CrCoNi medium-entropy alloy was subjected to high-energy shot peening (HESP) to fabricate a gradient nanostructure (GNS) in this work. The microstructures of the GNS samples at different depths within the deformed layer were thoroughly investigated. The microstructure exhibited a transformation from unstressed coarse grains to deformed coarse grains, followed by the formation of ultrafine grains, and ultimately reaching a final nanocrystalline structure with a uniform size of approximately 50 nm. Detailed structural analysis indicated that the deformation process was primarily influenced by the interaction between dislocations and deformation twins, which was attributed to the low stacking fault energy (SFE) of the alloy. The nanocrystalline mechanism was divided into three stages. In the coarse-grained deformation stage, the dislocation band divided twin/matrix lamellae into sub-segments, and the cross twin divided coarse grains into ultrafine grains simultaneously. In the ultrafine grain deformation stage, dislocations were arranged around the deformation twins in order to break the twins to form incoherent boundaries, destroying the coherent relationship between the twin and matrix. Finally, in the nanocrystalline deformation stage, the nanocrystalline structure was further divided into smaller segments to accommodate additional strains through the interaction between dislocations and twins.

A New Structure with Localized sp2 Bonding for Fivefold Twinning in Diamond.

Changes in atomic bonding configuration in carbon from sp3 to sp2 are known to exist in certain structural defects in diamond, such as twin boundaries, grain boundaries, and dislocations, which have a significant impact on many properties of diamond. In this work, the atomic structure of fivefold twinning in detonation synthesized ultra-dispersed diamonds is investigated using a combination of techniques, including spherical aberration-corrected high-resolution electron microscopy (HREM), HREM image simulations, and molecular mechanics (MM) calculations. The experimental HREM images reveal clearly that the fivefold twinning in diamond has two distinct structures. In addition to the concentric fivefold twins, where the core structure is the intersection of five {111} twinning boundaries, a new extended core structure with co-hybridization of bonding is identified and analyzed in fivefold twinning. The atomic structure forming these fivefold twinning boundaries and their respective core structures is proposed to involve both the tetrahedral sp3 and planar graphitic sp2 bonding configurations, in which a co-hybridized planar hexagon of carbon serves as a fundamental structural unit. The presence of this sp2 -bonded planar unit of hexagonal carbon rings in general grain boundaries is also discussed.

Twin density and twin thickness evolution in sputtered Al–Mg alloys

Twinned Al–Mg alloys have been reported. However, the role of Mg solute in facilitating the formation of growth twins remains unclear. By using a precession-assisted crystal orientation mapping technique (PACOM) coupled with transmission electron microscopy (known as ASTAR), we examined the evolution of twin boundaries in Al, Al–1Mg, and Al–2.2Mg (at. %) films. The twinned grain fraction elevates with increasing film thickness until it reaches a peak when the film thickness is 120–160 nm. The Al–Mg alloys exhibited greater twinned grain fractions than pure Al. To investigate the fluctuation of twinned grain fraction, two types of twin boundaries were classified including intergranular and intragranular twins. The initial increase in twin density is attributed to the impingement of twinned grains during island coalescence and the twinned grains are more likely to survive during the grain growth process. Whereas the decrease in twinned grain fraction in thicker films is related to the removal of intragranular twins, and a lack of formation mechanisms of new twins.

Influence of additives induced microstructural parameters on mechanical behavior of (111)-oriented nanotwinned microcrystalline copper

Depth sensing nanoindentation experiments were performed to investigate the strain rate dependent-independent behaviors of nanotwinned microcrystalline Cu. The specimens were electro-deposited using three different additives, which resulted in controlled variations in microstructural parameters, viz., grain size, and twin spacing. One of the three samples with the lowest twin lamella spacing showed a lower stress exponent, which might result in enhanced ductility. The stress exponent and activation volume increased with increasing twin spacing. In contrast, the hardness revealed a dependence on the effective length scale considering the weighted average of the twin spacing and grain size. The mechanical properties obtained by introducing nanotwins in coarser initial grain size are comparable to nanotwinned nanocrystalline materials. Thus, the possibility of using a coarser initial grain size for nanotwinned materials paves the way for a new paradigm with a broader range of nanotwinned materials.

Boosting the power factor and thermoelectric performance in eco-friendly Cu3SbS4 by twin boundary and grain boundary phase

Te/Se-free Cu3SbS4 is one of the most promising thermoelectric materials because of its earth-abundant, low-cost and environment-friendly characteristics. However, its thermoelectric performance is relatively low due to the high electrical resistivity. In this work, dense twin boundaries and grain boundary phases were constructed in the Ge-doped Cu3SbS4 materials by the melting reaction, sulfuration process, and plasma activated sintering. Density Functional Theory (DFT) calculations demonstrate that the Ge doping makes the Fermi level (EF) move from the forbidden-band into the valence band, thus significantly increasing carrier concentration. These twin boundaries don’t degrade the carrier mobility, leading to the enhanced electrical conductivity, thus obtaining a higher power factor of ∼ 14.4 μW cm−1 K−2 at 623 K. As a result, a peak ZT value of ∼ 0.78 at 623 K is achieved in the Cu3Sb0.975Ge0.025S4 sample, which is much higher than those of most metal sulfides. Moreover, high-density twin boundaries and grain boundary phases can inhibit the propagation of fatigue crack and enhance the fracture toughness. This modified approach can efficiently construct dense nanotwin boundaries in metal sulfides for high-performance thermoelectrics.

Effect of twin assisted precipitation on the microstructure and mechanical properties of gradient structure Mg-Gd-Zn-Zr alloy

Nano-precipitation can effectively pin the dislocations and inhibit the dislocations’ motion, making precipitation strengthening one of the most crucial strengthening mechanisms in magnesium (Mg) alloys. Consequently, controlling the segregation of elements at the coherent interfaces to induce the phase precipitation, has become an innovative way of designing Mg alloys. In this study, a novel twin-precipitation composite gradient structure GZ151K (Mg-15Gd-1Zn-0.4Zr) alloy was prepared using ultrasonic surface rolling process (USRP) and heat treatment. The results indicated that the USRP-treated samples subjected to aging treatment demonstrated high thermal stability without significant twin degradation. Additionally, the twin boundaries and grain boundaries could act as the nucleated points for the (Mg, Zn)5Gd phases during the subsequent aging treatment, and further formed the twin-precipitation composite gradient structure. This unique composite gradient structure of alloys treated by USRP and aging treatment was caused by an extra promotion-precipitation strengthening mechanism. Under the effects of promotion-precipitation strengthening, strain hardening, and precipitation strengthening, the mechanical properties of the GZ151K alloy were significantly improved.

Effect of twin grain boundary on the diffusion of Cu in bulk β-Sn

For Cu is the dominant diffusing species in the Cu/Sn diffusion couple, the diffusion behaviors of Cu in the β-Sn grain and at the twin grain boundary (TGB) have been studied in this paper by combining the first-principles calculation and molecular dynamics simulation, which has a significant impact on solder joint reliability. The results show that the energy barrier for Cu bulk diffusion in β-Sn depends on the diffusion path, ranging from 0.01 eV to 0.33 eV. Among the three possible diffusion paths at the (101) TGB, the primary diffusion path is along the [010] direction of the TGB plane. However, this diffusion path along the TGB still presents a relatively high energy barrier of 0.26 eV compared to the energy barrier for bulk diffusion. Furthermore, due to the segregation of Cu atoms at the β-Sn TGB and the inhibition effect of multiple energy barriers for diffusion crossing the TGB, long-range diffusion of Cu across the TGB between the two β-Sn bulk grains is difficult. As a result, the (101) TGB actually inhibits the diffusion of Cu in β-Sn. It can be concluded that increasing the number of TGBs inside a Cu/Sn solder joint can effectively suppress the interdiffusion between Cu and Sn, which will prevent the growth of Cu-Sn intermetallic compounds and improve the reliability of the solder joint.

Formation mechanism of mirror twin grain boundaries in molecular beam epitaxy grown monolayer WSe2–MoSe2 lateral heterojunctions

Mirror twin grain boundary (MTB) defects, being a special type of high-symmetry one-dimensional (1D) defects in two-dimensional atomically thin transition metal dichalcogenides (TMDCs), have received considerable interest due to their unique structures and intriguing 1D properties. However, formation and distribution of MTBs in hybrid TMDC materials such as heterojunction remain scarcely studied. Herein, we investigate the spatial distribution, lattice registry and formation mechanism of MTBs in molecular beam epitaxy grown monolayer WSe2–MoSe2 lateral heterojunctions using atomic-resolution annular dark-field scanning transmission electron microscopy (ADF-STEM). MTBs manifest a much higher density in MoSe2 than in WSe2 domains with a few of them spanning coherently across the domain interface. Compositionally, a Mo-dominant rather than W-dominant configuration was observed in those MTBs located in WSe2 domains and its origin can be attributed to the preferable Mo substitution to W along the MTBs occurring at the later MoSe2 growth period. This proposed mechanism is supported by ab-initio density functional theory calculations and substitution dynamics captured by in-situ ADF-STEM. The present study deepens our understanding of MTBs in heterostructured TMDCs, which may also serve as an excellent platform for the exploration of intriguing 1D physics.

Effect of initial orientation on microstructure and mechanical properties of AZ31 Mg alloy sheets via accumulated extrusion bonding

In this study, effects of initial orientation on microstructure evolution and mechanical properties of AZ31 Mg alloy sheets via accumulated extrusion bonding (AEB) was systematically studied. The samples with RD and TD parallel to extrusion direction (ED) were labeled as RED and TED, respectively. RD and TD pieces alternately stacked was named as RTED. The results revealed that under three-dimensional compressive stress, {10-12} tensile twinning dominated the first stage deformation in container. As the plunger continuous press down, dynamic recrystallization (DRX) occurred, and the newly fine DRXed grains along original and twin grain boundaries gradually consumed the matrix and twin grains. The microstructure was completely transformed into recrystallized grain structure at sizing band with an average grain size of ∼0.9 ​μm in TED sample, smaller than that of RED sample. After the alloys extruded out of the die, DRXed grains significantly grew to ∼4.5 ​μm and 3.5 ​μm for RED and TED samples, respectively. A laminated microstructure was obtained with average grain sizes of∼4.4 ​μm in RD layers and 3.5 ​μm in TD layers for RTED sample. The AEB processed samples exhibited an ED-tilt double-peak basal texture with similar texture intensity. The tensile tests results indicated that attributed to the combined effect of grain refinement and texture evolution, the yield strength and fracture elongation of RED and TED samples was significantly improved. The heterogeneous microstructure in RTED sample induced an extra work hardening (HDI-work hardening) in tensile deformation and resulted in a further improved elongation of 32.0%.