Dislocation Motion Research Articles

Excellent energy-storage performance in BNT-BT lead-free ceramics through optimized electromechanical breakdown

Dielectric capacitors with high recoverable energy storage density (Wrec) are in urgent demand for clean energy technologies. However, their lower breakdown strength (Eb) strongly limits their energy storage performance. We, herein, propose a facile method to enhance Eb by enhancing mechanical strength via second phase modulation. The efficiency of this method is validated in the (1-x)(0.94Na0.5Bi0.5TiO3-0.06BaTiO3)-xSr(Ta0.5Sb0.5)O3 ((BNT-BT)-xSTS, x = 0.1, 0.15, 0.2, 0.25, and 0.3) ceramics. The introduction of Sr(Ta0.5Sb0.5)O3 (STS) increases the relaxor degree, refines grain size, and most importantly, creates a second phase of BiSb2O7, which hinders dislocation movement and improves mechanical strength. Our results show that the breakdown strength strongly depends on the mechanical strength. The highest hardness of 7.42 GPa accompanied by the largest Eb of 620 kV/cm was obtained in (BNT-BT)-0.25STS sample. The sample exhibits the best energy storage properties of a large Wrec = 8.3 J/cm3, a high efficiency of 82.3 %, and excellent temperature/frequency stability. Furthermore, the sample also exhibits good charge/discharge stability and ultra-fast transient discharge time (62.26 ns). This work provides a theoretical guidance for developing lead-free dielectrics with superior energy-storage performance.

Effects of laser non-uniform shock peening on the microstructure and fatigue performance of SUS 304 stainless steel welded joints

Residual tensile stresses and microstructural heterogeneity in welded joints are primary factors that degrade fatigue performance. As an effective post-weld strengthening method, Laser Shock Peening (LSP) can address these issues. Given the personalized demands of different regions, this paper proposes a Laser Non-Uniform Shock Peening (LNUSP) technique. This approach involves applying high pulse energy to the weld zone (WZ) and heat-affected zone (HAZ), and low pulse energy to the base material (BM) to achieve a more uniform residual compressive stress distribution and refined microstructure, thereby improving overall performance and service life. Results indicate that LSP transforms residual tensile stresses on the weld surface into compressive stresses. Compared to original samples, uniformly LSP-treated samples, and locally LSP-treated samples, LNUSP-treated samples exhibit more uniform residual stress and hardness distribution. LSP significantly increases surface hardness, with LNUSP-treated samples achieving a maximum hardness of 308.3 HV. Additionally, LSP induces martensitic transformation in the BM and WZ areas, alters grain orientation, and promotes dislocation movement, resulting in dislocation tangling, dislocation networks, and twin structures, which refine grain size. Consequently, LSP markedly improves joint fatigue performance, delaying fatigue crack propagation. The fatigue life of LNUSP-treated samples is increased by 25.41 % compared to original samples, reaching levels comparable to high-energy large-area impacts. This enhancement is attributed to the uniform residual stress and refined grain structure.

Enhanced creep resistance and microstructure in 7055 Al alloy by in-situ (Al2O3 + ZrB2) nanoparticles and Al3(Er, Zr)

This work put forward a novel approach to enhancing high-temperature creep resistance of 7055 Al alloy via inducing reinforcements including in-situ (Al2O3 + ZrB2) nanoparticles and Al3(Er, Zr). Creep steady-state rates of the (Al2O3 + ZrB2)/7055 Al composites enhanced by synergistic Er, Zr were lower than the (Al2O3 + ZrB2)/7055 Al composites, indicating the values of stress component (n) and apparent activation energy (Q) were 1.05–1.15 and 1.06–1.08 times of (Al2O3 + ZrB2)/7055 Al composites. The dislocation climb mechanism was considered as the dominative creep mechanism because the suitable true stress exponent was 5. The uniform distribution of Al2O3 and Al3(Er, Zr) and their influence on improving the distribution of ZrB2 aggregates gave rise to relative uniform deformation during creep process. The built interface models that induce Al3(Er, Zr) between the nanoparticle/Al interface could lead to a more thermodynamically stable and well-bonded interface. The coupling effect of attachments containing Al2O3, ZrB2 and Al3(Er, Zr) played an enhanced role in terms of vacancy-trapping, precipitate-nucleation, dislocation motion during creep process. The nanoparticles and Al3(Er, Zr) together with nuclear sites containing solute atoms, vacancies and misfits nearby, caused an expansion-inhibiting effect on the helical dislocations. The in-grain misorientation axes (IGMA) concentration of some deformed grains associated with Schmid factor (SF) analysis were related to 〈001〉, 〈111〉 and 〈101〉 direction. The calculation of creep resistance mechanisms was quantified. Additionally, typical intergranular fracture appeared within the composites due to the relatively uniform reinforcements, resulting in the complex crack path.

Unveiling the microstructure evolution and the short-time tensile creep behavior in the CuCrZr alloy

The creep behavior of the CuCrZr alloy, used in the combustion chamber wall of reusable launch vehicles, was investigated at 600-700 K and 150–250 MPa. Results demonstrate significant differences in the creep behavior, microstructure evolution including texture component, dislocation configurations and cavities at 600 K and 700 K, particularly at 250 MPa. At 600 K, the creep life exceeds 400 h, while at 700 K it drops below 0.5 h. The texture components evolved at 600 K-250 MPa are more balanced between hard texture (Brass texture) and soft texture (Goss texture and R-Goss texture). The texture components at 700 K-250 MPa are integrally biased toward the soft texture, implying that it is favorable to accelerate creep strain. The introduced high-density dislocations before creep act as an effective barrier to impede the motion of dislocations at 600 K, inducing long-time dynamic balance of work hardening and recovery softening, while the pre-creep dense dislocations act as an energy supplement at 700 K, promoting that recovery softening dominates the creep process. The dense dislocations can provide energetic support for cavity nucleation.

Role of TiC on the microstructure, tensile property and thermal stability of laser powder bed fusion fabricated AlSi10Mg alloy

The incorporation of ceramic particles is frequently employed to enhance the printability of high-strength aluminum (Al) alloys in laser powder bed fusion (L-PBF). However, their influence on the long-term thermal stability and elevated temperature mechanical properties of the Al alloy is seldom studied. This study aims to address this gap by investigating the role of micron-sized TiC particles in the microstructure, long-term thermal stability, and elevated temperature tensile properties of an L-PBF-fabricated AlSi10Mg alloy. The results indicate that highly dense TiC/AlSi10Mg samples can be fabricated, featuring fine equiaxed grains rather than the coarse columnar grains typically formed in the AlSi10Mg alloy. This columnar-to-equiaxed transition is attributed to the residual TiC and the in-situ formation of Al3Ti particles, which act as nucleating agents. Moreover, the incorporation of TiC significantly enhances the long-term thermal stability of the AlSi10Mg alloy by improving the stability of the eutectic network and retarding the coarsening of Si particles by inhibiting Si diffusion. Compared to the AlSi10Mg counterpart, the TiC/AlSi10Mg composite exhibits a higher hardness retention ratio (56 % versus 45 %) after thermal exposure at 400 °C for 192 h. The residual TiC and in-situ formed Al3Ti have high thermal stability and can impede dislocation motion, thereby contributing to enhanced long-term thermal stability. This study offers insights into the design of L-PBF ceramic-reinforced Al alloys with improved thermal stability for high-temperature applications.

Effect of ultrasonic micro-forging on the microstructure and properties of GH4169 superalloy deposited by laser direct energy deposition

Nickel-based superalloys prepared by laser direct energy deposition (DED) still suffer from microstructure, elemental segregation, and unstable strength, which cannot meet the service requirements and severely limit the application and development of DED-prepared superalloy parts. In this paper, GH4169 specimens with high density and free of defects were prepared by ultrasonic micro-forging assisted laser direct energy deposition (UMT-DED) method. The effects of UMT on the dendritic structure, elemental segregation, and precipitation phases of the alloys prepared by UMT-DED, as well as the densities, hardness, and tensile properties at room and high temperatures were investigated. The contribution of the strengthening mechanism to the yield strength was estimated. The grain refinement mechanism and the deformation mechanism of the Laves phase in GH4169 were discussed. The results show that UMT achieves grain refinement by forming fine crystal strips between the layers. The proportion of fine grain areas increased from 5.9 % to 33.3 %. The plastic deformation introduced by UMT provides external work for dislocation movement to break and fracture part of the Laves phase during plastic deformation, with dislocation cutting of the Laves phase being the main deformation mechanism. The residual stresses was transformed into residual compressive stress. The tensile strength, yield strength, and elongation of the specimens at 25 °C were increased by 4.23 %, 6.42 %, and 23.67 % respectively.

In-situ electron channeling contrast imaging of local deformation behavior of lath martensite in low-carbon-steel

In-situ tensile Electron Channeling Contrast Imaging (ECCI) observations were performed on low-carbon steel lath martensite to elucidate its plastic deformation mechanisms under tensile loading. Two key deformation processes were identified through the direct observation of dislocation activity: intra-lath crystallographic slip and boundary sliding. The initial dislocation movement signifies the onset of microscopic yielding in lath martensite. As the applied stress increases, macroscopic yielding occurs when dislocations overcome internal barriers like high-dislocation-density walls within the laths. Out-of-lath-plane slip systems exhibit a strong interaction with the lath boundary, hindering their activity after initial small plastic deformation. In contrast, in-lath-plane slip systems predominantly contribute to plastic deformation. After the work hardening triggered by these slip systems, boundary sliding takes over the plastic deformation. The boundary sliding was found to accompany a rapid movement of the dislocation network adjacent to the boundary.

Microstructural evolution and mechanical behaviors of rock salt in energy storage: A molecular dynamics approach

The microstructure of rock salt significantly influences its macroscopic mechanical behaviors and deformation phenomena. Understanding the deformation and failure characteristics of rock salt at multiple scales is crucial for the secure and efficient functioning of energy storage in salt caverns. Although the macroscopic behaviors of rock salt are well understood, the microstructural changes occurring under uniaxial compression have not been thoroughly investigated. This study aims to investigate the evolving laws of rock salt microstructure and mechanical properties during the damage process, thereby providing a fundamental understanding of the underlying mechanism. To achieve this, a series of characterization tests on rock salt were conducted to study its microstructure and defects. Building on this, molecular dynamics simulations were utilized in rock mechanics to elucidate the mechanical behaviors, structural evolutions, and dislocation motions of rock salt during the damage process. The rock salt obtained from Qianjiang, China, is a polycrystalline material with an average subgrain size of 46 nm and an average dislocation density of 4.76 × 10−6 1/Å2. Uniaxial compression of single crystal NaCl exhibits three stages: elastic compression, rapid dislocation multiplication, and forest hardening. Scattered dislocations and isolated dislocation tangles trigger further plastic slippage, leading to decreased strength. Later, the formation of a dislocation cell network hinders further slip and reinforces the strength of rock salt. Plastic deformation is found to be a dominant factor in grain refinement and the formation of polycrystalline structures. Intergranular cracking results from cross-patterned dislocation lines on grain boundaries, which become vulnerable areas of the structure. This study describes the evolutionary process of rock salt microstructures and mechanical properties, identifies the micro-destruction mechanisms during the damage process at the ionic level, and provides insights into the micro-mechanical behavior of rock salt and for the design and stability assessment of underground energy storage facilities.

Microstructure and Mechanical Properties of GH3535 Superalloy Joints Using Electron Beam Welding

To achieve low‐stress and small‐deformation welding of thin‐walled structures for the fourth‐generation nuclear power plants, GH3535 superalloy is welded to itself using electron beam welding (EBW). The microstructure and mechanical properties of the GH3535 superalloy joints are characterized by scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), hardness, and tensile tests. A macro‐defect‐free GH3535 superalloy joint could be obtained using 22 mA welding beam current under 65 kV welding voltage with 300 mm min−1 welding speed. Different sub‐grain solidification morphologies are observed at the weld zone because of the influence of composition undercooling on the solidification process of the weld pool. The M6C type carbides formed in the weld zone and heat‐affected zone (HAZ) take place eutectic transformation under the action of thermal cycles. The eutectic transformation of M6C has no significant effect on the strength of HAZ, whereas this phenomenon is beneficial to hinder the dislocation movement in the weld zone, thereby promoting the tensile strength of the joint. The Vickers hardness of weld zone and HAZ are 250.6 and 252 HV0.5, remarkably lower than that of the base metal (261.2 HV0.5), due to the grain coarsening in those zones. The tensile strength of the GH3535 superalloy joint reached 791 MPa, ≈93.5% that of the base metal, with the fracture occurred in the weld zone.

Learning dislocation dynamics mobility laws from large-scale MD simulations

By dispensing with all the atoms and only focusing on dislocation lines, the computational method of Discrete Dislocation Dynamics (DDD) gains greatly over Molecular Dynamics (MD) in simulation efficiency of metal plasticity. But whereas in MD dislocations follow natural dynamics of atomic motion, DDD must rely on a dislocation mobility function to prescribe how a dislocation line should respond to the driving force exerted on it. However, reflecting our still incomplete understanding of ways in which dislocations move, mobility functions presently employed in DDD simulations entail simplifications and approximations of limited or, worse still, unknown accuracy and applicability. Here we introduce a data-driven approach in which the dislocation mobility function is modeled as a graph neural network (GNN) trained on large-scale MD simulations of crystal plasticity. We apply our proposed approach to predicting plastic strength of body-centered-cubic (BCC) metal tungsten and show that, once implemented in a DDD model, our GNN dislocation mobility function accurately reproduces the challenging tension/compression asymmetry of plastic flow observed both in ground-truth MD simulations and in experiment. Furthermore, subsequently validated by MD simulations, the same function accurately predicts plastic response of tungsten under conditions not previously seen in training. By demonstrating its ability to learn relevant physics of dislocation motion, our DDD+ML approach opens a promising avenue to bringing fidelity of DDD models closer in line with direct MD simulations at a much reduced computational cost.

The effects of forging strategies on microstructures and mechanical properties of a Ti–5Al–5Mo–5V–1Cr–1Fe near β-titanium alloy

The present study systematically investigated the influence of forging strategies on the microstructures and deformation behaviors of a Ti–5Al–5Mo–5V–1Cr–1Fe (Ti-55511) near β-titanium alloy. Microstructure analysis revealed that the as-cast alloy exhibited a basket-weave structure predominantly comprised of lamellar α phase. Three different forging methods were used to adjust the morphology of the primary α phase during the final two forging steps: 1) dual-phase (825 °C) + dual-phase forging (DD sample), 2) dual-phase + β single-phase (900 °C) forging (DS sample), and 3) β single-phase + dual-phase forging (SD sample). After standard heat treatment, the DD samples exhibited equiaxed αp due to sufficient deformation in dual-phase region, while DS samples transformed into lamellar shape due to αp reforming during the cooling process of the final β single-phase forging. In contrast, the SD samples displayed short-rod αp due to reforming in the single-phase region and insufficient deformation in the dual-phase region. All these αp and βt phases can hinder dislocation motion to enhance the strength. The DS sample exhibited the best combination of strength, ductility, and impact toughness. During tensile deformation, planar dislocation slip and {101‾1} twinning were dominant in the αp, while wave slip occurred in the βt phase. The fracture behavior transitioned from brittle to ductile after forging and heat treatment. Overall, this study offers a practical and effective method for optimizing the mechanical properties of the Ti-55511 alloy.

Martensite-austenite transformation during dual-stage tempering and work-hardening characteristics of Mn–Ni–Mo steel

The reactor pressure vessel (RPV) in nuclear power plants is mainly made from Mn–Ni–Mo steel. After quenching, this steel's microstructure features martensite-austenite (M-A) islands. Tempering above 600 °C poses a challenge, as these islands can transform into harmful rod-like Fe3C precipitates, adversely affecting the RPV's tensile properties and structural integrity. This study explores the effects of M-A transformation during dual-stage tempering on the tensile properties of Mn–Ni–Mo steel, focusing on two austenite grain sizes (5 μm and 47 μm). The dual-stage tempering included a 2-h pre-tempering phase at temperatures between 200 °C and 500 °C, followed by a 3-h final tempering stage at 650 °C. Results showed that M-A transformation is temperature-dependent. At 200 °C, partial transformation led to rod-like Fe3C precipitates during final tempering, similar to direct tempering at 650 °C, reducing the critical stress for micro-crack nucleation and compromising tensile properties. Pre-tempering at 350 °C produced fine, aggregated precipitates that improved tensile strength by inhibiting dislocation movement. Conversely, pre-tempering at 500 °C resulted in coarser, more dispersed spherical precipitates. The Kocks-Mecking plot indicated that the 350 °C pre-tempered sample had the highest strain hardening capacity, with the strain hardening rate curve farthest from the origin and the shallowest slope during stage IV hardening. However, the 500 °C pre-tempered samples showed the best combination of elongation and strength.

Preparation and research on the structure and properties of high damping Mn-Cu alloy

With the widespread application of Mn-Cu alloys with high damping capacity in industry, higher requirements of mechanical were properties proposed. In this work, the microstructure, damping properties, and mechanical properties of Mn-Cu alloys prepared by three different heat treatment processes were investigated. The results showed that the Mn-Cu alloy had good comprehensive properties after forging, rolling, solid solution at 850 °C for 2.5 h, and aging for 8 h. The yield strength, tensile strength and specific damping capacity were 336 MPa, 620 MPa, and 44 %, respectively. It was found that after aging treatment, the Mn-Cu alloy occurred spinodal decomposition and martensitic transformation, resulting in Mn-rich regions and lattice distortion that prevented dislocation movement and improved mechanical properties. At the same time, martensitic transformation promoted the formation of high-density twins and improved damping capacity. This work could provide experimental guidance for the production of Mn-Cu alloys with good comprehensive properties.

Microstructure-Based Flow Stress Model to Predict Machinability of Inconel 718.

Due to its exceptional mechanical and chemical properties at high temperatures, Inconel 718 is extensively utilized in industries such as aerospace, aviation, and marine. Investigating the flow behavior of Inconel 718 under high strain rates and high temperatures is vital for comprehending the dynamic characteristics of the material in manufacturing processes. This paper introduces a physics-based constitutive model that accounts for dislocation motion and its density evolution, capable of simulating the plastic behavior of Inconel 718 during large strain deformations caused by machining processes. Utilizing a microstructure-based flow stress model, the machinability of Inconel 718 in terms of cutting forces and temperatures is quantitatively predicted and compared with results from orthogonal cutting experiments. The model's predictive precision, with a margin of error between 5 and 8%, ensures reliable consistency and enhances our comprehension of the high-speed machining dynamics of Inconel 718 components.

Anomalous temperature-dependent strength of copper alloy manufactured by laser-beam powder bed fusion

This study reports an anomalous temperature-dependent tensile behavior of laser-beam powder bed fusion (PBF-LB) processed Cu–Cr–Zr alloy. The yield strength of the alloy initially decreases as the temperature increases to 200±5 MPa and then increases to 350±11 MPa at 500°C before reducing to 234±6 MPa at 600°C. The microstructure consists of elongated Cu grains with a high concentration of Cr solute (∼1 mass%), resulting from rapid solidification during the PBF-LB process. Transmission electron microscopy for the specimens deformed at 500°C revealed the presence of numerous nanoscale Cr-rich particles embedded inside the supersaturated solid solution of the Cu matrix. Nanoscale particles can act as barriers to dislocation motion, leading to an increase in internal stress during plastic deformation at elevated temperatures. This work provides the high potential of post heat treatments for achieving superior mechanical performance using high solute supersaturation formed by the PBF-LB process.

Microstructure evolution and mechanical properties of a lamellar AlCoCrFeNi2.1 eutectic high-entropy alloy processed by high-pressure torsion

High-pressure torsion (HPT) was applied to a lamellar eutectic high-entropy alloy (EHEA) to study the effect of severe plastic deformation (SPD) on the composite structure and mechanical properties. We found that the existence of multiple phases affects defect distribution and the fragmentation process during HPT. Structural evolution shows orientation dependence with respect to the shear plane, which finally leads to a refined multiphase structure with nanograins and vortex clusters after a shear strain γ of 24. In nanograins, dislocation-mediated deformation prevails. The high density of grain boundaries, forest dislocations, and the generation of deformation twins restrict dislocation movement. As a result, an EHEA with a yield strength of 1.75 GPa, an excellent tensile strength of 2.20 GPa, and an appreciable failure strain of 5 % is realized. Our results demonstrate that the HPT deformation process of the lamellar EHEA is significantly affected by the structure. SPD on the multiphase structure is a suitable route for designing high-strength yet ductile alloys.

A New Cu-Alloyed Precipitation-Hardening Hot-Work Tool Steel

Abstract Hot-work tools for hot forming processes, such as die forging, are subject to complex loads that can often lead to premature tool failure. The present study attempts to reduce the main cause of failure of high-speed hot working tools, i. e., wear due to surface breakdown, with the aid of damage-tolerant microstructures. For this purpose, a hot-work tool steel is specifically alloyed with copper in order to form copper precipitations during the subsequent heat treatment, which increase the strength of the material and contribute to the reduction of dislocation movements. The aim is to close the property gap between the standardized direct-hardening and secondary-hardening hot-work tool steels.

Effects of stabilized heat treatment on stress rupture properties of high Si-bearing austenitic stainless steel weld metal

The 15Cr-9Ni-Nb austenitic stainless steel weld metal with a Si content of 3.5 wt% was prepared via gas tungsten arc welding and then held at 900 °C for 3 h for the stabilized heat treatment (SHT). The stress rupture properties of the as-welded (AW) and SHT weld metals at 550 °C were evaluated via the Larson-Miller parameter. The microstructure evolution was discussed during the 550 °C stress rupture process. The coarse σ-phase and relatively fine G-phase formed on the δ-ferrite during aging at 550 °C. In the AW weld metal, the continuous δ-ferrite with a large amount of coarse σ-phase led to the formation and expansion of cracks during the stress rupture process, which accelerated the eventual rupture and damaged the stress rupture properties. The SHT decreased the δ-ferrite content and formed a large amount of nanoscale NbC precipitated in the matrix. The decreased δ-ferrite content avoided the rapid formation and expansion of cracks and the nanoscale NbC blocked the dislocation movement during the stress rupture process, which improved the stress rupture properties.

The dual effects of phosphorus on the hot deformation behavior in an as-cast Ni-Fe-Cr based alloy

The effect of phosphorus on dynamic recrystallization (DRX) and hot deformation behavior of an as-cast Ni-Fe-Cr based was investigated by performing hot compression deformation. The results indicate that phosphorus can play dual roles during hot deformation. The phosphorus dissolving in matrix and segregating to the dislocation can hinder the dislocation motion and DRX nucleation, which acts as the solute drag effect. And phosphorus-addition can increase the size and number of MC carbides. MC carbides increase the local dislocation density around them and promote the DRX nucleation and exhibit the particle stimulated nucleation effect. Influenced by the dual effects of phosphorus, the DRX fraction and grain size decrease first and then increase with the increase of phosphorus content. The solute drag effect of the substitutional phosphorus atoms on dislocation motion decreases the DRX nucleation rate and increase the possibility of instability at low deformation temperature in phosphorus-addition alloys. In the early stage of deformation, more MC carbides in the high-phosphorus alloy hinder the motion of dislocation and result in the stress concentration, which leads to the intergranular cracks at high temperature and high strain rate.

Insight into microstructure evolution of in-situ ZrB2np/AA6111 composites with both excellent room and high temperature properties

With rapid development of aerospace and automobile, the service condition of structural components becomes increasingly harsh, diverse and high mechanical properties are urgently needed. In this work, an advanced in-situ ZrB2np/AA6111 composite with excellent room and high-temperature properties was prepared via Al-Zr-B reaction system. The microstructure revealed that ZrB2 nanoparticles with regular polygonal shapes ranged from 10 to 150 nm approximately, and a good α-Al/ZrB2 interface bonding was confirmed. The agglomeration of ZrB2 particles was significantly weakened during hot extrusion. Due to the inhibiting effect on dislocation motion and pinning effect on grain boundaries induced by ZrB2 particles, fine recrystallized grains were obtained in ZrB2/AA6111 composites after extrusion. Compared with AA6111 matrix alloy, the yield strength, ultimate tensile strength and elongation of AA6111-3ZrB2 composites at room temperature and 300 °C were increased by 18.9 %, 12.7 %, 50.0 % and 34.1 %, 22.4 %, 42.4 %, respectively. An excellent room and high-temperature strengthening effect was simultaneously achieved assisted by ZrB2 particles. The improvement of room-temperature properties was mainly owed to the Grain refinement and Orowan bypassing strengthening mechanisms. The good high-temperature properties mainly relied on the superior coarsening resistance of ZrB2 particles which could fully played the Orowan strengthening effect and well protected the grain boundaries at high temperature. This work provides a feasible approach to design high-performance aluminum alloys and is beneficial for the further development of modern industries.