Dislocation Activity Research Articles

A crystal plasticity based strain rate dependent model across an ultra-wide range

Numerous studies have investigated the strain rate sensitive behaviors of materials, consistently reporting enhanced stress values and increased dislocation density with rising strain rates. Behind these phenomena lies the intrinsic nature of dislocation activity. In this context, we introduce an analysis method within a crystal-plasticity (CP) framework, incorporating molecular dynamics insights for a comprehensive range of strain rates (7.5 × 10−5/s to 5 × 107/s). This approach offers a refined understanding of strain rate sensitive behaviors, mainly influenced by dislocation movement laws and strain-rate-dependent saturation of dislocation density. We elucidate the impact of deformation loading conditions on Schmidt factors and active slip systems, which are also crucial for understanding variations in SRS. Ultimately, this study underscores the CP method's effectiveness in comprehensive SRS analysis, seamlessly integrating experimental observations with theoretical predictions for advanced material characterization.

Unraveling factors affecting the reversibility of martensitic phase transformation in FeNiCoAlTi shape memory alloys: Insights from HR-EBSD and acoustic emission analysis

The reversibility of the martensitic phase transformation is crucial for the functionality of shape memory alloys. The easier the martensite is pinned, the faster occurs the degradation of the shape memory effect during cyclic loading. This study investigates various factors influencing the stabilization of martensite during deformation of single-crystalline states of the FeNiCoAlTi system. In order to investigate early pinning mechanisms in more detail a 〈011〉 single crystal with expected lower reversibility of the martensitic phase transformation and, consequently, lower superelasticity was chosen. Complementary in situ methods, such as digital image correlation and acoustic emission, were used to characterize the evolution of the martensitic phase transformation and the role of dislocations during deformation. Thereby, experimental evidence was elaborated pointing at mechanisms that so far have been underrated in terms of functional degradation. Beside interaction of martensitic variants and dislocation activity, detwinning, associated with high acoustic energy, was identified as an additional significant factor contributing to reduced reversibility and superelasticity. In addition, high-resolution electron backscatter diffraction measurements were applied for the first time to identify residual stresses in the austenitic matrix, which significantly contribute to the reverse transformation. Micro-mechanical experiments using pillar compression tests were carried out to study the influence of residual back stresses of the austenitic matrix on the reversibility of the martensitic phase transformation. A reduced reversibility was found in the case of the absence of back stresses. The findings from this study foster the understanding of pinning mechanisms during loading of the FeNiCoAlTi shape memory alloy eventually enabling targeted optimization for enhanced superelastic material behavior.

Co‐Deformation in Severely Deformed Multiphase Crystalline Metallic Systems: Physical Mechanisms, Intrinsic Limitations, and Perspectives

The deformation of multiphase metallic alloys or metal matrix composites by forming processes such as rolling or drawing requires the co‐deformation of phases. Severe plastic deformation processes have extended possibilities with the ability to produce unique nanoscaled structures through the formation of strain‐induced super‐saturated solid solutions, the fragmentation of second‐phase particles, or ultimate co‐deformation down to the nanoscale. Based on various examples, the fundamental mechanisms of co‐deformation are addressed. Parameters that promote the extension of existing heterophase boundaries are discussed, and some possible atomic‐scale mechanisms involving dislocation activities based on geometrical and energetic considerations are proposed. Finally, the physical limits of co‐deformation are considered before suggesting research directions that might be followed to develop fundamental knowledge and applications in the field.

Evidence of Dislocation Mixed Climb in Quartz From the Main Central and Moine Thrusts: An Electron Tomography Study

AbstractIn this study we apply electron tomography to characterize 3D dislocation microstructures in two quartz mylonite specimens from the Moine and Main Central Thrusts, both of which accommodated displacements by dislocation creep in the presence of water. Both specimens show dislocation activity with dislocation densities of the order of 3–4 × 1012 m−2 and evidence of recovery from the presence of subgrain boundaries. 〈a〉 slip occurs predominantly on pyramidal and prismatic planes (〈a〉 basal glide is not active). [c] Glide is not significant. On the other hand, we observe a very high level of activation of 〈c + a〉 glide on the , , (n = 1,2) and even planes. Approximately 60% of all dislocations show evidence of climb with a predominance of mixed climb, a deformation mechanism characterized by dislocations moving in a plane intermediate between the glide and the climb planes. This atypical mode of deformation demonstrates comparable glide and climb efficiency under natural deformation conditions. It promotes dislocation glide in planes not expected for the quartz structure, probably by inhibiting lattice friction. Our quantitative characterization of the microstructure enables us to assess the strain that dislocations can generate. We show that glide systems indicated by the observed dislocations are sufficient to accommodate any arbitrary 3D strain by themselves. Although historically dislocation glide has been regarded as being primarily responsible for producing strain, activation of climb can also directly contribute to the finite strain. On the basis of this characterization, we propose a numerical modeling approach for attempting to characterize the local stress state that gave rise to the observed microstructure.

Microstructure evolution and surface strengthening behavior of 300 M ultrahigh strength steel under engineered surface treatments

In this work, the most efficient and common engineered surface treatments identified as shot peening (SP), laser shock peening (LSP) and ultrasonic surface rolling process (USRP) were utilized to impact the 300 M ultrahigh strength steel. The microstructure evolution and surface strengthening behavior were carefully characterized and compared for the 300 M steel with various treatments. Besides, the underlying mechanisms for difference in grain refinement and surface mechanical properties by the arranged surface modification are revealed. The results indicate that a gradient structure with nano-grains at the top surface appears on the SP, LSP and USRP treated samples. The process of repeated impacts of SP in different directions leads to activation of multiple slip systems, which thus causes severe refined grains and dense dislocation activities associated with dislocation tangles, dislocation walls and blocking. A great variety of strikes per unit area by USRP results in higher plastic deformation compared to SP where numerous dislocation cells and deformation bands were generated. In contrast to SP and USRP, fewer slip systems are activated in LSP due to the short duration time which thus leads to the thinnest refined grain layer and more planar dislocation structures. The combination of low plastic work and lack of refined grains leads to lowest full width at half maximum (FWHM) in case of LSP samples whereas high FWHM comes from both fine grain size and dislocations in SP. The grain refinement and plastic work contributes to an increase of microhardness and compressive residual stress for the SP, LSP and USRP samples. To be clear, SP leads to a higher magnitude of hardness and compressive residual stress on the topmost surface while LSP causes a larger depth due to that shock wave in LSP influences material to much greater depth. Besides, the decomposition and redistribution of carbides are also noticed on the surface layer after the surface treatments with very high strain rate where numerous dislocations accumulate at the border of the carbides and slip trace is also observed within the carbides.

Atomic-scale compositional complexity ductilizes eutectic phase towards creep-resistant Al-Ce alloys with improved fracture toughness

Hierarchical microstructures spanning from micro-sized eutectic structure to nano-sized precipitates are promisingly engineered in lightweight Al alloys to improve the high-temperature creep resistance that is increasingly required for rapid industrial development. However, the intrinsically-brittle eutectic phase is ready to fracture upon applied loading, which, dramatically reducing room-temperature ductility and fracture toughness, greatly hampers practical applications of the creep-resistant Al alloys. Here, through the combination of Sc microalloying with sub-rapid solidification, we observe the ductilization of Al11Ce3 eutectic phase in cast heat-resistant Al-Ce-Sc alloys due to the formation of atomic-scale compositional complexity. High-concentration Sc atoms are frozen within the Al11Ce3 intermetallic phase by the sub-rapid solidification, which then assemble into unusual atomic-scale compositional dipoles with the Sc atoms enriched at one pole and the Al atoms at the opposite during subsequent heat treatment. The dispersed Sc-Al compositional dipoles induce local lattice distortions that stimulate dislocation activities, as temporally and spatially visualized by in-situ neutron diffraction tensile test and microstructural characterizations. The unexpected plastic deformation triggered in Al11Ce3 improves the deformation compatibility between the eutectic phases, enabling the sub-rapidly-solidified Al-Ce-Sc alloy to reach a room-temperature tensile elongation 3 times and fracture toughness over 8 times of its counterpart derived from traditional solidification. In addition, the sub-rapidly-solidified Al-Ce-Sc alloy exhibits an excellent creep resistance at 300 °C, achieving a tensile creep stress threshold of ∼ 70 MPa. These findings provide new perspectives on the design of ductile intermetallic phases and the development of creep-resistant Al alloys with application-level ductility.

Effects of broken β-Mg17Al12 and non-uniform Mg2Pb particles introduced by EX-ECAP on microstructure and mechanical properties of Mg–Pb-9.2Al-0.8B alloys

Mg–Pb-9.2Al-0.8B alloy faces a significant strength-toughness balance problem in applications. To address this challenge, severe plastic deformation was applied to the alloy by combining traditional extrusion with multi-pass equal channel angular pressing (EX-ECAP). The research delves into the microstructural evolution (including broken β-Mg17Al12 and non-uniform Mg2Pb particles) and its effects on the strengthening mechanisms. The synergistic effects of grain boundary strengthening due to grain refinement, Orowan strengthening, geometrically necessary dislocation strengthening, and coefficient of thermal expansion strengthening collectively enhance the tensile strength and ductility, balancing the strength and toughness. The results showed an impressive ultimate tensile strength of 386.1 MPa, which is 2.33 times that of the as-cast alloy. The altered fracture morphology clearly indicated a transformation from brittle to brittle-ductile fracture mode. During the deformation process, the activation of dislocations, atomic-scale ripples, minor lattice distortions, and kink bands enhance the macroscopic flexibility. The introduction of EX-ECAP presents new opportunities for manufacturing high-performance Mg–Pb-9.2Al-0.8B alloy through integrated deformation techniques.

In situ studies on temperature‐dependent deformation mechanisms of Al2O3 prepared by spark plasma sintering

AbstractAlumina (α‐Al2O3) is one of the most versatile engineering ceramics, and its mechanical properties have been extensively studied. However, the micromechanical properties of Al2O3 with a fine microstructure are less well understood. Here, we present one of the first investigations that probe the micromechanical properties of fine‐grained polycrystalline Al2O3 fabricated via spark plasma sintering, employing in situ microcompression tests inside a scanning electron microscope. This study explores the influence of temperature variations on the deformation mechanisms, particularly the involvement of microcracks and dislocation activities throughout the deformation process. As temperature rises, substantial deformability occurs in the inherently brittle Al2O3 at intermediate temperature, where the improved plastic deformability mainly arose from prominent dislocation activities accompanied by grain boundary sliding. This study sheds light on understanding the relationship between defect evolution and mechanical behavior in Al2O3 with fine grain sizes.

Revealing dislocation activity modes during yielding and uniform deformation of low-temperature tempered steel by acoustic emission

Revealing dislocation activity modes during yielding and uniform deformation of low-temperature tempered steel by acoustic emission

Additive manufacturing of an ultrastrong, deformable Al alloy with nanoscale intermetallics

Light-weight, high-strength, aluminum (Al) alloys have widespread industrial applications. However, most commercially available high-strength Al alloys, like AA 7075, are not suitable for additive manufacturing due to their high susceptibility to solidification cracking. In this work, a custom Al alloy Al92Ti2Fe2Co2Ni2 is fabricated by selective laser melting. Heterogeneous nanoscale medium-entropy intermetallic lamella form in the as-printed Al alloy. Macroscale compression tests reveal a combination of high strength, over 700 MPa, and prominent plastic deformability. Micropillar compression tests display significant back stress in all regions, and certain regions have flow stresses exceeding 900 MPa. Post-deformation analyses reveal that, in addition to abundant dislocation activities in Al matrix, complex dislocation structures and stacking faults form in monoclinic Al9Co2 type brittle intermetallics. This study shows that proper introduction of heterogeneous microstructures and nanoscale medium entropy intermetallics offer an alternative solution to the design of ultrastrong, deformable Al alloys via additive manufacturing.

Influence of interface on deformation compatibility of a heterogeneous Cu/Al nanoscale multilayer

The incompatible deformation among the constituent layers usually results in extremely low tensile ductility in nanoscale metallic multilayers. Here, molecular dynamics simulations are conducted to investigate the deformation compatibility of a heterogeneous Cu/Al multilayer with various interface structures under compression. The results show that, by selecting an appropriate thickness of thin interlayers, the heterogeneous Cu/Al nanolayered composite with (1¯1¯1) interface exhibits excellent compatible deformation between the soft and hard layers while the homogeneous counterpart does not. The excellent deformation compatibility of the heterogeneous structure originates from the presence of the thin soft interlayer that can enhance the dislocation activities of the hard layers. Unlike that with (1¯1¯1) interface, the heterogeneous structure with (001) interface shows obvious incompatible deformation among the component layers. Such a scenario can be attributed to the stable dislocation network on the (001) interface, making the dislocations in the soft phase difficult to cross the interface into the hard phase. These findings may deepen our understanding of the deformation compatibility of heterogeneous nanoscale metallic multilayers.

New insights into ductility improvement of a nickel-based superalloy through grain boundary engineering

Grain boundary engineering (GBE) provides an effective approach for tailoring the properties of metallic materials by optimizing the grain boundary character distribution (GBCD). Using electron backscatter diffraction analysis and uniaxial tensile testing at room temperature, this study systematically investigated the impact of GBE-type thermomechanical processing (TMP) on the ductility of an Incoloy 925 nickel-based superalloy (Alloy 925). The results showed that at 5 % strain, increasing the annealing temperature results in larger twin-related domains through strain-induced boundary migration. The GBE-type TMP can significantly enhance the ductility of Alloy 925, namely, the elongation increases from 64.22 % to 95.86 % as the annealing temperature increases from 1050 °C to 1100 °C. The evolved GBCD introduces more coherent twin boundaries with lower average residual Burgers vectors, enabling efficient dislocation transmission. In addition, extensive twinning can widely generate some soft orientations and simultaneously reduce the average Taylor factor within the twin clusters, leading to easy dislocation activation during plastic deformation. The synergistic effect of the optimized grain boundary network and twin cluster anisotropy promotes uniform plastic flow and effective strain accommodation, thereby enhancing the ductility of Alloy 925. Overall, this work provides novel and valuable insights into ductility optimization through GBE in a nickel-based superalloy.

Tensile deformation of fine-grained Mg at 4K, 78K and 298K

The impact of grain size, ranging from 0.9 μm to 9 μm, on the mechanical properties of commercially pure Mg is investigated at temperatures of 4K, 78K, and 298K. The mechanisms governing plastic flow are influenced by both grain size and temperature. At 4K and 78K, dominant deformation modes in Mg involve dislocation glide and extension twinning, regardless of grain size. The interactions between basal and non-basal dislocations and dislocations with grain boundaries promote an unusually high rate of work hardening in the plastic regime, leading to premature failure. The yield stress follows the Hall-Petch relationship σy∼k/d, with the slope k increasing with decreasing temperature. At 298K, in addition to dislocation glide and twinning, grain boundary sliding (GBS) becomes significant in samples with grain sizes below 3 μm, considerably enhancing the material’s deformability. GBS activation provides an additional recovery mechanism for dislocations accumulating at grain boundaries, facilitating their absorption during sliding and rotation. Analysis of σΘ relationship suggests that the basal slip is the dominant dislocation mode in Mg at 298K. Decreasing grain size suppresses dislocation activity and twinning and increases GBS, resulting in lower Θ and σΘ values. Suppressing conventional deformation modes coupled with enhanced GBS yields stress softening, breaking down the Hall-Petch relationship in Mg below 3 μm grain size, leading to an inverse Hall-Petch behaviour. The work reports new data on the strength, ductility, work hardening and fracture behaviour, and their variations with Mg grain size across different temperature regimes.

Influence of high-strain-rate compression and subsequent heat treatment on (TiNbZr)89(AlTa)11 refractory high-entropy alloys: Dynamic-mechanical behavior and microstructural changes

This study explored the dynamic-mechanical behavior of a novel low-density (TiNbZr)89(AlTa)11 refractory high-entropy alloy (RHEA) across strain rates ranging from 1.0 × 103 to 3.5 × 103 s−1. A significant increase in the yield and ultimate compressive strengths with rising strain rates up to 3.0 × 103 s−1 was observed and attributed to enhanced dislocation activities and stress-induced microstructural transformations. The formation of the B2 phase and Zr5Al3 precipitates was found to be crucial in bolstering the alloy strength at high strain rates. Beyond strain rates of 3.0 × 103 s−1, a decrease in strength occurred due to thermal softening and strain localization. Microstructural analyses at 3.5 × 103 s−1 revealed grain refinement, the development of micro shear bands, and dislocation tangles, which were indicative of dynamic recrystallization. Besides, the findings also revealed that the post-dynamic compression heat treatment further enhanced the hardness and microstructural stability of the alloy. These results highlight the potential of the (TiNbZr)89(AlTa)11 RHEA for applications requiring materials with high strength-to-weight ratios, particularly in dynamically loaded environments. It is expected that the results of this study will further advance our fundamental understanding of the behavior of RHEAs under extreme conditions, thereby opening new avenues for material innovation.

Molecular dynamics simulation of the scratching process of GaAs with different crystal orientations.

In order to further improve the manufacturing technology of resonator facet of GaAs (gallium arsenide)-based laser, the scratching process of GaAs was simulated by molecular dynamics. Models of GaAs crystals with different orientations, including GaAs [100], GaAs [110], and GaAs [111], were generated, followed by scratch simulations on these models. The surface characteristics of scratches, damage width, subsurface damage, stack height, and the distribution and activity characteristics of dislocations were analyzed based on the simulation results. The results show that there are obvious anisotropy in the deformation of different crystal orientation during the scratching process of GaAs. Surface features, damage width, subsurface damage, and dislocation dynamics during scraping in GaAs crystals strongly depend on crystal orientation. It was also observed that GaAs exhibits distinct characteristics of dislocation activity during the scratching process, depending on its crystal orientation. In addition, GaAs [110] crystal direction has the smallest maximum damage width and subsurface damage depth. The maximum of maximum damage width is in GaAs [100] crystal direction, and the maximum subsurface damage depth is in GaAs [111] crystal direction. In addition, the stacking height is maximum when GaAs [100] is scraped and minimum when GaAs [110] is scraped. The engraving quality of GaAs materials was investigated utilizing the LAMMPS software through molecular dynamics simulations, while observations were facilitated using the OVITO software. The MD simulation was conducted employing the NPT ensemble, with the temperature fixed at 300 K. A time step of 2 fs was utilized, and the total duration of the MD simulation spanned 600 ps.

Preparation of the gradient nanostructured layer in uranium by surface shot peening treatment

The gradient nanostructured layer in uranium prepared by surface shot peening treatment has been systematically characterized. The evolution of the ultrafine grains within the surface layer of uranium was derived. The results show that the gradient microstructures from the surface to the matrix could be divided into four zones including ultrafine grains, high density deformation twins, low density deformation twins, and coarse grain matrix. The deformed microstructures in the four zones were characterized in detail. Twinning and dislocation activities were found to be the main plastic deformation mechanisms for grain refinement in uranium. Four types of twinning and dislocation slip modes were distinguished. The formation of fine equiaxed grains on uranium surface should be attributed to the combined action of the twinning, dislocation activities, and the subgrain rotation driven by mechanical-thermal coupling. The surface microhardness and the natural oxidation corrosion performance were improved in uranium prepared by shot peening treatment.

Revealing the exceptional cryogenic strength-ductility synergy of a solid solution 6063 alloy by in-situ EBSD experiments

A solid solution 6063 aluminium alloy features an exceptional combination of strength and ductility at 77 K. Here, the deformation mechanisms responsible for superior strength-ductility synergy and excellent strain hardening capacity at a cryogenic temperature of the alloy were comparatively investigated by in-situ electron backscatter diffraction (EBSD) observations coupled with transmission electron microscopy (TEM) characterization and fracture morphologies at both 298 and 77 K. It is found that kernel average misorientation (KAM) mappings and quantified KAM in degree suggest a higher proportion of geometrically necessary dislocations (GNDs) at 77 K. The existence of orientation scatter partitions at 77 K implies the activation of multiple slip systems, which is consistent with the results of potential slip systems calculated by Taylor axes. Furthermore, dislocation tangles characterized by brief and curved dislocation cells and abundant small dimples have been observed at 77 K. This temperature-mediated activation of dislocations facilitates the increased dislocations, thus enhancing the strain hardening capacity and ductility of the alloy. This research enriches cryogenic deformation theory and provides valuable insights into the design of high-performance aluminium alloys that are suitable for cryogenic applications.

Enhancing surface strength of tungsten by gradient nano-grained structure

A gradient nano-grained (GNG) structure demonstrates satisfactory surface strength. However, the underlying mechanism responsible for its strengthening lacks sufficient research. To explain how gradient nano-grained structures improve surface strength in detail, large-scale parallel molecular dynamics simulations are utilized in this study to investigate the mechanical deformation behavior of BCC tungsten with varying grain sizes during spherical nanoindentation. The findings suggest that a well-designed gradient structure can promote rational plasticity and an appropriate distribution of internal atomic stress. The critical point of maximum stress and hardness is observed when the initial grain size is 4.5 nm, with an average grain size of 7.1 nm. The interaction between grain boundary slip and migration in small grains, along with the enhanced activity of grain boundary dislocations in large grains, collectively contributes to the enhancement of the strength and hardness of the GNG structure. Compared with a homogeneous nano-grained structure, the gradient nano-grained structure exhibits a more rational distribution of dislocations and stress relaxation effects to enhance strength. The present work utilizes the molecular dynamics nanoindentation method to study GNG materials, providing a methodology for investigating the surface strengthening effects of GNG structures at the atomic scale and effectively revealing potential mechanisms for resisting surface deformation in GNG structures.

Making titanium alloys ultrahigh strength and toughness synergy through deformation kinks-mediated hierarchical α-precipitation

Titanium alloys engineered in structural applications achieve ultrahigh strength primarily through precipitation strengthening of secondary α-phase (αs) during aging, while they often experience compromised ductility and toughness due to traditional strength-toughness tradeoff. In this study, we propose a novel strategy to address this conflict by introducing deformation kinks prior to conventional cold rolling (CR) and aging processes. These kinks are produced by cold forging (CF) to create macroscopic lamellar structures in β-grains, which alter strain partitioning during subsequent CR and ultimately tailor αs-precipitation upon aging. As a result, an ultrafine duplex (αe + β)-structure is formed within kink interiors, while hierarchical αs-precipitates are generated in the external β-matrix. This unique microstructure effectively enhances dislocation activity, promotes uniform plastic strain distribution and impedes crack propagation. Consequently, a simple Ti-V binary titanium alloy exhibits exceptional properties with ultrahigh strength ∼1636 MPa, decent ductility ∼5.4 % and appreciable fracture toughness ∼ 36.1 MPa m½. The synergetic properties surpass those obtained through traditional CR and aging processes for the alloy and even outperform numerous multielement engineering titanium alloys reported in literature. Our findings open up a new avenue for overcoming the strength-toughness tradeoff of ultrahigh-strength titanium alloys, and also offer a facile production route towards structural materials for advanced performance.

Damage accumulation and its effect during thermal- and stress-induced cycling martensite transformation of laser powder bed fused (LPBF) NiTi alloy

Nitinol (NiTi) alloys produced via laser powder bed fusion (LPBF) exhibit promising potential across diverse applications, yet their suitability for devices necessitating repetitive actuation confronts uncertainties regarding microstructural evolution and functional fatigue behavior during cyclic operation. Our investigation unveils that damage accumulation during thermal cycling stems from geometrically necessary dislocations formed through finite-scale incoherent interface motion, contrasting earlier hypotheses attributing LPBF-induced dislocation activity as the primary contributor. Damage accumulation during mechanical cycling is characterized by notable multiplication of cyclic dislocations via dislocation twinning interactions, rather than severe martensitic plasticity. These newborn cycling dislocations inherit martensite's crystallographic characteristics, exerting a distinct influence mechanism on martensite transformation and deformation behavior, diverging from conventional studies on function fatigue in traditional NiTi alloys. This study sheds light on the distinctive damage accumulation traits observed during thermal/mechanical cycling of LPBF-fabricated NiTi alloys, offering valuable insights for probing their functional fatigue mechanisms.