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Internal strain and its distribution within the crystal lattice play crucial roles in modulating dislocation activities, thereby affecting mechanical properties of materials. Through the synergistic application of integrated differential phase contrast, in situ transmission electron microscopy characterizations, and computational simulations, a method is unveiled for homogenizing dislocation pinning in NiCoCr multi‐principal element alloy (MPEA) through the introduction of a high concentration of oxygen atoms with high diffusion mobility. The doping of massive oxygen atoms creates a high density of strong local pinning points for dislocation motion. Notably, oxygen interstitials exhibit remarkable diffusion and mobility across different octahedral and tetrahedral sites within the distorted crystal lattice of NiCoCrO alloy, even at room temperature. The capability allows for the release of severe stress concentrations arising from dislocation entanglement and the establishment of new strong local pinning points at alternative locations in a uniform way, enabling the material with high strength and outstanding deformability. These findings suggest that interstitial atoms can exhibit significant mobility, even at ambient temperature, in complex MPEAs with spreading lattice distortion, opening new possibilities for dislocation engineering. [ABSTRACT FROM AUTHOR]

High-content interstitial carbon addition has been proven to regulate the dislocation behavior in high-entropy alloys (HEAs) during deformation, thereby overcoming the strength-ductility trade-off, but its exact mechanism is still unclear. In this work, the dislocation generation at incipient plasticity in a high carbon-added Al10(FeNiCoMn)90 HEA was investigated by instrumented nanoindentation. It was found that interstitial carbon addition could increase the atomic cohesion, which induced a higher maximum shear stress and activation volume required to trigger the incipient plasticity related to dislocation nucleation. However, it decreased the activation energy for a critical dislocation loop during deformation, increasing the dislocation density and facilitating dislocations multiplication. Consequently, the plastic zone underneath the indenter expanded and pop-in events occurred in the carbon-added HEAs. These findings contribute to understanding how interstitial carbon regulates the dislocation behavior in HEAs.

Significantly enhancing the mechanical properties of Mg–Bi base series alloys is of great significance for their wider application prospects. Here, a new Mg–3Bi–1Ca (BX31, wt.%) alloy showing high tensile yield strength of ∼397.1 MPa, high ultimate tensile strength of ∼416.2 MPa and acceptable elongation of ∼8% was fabricated by low-temperature extrusion. The analysis of microstructure showed that the enhanced yield strength was attributed to the ultra-fine grain structure (∼0.84 μm), nano-scale second phases including Mg2Bi2Ca and Mg2Ca, and Ca segregation at grain boundaries. In addition, a weak basal fiber texture to some extent provided more basal slip activities, guaranteeing the suitable ductility. The dynamic recrystallization and dislocation behavior of the BX31 billet during extrusion were also systematically observed and discussed to reveal the formation mechanisms of the ultra-fine grain structure with the weak basal fiber texture in the as-extruded BX31 alloy. As mentioned above, this work offers a new insight to obtain high-performance Mg–Bi base series alloys.

Multilayer laminates with a 304 stainless steel as surface layers and with a low C steel and a medium-Mn steel as alternating central layers have been developed in the present study. The number of interfaces and the layer thickness have been varied, while maintaining the similar microstructures for each layer. The uniform elongation is observed to increase from 10.1% to 37.8%, and the product of strength and elongation is found to increase from 13.6 GPa·% to 36.8 GPa·% monotonically with decreasing layer thickness in multilayer laminates, while the yield stress remains almost constant. Firstly, deformation-induced martensite transformation is significantly promoted with decreasing layer thickness. Secondly, the more interfaces can induce the accumulation of higher density of geometrically necessary dislocations, resulting in better mechanical properties. Lastly, the main cracks nucleate and propagate at the interfaces of low C steel layers and medium Mn steel layers, thus the samples with smaller layer thickness have more interfaces and require more energy consumption during the micro-fracture process, resulting in better tensile performance.

In this research, a repetitive bending and straightening process was carried out on the Ti–3Al–4Cr–Mo alloy for 20 passes. The changes in mechanical properties of the titanium alloy before and after repetitive bending and annealing were studied. The microstructure evolution and deformation mechanism were analyzed. The results show that after the repetitive bending and straightening process, the microstructure of the Ti–3Al–4Cr–Mo alloy is obviously refined, and, simultaneously, the yield strength is significantly improved. After annealing at 850 °C, the plastic ductility of the material was improved. The combined effects of grain refinement and dislocation behavior were the main reasons for the improvement in mechanical properties of the Ti–3Al–4Cr–Mo alloy. Twinning rarely occurred during plastic deformation of the Ti–3Al–4Cr–Mo alloy. The fine grains strongly inhibited the formation of twins. In addition, a small amount of α to β phase transformation was observed during the repetitive bending and straightening process of the material, which may have been induced by strain accumulation. [ABSTRACT FROM AUTHOR]

This study presents a design strategy to enhance the high-temperature creep resistance of Ni-based superalloys. This strategy focuses on two principles: (1) minimizing the dimensions of γ/γ′ interfaces and γ channels by reducing the size of the γ′ phase; (2) key alloy composition control to strengthen the heterostructure γ/γ′ interfaces. This strategy proved very effective by the designed three superalloys’ prolonged creep lives. An alloy exhibits ultra-long creep life by 388 h at 1100°C/137 MPa, which runs at the highest level among those alloys without Ru addition. With Ru addition, an alloy that lasted for 748 h with a creep strain of ~6% at 1110°C/137 MPa is developed. This study provides a new route of high-temperature creep lives through heterostructure interfacial design with size effects and key alloying elements.

This study investigates the impact of machining parameters on forces, thermal dynamics, dislocation behavior, and crystalline structure changes in FeCoNiCrAl high-entropy alloys during nanoscale material removal using molecular dynamics (MD) simulations. Utilizing Embedded Atom Method (EAM) and Tersoff interaction potentials, simulations were performed at cryogenic (73 K) and room (293 K) temperatures. Novel findings reveal that at 73 K, cutting velocities below 200 m/s produced the highest forces along the [001] direction, whereas at 200 m/s, the peak force shifted to [100]. Increasing velocity decreased the force along [001], while [100] exhibited an inverse relationship. At 293 K, the force remained highest along [001] across all velocities. Notably, forces at 73 K were 1.82, 1.79, and 1.58 times higher than at 293 K for velocities of 100, 200, and 300 m/s, respectively. Dislocation density, particularly 1/6<112> (Shockley) dislocations, peaked under all machining conditions, with a slight initial decrease at 293 K before a significant drop with higher cutting speeds. At 293 K and a cutting speed of 100 m/s, dislocation densities for depths of 5, 10, 15, and 20 Å were approximately 1.04, 1.54, 1.17, and 1.14 times greater than those at 73 K, respectively. • The effect of temperature on the machining mechanism of nano-cutting is studied. • The synergistic effect of temperature effect and thermodynamic coupling mechanism is studied. • The influence of temperature effect on the evolution of high-entropy alloy nanocutting dislocations was studied. [ABSTRACT FROM AUTHOR]

Refractory complex concentrated alloys (RCCAs) have emerged as a promising class of structural materials, demonstrating exceptional mechanical performance in aggressive environments. However, the complex atomic environments, significant lattice distortion, and vast compositional space of RCCAs present challenges to understanding the mechanisms that govern structure-property relationships. In this study, we explore the dislocation mechanisms in three model quaternary RCCAs, namely Mo25Nb10Ta25W40 (at. %), Mo25Nb25Ta25W25, and Mo25Nb40Ta25W10 using large-scale atomistic simulations and machine learning based Spectral Neighbor Analysis Potential. Our atomistic simulations examine how the chemical composition and local ordering influence the mobility of both edge and screw dislocations, and how lattice distortion and diffuse anti-phase boundary energy (DAPBE) affect dislocation behaviors during nanostructural evolution. Notably, with the increase in Nb concentration in the model RCCAs, both DAPBE and lattice distortion are simultaneously enhanced as the chemical short-range order (CSRO) evolves into nanoscale B2 precipitates. This evolution results in high lattice distortion due to the lattice mismatch between B2 precipitates and the random matrix. Consequently, B2 nanoprecipitates provide a stronger pinning effect, hindering edge dislocation motion while promoting cross-slip of screw dislocations, leading to a reduced screw-to-edge ratio in slip resistance and mobility discrepancy. These findings offer valuable insights into dislocation behaviors and interactions with ordered precipitates, highlighting the importance of exploring non-equiatomic compositions and advancing beyond CSRO in RCCAs. This study has implications for optimizing alloy compositions and processing methods for superior performance in aggressive environments. [Display omitted] [ABSTRACT FROM AUTHOR]

Interphase boundary plays a dominant role in the mechanical properties of dual-phase titanium alloys, and therefore mechanistic understandings of the effect of phase boundary on the plasticity is significant to tailor the microstructure for desired performance. In this study, compression tests were conducted on the Ti-6Al-4V micro-pillars with a dual-phase lamellar structure and designated crystallographic orientation of pillars and the number of interphase boundaries in the pillars were achieved by elaborate processing, to reveal the interphase boundary and its quantity on the dislocation behavior and critical resolved shear stress (CRSS) of the alloy. Transmission electron microscopy was employed to characterize the dislocations and their interplay with interphase boundaries during compression. It is found that in the pillars oriented for prismatic 〈a 3 〉 slip, which represents the hard mode for dislocation transmission through the α/β phase boundary, the strain distribution in the pillars was significantly delocalized by introducing interphase boundaries. More dislocation slip bands are generated owing to the strengthening effect from the interphase boundaries during compression, which results in a more homogeneous strain distribution. Moreover, quantitative analyses of the contribution of interphase boundaries on the CRSS were performed, and the interphase strength for prismatic 〈a 3 〉 slip was experimentally determined to be ∼ 50 MPa. The effect of pillar size on the CRSS value was also assessed. Although the CRSS of the pillars increases when their size decreases, the size dependency becomes much less pronounced when more interphase boundaries are introduced into the pillars. The underlying mechanisms for these phenomena were discussed based on the experimental results and finite element modeling. These results provide some new insights into the plasticity and strengthening mechanism of Ti-6Al-4V alloy, which are also applicable to other dual-phase titanium alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

The origin of the indentation size effect has been extensively researched over the last three decades, following the establishment of nanoindentation as a broadly used small-scale mechanical testing technique that enables hardness measurements at submicrometer scales. However, a mechanistic understanding of the indentation size effect based on direct experimental observations at the dislocation level remains limited due to difficulties in observing and quantifying the dislocation structures that form underneath indents using conventional microscopy techniques. Here, we employ precession electron beam diffraction microscopy to "look beneath the surface," revealing the dislocation characteristics (e.g., distribution and total length) as a function of indentation depth for a single crystal of nickel. At smaller depths, individual dislocation lines can be resolved, and the dislocation distribution is quite diffuse. The indentation size effect deviates from the Nix-Gao model and is controlled by dislocation source starvation, as the dislocations are very mobile and glide away from the indented zone, leaving behind a relatively low dislocation density in the plastically deformed volume. At larger depths, dislocations become highly entangled and self-arrange to form subgrain boundaries. In this depth range, the Nix-Gao model provides a rational description because the entanglements and subgrain boundaries effectively confine dislocation movement to a small hemispherical volume beneath the contact impression, leading to dislocation interaction hardening. The work highlights the critical role of dislocation structural development in the small-scale mechanistic transition in indentation size effect and its importance in understanding the plastic deformation of materials at the submicron scale. [ABSTRACT FROM AUTHOR]

By investigating the relation between microstructure evolution and tensile responses of a BTi6431S alloy at room temperature and 700 °C, this study finds an annealing-induced ductile-to-brittle transition phenomenon in the near-α titanium alloy. The high strength of the as-received BTi6431S alloy is rooted in the high dislocation density and collective dislocation behavior in the α phase. However, after annealing at 650 °C, the alloy exhibits brittle fracture at room temperature. Using multiscale characterization methods, we attribute this phenomenon to the presence of brittle precipitates and the change of dislocation structures near the surface of BTi6431S titanium alloy. Silicides are generated in the vicinity of the surface region due to the relatively high dislocation density on the surface after the rolling process. The room temperature ductility can be restored by removing the near-surface area. When the alloy is tested at 700 °C, most of the near-surface brittle silicides and dislocations can be eliminated, because the deformation is mainly affected by dislocation behavior and dynamic recrystallization at high temperature. The findings in this study can advance the understanding of alloying effects on the mechanical behavior of high-temperature titanium alloy. The results suggest that inhibiting silicide formation and controlling the environmental temperature during service are potential approaches for maintaining the mechanical performance of this near-α titanium alloy. [ABSTRACT FROM AUTHOR]

Refractory multi-principal element alloys (RMPEAs) are promising candidate materials for use in high temperature structural applications and other extreme environments. Recent experiments and simulations have highlighted the unusual deformation behavior of an equiatomic MoNbTi alloy, which exhibits slip on higher order planes and sluggish edge dislocation mobility. In this work, we utilize micropillar compression and postmortem transmission electron microscopy to elucidate the orientation dependent deformation behavior of MoNbTi. Our results suggest that deformation in this system is largely mediated by kink-migration of screw dislocations, as evidenced by the presence of long screw dislocations and significant dislocation debris in postmortem observations, and the absence of twinning/anti-twinning asymmetry. Moreover, we report an unusual orientation dependence of the yield strength, owing to the high stress required to facilitate slip on {110} type planes. These results further demonstrate the unconventional plasticity in BCC RMPEAs, and provide experimental verification that kink-migration is the rate limiting feature in this alloy at low temperatures. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract Large scale, atomistic simulations of the core structure and mobility of ½[111] screw, edge and mixed dislocations in ternary multicomponent alloys (e.g. High Entropy alloys), NbTiZr, Nb 1.5 TiZr 0.5 and Nb 0.5 TiZr 1.5 , are presented. The core structure of ½[111] screw dislocations continuously varies from compact to 3-fold with decreasing Nb content. The screw dislocation core structures in NbTiZr and Nb 1.5 TiZr 0.5 are calculated using Embedded Atom Potentials (Johnson-Zhou) and compared with first-principles calculations of the screw dislocation in a quasi-random structure. In both simulations the dislocation core spreads on different (110) glide planes as the composition varies along the dislocation line in stoichiometric NbTiZr. The Nb-rich composition Nb 1.5 TiZr 0.5 shows a compact core with very little core structure variation along the dislocation line in both First Principles and atomistic simulations. The screw dislocation deposits interstitial and vacancy dipole debris as it moves under stress. Average solute-dislocation core interaction energies in NbTiZr, Nb 1.5 TiZr 0.5 and Nb 0.5 TiZr 1.5 are derived from the average interatomic potential derived for each of the three systems. The interaction energies are used to determine the critical stress for the motion of ½[111] screw dislocations in the three systems as a function of temperature using the Suzuki model of kink migration controlled mobility developed for concentrated BCC random alloys. This analysis shows that the relatively high barrier for kink migration caused by fluctuations in solute concentration along the screw dislocation line and the dipole dragging stress associated with the screw dislocation motion results in a shallow fall-off of critical stress with temperature in these alloys as compared to simple BCC metals. Finally, the screw dislocation to edge and mixed dislocation critical stress ratio in NbTiZr are shown to be low, ∼2 at 5 K, in contrast to simple BCC metals, where it could be as high as 100–1000. Graphical abstract Fig.: A comparison of Suzuki model results, without (a) and with (b) interstitials at an applied strain rate of 0.001/s with experimental yield stress data as a function of temperature in stoichiometric NbTiZr. Image 1 [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • A bimodal microstructure with nanocrystalline network separating each micrograin. • Reversible motion of fatigue dislocations leads to fatigue resistance metals. • Nanocrystalline network effectively "absorbs" the dislocation stress. Abstract We propose a new bimodal microstructure with promising potential for developing fatigue-resistant metals taking advantage of molecular dynamics (MD) simulations. In the proposed bimodal microstructure, each micrometer-sized grain is separated by a network of ultrafine nanocrystalline structure. Then, MD simulations of low cycle fatigue deformation of the proposed bimodal microstructure were performed. Modeling results suggest that most of the dislocations generated by the repeated plastic deformation (or the low cycle fatigue) are absorbed by the nanocrystalline network. No significant accumulation of dislocations is observed within the micrograins. This work suggests a potential path to achieve high fatigue resistance in metals as compared with their microcrystalline counterparts, which would be useful in nearly every engineering setting. [ABSTRACT FROM AUTHOR]

Microcrystalline Ni-W alloy layers with different thickness were electroplated on the surface of Cu substrate. The mechanical properties of coating samples with different thickness were analyzed. The microstructure and fracture morphology between the substrate and the coating were observed by SEM (scanning electron microscopy) and EDS (energy dispersion spectrum). The results show that the coating thickness increases linearly from 0.031 μ m to 7.77 μ m with the increase of electroplating time. With the Ni-W coating thickness increasing, the microstructure of fracture changes from cross cracks to straight cracks, the number of cracks per unit area decreases, and the crack spacing increases from 0.79 ± 0.35 μ m to 153.56 ± 35.16 μ m. The strength and plasticity of samples with Ni-W coating are higher than those of Cu substrate film because of the restriction in dislocation movement and surface work hardening. At the same time, the coating cracks hinder the dislocation movement, absorb energy and restrain the crack growth. When the coating breaks, the dislocation slip behavior will change into plastic deformation, and there is a synergistic mechanism of interface strengthening between the substrate and the coating.

Press forming and paint curing are necessary in automobile production. However, strain and baking effects induced by these processes cannot be ignored in steel plates. Previously researchers have focused on the baking hardening effect but neglected the embrittlement issue. Baking embrittlement occurs when the elongation of ferrite-martensite dual-phase (DP) steel decreases significantly after strain-baking. In this work, the baking embrittlement value (ΔE) was defined for the first time as an evaluation index, and the baking embrittlement mechanism of DP steels was clarified. The baking embrittlement process of DP steels was divided into two stages. The first stage consisted of dislocations pinned by C clusters or low-temperature carbide, while the second stage was the tempering effect of martensite. Furthermore, the volume contraction of martensite led to the release of internal stress, resulting in an increase in the critical stress for dislocation movement. Both processes reduced dislocation mobility. In addition, by calculating the geometrically necessary dislocation (GND) densities before and after strain-baking, we found that the multiplication ability of dislocations was weakened. • Baking embrittlement value was defined for the first time as an evaluation index. • The baking embrittlement mechanism of dual-phase steel was clarified. • Two stages of baking embrittlement process reduced the mobility of dislocations. • Strain-baking weakened the multiplication ability of dislocations. [ABSTRACT FROM AUTHOR]

In this work, molecular statics and molecular dynamics simulations of a/2<110> dislocation behavior for a model FCC Co 30 Fe 16.67 Ni 36.67 Ti 16.67 alloy are discussed. It is shown that the single FCC phase is elastically stable in this alloy. Local stacking fault energies for the FCC alloy are determined as a function of average composition. The core structure of a/2<110> screw and edge dislocations in the FCC Co 30 Fe 16.67 Ni 36.67 Ti 16.67 alloy is shown to be planar with significant variations in the Shockley partial splitting along the dislocation line (factor of ∼3) due to concentration fluctuations. The correlation lengths for dislocation line fluctuations in this alloy are determined and discussed. The critical stress to move both a/2<110> screw and edge dislocations at 0 K in the model FCC Co 30 Fe 16.67 Ni 36.67 Ti 16.67 alloy is of the order of 0.0025–0.005μ, where μ is the (111) shear modulus, and is significantly higher than that of pure FCC Ni. Molecular dynamics simulation results on the critical stress to move a/2<110> screw and edge dislocations in the model FCC concentrated solid solution alloy show that it decreases with increasing temperature, similar to solid-solution strengthened FCC metals. These molecular dynamics simulation results are in reasonable agreement with experimental tensile yield strength data for an analogous FCC concentrated solid solution alloy. It is also shown that local fluctuations in the concentration of solutes has a strong effect on the effective cross-slip activation energy of screw dislocations in the random alloy. [ABSTRACT FROM AUTHOR]

The effects of crystal orientation on the plastic deformation behavior for typical CoCrNi medium entropy alloy under tensile and compressive loading were investigated by molecular dynamics simulations. The Lomer-Cottrell (L-C) structure and parallel hexagonal close packed (HCP) bands are produced in the [111] and [1 1 ̅ 0] orientations at the initial stage of tensile deformation, respectively. However, perfect dislocations and two types of non-slip dislocations (Hirth and Stair-rod) are generated in the [111] and [1 1 ̅ 0] orientations during compressive deformation, respectively. The L-C structure produced by tensile loading in the [111] orientation and Hirth/Stair-rod dislocations produced in the [1 1 ̅ 0] orientation under compression act as a dislocation pinning and limit the continued glide of the dislocations. The dislocation reactions under various conditions were analyzed, which is helpful to understand the plastic deformation mechanism of CoCrNi alloy. • Dislocation nucleation is preferentially located at defects. • Dislocation behavior is obviously dependent on crystal orientations. • L-C structures are induced in the [111] orientation during tensile deformation. • Hirth and Stair-rod dislocation are formed in [1 1 ̅ 0] orientation for compression. • L-C structure and Hirth/Stair-rod dislocations hinder the glide of dislocations. [ABSTRACT FROM AUTHOR]

In view of the precipitate deficiency and mechanical property deterioration, Sc was added into the weld of aluminum-lithium alloy electron beam welded joint to achieve precipitate compensation and the consequent mechanical property optimization. Quantities of intragranular angular Al 3 Sc precipitated inside the weld after adding Sc, corresponding to the grain refinement effect and dispersoid strengthening effect. The continuous brittle T 2 (Al 6 CuLi 3) quasi-crystal at grain boundaries of weld was transformed into intermittent state. The tensile strength of the joint was optimized from 335 MPa (61% of base metal) to 426 MPa (78% of base metal) as Sc was added into the weld, which was mainly due to the dispersoid strengthening effect of Al 3 Sc and morphology transformation of brittle T 2 (Al 6 CuLi 3) quasi-crystal. Dislocation clusters interacted with Al 3 Sc, indicating a pronounced inhibitory effect on dislocation movement. The novel phenomena of four-chain-shaped dislocations and dislocation loops were found at Al 3 Sc corners deriving from the attraction to dislocations. The formation model was proposed for the two kinds interaction between Al 3 Sc/α-Al interface and dislocations. • Precipitate compensation is achieved for Al–Li alloy welded joint by adding Sc. • Mechanical properties of welded joint are considerably optimized after adding Sc. • The novel phenomenon of four-chain-shaped dislocations is found at Al 3 Sc corners. • Interaction between Al 3 Sc/α-Al interface and dislocations is analyzed in detail. [ABSTRACT FROM AUTHOR]

Zigzag-shaped {112}〈111〉 mechanical twins are formed in a cold-compressed Ti–24.5 Nb–0.7 Ta–2 Zr–1.4 O β alloy. The formation process involves a 1/2〈111〉 screw dislocation dissociation, movement of three partials on adjacent {112} planes to form microtwins, constriction of blocked extended dislocation to form a new screw dislocation, as well as cross-slip of such dislocation onto another {112} plane in repetitive sequences. Obstacles to extended dislocation and appropriate stacking fault energy of the β phase should be the key factors for the formation of such twins. [ABSTRACT FROM AUTHOR]

Manipulating the grain size and the distribution of precipitates has been widely used for tailoring mechanical properties of magnesium alloys. In this work, we systematically studied the influence of LPSO lamellas, Al 2 Y particles as well as grain size on the mechanical properties of the Mg-11Y-1Al alloys. The LPSO phase has little effect on the strength and ductility of the Mg-11Y-1Al alloys. Grain refinement gives rise to the simultaneous improvement of the strength and ductility. The yield strength is improved from 160 MPa to above 300 MPa and the elongation is increased from 4% to more than 13% with the grain refinement from 40 μm to 5 μm. Based on the CPFEM simulation results, it is found that stress concentration generally increases at the adjacent of the Al 2 Y particle at the scenario of the 40 μm grain size. Cracks thus nucleate at the adjacent area of the Al 2 Y particles, which results in poor ductility. With the reduction of grain size to 5 μm, the ductility is enhanced with alleviating the stress concentration around micrometer-size Al 2 Y particles and subsequent activating < c+a > dislocations. Such a finding provides a guideline in tailoring mechanical properties of other Mg-RE-Al alloys for demanding applications. • Yield strength and elongation of WA111 alloy is both enhanced by grain refinement. • LPSO phase is beneficial for homogeneous recrystallization during hot extrusion. • Ductility could be enhanced by relieving stress concentration around Al 2 Y particles. [ABSTRACT FROM AUTHOR]

Recent studies of complex concentrated alloys suggest unusual dislocation core structure and motion, thanks to a collective concentration/structural inhomogeneity. Less is known whether these effects also work in extreme conditions where the solid solution effect is overwhelmed by the large driving force. Here, we investigate the dislocation structure behind a shock-wave front in bcc high-entropy alloys (HEAs) using large-scale molecular dynamics (MD) simulations. In contrast to bcc elemental metals, we find anomalous "extended" edge dislocation structure (6 ~ 8 Burgers vector) with high stability in shock compressed TiZrNb and NiCoFeTi HEAs. The unique dislocation structures can facilitate faster dislocation motion, thus deterring the early nucleation of deformation twins. Combined with continuum elasticity theory, we show that the "extended" dislocation structure can be attributed to the presence of local structures with low elastic stability that are imparted by the nanoscale chemical heterogeneities in HEAs. We also show how the dislocation structures are affected by the interatomic potentials. [Display omitted] [ABSTRACT FROM AUTHOR]

The origin of the indentation size effect has been extensively researched over the last three decades, following the establishment of nanoindentation as a broadly used small-scale mechanical testing technique that enables hardness measurements at submicrometer scales. However, a mechanistic understanding of the indentation size effect based on direct experimental observations at the dislocation level remains limited due to difficulties in observing and quantifying the dislocation structures that form underneath indents using conventional microscopy techniques. Here, we employ precession electron beam diffraction microscopy to "look beneath the surface," revealing the dislocation characteristics (e.g., distribution and total length) as a function of indentation depth for a single crystal of nickel. At smaller depths, individual dislocation lines can be resolved, and the dislocation distribution is quite diffuse. The indentation size effect deviates from the Nix-Gao model and is controlled by dislocation source starvation, as the dislocations are very mobile and glide away from the indented zone, leaving behind a relatively low dislocation density in the plastically deformed volume. At larger depths, dislocations become highly entangled and self-arrange to form subgrain boundaries. In this depth range, the Nix-Gao model provides a rational description because the entanglements and subgrain boundaries effectively confine dislocation movement to a small hemispherical volume beneath the contact impression, leading to dislocation interaction hardening. The work highlights the critical role of dislocation structural development in the small-scale mechanistic transition in indentation size effect and its importance in understanding the plastic deformation of materials at the submicron scale., Competing Interests: The authors declare no competing interest.

• Variation of microstructure in the weld zone and pile-up phenomenon were observed. • Hardness depended not only on indentation size but also on strain rate indentation. • Both hardness and SRS of WM, HAZ, and BM showed the depth-dependent behavior. • SRS value of WM is lower than those in BM and HAZ at larger indents. • Continuous SRS in the weld zone is attributed to the change in dislocation behavior. In this study, a series of experiments consisting of continuous stiffness measurement (CSM) nanoindentation experiments, optical microscope (OM), atomic force microscopy (AFM) examinations, and finite element (FE) analysis were performed to study the microstructures, the indentation size effects, and the rate-dependent behavior of mechanical properties in three phases of SM490 structural steel weld joint. The microstructures of base metal (BM), heat-affected zone (HAZ), and weld metal (WM) were observed using OM examinations. The size-dependent behaviors of mechanical properties in BM, HAZ, and WM were characterized and interpreted through the strain gradient theory. The CSM nanoindentation experiments were carried out in the strain rate range of 0.01–0.1 s−1 to investigate the rate-dependent behavior of indentation hardness and continuous strain rate sensitivity. The results indicated that indentation hardness depended on not only the indentation size but also the strain rate of the indentation level. The strain rate sensitivity (SRS) of BM, HAZ, and WM showed the depth-dependent behavior. The SRS of WM is quite high, over 0.4 at the indentation depth of 200 nm, quickly drops to 0.1, and finally is around 0.0298 at large indents. Similarly, the SRS behavior in the case of HAZ is the same, however, the SRS values at larger indents are higher than those obtained in the WM region. At larger depths, the SRS values of the WM region are lower than those of BM and HAZ, while BM has the highest SRS value compared with WM and HAZ. The continuous SRSs of WM, HAZ, and BM are attributed to the change in the dislocation behaviors during the indentation process. The results of the present study can be used to access and understand the rate- and the size-dependent behaviors in structural steel weld zone. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Molecular dynamic simulations based on a recently constructed potential reveal that quasi-repeating patterns could appear in both Fe(110)/W(110) and W(110)/Fe(110) interfaces, and that three kinds of atomic displacements of Fe atoms because of the Fe-W interaction intrinsically bring about the interesting quasi-repeating patterns of the Fe-W interfaces. It is also found that the Fe-W interface becomes more brittle with less critical strains under tensile loading than pure Fe or W, which is fundamentally attributed to the movement of the interface dislocations as a result of the lattice mismatch between Fe and W. Interestingly, the dislocation loops could be formed in the Fe-W interface under tensile loading due to the pinning of the100edge dislocations by the edge dislocations of1/2111, whereas no dislocation loop would be generated in pure Fe or W., (© 2021 IOP Publishing Ltd.)