Formation Of Dislocations Research Articles

Changes in the structural properties of polycrystalline oxide La<sub>2</sub>O<sub>3</sub> at heating it in air and in vacuum

Changes in the phase’s structure of polycrystalline La2O3 upon heating in air and vacuum have been established. For the first time, on La2O3 samples after annealing in the range 700‒1900 оC in air, the microstructure of a monoclinic phase of type B and a hexagonal phase of type A with the presence of texture was obtained. For the first time, after annealing La2O3 samples at 1900 °C in air, the formation of loops and screw dislocations on La2O3‒х grains was discovered, through which the evaporation of crystals of different colors and the decomposition of hexagonal phase grains into cubic phase crystals of type F with an fcc lattice and type C with a bcc lattice occurred. For the first time, it has been theoretically substantiated and experimentally confirmed that heating lanthanum oxide samples at 1750 and 1900 °C leads to the decomposition of the hexagonal phase of type A with the presence of texture into two cubic phases: type F with the content of anionic vacancies and a crystal lattice parameter a = 0.5677 nm (Fm3m) and type C containing color centers and a = 1.14838 nm (Ia3). The refractive indices of these phases were determined. The energy of formation of anion vacancies in the lattice of the hexagonal phase of type A is calculated and the critical concentration of anion vacancies leading to the decomposition of this phase is determined.

Enhancing strength-ductility synergy in nano-sized TiB2/Al composite via rapid solidification and thermo-mechanical processing

The Al-Cu-Mn (AA2219) composite reinforced by 5 wt% nano-sized TiB2 particles were fabricated by hot isostatic pressing (HIP) combined with thermal-mechanical processing to achieve a superior strength-ductility combination. Coupling the comprehensive characterizations and microstructure-based analysis, the strength-ductility mechanisms were deeply understood. Results reveal that the uniform dispersed nano-sized TiB2 particles under the current preparation process helps to improve the mechanical properties of the composite. The introduction of pre-stretch before artificial aging produced a fine θ' and S precipitates. By combining high-resolution TEM and crystal structure analysis, Al(Cu) and S phase at TiB2/Al interface were identified, and their mismatch (δ) with Al were 3.8 % and 4.8 %, respectively. Mechanisms related to pre-stretch effects on the formation of dislocation, θ' and S precipitates and corresponding structures are discussed, as well as the implications of TiB2/Al interface characteristics on strength and ductility.

Characterisation of AZ31/TiC composites fabricated via ultrasonic vibration assisted friction stir processing

In this study, the effect of ultrasonic vibration during Friction Stir Vibration Processing (FSVP) on the microstructure and mechanical behaviour of AZ31/TiC surface composites was investigated. Specifically, Titanium Carbide (TiC) particles were introduced as a reinforcement (15 vol%) into the magnesium alloy AZ31 using both Friction Stir Processing (FSP) and FSVP. Comprehensive examinations were carried out to analyse the microstructure, hardness, and tensile behaviour of the resulting composites. The study revealed significant improvements in mechanical properties due to the application of ultrasonic vibration during FSP. Firstly, the stir zone region was found to be free from voids, enhancing material flow and promoting even dispersion of TiC powders within the matrix. Secondly, refinement of grains was observed due to dynamic recrystallization and the pinning effect imposed by TiC particles, leading to the formation of more dislocations in the composite and indicating a considerable alteration in the material’s structure. Importantly, the vibration during FSP introduced an auxiliary energy source, resulting in a remarkable enhancement in both hardness and tensile strength. Compared to the AZ31/15 vol% TiC FSP composite, the composites produced via FSVP exhibited a grain size reduction of about 64% and improvements in hardness and ultimate tensile strength (UTS) of about 55% and 21%, respectively. Notably, these improvements were achieved without compromising the ductility of the composite, which remained at appreciable levels.

The tensile behaviors of Ag-Cu alloys with different Cu contents: Molecular dynamics simulations study

The Ag-Cu alloys have received attention due to their excellent electrical and mechanical properties. However, the analytical research on why the mechanical properties of the Ag-Cu alloys increase after a small amount of Cu atoms are added is still lacked. In this work, the classical molecular dynamics (MD) simulations method was used to establish the Ag-Cu alloy models with different Cu contents. The tensile deformation mechanism of Ag-Cu alloy at room temperature was revealed from the atomic scale through the analysis of the stress-strain behavior and the evolution of dislocations. During the simulations, the embedded atom method (EAM) potential was used to describe the interaction between Ag atom and Cu atom. The uniaxial tensile deformation and nanostructure evolution of Ag-3 at.% Cu, Ag-6 at.% Cu and Ag-10 at.% Cu alloy models at the temperature of 300 K and the strain rate of 0.02 Å/ps were studied in detail. Results show that the internal stress was generated due to the large lattice mismatch among Ag atom, Cu cluster and the grain boundary. It results in the formation of dislocation and the negative initial stress at the beginning stage of Ag-Cu alloy models under tension. The dislocation firstly nucleates at the grain boundary and the growth rate of dislocation is slow before the crack propagation, and the complex dislocation reactions also exist. With the increase of Cu content, the tensile strengths of Ag-Cu alloys also increase. Cracks mainly form at the intersection of multiple grains, and the fracture mode changes from the intergranular fracture to the intergranular fracture and transgranular fracture after the unstable propagation of cracks. Meanwhile, a plenty of Cu atoms are distributed in the form of clusters in Ag matrix. They hinder the slip of dislocation and slow the relative sliding between atomic layers, and thus finally improves the deformation resistance to the plastic deformation of Ag-Cu alloy. The study on the tensile deformation behavior of Ag-Cu alloy at room temperature has certain guiding significance for the practical production and application of the design of Ag-Cu alloy.

Influence of different rolling passes and deformation amounts on the microstructure evolution and strengthening mechanism of Cu-Ni-Si alloy

Under the condition of controlling the total rolling deformation to 80.0%, the effects of multi-pass rolling, the ratio of deformation between the first and second stages in two-stage rolling, and aging parameters on the microstructural evolution and property enhancement of Cu-Ni-Si alloy were investigated. The results showed that the second-stage rolling promotes the formation of high-density dislocations, significantly enhancing dislocation strengthening. Simultaneously, the high-density dislocations facilitate the precipitation of the second phase during aging, and concurrently improving the strength and electrical conductivity of the alloy. In the two-stage rolling and aging process, a ratio of first-stage to second-stage rolling deformation less than 1 yields superior comprehensive properties. Specifically, when the first-stage rolling deformation is 35.0% and the second-stage deformation is 68.8%, the alloy achieves an optimal electrical conductivity of 49.53% IACS, hardness of 272.9 HV0.1, tensile strength of 773.0 MPa, and yield strength of 723.3 MPa. This is because a rolling ratio less than 1 results in higher dislocation density; the δ-Ni2Si precipitates have a coherent relationship with the Cu matrix, the interparticle spacing of precipitates is smaller. The average grain size is refined from 1.29 μm to 1.08 μm, significantly enhancing grain boundary strengthening. Dislocation strengthening and second-phase strengthening are the primary strengthening mechanisms of the alloy, and the calculated alloy strength agrees well with the experimental values, with errors less than 3.4%. Additionally, the Avrami equation in phase transformation kinetics was utilized to study the variation law of the volume fraction of the δ-Ni2Si phase with aging time.

Microstructure evolution and mechanical behavior of 2024 aluminum alloy under electric pulse treatment

This study examined the effects of electric pulse treatment (EPT) on solution-treated 2024 alloy. We found that the combined effect of non-uniform stress field caused by thermal expansion and localized Joule heating promoted the formation of helical dislocations. During the subsequent aging process, these severely deformed helical dislocations formed a network configuration, which accelerated the coarsening of the S phase, and the S phase also retained a helical shape. Compared to conventional solution-treated samples, the yield strength of the EPT samples increased by 24 % with almost no loss of fracture elongation. The increase in strength is primarily due to the increase of volume fraction of rod-shaped S phase，enhancing the matrix's resistance to deformation. This study proposes a simple and rapid method to adjust dislocation configurations and aging precipitation through electric pulses, offering new insights for the industrial application of aluminum alloys to swiftly enhance mechanical properties.

On the stability of the dislocation structure of speckle fields

The features of propagation in free space of light beams with a dislocation structure of the wavefront are considered. The stability to the influence of diffraction of small-scale dislocation systems providing spatial super-resolution is analyzed. Based on a comparison of the characteristics of various dislocation formations, a conclusion is made about the greatest structural stability of systems with special type screw dislocations. Such dislocations can be obtained by azimuthal rotation of edge dislocations characterized by a sharp phase jump of light oscillations along the singular line. The obtained results will find application in optimizing the characteristics of optical devices with ultra-high spatial resolution, as well as in developing software for the operation of spatial light modulators forming beams with a specified amplitude-phase profile.

Gradient structured Al alloy with a synergistic improvement of strength-ductility obtained by friction stir processing

One Al 6061 with gradient microstructure is prepared by friction stir processing (FSP). Microscopic characterization indicates that the regulation of grain size gradient could be achieved by adjusting the process parameters of FSP. The mechanical property test indicates that one gradient Al 6061 combines an ultimate tensile strength of 266 MPa with a uniform elongation of 27 %, and the other comes 258 MPa with 29 %, which provides an excellent combination of strength and ductility. Moreover, the molecular dynamics (MD) method is employed to reconstruct and simulate the mechanical behavior of the gradient region and the dynamic evolution of the microstructure. When the stacking faults (SFs) with antisymbolic Burgers vector meet and detwinning occurs, grain refinement will induce localized strengthening. When the Shockley partials occur cross-slip, numerous sessile dislocations obstruct the dislocation motion more facilitate strain hardening. Meanwhile, a unique interaction mechanism between shear bands (SBs) and intergranular cracks in Al 6061 is discovered in plastic deformation. The coarse net-like SBs in the small-grain region are highly susceptible to catastrophic fracture of the material, and the large cracks formed within the severely band-like SBs in the large-grain region weaken the strength substantially. However, the formation of sessile dislocations in the gradient region effectively mitigates strain localization and inhibits the nucleation of large cracks. The results interpret the dynamic evolution process of gradient microstructure prepared by FSP and the mechanism of strengthening and toughening and provide process and theoretical references for the research and development of new engineering materials.

Hot oscillatory pressing of SiC ceramics with B4C and C as sintering additives

SiC ceramics with 0.5 wt% B4C and 0.5 wt% C as sintering additives were prepared by hot oscillatory pressure (HOP) and hot pressure (HP). The mechanical properties and microstructures of the resultant ceramics were investigated. The results showed that the oscillatory pressure could effectively suppress the abnormal growth of SiC grains and promote the formation of high-density dislocation and stacking faults. The hardness of SiC ceramics made by HOP at 2000oC reached 28.9 GPa, which was much higher than that (25.9 GPa) of the sample made by HP at the same temperature. The current study suggests that HOP is a promising technique for preparing high-performance SiC ceramics.

Effect of phase composition on the crystal structure and microwave dielectric properties of ZnMg2TiO5 ceramics

Novel microwave dielectric ceramics ZnMg2TiO5 were synthesized by traditional solid-state method. Crystal structure refinement, transmission electron microscopy (TEM), and Raman spectroscopy reveal the coexistence of a secondary phase, MgO, with the predominant cubic spinel phase (Fd-3m) in ZnMg2TiO5 ceramics. The ceramics with dense microstructures exhibit excellent microwave dielectric properties at 1260 °C: εr = 15.6, Q×f = 92,899 GHz (at 10.8 GHz), and τf = −56.9 ppm/°C. The content of MgO, driven by thermodynamic factors, influences significantly impact the microstructure and microwave dielectric properties of the ceramics, particularly inducing the generation of localized lattice distortions and the formation of dislocations. The εr follows the same trend as the molecular polarizability of the actual individual crystal cells. Notably, a near-zero τf (−7.4 ppm/°C) was realized by two-phase composites and excellent comprehensive properties were obtained (εr = 17.9, Q×f = 57,546 GHz).

Role of grain-level chemo-mechanics in composite cathode degradation of solid-state lithium batteries

Solid-state Li-ion batteries, based on Ni-rich oxide cathodes and Li-metal anodes, can theoretically reach a high specific energy of 393 Wh kg−1 and hold promise for electrochemical storage. However, Li intercalation-induced dimensional changes can lead to crystal defect formation in these cathodes, and contact mechanics problems between cathode and solid electrolyte. Understanding the interplay between cathode microstructure, operating conditions, micromechanics of battery materials, and capacity decay remains a challenge. Here, we present a microstructure-sensitive chemo-mechanical model to study the impact of grain-level chemo-mechanics on the degradation of composite cathodes. We reveal that crystalline anisotropy, state-of-charge-dependent Li diffusion rates, and lattice dimension changes drive dislocation formation in cathodes and contact loss at the cathode/electrolyte interface. These dislocations induce large lattice strain and trigger oxygen loss and structural degradation preferentially near the surface area of cathode particles. Moreover, contact loss is caused by the micromechanics resulting from the crystalline anisotropy of cathodes and the mechanical properties of solid electrolytes, not just operating conditions. These findings highlight the significance of grain-level cathode microstructures in causing cracking, formation of crystal defects, and chemo-mechanical degradation of solid-state batteries.

Low cycle fatigue and cycle hardening behavior of heterogeneous Ni2Co1Fe1V0.5Mo0.2 medium entropy alloy

Low-cycle fatigue behavior of high-entropy alloy (HEAs) and medium-entropy alloys (MEAs) have rarely been reported in literature though it is critical for industrial applications. In our previous work, a non-equiatomic Ni2Co1Fe1V0.5Mo0.2 MEA was designed by adding V and Mo elements with bigger atomic size to heighten solution strengthening effect. Due to larger atomic size mismatch and more severe lattice distortion, the Ni2Co1Fe1V0.5Mo0.2 MEA exhibits stronger strain hardening effect than that in CoCrFeMnNi HEA. In present work, the low-cycle fatigue behavior of the non-equiatomic Ni2Co1Fe1V0.5Mo0.2 MEA with heterogeneous grain structures was further investigated. The heterogeneous Ni2Co1Fe1V0.5Mo0.2 MEA exhibits high fatigue resistance at 0.25 % strain amplitude (51285 N), attributed to the pronounced dislocation planar slip and formation of stacking faults. At 0.3 % and 0.5 % strain amplitudes, dislocation interactions (including tangles and microbands) induced by extensive dislocation cross-slip result in obvious cyclic hardening but reduced lifetime. The findings assess the effect of solid solution strengthening on the fatigue behavior of MEAs. The fatigue cracks form either along slip bands in large grains or grain boundaries of small grains.

Dislocation Effect Boosting the Electrochemical Properties of Prussian Blue Analogues for 2.6 V High-Voltage Aqueous Zinc-Based Batteries.

Prussian blue analogues (PBAs) have attracted increasing attention in aqueous zinc-based batteries (AZBs) with the advantages of an open framework, adjustable redox potential, and easy synthesis. However, they exhibited a low specific capacity and a poor cycle performance. In this work, crystalline potassium iron hexacyanoferrate (FeHCF) with dislocation was designed and prepared by a poly(vinylpyrrolidone) (PVP) additive. The metastable state provided by PVP would cause an electrostatic interaction between cyanogen and water molecules. The reduced force increases the steric resistance of the water molecules entering the crystal. The low content of crystal water in FeHCF is associated with the formation of dislocation. The dislocation effect effectively improves the electrochemical reactivity and reaction kinetics of FeHCF. Thus, it presents a high reversible capacity of 131 mAh g-1 with a superior capacity retention of 85% after 550 cycles at 0.5 A g-1. When used as a cathode, the AZBs display a high voltage of 2.6 V, a fast charging capability (<5 min), and a satisfactory cycle stability with a capacity retention of 82% after 400 cycles at 0.2 A g-1 in decoupling electrolytes. This work provides an effective strategy for the design of high-performance PBA-based cathodes for 2.6 V AZBs.

Near-α titanium alloy dwell load-induced deformation twinning to coordinate the deformation mechanism associated with crack initiation

In this study, we identified a specific phenomenon of coordinated deformation of twins in near-α titanium alloys during dwell fatigue (DF). The main crack source regions and internal cracks in the micro-texture region (MTR) and no-MTR samples with inconsistent orientation characteristics were characterized. The results demonstrate that the main cracks in all specimens are aligned with the (0001) basal plane, which is associated with basal slip. Notably, twins are found and confirmed to be involved in the DF crack initiation process in high Al content near-α Ti60 alloys where deformation twins are rare, and tend to nucleate at the DF basal cracks. Further in-situ dwell investigation reveals and proposes that the dislocation pile-up and prismatic-basal (PB) interfacial features between soft/hard grains lead to deformation twin nucleation and growth from hard grain boundaries. Concurrently, the pyramidal 〈c+a〉 dislocation slip is observed to be in concurrent operation with the induced twins. These findings suggest that deformation twins not only coordinate basal grain deformation but also hinder the initiation and propagation of basal cracks caused by basal 〈a〉 slip. Moreover, a high density of pyramidal 〈c+a〉 dislocations within the twin and their dissociation to form basal stacking faults (SFTs) with a specific nanometric spacing make the twinning process accompanied by significant lattice distortions inside the twin. These newly formed nanotwin boundaries and SFTs act as barriers to dislocation motion, enhancing the strength and DF lifetime of near-α Ti60 alloys and effectively reducing the dwell sensitivity of no-MTR alloys. Our findings extend the understanding of the coordinated roles of dislocation slip, twin nucleation and formation of basal SFTs in near-α titanium alloys in dwell fatigue.

Elastodynamic behaviors of steady moving straight dislocation within thin nano film

The elastodynamic dislocation behaviors are of great interest for understanding the performances of structural alloys under intense dynamic loading conditions. The formation, propagations, and interactions of dislocations (such as injected dislocation, accelerating dislocation, steady moving dislocation at high constant speed) are quite different from static dislocations. For steady-moving dislocation within the isotropic infinite medium, the effects of surface and interface on steady-moving dislocations within limited space are still known. In this paper, we investigate the elastodynamic image stress simulation of steady moving dislocation within film of limited thickness at constant speed using Eigenstrain theory, Lorentz transformation, and steady dynamic equilibrium equations. We propose an efficient solution method that involves complex Fourier series, transforming partial differential equations into ordinary differential equations, and ultimately into a set of algebraic equations in spectral space. The effects of dislocation speed and position near the free surface on the image stress of steady-moving climbing and gliding dislocations within the thin film are examined. The results show that relativistic effects are significant for certain dislocation configurations and stress components, whereas other stress components are less sensitive to relativistic effects near the transonic speed region.

Structural features and electrical properties of si(al) thermal migration channels for high-voltage photovoltaic converters

The results of a study of the structural features and electrical properties of Si(Al) through thermomigration p-channels in a silicon wafer are presented. Structural studies were performed using X-ray methods of projection topography, diffraction reflection curves and scanning electron microscopy. It is shown that the channel-matrix interface is coherent without the formation of mismatch dislocations. The possibility of using an array of thermomigration p-channels of 15 elements to form a monolithic photovoltaic solar module in a Si(111) silicon wafer based on p-channels with a width of 100 microns with walls in the plane is shown. The monolithic solar module has a conversion efficiency of 13.1%, an idle voltage of 8.5 V and a short-circuit current density of 33 mA/cm².

Achieving superior high-temperature strength in an additively manufactured high-entropy alloy by controlled heat treatment

High-entropy alloys (HEAs) usually exhibit high strengths at ambient and low temperatures but rapidly degraded tensile properties with increased temperature. In this study, a high-strength HEA, (CoCrNi)94(TiAl)6, is selected and subjected to laser powder bed fusion (L-PBF) and ageing treatment. The microstructural evolution and mechanical property development of the material over a wide range of temperatures are thoroughly investigated. It is found that the as-printed microstructure is dominated by numerous ultrafine cellular structures (∼1 μm) with cell boundaries decorated by Al2O3 nanoparticles, leading to high 0.2% yield strength (YS = 725∼750 MPa) and excellent elongation (>28%) at room temperature. The cellular structures remain up to 700 °C but disappear at or above 800 °C. Ageing at or above 600 °C leads to significant γ′ precipitation with the particle size increasing constantly with increased temperature. The samples containing both cellular structures and coarsened γ′ precipitates (aged at 700 °C) exhibit the highest YS (∼1227 MPa) and ultimate tensile strength (UTS∼1539 MPa) at room temperature and display unprecedented YS at high temperatures, i.e., 949 MPa at 600 °C and 728 MPa at 700 °C, respectively. The exceptional tensile strengths are mainly due to the γ′ precipitates and cell boundaries decorated by Al2O3 nanoparticles which may have acted as strong barriers for dislocation motion. At room temperature, the sample deforms mainly by dislocation slip and formation of stacking faults while at elevated temperatures, deformation becomes increasingly planar as characterized by the formation of increased number of stacking faults and the activation of twinning.

Atomistic understanding of ductile-to-brittle transition in single crystal Si and GaAs under nanoscratch

Ensuring ductile removal in a grinding process is crucial for achieving the desired finish on a hard and brittle single crystal. This study provides new insights into the material removal processes in Si and GaAs single crystals, exploring their deformation behaviour using Berkovich and Conical tips to replicate contact from a fixed abrasive grit. Experimental observations are compared with Molecular Dynamic (MD) simulations to uncover the atomistic deformation mechanisms during the ductile-to-brittle transition (DBT). Notable plastic deformation and minimal cracking were consistently observed in Si, irrespective of the tips used. MD simulations supported this observation, revealing pronounced chip formation indicative of ductile material removal. The resistance to cracking in Si was attributed to amorphization induced by localized high contact stresses. In contrast, GaAs showed a propensity for cracking, with MD simulations revealing dislocation and slip band formation, and cracks emerging in the areas of substantial plastic deformation. These findings address phenomena not previously discernible in experimental studies due to the challenge of real-time observation. Moreover, the tip geometry was shown to significantly influence stress distribution, impacting deformation and crack formation in GaAs. This study also reveals limitations in predicting the critical depth for DBT in both Si and GaAs throught the amended Bifano, Dow, and Scattergood (aBDS) models and MD simulation, offering nuanced insights into these challenges that have not been extensively explored. It was found that the experimental results exceeded predictions by an order of magnitude. These discrepancies underscore the aBDS model's disregard for essential material properties and tip geometry, while the disparities between MD simulation and experiment are primarily attributed to the inherent limitations in the simulated length scales and challenges in detecting initial subsurface cracks.

The Role of Air-Pocket in Crucible Structure for High Quality SiC Crystal Growth

The modified design using an air-pocket existing on inside of crucible has been proposed for the growth of 6-inch SiC single crystal. The actual growth has been performed for conventional, focus ring design and modified hot-zone designs under the same growth conditions and then three SiC crystals were systematically compared in terms of crystal quality. Since stacking faults and polytype inclusions, which could cause dislocation formation, were suppressed by the suitable C/Si ratio control, it was possible to grow SiC ingots with reduced defect density and excellent crystal quality with crucible structures proposed in this study.

Evaluating the electromigration effect on mechanical performance degradation of aluminum interconnection wires: A nanoindentation test with molecular dynamics simulation study

Electromigration (EM) is a crucial failure mode in Aluminum (Al) interconnection wires those are widely used in high density semiconductor packaging. This study systematically investigated the influence of EM on the mechanical properties of Al interconnects via nanoindentation experiments and molecular dynamics (MD) simulations. The results are list as follows. (1) The indentation depth gradually increases with the increase in indentation load, resulting in a gradual increase and stabilization of the Young’s modulus and hardness of the structure. Within a specific range, the influence of the loading rate on the indentation depth and mechanical properties is relatively small. (2) The region where Young’s modulus of the interconnect decreases correlates with the location where EM-induced voids initiate. EM-induced voids have a direct impact on the material’s mechanical properties, particularly the decrease in Young’s modulus. (3) These EM-induced voids affect the nucleation and formation of dislocations. With the increase in void concentration and indentation depth, The generation and slip of dislocations increase as the void concentration and indentation depth increase, leading to a decrease in the material’s mechanical properties over time. This comprehensive findings expand the knowledge on mechanical behavior degradation of Al interconnects when EM failure occurs, providing a scientific basis for designing and optimizing high density semiconductor packaging.