Dislocation Annihilation Research Articles

Achieving high strength and ductility of Al-Zn-Mg-Cu alloys via laser shock peening and spray forming

In order to further enhance the strength and ductility of ultra-high strength aluminum alloys, the laser shock peening technology was applied to ultra-high strength Al-Zn-Mg-Cu alloy. The ultimate tensile strength, elongation and hardness can reach to 751 MPa, 11 % and 208.3 HV by combining spray forming, secondary extrusion, solid solution, retrogression and reaging as well as laser shock peening. The high strength and hardness of the alloys is mainly attributed to the fine-grained layer on surface as well as new grain boundaries, dislocation cells and high-density dislocations introduced by laser shock peening, uniform nano-sized strengthening phases with high density precipitated during heat treatments. The excellent ductility of the alloys is mainly ascribed to multiple structures including fine-grained layer on surface and slip lines inside different grains introduced by laser shock peening, smaller size of fibrous grains and Al7Cu2Fe phase produced by secondary extrusion and spray forming. The aging treatment after laser shock peening can lead to the annihilation of high-density dislocations as well as significantly promote the formation of stable and coarse η phase, which can greatly reduce the strength of the studied alloys.

Nano-tribological behavior and structural evolution of 304 stainless-steel by molecular dynamics simulation and experiment

304 stainless-steel has excellent mechanical properties and is commonly used in ultra-precision instrumentation. Nevertheless, the research on the intricate nano tribological behavior of 304 stainless-steel during ultra-precision machining remains limited. In this work, the tribological behavior of 304 stainless-steel is investigated at different indentation depths and scratching velocities by molecular dynamics (MD) simulations and nano-scratch tests. The results show that higher indentation depths lead to more serious damage on the surface and subsurface of 304 stainless-steel. The friction force and friction coefficient also increase significantly with higher indentation depths. It is worth noting that the dislocation density is much smaller at the low-indentation depth, which is due to the large-scale dislocation annihilation at this depth. As the scratch velocity increases, the subsurface shear stress decreases, the dislocation length rises, and it is accompanied by the generation of V-shaped dislocations, which are caused by dislocation entanglement within the substrate. In addition, nano-scratch tests were performed and similar trends were obtained by comparing the results with simulations. This work provides theoretical guidance for a deeper understanding of the deformation behavior of 304 stainless-steel during nano-scratching and its engineering applications at the micron and nano-meter scales.

Deformation Behavior and Microstructural Evolution of Ti–6.5Al–2Zr–1Mo–1V Alloy during Isothermal Hot Compression

In this investigation, the effects of different deformation passes—namely, single pass and multipass—on the microstructural evolution and deformation mechanisms in the Ti–6.5Al–2Zr–1Mo–1V alloy are analyzed. It is observed that following a single‐pass hot compression, the extent of lamellar α phase spheroidization enhances as the temperature increases. During multipass hot compression, there is a gradual reduction in the size and concentration of equiaxed α phase, alongside an increase in spheroidization. Dislocation density escalates to 15.88%, while the proportion of high‐angle grain boundaries (HAGBs) diminishes to 75.24%. Static recrystallization occurring during the holding process facilitates dislocation annihilation. The dynamic phase transformation mechanism manifests through interfacial permeation at the primary α phase. Strain localization at the boundaries or sub‐boundaries of the primary α phase, which exhibit minimal curvature, induces elevated shear stress, thereby promoting the shearing of the primary α phase and reducing its presence. Texture components predominantly observed are <‐12‐10>//Z0 and <0001>//Z0, transitioning from <‐12‐10> to <0001> with increasing strain.

Investigation on fatigue performance and residual stress release mechanism of split sleeve cold expansion holes of heterogeneous 7050-T7451 aluminum alloy

7050-T7451 aluminum alloy is widely applied in aircraft structural components due to good properties, these components majority connected with hole structures. However, stress concentration occurs around structural holes during application, which leads to fatigue cracking and ultimately resulting in low fatigue performance of these parts. The residual stress generated by cold expansion can provide sufficient strength to prevent fatigue crack initiation. However, this protective effect is lost when the residual stress is released during fatigue. Consequently, it is crucial to investigate the release of residual stress in hole structures during the fatigue process. In this work, the residual stress release process and microstructure evolution under to different cyclic loads were investigated. The back stress induced by the heterogeneous structure on residual stress release and the effect of heterogeneous structure on crack propagation were discussed. The findings indicate that the back stress generated by GNDs of heterogeneous structures can resist the applied load in early stages, delay residual stress release. As the cycle number increases, potential mechanisms for the release of residual stress include dislocation annihilation, dislocation rearrangement, and dislocation shearing. The heterogeneous grain structure with high tortuosity and the deflection and branching of cracks can reduce the driving force of crack growth.

Effect of strain ratio and pre-strain on the low-cycle fatigue behavior of 4130X steel at different strain amplitudes

This study investigates the impacts of strain ratio and pre-strain on the low-cycle fatigue properties and microscopic damage mechanisms of 4130X steel across varying strain amplitudes. Under symmetric loading, initial cycles at low strain amplitudes demonstrate clear peak/valley stress hardening followed by cyclic softening in later stages of fatigue. In contrast, the hardening behavior vanishes at higher strain amplitudes. Increasing strain ratio and pre-strain cause the initial hardening behavior at low strain amplitudes to gradually disappear. Additionally, at lower strain amplitudes, fatigue life decreases as strain ratio and pre-strain rise, attributed to substantial tensile mean stress. Microscopic examination reveals that symmetric cyclic loading at low strain amplitudes releases stress concentrations caused by quenched and tempered processing and reduces both dislocation density and localized plastic strain. Conversely, at higher strain ratios and pre-strains, increased tensile mean stresses not only intensify dislocation multiplication, resulting in high dislocation density, but also activate multi-slip system, leading to dislocation cross-slip. Moreover, at a high strain amplitude of 0.45 %, pre-strain aids in martensite recovery and promotes dislocation annihilation during recovery, thus inducing cyclic softening. Finally, the fatigue lives under varied loading conditions are compared with the fatigue design curve prescribed by ASME VIII-II code.

Stress sensitivity and its influence on high-temperature creep behavior of a novel 2nd-generation low-cost nickel based single crystal superalloy

To meet the application requirements under complex service conditions for aero engine blades, the influence of stress-state on creep behavior of a self-designed Re-low Ni-based single crystal superalloy at 1038 °C over a wide stress range of 137–207 MPa was investigated. With applied stress decreasing from 172 to 137 MPa, the creep life rapidly extends by a factor of 2.41, showing strong stress dependence. Further, the creep mechanisms under different stresses are discussed based on microstructural characterization and threshold stress calculation results. With the increasing applied stress, interfacial dislocation networks exhibit sparse and irregular shapes. Particularly, in necking zone of the fractured specimen at 1038 °C/137 MPa, due to the long-duration accumulation of deformation storage energy, dislocation arrays within γ′ rafts gradually evolve into dislocation walls through recombination and annihilation of dislocations, then develop into subgrain boundaries. Finally, some recrystallization grains with large misorientations (>10°) form by subgrain-rotation. Moreover, based on the calculated threshold stress, dislocations climbing can play a major role in creep deformation throughout the steady-state stage under a stress range of 137–207 MPa. Nevertheless, with the arrival of tertiary stage, the increasing actual stress resulted from necking deformation can also promote dislocations to cut or bypass γ′ rafts under lower stresses (137 and 172 MPa), which is consistent with the final evolution of dislocation substructures. Overall, the findings from this work are helpful to understand the high-temperature creep mechanism of low-cost single crystal superalloys, so as to improve the creep resistance of materials.

Melting and freezing of a skyrmion lattice

We report comprehensive Monte-Carlo studies of the melting of skyrmion lattices (SkL) in systems of small, medium, and large sizes with the number of skyrmions ranging from 103to over 105. Large systems exhibit hysteresis similar to that observed in real experiments on the melting of SkLs. For sufficiently small systems which achieve thermal equilibrium, a fully reversible sharp solid-liquid transition on temperature with no intermediate hexatic phase is observed. A similar behavior is found on changing the magnetic field that provides the control of pressure in the SkL. We find that on heating the melting transition occurs via a formation of grains with different orientations of hexagonal axes. On cooling, the fluctuating grains coalesce into larger clusters until a uniform orientation of hexagonal axes is slowly established. The observed scenario is caused by collective effects involving defects and is more complex than a simple picture of a transition driven by the unbinding and annihilation of dislocation and disclination pairs.

Enhancing microstructural integrity and mechanical performance of thick wire-arc directed energy deposited eutectic aluminum-silicon solid structure through continuous multi-pass friction stir processing

In wire-arc directed energy deposition (waDED), fabricating larger 3D components with single-layer tracks is challenging due to limitations in track thickness, requiring the development of bead overlapping strategies. However, fusion-based 3D-printed aluminum structures often show porosity and coarse-grained microstructures, limiting their industrial applications. To address these limitations, this study employs continuous multi-pass friction stir processing (FSP) on a waDED-produced aluminum block (140 mm × 150 mm × 28 mm). The severe plastic deformation induced by FSP triggers dynamic recrystallization (DRX), promoting the formation of equiaxed grains with an average grain size 8.75 times smaller than the as-fabricated waDED sample (67.03 μm vs 7.66 μm). Transmission Electron Microscopy (TEM) revealed decreased dislocation loops post-FSP, indicating DRX and dislocation annihilation. X-ray micro-computed tomography (X-CT) examination showed complete elimination of porosity in the waDED sample, owing to the intense mechanical stirring induced during FSP. Scanning electron microscopy (SEM) images of the waDED samples showed fibrous eutectic networks disrupted by FSP, leading to a uniform distribution of broken Si particles within the α-Al matrix. Hardness decreased by approximately 38 % (86 HV0.1 vs 53 HV0.1), while the average ultimate tensile strength decreased by about 4 % in the FSP-treated samples (159.19 MPa vs 153.02 MPa). Additionally, the formation of alpha fiber and weak cube texture components during FSP enhances ductility, as evidenced by a sixfold improvement in elongation (1.93 % vs 11.59 %). This textural transformation contributes to the characteristic deeper and more uniform dimples in the FSP-treated samples, indicative of ductile fracture, compared to the premature failure patterns in as-deposited samples.

Study of strengthening mechanisms in Al-Cu and Al-Cu-Mg systems subjected to plastic deformation and T6 treatment

ABSTRACT A series of Al-4Cu and Al-2Cu-2Mg wt.% alloys were synthesized through casting and subjected to plastic deformation to evaluate the interaction and contribution of the different strengthening mechanisms since the experimental Vickers microhardness test. The plastic deformation was performed by cold-rolling to achieve a 50% thickness reduction. The results show that solid solution strengthening is the main mechanism responsible for the total hardness enhancement of the alloys, and the Mg presents a higher hardening capacity for the solid solution strengthening than the Cu. For Al-4Cu alloy, competition between the annihilation of dislocations and the formation of precipitates generates an apparent low response precipitation hardening. Nevertheless, for Al-2Cu-2Mg alloy, the Mg addition allowed retard the dislocation annihilation induced an apparent higher response precipitation hardening.

Effect of High Deformation without Preheating on Microstructure and Corrosion of Pure Mg

In this study, the relationship between the extrusion ratio and the corrosion resistance of pure Mg deformed using extrusion with an oscillating die (KoBo) without preheating of the initial billet was investigated. The materials investigated in this study were extruded at high deformation ratios, R1 5:1, R2 7:1, and R3 10:1, resulting in significant grain refinement from the very coarse grains formed in the initial billet to a few µm in the KoBo-extruded samples at room temperature, which is not typical for hexagonal structures. Our research clearly shows that KoBo extrusion improves the corrosion performance of pure Mg, but there is no straightforward dependence between the extrusion ratios and corrosion resistance improvement. Although it was expected that the smallest grain size should provide the highest corrosion resistance, the dislocation density accumulated in the grain interiors during deformation at the highest extrusion ratio, R3 10:1, supports dissolution reactions. This, in turn, provides the answers for the greater grain size observed after deformation at R2 7:1, where dynamic recovery prevailed over dynamic recrystallization. This situation led to the annihilation of dislocation, leading to better corrosion resistance of the respective alloy. Therefore, the alloy with the greatest grain size has the best corrosion resistance.

Strain-rate and size dependence of gradient lamellar nickel investigated by in-situ micropillar compression

Strain rate plays a nonnegligible role in the plastic deformation behavior of metallic materials with heterogeneous nanostructure, while little research focuses on the topic. In-situ compression tests at various strain rates were conducted on the micropillars located at different depths from the gradient lamellar Ni. These tests intended to reveal the strain-rate dependence of micropillars with different lamellar thicknesses and illustrated the competitive relationship between strain rate and intrinsic grain size on the plastic deformation behavior of metallic materials. In addition, due to the strain rate sensitivity related to not only intrinsic size but also external size, single Ni micropillars with different diameters were also compressed at various strain rates in order to clarify the competitive relationship between intrinsic size and external size on the strain rate sensitivity. It is found that lamellar thickness rather than strain rate has more effect on plastic deformation, and external size exhibits a more obvious impact on the strain rate sensitivity. The plastic deformation behaviors of different micropillars are mainly explained by transitions in the effect of lamellar grain boundaries on the dislocation motion at various strain rates and over different lamellar thicknesses, resulting in the change of dislocation multiplication and annihilation rate.

Effective InAsP dislocation filtering layers for InP heteroepitaxy on CMOS-standard (001) silicon

In this work, we report InAsP-based dislocation filter layers (DFLs) for InP heteroepitaxy on CMOS-standard (001) Si substrates, demonstrating a threading dislocation density of 3.7 × 107 cm−2. The strain introduced by InAsP induces dislocation bending at the InAsP/InP interface, thereby facilitating the reaction and annihilation of dislocations during their lateral glide. Concurrently, the InP spacer exhibits tensile strain, leading to the formation of stacking faults (SFs). With a comprehensive analysis utilizing x-ray diffraction, electron channeling contrast imaging, and transmission electron microscopy, the effects of DFL-induced strain on dislocations and SFs are investigated. Fine-tuning the strain conditions allowed low-dislocation-density while SF-suppressed, anti-phase boundary free InP on Si. This work, therefore, provides a useful buffer engineering scheme for monolithic integration of InP-based electronic and photonic devices onto the industry-standard silicon platform.

The influence of high strain rates on mechanical properties and microstructure of CuCrZr alloy under strong current carrying condition

The extreme condition of high-speed metal deformation under intense current flow is commonly encountered in the realms of national defense, military, and industry. However, there has always been a lack of effective research method for the micro deformation mechanism of metal under this extreme condition. We have developed a high current and high strain rate tensile (HCHST) testing platform based on RC circuits, which is used to research the micro-deformation mechanisms of CuCrZr alloy under this extreme condition. The dynamic tensile tests of CuCrZr alloy at different high strain rates (1401s−1, 2957 s−1, 5503 s−1, 8012 s−1) under the high current density (6550 A/mm2) condition were conducted using the HCHST experimental platform. The mechanical properties and microstructure of CuCrZr alloy under the HCHST were analyzed by combining TEM and EBSD. The research results indicate that the microhardness of the alloy in the HCHST #1 environment (6550 A/mm2,1401s−1) is almost the same as the original state. This is attributed to the strong current promoting the movement of dislocations and reducing the accumulation of dislocations. Under condition like HCHST#1, the dislocation annihilation rate and dislocation formation rate of CuCrZr alloy almost reach equilibrium. At higher strain rates, the dislocation multiplication rate is much higher than the dislocation annihilation rate caused by strong current. This imbalance leads to dislocation entanglement and the formation of dislocation cells, resulting in an increase in the microhardness of the material. Additionally, EBSD was used to analyze the changes in grain size, average volume fraction of LAGBs, average KAM value, and texture of the samples, further verifying the dislocation evolution and microhardness changes of copper alloys under HCHST conditions. Our research results provide strong theoretical for the application of CuCrZr in the field of electrical thermal mechanical coupling.

Investigation of the Dislocation Behavior of 6- and 8-Inch AlGaN/GaN HEMT Structures with a Thin AlGaN Buffer Layer Grown on Si Substrates

Developing cost-effective methods to synthesize large-size GaN films remains a challenge owing to the high dislocation density during heteroepitaxy. Herein, AlGaN/GaN HEMTs were grown on 6- and 8-inch Si(111) substrates using metal–organic chemical vapor deposition, and their basic properties and dislocation evolution characteristics were investigated thoroughly. With the insertion of a 100 nm thin AlGaN buffer layer, bow–warp analysis of the epitaxial wafers revealed excellent stress control for both the 6- and 8-inch wafers. HR-XRD and AFM analyses validated the high crystal quality and step-flow growth mode of GaN. Further, Hall measurements demonstrated the superior transport performance of AlGaN/GaN heterostructures. It is worth noting that dislocations tended to annihilate in the AlN nucleation layer, the thin AlGaN buffer layer, and the GaN buffer layer in the initial thickness range of 200–300 nm, which was indicated by ADF-STEM. To be specific, the heterointerfaces exhibited a significant effect on the annihilation of c-type (b = <0001>) dislocations, which led to the formation of dislocation loops. The thin inserted layers within the AlGaN buffer layer played a key role in promoting the annihilation of c-type dislocations, while they exerted less influence on a-type (b = 1/3<112¯0>) and (a+c)-type (b = 1/3<112¯3>) dislocations. Within an initial thickness of 200–300 nm in the GaN buffer layer, a-type and (a+c)-type dislocations underwent strong interactions, leading to considerable dislocation annihilation. In addition, the EELS results suggested that the V-shaped pits in the AlN nucleation layer were filled with the AlGaN thin layer with a low Al content.

Low-cycle fatigue behavior and microstructure evolution of ODS steel pipes at high temperatures

Oxide-dispersion-strengthened (ODS) steels are candidate materials for application in advanced nuclear reactors. In this study, the low-cycle fatigue performances of 13Cr-ODS ferritic steel pipes were investigated at 600, 700, and 800 °C. Cyclic softening was observed at high strain amplitudes with an increase in the number of fatigue cycles. However, cyclic hardening appeared first, and then cyclic softening occurred at a low strain amplitude with the increase in the number of fatigue cycles. By comparing the cyclic stress–strain curves and the monotonic stress–strain curves, it was found that cyclic softening occurred regardless of the strain amplitude. The Coffin–Manson and Basquin equations were used to predict the fatigue of the pipes. Microstructure analysis indicated that cyclic softening was induced by the dynamic recovery and recrystallization, which reduced the number of low-angle grain boundaries in the deformed grains by promoting dislocation annihilation and reorganization. A complex multi-layer core–shell structure with a large size (∼500 nm) was observed.

Stress effects on interaction modes and cross-slip annihilation distance of screw dislocation interactions

Phase field microelasticity model is used to study the cross-slip and annihilation of screw dislocations in face-centered cubic crystals, revealing the non-negligible effects of Escaig stress on dislocation interactions. We find that the Escaig stress (in the direction that expands the stacking fault width) promotes the partial dislocation interactions and the Fleischer cross-slip mechanism. In addition to three fundamental dislocation interaction modes, three new dislocation interaction modes emerge with the formation of parallel stacking fault bands. Furthermore, with the increasing of Escaig stress, the critical annihilation distance in dislocation interactions can be appropriately reduced. This research provides novel insights for manipulating strain hardening and understanding the formation of dislocation patterns.

Improved elevated-temperature strength and thermal stability of additive manufactured Al–Ni–Sc–Zr alloys reinforced by cellular structures

Laser powder bed fusion (LPBF) processed Al–Ni alloys typically exhibit cellular structures, contributing to superior strength at ambient temperature as the cell walls can effectively impede dislocation motion during plastic deformation. However, it is still unclear whether cellular structures can lead to excellent elevated-temperature performance. In this study, we conducted a systematic investigation into the elevated-temperature mechanical properties and thermal stability of an LPBF Al–Ni–Sc–Zr alloy with cellular structures. The as-built alloy exhibits a yield strength of 368 MPa at 25℃ and 332 MPa at 250℃, with an exceptional strength retention rate of ∼90 % at 250℃. Moreover, the alloy maintains a hardness of ∼146 HV following exposure at 250℃ for 100 h and shows no apparent decrease despite increasing the exposure time to 500 h. The excellent elevated-temperature performance can be attributed to the cellular structures. The cell walls effectively confine dislocations at elevated temperatures, suppressing the dislocation annihilation and leading to exceptional strength retention. On the other hand, even if the cell walls decompose into nanoparticles after prolonged thermal exposure, the reduction in strength due to the decomposition can be compensated by the strengthening of the nanoparticles, resulting in excellent thermal stability. By studying the influence of the cellular structures on elevated-temperature performance, this study could provide valuable insights into developing high-performance Al alloys for elevated-temperature applications by additive manufacturing.

Revealing and predicting the mechanical and strain hardening behaviors of Cu-Al alloys based on microscopic mechanism

In order to reveal and predict the strain hardening behaviors of metallic materials, mechanical properties of Cu-Al alloys processed by rolling were investigated by tensile tests. The microstructure evolution of Cu-Al alloys processed by rolling were characterized. The true stress-strain curves were fitted by an exponential strain-hardening (ESH) model. Based on the microscopic mechanism of dislocation growth and annihilation, the true stress strain curves were predicted through macroscopic mechanical properties. The strengthening effects on Cu-Al alloys of different methods were analyzed quantitively by hardening exponent z and saturation strength σs which were derived from the ESH model. The fundamental assumptions and physical meaning of parameters have been validated by investigating strain hardening behaviors of Cu-Al alloys. The limited applicable materials and future research of ESH model are also proposed for the further development. This study could provide guidance to understand work-hardening behavior and predict tensile performance of Cu-Al alloys.

Effects of thermal exposure on microstructure deformation and evolution of shot peened nickel-based single crystal superalloy

Microstructure deformation and thermally activated microstructural evolution of nickel-based single crystal (NBSC) superalloy subjected to shot peening (SP) were investigated. SP induced high-density dislocations and activated the octahedral slip system {111} 〈110〉 of NBSC superalloy. Under 0.25 mmA SP intensity, there appeared a severely deformed layer with a depth of 2.5–3.0 μm beneath the surface. High SP intensity resulted in rougher surface morphology, more dense slip bands, deeper cold work layer and larger mean geometrically necessary dislocation (GND) density. However, the deformed microstructure was thermodynamically unstable. Dislocation annihilation and rearrangement were the main thermally activated mechanism of microstructural evolution, and high temperature promoted dislocation movement and consumption. Under 0.25 mmA SP intensity after 24 h thermal exposure, GND density decreased from 4.86 × 1014 m−2 to 2.72 × 1014 m−2, and the depth of cold work layer decreased from 67 μm to 62 μm. In addition, SP treatment provided rapid diffusion channels for alloy elements, while thermal exposure further aggravated element diffusion.

Size effect and underlying microstructural mechanism in cyclic deformation behavior of nickel-based superalloy sheet

The nickel-based GH4169 superalloy is widely used in the aerospace field, which has desirable oxidation resistance and strength in extreme environments. However, the size effect and underlying microstructural mechanism in the cyclic deformation of the GH4169 sheet are still not fully elucidated, which affects the forming accuracy precision of thin-walled components. To this end, the cyclic mechanical properties and microstructural evolution of the GH4169 sheets with different thicknesses and grain sizes were systematically investigated via cyclic shearing tests. The superalloy sheet exhibits obvious early re-yielding, instantaneous softening and permanent softening during the cyclic deformation and these cyclic deformation behaviors are obviously affected by size effect. To rationalize the cyclic mechanical responses and size effect, transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) tests were conducted. The results indicated that the dislocation slip patterns are principally planar array slip and cross slip in the cyclic shearing deformation. Meanwhile, the annihilation of unstable dislocation and the presence of carbides led to the instantaneous softening. Moreover, the change of loading direction, the reduction of texture strength, the proportion of low-angle grain boundary (LAGB), and geometrically necessary dislocation (GND) density together result in the occurrence of permanent softening. Based on the comparative analysis of the microstructure evolution of superalloy sheets with different size factors, the underlying mechanism of size effect on cyclic deformation was clarified. The findings in the research combine the cyclic mechanical response and microstructure evolution closely in the cyclic deformation of superalloy sheets and deepen the understanding of size effect under complex loading paths.