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Nanoindentation measurements were conducted to investigate the high-cycle response of 316L stainless steel in bending fatigue. Hardness variation owing to the gradient flexure stress amplitude for different curvatures was plotted along with the thickness and length, respectively. Scanning electron microscopy (SEM) was subsequently conducted to explore the deformation characteristics in multiple layers, which had cyclic gradient stress, on the cross-section of specimens. The nanoindentation results indicated that the cyclic hardening response of 316L stainless steel is correlated with the level of stress amplitude in the high-cycle fatigue (HCF) regime. Furthermore, an analytical model was proposed to clarify the relationship between nanohardness and stress amplitude. Finally, the evolution of damage accumulation due to irreversible plastic deformation is continuous during stress reduction up to the neighboring zone at the neutral surface of the flexure beam in some individual grains. [ABSTRACT FROM AUTHOR]

Symmetrical high‐temperature low‐cycle fatigue tests were performed in this study to examine the influence of temperature on the fatigue failure mechanism of the additively manufactured Inconel 625. According to the fracture analysis, cracks initiate from the crystallographic plane at room temperature. At 600°C, oxidation of carbides leads to crack initiation. The strengthening effect of γ″ precipitation prevents crack propagating within the matrix. At 750°C, the crack becomes transgranular propagating, as a consequence of γ″‐δ transformation. Geometrically necessary dislocations distribution on the fracture supports this result. Furthermore, a modified life prediction formula considering effect of temperature and strain amplitude was established. It was observed that the modified model predicted the fatigue life of the studied materials well in both test conditions. Highlights: Cracks initiate from favorable slip plane at room temperature.Precipitation of γ″ at 650°C prevents dislocation shearing into the matrix.Cyclic plastic deformation accelerates the γ″‐δ transformation at 700°C.Life prediction model considering strain amplitude and temperature shows good accuracy. [ABSTRACT FROM AUTHOR]

The study on the interaction between defects and dislocation emission is of significance to predict the behaviors of fracture and plastic deformation in crystalline metals; however, the multiscale interaction mechanism has not been well understood. In this work, based on molecular dynamics simulation and distributed dislocation technique, the interaction mechanism between dislocation emission and a crack or a void is studied at the atomic and continuum scales. The results show that dislocation emission can change the mode of crack growth, i.e., from brittle to ductile, and the emitted dislocations cause crack‐tip blunt and decrease stress concentration near the defects. The increment of void growth is completely contributed by dislocation emission, and it can be predicted by the characteristics of dislocation emission. This work reveals the influence mechanism of dislocation emission on failure behaviors and deepens the understanding of the interaction between dislocation emission and defects. Highlights: Interaction between defects and dislocation emission is analyzed at different scales.Dislocation emission changes crack growth mode and improves the resistance of fracture.Mechanism of void growth by dislocation emission is proposed based on MD simulations.Dislocation density and stress field are obtained by distributed dislocation modeling. [ABSTRACT FROM AUTHOR]

Densifying transition metal carbide (TMC) ceramics is challenging due to their low diffusion coefficient and strong covalent bonding. Ultra-high-pressure sintering (UHPS) is a promising technique for preparing dense ceramic materials at low temperatures, but it is underutilized in TMC ceramics. Herein, we report the successful employment of UHPS at 1500 °C for 3 min with an auxiliary pressure of 7 GPa, yielding TaC, NbC, TiC, and ZrC ceramics with relative densities of 99.3%, 98.9%, 98.2%, and 96.4%, respectively. We demonstrate that the densification mechanism of the TMC ceramic in this investigation encompasses plastic deformation and high-pressure assisted atomic diffusion. Furthermore, we determine the Vickers hardness and fracture toughness of TaC, NbC, TiC, and ZrC ceramics to be 16.5, 15.9, 24.0, and 17.7 GPa and 2.5, 2.9, 2.8, and 1.8 MPa m1/2, respectively. Moreover, with increasing temperature from room temperature to 500 °C, the hardness and elastic modulus of all four TMC ceramics gradually decrease, with the hardness declining by approximately 17–25%. [ABSTRACT FROM AUTHOR]

Developing ultrahigh strength steels that are fracture resistant and economical would be appealing for structural applications. In this work, an ultra strong-and-tough steel was fabricated by introducing fibrous ferrite into martensite matrix. This steel exhibited an ultrahigh yield strength and tensile strength, 1560 and 2103 MPa respectively, along with a 7.3 % uniform elongation. The fracture toughness and smoothly rising R-curve were measured through the elastic-plastic unloading compliance method using miniaturized side-grooved compact tension (CT) specimens. A high fracture toughness (K I C ≈ 105 MPa m1/2) that is superior to many steels at the same strength level was achieved. The heterogeneous microstructure of fibrous ferrite embedded in a martensitic matrix allows for high strain hardening rate and adequate micro-void formation, consequently leading to higher fracture strain and the occurrence of localized necking. The toughening mechanisms of CT samples are discussed according to the energy criterion for fracture. The expanded plastic deformation area (intrinsic toughening mechanism), deflecting and bridging the primary crack propagation as well as interface delamination between fibrous ferrite and martensite (extrinsic toughening mechanisms) significantly improve the overall fracture resistance. The current findings promote the development of high-strength and high-toughness medium-Mn steels for structural applications. [ABSTRACT FROM AUTHOR]

• To improve the computational efficiency and accuracy of SP simulation, a coupled DEM-FEM model was established. • Dislocations are plugged in the γ matrix to form subgranular boundaries that can fix γ' to form small-sized grains, and in addition some of the dislocations shear γꞌ further leading to grain refinement. • The SP treatment produced a high density of dislocations, work hardening, and residual compressive stresses, also reduces porosity defects left on the surface. The effects of shot peening on the surface microstructure, tensile properties, and high-cycle fatigue properties of MAR-M247 nickel-based superalloy were investigated. The predicted surface roughness, magnitude of residual stress, and depth of plastic strain in the surface layer were simulated by the numerical simulations coupled Finite Element Method-Discrete Element Method (FEM-DEM). The results show that the shot peening treatment induced significant plastic deformation in the surface layer, leading to the accumulation of a large number of dislocations in the γ matrix, forming dislocation walls, resulting in the formation of fine grains. A high density of dislocations, work hardening, and residual compressive stresses occur after the shot peening treatment. These beneficial changes enhanced the resistance to crack initiation and propagation, thereby inhibiting surface crack initiation, and increasing the fatigue life and tensile strength of the alloy. [ABSTRACT FROM AUTHOR]

• A thermo-mechanically coupled, grain-size-dependent and crystal-plasticity-based phase field model is proposed. • The functional properties and plastic deformation of porous NiTi SMAs were simulated. • The effects of porosity, pore diameter, pore aspect ratio, pore orientation, and pore distribution, as well as the interaction between pore and grain size, are revealed and discussed. Porous NiTi shape memory alloys (SMA) have been developed into important biomedical and damping materials. However, the effect of pore on the martensitic microstructure evolution, functional properties, and plastic deformation has not been fully clear yet. In this work, to reveal the functional properties including superelasticity (SE), elastocaloric effect (eCE), one-way shape memory effect (OWSME), and stress-assisted two-way shape memory effect (SATWSME), as well as plastic deformation of porous NiTi SMAs, the polycrystalline SMA system is considered as a composite containing the grain-interior (GI) and grain-boundary (GB) phases, a thermo-mechanically coupled, grain-size-dependent and crystal-plasticity-based phase field model is proposed for the GI phase, and an elasto-viscoplastic constitutive model is established for the GB phase. The effects of porosity, pore diameter, pore aspect ratio, pore orientation, and pore distribution, as well as the interaction between pore and grain size, are comprehensively revealed and discussed. The simulations demonstrate that the pores cause a heterogeneous stress field, and the interaction among pores and that between pores and grain boundaries can enhance such heterogeneity, which can promote the initiation of martensitic transformation and reorientation. Moreover, the geometrical constraints caused by the pores can significantly affect the morphology and evolution of martensite microstructures. However, due to the diverse microstructure evolutions during SE, OWSME, and SATWSME, their interactions with the pores are quite different, resulting in various dependencies of each functional property and plastic deformation on the pore, and such dependencies are strongly influenced by porosity, pore diameter, pore aspect ratio, pore orientation, and pore distribution. The simulated results in this work are helpful to comprehensively and deeply understand the pore effect and its microscopic mechanism. [ABSTRACT FROM AUTHOR]

[Display omitted] • The evolution of the deformation mechanism with stress loading is analyzed. • The alteration of atomic arrangement due to dislocation motion is illustrated in conjunction with HRTEM. • The magnitude of the SFE of AM 939 at intermediate temperature is estimated from the critical shear stresses. Tensile interruption experiments with different strains were conducted on additively fabricated nickel-based Inconel 939 superalloys at 700 ℃ to investigate the evolution of multiple deformation mechanisms. The microstructures of different strain groups captured by transmission electron microscopy (TEM) were analyzed, and it was concluded that a total of four various deformation mechanisms were identified, of which the first to be activated was the Orowan loop bypassing γ', and a tendency of transition from double to single loop was observed. Anti-phase boundary (APB) shearing was subsequently initiated, with a needed critical resolved shear stress level of approximately 195.82 MPa. Stacking faults (SFs) and deformation twinnings (DTs) were observed at higher stress, accompanied by the dissociation of dislocations. In addition, the magnitude of the stacking fault energy of AM 939 superalloys at intermediate temperature was calculated and estimated by matching the experimental phenomena with the critical shear stress. [ABSTRACT FROM AUTHOR]

Translucent nanocrystalline Gadolinium Zirconate (Gd 2 Zr 2 O 7) ceramic with grain size below 10 nm was successfully fabricated using high pressure sintering technology at 5 GPa and low temperature of 550 °C. The bilinear compressive stress-strain relationship of this sample was firstly extracted from nanoindentation procedure based on the partial unloading method. It indicated that the plastic deformation did not originate from dislocation sliding and was controlled by the grain boundary sliding. Moreover, the compressive yield strength, σ y = 7.8 GPa of the translucent nanocrystalline Gd 2 Zr 2 O 7 ceramic was also determined by the intersection of the theoretical elastic line and the indentation compressive stress-strain line. [ABSTRACT FROM AUTHOR]

Optimizing strength and ductility by uniting microstructural heterogeneity and profitable crystallographic orientation is a challenge due to processing difficulties. Here, we synthesize a new kind of highly-oriented heterostructure (HOH) in pure titanium (Ti) by controlling hot extrusion, cold swaging, and annealing processing. The processed microstructure is characterized by (i) obvious grain size heterogeneity and (ii) highly-preferred crystallographic orientation with the c-axis of most grains perpendicular to the rolling direction (RD). The HOH Ti exhibits superior strength-ductility synergy in the RD, while higher strength but lower ductility in the transverse direction (TD). When the sample is stretched along the RD, obvious geometrically necessary dislocation accumulation and strain gradients near grain boundaries in a soft zone are experimentally verified, inducing extraordinary strengthening/hardening by producing hetero-deformation-induced stress and promoting dislocation storage. In the TD, {10–12} twins are observed in a hard zone while no plastic deformation occurs in the soft zone. It is further revealed that both the hard and soft zones exhibit high Schmid factors (m) for the prismatic, pyramidal slip and pyr1/pyr2 slip in the RD, indicating a soft slip mode. On the other hand, the soft zone becomes "hard" due to very low m values for the prismatic slip in the TD, resulting in fewer slip systems in the soft zone. These observations suggest that the heterostructure design with profitable crystallographic orientation is an effective approach to achieving high strength-ductility synergy. [ABSTRACT FROM AUTHOR]

Traditional micro-sized and nano-sized YAG transparent ceramics were tested by nanoindentation at different peak loads. The micro-sized YAG transparent ceramics show a marked indentation size effect (ISE). However, for the nano-sized YAG transparent ceramics, the hardness was constant in the whole investigated range without any evidence of ISE. We show that the absence of indentation size effect for nano-sized YAG transparent ceramics can be accurately modeled using the plastic deformation mechanism of grain boundary sliding. [ABSTRACT FROM AUTHOR]

The elastic-plastic deformation behaviors of nano-grained and coarse-grained yttrium aluminum garnet (YAG) transparent ceramics are investigated using nanoindentation. An inverse Hall-Petch relation is observed for the nano-grained YAG ceramic and a forward Hall-Petch relation is observed for the coarse-grained YAG ceramic. In addition, the plastic work ratio as a function of applied load for the nano-grained YAG ceramic shows a different trend than that for the coarse-grained YAG ceramic. These observations suggest that the plastic deformation of the nano-grained YAG ceramic cannot be attributed to the normal dislocation mechanism and is controlled by grain boundary sliding. A generalized self-consistent model for studying the mechanical behavior of the nano-grained YAG ceramic is developed and validated by experimental results. The stress-strain relationship predicted by this model is embedded in finite element simulations which confirmed that the plastic deformation of the nano-grained YAG ceramic indeed can be attributed to grain boundary sliding. [ABSTRACT FROM AUTHOR]

• Influence of induction hardening on the fatigue strength of EA4T specimens was studied. • Artificial impact damages were introduced on as-received specimens and induction hardened specimens. • Both damage modes and failure mechanisms of as-received and hardened specimens were analyzed. • Positive effect of induction hardening on the damage tolerance of EA4T axles was discussed. Induction hardening, a more efficient and effective surface hardening technique of railway axles, has received considerable attention for its potential of replacing deep rolling to be the surface strengthening technique of EA4T axles. In order to establish guidance for industrial utilization of induction hardened railway axles, damage tolerance assessment on induction hardened EA4T axles is a necessity. The purpose of this paper is to evaluate the influence of induction hardening on the damage tolerance of EA4T axles subjected to flying ballast impact damage. Impact damages were introduced artificially by means of shooting balls of 2 mm in diameter to induction hardened specimens and untreated ones. The damage modes of two kinds of specimens were analyzed and the resultant loading capacity debit was evaluated. Based on the characterization of damages and fracture surfaces, failure mechanisms were discussed. Due to the high strength of hardened layer, it is more significant to resist plastic deformation under a moderate impact. Under ballistic impact, microcracks, material loses and adiabatic shear bands (ASBs) present in damage of hardened specimen. Fatigue strength of smooth hardened specimen was improved by 61 % in comparison with that of untreated one. As a result of stable and considerable compressive residual stress of induction hardened (IH) specimen, fatigue strength of damaged hardened specimen is markedly larger than the as received (AR) ones with similar defect size. [ABSTRACT FROM AUTHOR]

The elastic and plastic deformation behaviors of ytterbium-doped yttrium aluminum garnet (Yb:YAG) transparent ceramics which affect its machinability were investigated by nanoindentation and in situ scanning probe microscopy (SPM) measuring method. This work aimed at studying the influence of applied load on the resistance to plasticity. It showed that the resistance to plasticity decreased with increasing the applied load, and the ductile machining became easier. But when the applied load exceeded 8 mN, microcracks were observed around the nanoindentation. Meanwhile, the indentation size effect was also analyzed for both measured nanoindentation hardness and Young’s modulus. The true hardness was determined based on the proportional specimen resistance (PSR). This study lends credence to the realization of manufacture for high surface quality for Yb:YAG transparent ceramic in abrasive machining. [ABSTRACT FROM AUTHOR]

• Investigating the mechanical behaviors and underlying micromechanics in TWIP steels. • Profuse deformation twins are introduced in TWIP steel at cryogenic temperature. • The cross GBs and TBs provide an easy mean for cracking in TWIP steel. We study the tensile deformation behaviour and microstructure evolution of twinning-induced plasticity (TWIP) steel at room and cryogenic temperatures. The TWIP steel shows the higher strain hardening capability, excellent strength and obvious ductility loss at cryogenic temperature. The TWIP steel has excellent strength at cryogenic temperature due to the synergistic effect of dislocation slip and deformation twinning. High-density nanotwin structure can cause strong strain hardening capability. In general, strong strain hardening leads to uniform plastic deformation, which provides high ductility. According to the microstructure characterization, the intersecting position of high-density twin-grain and twin-twin boundaries act as the nucleation sites for nanovoids, which were formed by dislocation pile up. Therefore, its ductility reduced at cryogenic temperature even with continuous steady strain hardening. [ABSTRACT FROM AUTHOR]

Ultrasonic fatigue tests were performed on friction stir welded AA6061 joint to investigate very high cycle fatigue (VHCF) behaviors. As a result, almost all the fatigue cracks are initiated from local plastic slip markings around the boundary between thermo-mechanically affected zone and heat affected zone. The fatigue strength decreases from the top to root of the welded joint, owing to the variation of plastic deformation history and temperature distribution through the thickness. In fractography, the fatigue crack initiation site is surrounded by a semicircular flat zone, of which the formation in VHCF regime accounts for more than 98% of the total fatigue life. [ABSTRACT FROM AUTHOR]

The sintering densification mechanism of nano grained Yttrium aluminum garnet transparent ceramic during high pressure sintering process can be divided into three period. The first sintering stage can be described as plastic deformation after yield point. The intermediate stage is conducted by grain boundary viscous sliding. The grain boundary diffusion is the significant contribution for the final sintering densification period. [ABSTRACT FROM AUTHOR]

The effect of temperature on the tensile behavior and deformation mechanisms in a single crystal superalloys CMSX-4 is addressed and deduced by transmission electron microscopy in the temperature range from room temperature to 1100 °C. It is found that the tensile yield strength reaches a peak at 800 °C. And then, the yield strength decreases with increasing temperature. At room temperature, anti-phase boundary shearing dominates the plastic deformation. From 800 °C to 850 °C, the plastic deformation mechanism is mainly controlled by stacking fault shearing. The Kear-Wilsdorf locks have also appeared. When the temperature reaches at 950 °C, dislocation loops with anti-phase boundary shearing of the γ ′ precipitates are presented. Above 950 °C, the plastic deformation mechanism is processed by the rafted structure of the γ ′ precipitates by-passing, i.e., Orowan by-passing and dislocation climb. Finally, according to the experimental results, the variety of stacking faults with temperatures and the relationship between the yield strength and plastic deformation mechanism are discussed. • The tensile deformation mechanism of CMSX-4 is dependent on the temperatures. • The Kear-Wilsdorf locks are responsible for higher tensile yield strength at 800 and 850 °C. • The temperature for SSFs formation under tensile tests is lower than that at compression tests. • The deformation mechanism at 950 °C is controlled by dislocation loops with APB cutting of γ ′ precipitates. • Above 950 °C, the deformation mechanism is addressed by rafted γ ′ phase by-passing. [ABSTRACT FROM AUTHOR]

The deformation microstructure of single crystal superalloys CMSX-4 was investigated during uniaxial tensile with a strain rate of 5 × 10−3 s−1 at room temperature. The objection of the current study is to evaluate the deformation twinning and investigate the deformation mechanism. Experimental results followed by transmission electron analysis show that deformation nanotwins and elemental segregation in single-crystal Ni-based superalloys were observed after tensile deformation at room temperature with a strain rate of 5 × 10−3 s−1. Furthermore, the relationship among deformation twinning, elemental segregation, and high local stress due to plastic deformation with increasing plastic strain was estimated and discussed. These findings provide valuable insights into understanding twinning and elemental segregation in the plastic deformation of single-crystal Ni-based superalloys. • Deformation nanotwins in CMSX-4 are initiated under tensile tests with a strain rate of 5 × 10−3 s−1 at room temperature. • Element segregation can be observed at the twin zone at room temperature. • Deformation nanotwins in CMSX-4 is not necessarily accompanied by an atomic reordering process at room temperature. [ABSTRACT FROM AUTHOR]

In this study, AZ31 magnesium alloy plates with fine grained microstructure were processed by cold dynamic extrusion followed by low temperature annealing. The corresponding microstructure evolution and mechanical properties were systematically investigated, with a great emphasis on the effect of high {10−12} extension twin density. Profuse {10–12} extension twin and nanograined structure was generated in various regions after 20% cold dynamic extrusion. Thus, pre-twinning greatly improved the strength and ductility in magnesium alloy. In addition, the introduction of high twin density can be an effective way to improve mechanical properties of magnesium alloy via twin-dislocation interaction during plastic deformation. With mechanical behavior properly described, high twin density increases the uniformity of localized dislocation plasticity, thereby improving the strain hardening property. Also, we conclude that high strain rate in cold dynamic extrusion can be considered as an effective method for preparation of pre-twinning magnesium alloy, which exhibits good mechanical properties. • Pre-twinning greatly improved the strength and ductility in magnesium alloy. • Twin density can be an effective way to improve mechanical properties of magnesium alloy. • High twin density increases the uniformity of localized dislocation plasticity. • Cold dynamic extrusion can be considered as an effective method for preparation of pre-twinning magnesium alloy. [ABSTRACT FROM AUTHOR]

The low cycle fatigue (LCF) behaviors of Sanicro 25 steel welded joint were investigated under different total strain amplitudes (Δεt/2) at 700 °C. The results revealed that the stress amplitude increased with an increase in Δεt/2, while the fatigue life decreased. In addition, the cyclic hardening ratio first increased and then decreased with increasing Δεt/2. This abnormal trend was attributed to the variation in the cyclic yielding stress, indicating the evolution of the dislocation morphology. The microstructure analysis revealed a significant change in the cyclic plastic deformation mechanism from the dislocation tangle (0.2%–0.4%) to the entanglement of dislocations on the precipitate (0.5%) with an increase in Δεt/2. Furthermore, the cyclic hardening models considering the entanglement of dislocations on the precipitate and the dislocation tangle were unified according to the modified Taylor stress. The validated results indicated that the unified cyclic hardening model accurately calculated the maximum cyclic stress during LCF for a 316H welded joint. [ABSTRACT FROM AUTHOR]

• Compression behavior from room temperature to 1100 °C is investigated by transmission electron microscopy. • Deformation mechanism of CMSX-4 is dependent on the temperatures. • The higher yield strength at intermediate temperatures is controlled by Kear–Wilsdorf locks within γ ′. • Stacking fault shearing is related to the temperature, which plays an important role in the plastic deformation at 950 °C. The compression behavior under a temperature range from room temperature to 1100 °C were conducted on the single crystal superalloys CMSX-4. The objective of this study is to identify the effect of the temperature on the compression behavior and understand the deformation mechanism by transmission electron microscopy after yielding. Experimental results show that with increasing temperature, the yield strength increases until 850 °C. After that, there is a opposite trend in yield strength. At room temperature, plastic deformation is dominated by anti-phase boundary shearing. From 700 °C to 850 °C, the major anti-phase boundary and minor stacking fault shearing is observed. Besides, Kear–Wilsdorf locks are frequently observed especially at 800 °C and 850 °C, resulting in the best yield strength at these temperatures. At 950 °C, the deformation mechanism is mainly controlled by stacking fault shearing. With temperature increasing to 1100 °C, the dislocation network and rafted structure are formed, where the deformation mechanism is Orowan by-passing and dislocation climb. Finally, the formation of stacking faults with temperatures and the relationship between yield strength and deformation mechanism under compression tests are deduced. [ABSTRACT FROM AUTHOR]

It is suspected early that the ultra-slow crack propagation in fine-granular-area (FGA) may be sensitive to the internal vacuum-like environment. In other words, fatigue failure in the very-high-cycle regime may be related to the internal vacuum-like environment. Since magnesium alloy is more susceptible to the environmental effect, it is selected as the research carrier in this study. A magnesium alloy is investigated in ambient air and vacuum environments comparatively, and the vacuum loading environment offers a valuable reference to investigate the possible environmental effect. The results show that plastic deformation mechanism in different loading environments keeps the same at crack initiation site. However, the different loading environments not only change the fatigue life, but also the failure mode. At higher stress levels, surface failure dominates in air loading environment, whereas internal failure dominates in vacuum loading environment and presents longer fatigue life. It is considered that failure mode in vacuum loading environment is mainly affected by the critical grains, whereas that in air loading environment is mainly affected by the competition between the surface air acceleration effect and the critical grains. Fatigue life is mainly affected by the internal vacuum retarding effect and the surface air accelerating effect. The different loading environments not only change the fatigue life, but also change the failure mode. Unlabelled Image • The intrinsic fatigue behavior in vacuum loading environment. • Air acceleration effect promotes premature surface failure. • Different loading environments change the failure mode. [ABSTRACT FROM AUTHOR]

• Microstructural evolution along the depth direction presents a gradient feature. • Four types of deformation twins are introduced into primary α phases. • High-angle grain boundaries are increased by deformation twins (~13%). • Strengthening mechanisms of extra-long life fatigue resistance are revealed. Ultrasonic fatigue tests were performed on notched specimens to investigate high and very high cycle fatigue behaviors in the laser shock peened region of a compressor blade titanium alloy (Ti–8Al–1Mo–1V). The plastic deformation and compressive residual stress, induced by the laser shock peening (LSP), present gradient distributions along the depth direction. High-angle grain boundaries (HAGB, >15°) are increased by deformation twins, which are introduced into primary α phases during the plastic deformation. The enhancement in fatigue resistance is ascribed to the comprehensive retardation effects of the compressive residual stress, high-density dislocations, and HAGBs. [ABSTRACT FROM AUTHOR]

• A sound ("defect-free") Mg-FSW joint is produced. • Unique texture and banded structure of Mg-FSW joint affect crack initiations. • A semi-quantitative mechanical model verifies the different failure mechanisms. Friction stir welding (FSW) produces heterogeneous microstructure in nugget zone of its joint, which should affect the fatigue responses. This work revealed the unique texture and banded structure of the Mg-FSW joint and their decisive roles on crack initiation behaviors. At higher stress levels, failures occur along the AS NZ/TMAZ boundary (AS: advancing side, NZ: nugget zone, TMAZ: thermo-mechanically affected zone) due to the inhomogeneous deformations near the boundary. Severe plastic deformations concentrate in the softer layers of banded structure. However, at lower stress levels, plastic deformations become weak and localized, but accumulate in the alternate layers of banded structure simultaneously due to the same soft texture orientation, and induce random failures in banded structure. The failure mechanisms are verified by semi-quantitative mechanical models. [ABSTRACT FROM AUTHOR]

In the current work, molecular dynamics (MD) simulations were employed to model the compressive property of FCC (face centered cubic) Al 0.25 CoFeNiCu 0.75 high entropy alloy (HEA) nanopillars. For comparison the binary, ternary and quaternary derivatives based on Ni–Cu–Fe–Co–Al system along with pure Ni were investigated. The twin nucleation and migration stress, atomic strain and stress and generalized planar faults energy (GPFE) of the compositions were calculated. The simulation results suggest that plastic deformation of all nanopillars is mediated by deformation twinning, but both the yield strength and flow stress decrease with the increase in the number of alloying elements, implying the decrease in the twin nucleation and migration stresses, respectively. The atomic strain and stress, increasing with the addition of alloying elements, provide direct evidences at atomic scale for the severe lattice distortion resulting in the decrease in stacking fault energy (SFE) and twin boundary energy as shown in the GPFE curves. The SFE of Al 0.25 CoFeNiCu 0.75 HEA is 16 mJ / m 2 . The twinnability of the studied compositions also increases with the addition of alloying elements, so twinning deformation dominates in the current nanocrystals. [ABSTRACT FROM AUTHOR]