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This study investigates the impact of initial temperature on the microstructure and mechanical properties of welded components, using molecular dynamics (MD). The stress–strain curves of the welded components, following various initial temperature treatments, revealed a double yielding phenomenon. Notably, there was a significant strain difference of 19.7% between the two yields. When the strain was loaded to the point of doubling yielding, stacking faults and twins covered the aluminum component part, while no such observations were made in the nickel component part. Additionally, tensile cracking occurred in the aluminum component part. The results indicate that treatment at varying initial temperatures alters the internal structure of the welded components. After the material yielded the first time, a significant number of disordered atoms and Shockley partial dislocations emerge, resulting in a substantial buildup of dislocation tangles and reduced dislocation migration rates. Consequently, the material exhibits a phenomenon of double yielding, with dislocation slip and deformation serving as the primary mechanisms. The optimal mechanical properties of the welded components achieved an initial temperature of 200 K. Additionally, the effect of tensile temperature on the mechanical properties of the welded components were analyzed, and similar observations of double yielding were made. The significant number of dislocation tangles served as a barrier to dislocation slip, effectively enhancing the material's mechanical properties. The simulation results provide theoretical support for the development of aluminum–nickel multilayer film self-propagation welding process. [ABSTRACT FROM AUTHOR]

Gradient nanostructured metals have become one of the research hotspots in the field of materials science due to their excellent mechanical properties and unique deformation behavior. In our contributions, the nanotwin gradient was introduced into the nanocrystalline Ni–Co alloy, and the influence of the gradient structure on the shear properties and microscopic deformation behavior was studied in order to reveal the strengthening mechanism. The gradient nanotwin Ni–Co alloy has elastic deformation and work hardening at the initial shear stage. The stress decreases after reaching the stress limit, and the uniform plastic deformation is maintained at the later stage. The excellent stability of fine twin layer and its effective hindrance to dislocation nucleation and movement can improve the strength of the material. In addition, the yield and work hardening of nanotwin gradient Ni–Co alloy are easy to occur at low temperature, and the stress–strain curve fluctuates obviously. At high temperature, the alloy is prone to plastic deformation, and the strength is reduced macroscopically. The higher the temperature is, the more difficult it is to yield. The results provide a solid theoretical basis for the design of Ni‐based alloys. [ABSTRACT FROM AUTHOR]

Grain boundary engineering is an effective and feasible metal strengthening strategy to enhance the properties of nanopolycrystalline alloys by changing the number, configuration, and connectivity of different types of grain boundaries, especially for the twin boundaries. In the present contribution, the effect of twin spacing and loading mode on the deformation behavior and mechanism of NiCoAl columnar polycrystalline alloy is investigated. The results show that the nanotwins can not only increase the bearing capacity of dislocations but also emit many dislocations, resulting in the coupling effect of dislocation strengthening and twin strengthening. When the twin spacing is large, intrinsic stacking faults occur and gradually transform into deformation twins. In this stage, Shockley partial dislocation controls plastic deformation. When the twin spacing is small, the high‐density twin layers and stacking faults are more likely to interweave, showing a combination action of Shockley partial dislocation and stair‐rod dislocation. With the loading changing to Z axis, the yield strength decreases due to reduced resistance to the dislocation and a weakened number of Shockley partial dislocations of the emission, leading to less strengthening of the twins. The insights provide a solid theoretical foundation for the further application of NiCoAl in industrial production. [ABSTRACT FROM AUTHOR]

Molecular dynamics was used to study the uniaxial tensile process of Ni–Co alloy with gradient nanotwin structure, and the microstructure and deformation mechanism of mechanical properties of gradient nanotwin Ni–Co alloy were studied from the micro perspectives of dislocation, twin layer and grain boundary movement. The results show that when the twin structure Ni–Co alloy is uniaxial stretched along the gradient direction and perpendicular to the gradient direction, the twin layer has different strengthening effects on the structure and strengthening mechanism, the stretching in different directions has greater influence on the dislocation movement and stacking fault nucleation of the nanogradient twin structure. The yield strength of Ni–Co alloy is 11.3 Gpa and 5.5 Gpa, respectively, after stretching along gradient and vertical gradient directions, Young's modulus reaches 221.16 Gpa and 127.90 Gpa. It is found that the gradient twin structure stretched along gradient direction has greater strength and the twin layer breaks into fragments and distributes in the system during the process of strain increase. At different temperatures, it was found that the strength and plasticity of the material decreased sharply with the increase of temperature, the dislocation density in the system also decreased with the increase of temperature, resulting in poor performance of the material. [ABSTRACT FROM AUTHOR]

The effects of gradient grain size and temperature on the mechanical properties of nanocrystalline nickel were studied by the molecular dynamics method, and the microstructure change process of size gradient nanocrystalline nickel under tensile load was characterized by Common Neighbor Analysis (CNA). The research results show that dislocations tend to generate stress concentration in small-sized grains during stretching, so the plastic deformation of the material always occurs in small-sized grains first. At the same time, through Dislocation Analysis (DXA), it was found that Shockley dislocation is the main factor affecting its mechanical properties, and the interaction between dislocations provides a certain contribution to the strength of the metal. In addition, yield strength, maximum tensile stress, and Young's modulus all decrease with increasing temperature, and the grain boundary atomic content and the degree of disorder are greatly increased. During the stretching process, the number of dislocations is greatly reduced, which leads to a reduction of their interactions, which is the reason for the reduction of the mechanical properties of the material. The above research results have positive meaning for the design of gradient nanocrystalline materials, and provide reference and help for the improvement of nanomaterial properties. [ABSTRACT FROM AUTHOR]

The plastic deformation modes of commercially pure titanium (CP-Ti) were studied using an in situ tensile test monitored by electron-backscatter-diffraction (EBSD) assisted slip trace analysis. The plastic strain was primarily accommodated by prismatic slip, followed by deformation twins and pyramidal slip. The slip transmission between two adjacent grains was predicted using the geometric compatibility factor m ′ , which influenced not only the degree of stress concentration but also the activity of dislocation slip systems. Stress concentration mainly occurred at GBs with an m ′ less than 0.5 and could be released by the activities of pyramidal slip or deformation twins with high critical shear stress (CRSS). [ABSTRACT FROM AUTHOR]

The mechanism of plastic deformation under tensile and compressive loading of hexagonal close-packed (HCP)/face-centered cubic (FCC) biphasic titanium (Ti) nanopillars at different temperatures (70 K, 150 K, 300 K and 400 K) and different FCC phase sizes (2 nm, 4 nm, 6 nm and 8 nm) was investigated by molecular dynamics (MD). The plastic deformation is mainly concentrated in the FCC phase during compression loading. The HCP/FCC interface is the main source of 1 6 〈 1 ̄ 2 ̄ 1 〉 Shockley partial dislocations. As the temperature increases, the dislocation nucleation rate increases and the surface dislocation source is activated. During tensile loading, it is more likely that the Shockley partial dislocations react with each other in the FCC phase to form Lomer–Cottrell sessile dislocations and stacking fault (SF) nets. When the temperature is reduced to 70 K, tensile twins are formed at the phase interface. The plastic deformation is dominated by twins and 〈 c + a 〉 dislocation slip occurs in the HCP phase. The effect of the FCC phase size on the plastic deformation mechanism of the nanopillar is strong. The FCC phase is transformed into the HCP phase when the FCC phase size in the nanopillar is reduced to 4 nm under compressive loading. However, twin deformation occurs at the HCP/FCC interface when the FCC phase size is reduced to 2 nm under tensile loading. [ABSTRACT FROM AUTHOR]

Gradient nanograined Ti (GNG Ti) samples were produced by a surface rolling treatment. The mechanical properties were evaluated by microhardness and tensile tests. The results indicated that GNG Ti achieved an optimized combination of strength and ductility relative to the free-standing nano/ultrafine-grained Ti. The deformation and fracture mechanisms were studied using transmission electron microscope (TEM) and scanning electron microscope (SEM). As the grain size reduced from the coarse-grained (CG) matrix to the nano/ultrafine-grained (NG/UFG) layer, the transition from conventional plastic deformation mechanisms, such as dislocation activity and twinning deformation, to grain boundary-mediated deformation mechanisms, such as stress-inducing grain growth, was identified. At the early stage of tensile deformation, the microcracks were first initiated in the CG and deformed grain (DG) regions rather than in the NG/UFG region and the CG/DG interface had a deflection effect on crack propagation. After tensile failure, the fracture morphology showed a mixture of ductile dimples and shear band-like regions. [ABSTRACT FROM AUTHOR]

Molecular dynamics (MD) simulations are performed to investigate the nanoscale tribology behavior of Ti-based FeCoNiTi High-entropy alloy (HEA) coating through nanoscratching. Three coating interface models, (001) HEA ||(0001) Ti , (001) HEA ||(11 2 ‾ 0) Ti , and (001) HEA ||(1 1 ‾ 00) Ti , are selected to explore the effects cutting depth (D) and cutting velocity (V) of abrasive particles during the nanoscratching process. The results show that the Ti-based HEA coating with the interface (001) HEA ||(0001) Ti has the smallest coefficient of friction (COF) making it most suitable as the coating interface. The increase in V resulted in the increase of the frictional force influenced by the Other structure composed of disordered atoms. Due to the existence of multiple phase structures in the limited area of the coating, dislocation nucleation is difficult and disordered atoms are formed. This is different from the strengthening caused by dislocation accumulation in the scratching region. The HEA coating plays an effective role in protecting the pure Ti substrate. With the protection of HEA coating, dislocation slip and deformation twinning were not observed in the pure Ti substrate. However, dislocation slip are the main plastic deformation mechanisms of uncoated pure Ti. These findings provide a theoretical reference for upgrading the wear resistance of pure Ti with HEA coating and expands the area of application of pure Ti. [Display omitted] • Cutting of depth and velocity affect the friction behavior of Ti-based HEA coating. • Other structure composed of disordered atoms strongly affects the frictional force. • With the protection of HEA coating, the Ti substrate has no plastic deformation. [ABSTRACT FROM AUTHOR]

Bulk micro/nanostructured 304 austenitic stainless-steel plates with bimodal grain size distributions were prepared by Alumina Thermite Reaction at various temperatures and extents of rolling deformation. Rolling cogging of the sheet was performed with a rolling reduction of 40% at 1000∘C followed by rolling reduction of 80% at 700∘C. The strength and plasticity of the resulting micro/nanostructured 304 stainless steels with bimodal grain size distribution achieved the best matching, with tensile strength, yield strength, and elongation of 1410 MPa, 723 MPa and 15.3%, respectively. To better understand the deformation mechanism of this micro/nanostructured stainless steel sample, an in situ scanning electron microscopy technique was adopted. The crack initiation, propagation, and fracture were dynamically observed and recorded during the tensile deformation. Our results revealed that a stress concentration near the preset notch served as the initiation source and that microcracks were formed in the grain boundaries between micro- and nano-grains and then spread to the microcrystalline region until passing through the microcrystalline region or until passivation occurred in the microcrystalline region. The microcracks not only caused serious damage to the specimen but also generated back stress, which could lead to hardening of material, thereby enhancing the global ductility. Finally, the mechanism responsible for the enhanced plasticity and strength of the micro/nanostructured 304 stainless steel with a bimodal grain size distribution was analyzed and combined with the fracture morphology. [ABSTRACT FROM AUTHOR]

Mechanical behavior and plastic deformation mechanism of a new type of Co30Fe16.67Ni36.67Ti16.67 high entropy alloys (HEAs) have been calculated by the molecular dynamics method. The results show that the polycrystalline Co30Fe16.67Ni36.67Ti16.67 HEA has remarkable tensile plasticity and anisotropy. When the crystallographic orientation of the grain is <001>, the plastic deformation mechanism is face‐centered cubic (FCC)→body‐centered cubic (BCC) transformation and deformation twins. Grain boundary and vacancy reduce the nucleation energy of FCC→BCC phase transition, making BCC phase nucleation easy and growing in a shear manner, eventually forming deformation twins in the BCC phase. When the crystallographic orientation of grain is <110> and <111>, the plastic deformation mechanism is stacking faults, FCC→hexagonal‐close‐packed (HCP) phase transformation, and deformation twins. The motion of Shockley dislocation leads to the stacking fault, intrinsic stacking fault leads to the FCC→HCP phase transition, extrinsic stratification fault leads to the twin deformation, and the grain refining occurs during the tension process. Temperature and strain rate also have strong effects on tensile strength and elastic modulus. These results will provide a theoretical basis for the development of the HEAs and expand their application. [ABSTRACT FROM AUTHOR]

Al/Cu/Ti/Cu/Al laminated composites were fabricated successfully by rolling with various rolling reduction at room temperature. It was found that the size of interior layer and Ti layer decreased gradually with the increasing of rolling reduction. The hardness values of Al, Cu and Ti layers increased linearly as the rolling reduction increased. The tensile tests showed that the larger rolling reduction was beneficial for the tensile strength increase, but the elongation decreased. The ultimate tensile stress of composite with 85% rolling reduction was 233 MPa. In-situ tensile test indicated that the crack initiated and propagated at Al/Cu interface. Due to the weak interface bonding strength and different plastic deformation capacity, the Al/Cu interface delaminated and the composites finally fractured. • Hardness function was proposed. • Tensile feature was in-situ investigated. • Fracture mechanism of composite was clarified. [ABSTRACT FROM AUTHOR]

FeCrVTi x medium-entropy alloys (MEAs) with BCC matrix and Laves precipitation have high yield strength (1097–1493 MPa), good fracture strain (27.1–47.7%) and excellent thermal conductivity (42.2–69.7 W/(m·K) at 673–1273 K), it is expected to be widely applied in the fields of advanced nuclear energy and aerospace. However, what is this kind of MEAs strengthening and high thermal conductivity mechanisms? According to the experimental results and theoretical calculations, a mixed-strengthening-mechanism model dominated by precipitation and fine-grain strengthening is proposed, it can well explain the increase in yield strength of FeCrV-based MEAs due to Ti-added. Meanwhile, the Laves-phase precipitation formed by adding Ti improves the compressive plastic deformation through grain refinement, crack bridging and crack deflection. Moreover, the high thermal conductivity of FeCrVTi x MEAs is primarily due to the inelastic scattering of phonons and electrons in the medium/high-temperature regions. Based on the above mechanisms, the mechanical and thermal properties of MEAs/HEAs can be regulated in the pre-design and post-treatment stages, which provide new ideas and methods for the design and performance-optimization of high-performance structural materials. [Display omitted] • The new criterion of Laves-phase formation in MEAs/HEAs is observed when δ ≥ 3.25% and ∆ χ Allen ≥ 6.957%. • A mixed-strengthening-mechanism model is proposed to explain well the strength increment of FeCrVTi x MEAs due to Ti-added. • Ti-added improves the compressive plastic deformation behavior of FeCrVTi x MEAs. • The high thermal conductivity in FeCrVTi x originates from inelastic scattering of phonons and electrons at 473–1273 K. [ABSTRACT FROM AUTHOR]

• Tensile response and deformation mechanism under different influencing factors. • When the Co content is 10%, the number of stair-rod dislocations is the largest. • When loading along the [111] direction, the dislocation density maintains a relatively high level. • The existence of small-volume voids is conducive to the nucleation and expansion of dislocations. In this present contribution, molecular dynamics simulation was performed to investigate the tensile response and deformation mechanism under different influencing factors, such as Co content, crystallographic orientation, rate, temperature and void. It is concluded that the number of stair-rod dislocation is the maximum during the deformation process of single crystal nanopillars with the Co content of 10%. It is interesting that the aggregation formed by five stacking fault tetrahedron, whose edge is composed of stair-dislocation formed by the combination of the stacking fault and another incomplete dislocation, is present, responsible for the rise of stress in the plastic deformation curve (0.1 < ε < 0.13). When loading in the [111] direction, the dislocation density keeps a relatively high level compared to the other two directions. In addition, the prismatic dislocation loops are formed by the interaction, reaction and separation of Shockley, stair-rod dislocations by the intersection of the slip planes at an acute angle and Hirth dislocations at an obtuse angle at 50 K, having a fixed effect on the dislocation, which also leads to the increase of strength. The presence of the smaller void volume is conducive to the nucleation and expansion of dislocations, yet when the void volume is too large, the failure process is not dominated by dislocation but by stress concentration at the voids. The above results are beneficial to explore and design of this fascinating single crystal alloy in the fields involving aerospace and electronic power. [ABSTRACT FROM AUTHOR]