Microstructural evolution and mechanical properties of FeCoCrNiCu high entropy alloys:a microstructure-based constitutive model and a molecular dynamics simulation study

High entropy alloys(HEAs)attract remarkable attention due to the excellent mechanical performance.However,the origins of their high strength and toughness compared with those of the traditional alloys are still hardly revealed.Here,using a microstructure-based constitutive model and molecular dynamics(MD)simulation,we investigate the unique mechanical behavior and microstructure evolution of FeCoCrNiCu HEAs during the indentation.Due to the interaction between the dislocation and solution,the high dislocation density in FeCoCrNiCu leads to strong work hardening.Plentiful slip systems are stimulated,leading to the good plasticity of FeCoCrNiCu.The plastic deformation of FeCoCrNiCu is basically affected by the motion of dislocation loops.The prismatic dislocation loops inside FeCoCrNiCu are formed by the dislocations with the Burgers vectors of a/6[112]and a/6[112],which interact with each other,and then emit along the<111>slip direction.In addition,the mechanical properties of FeCoCrNiCu HEA can be predicted by constructing the microstructure-based constitutive model,which is identified according to the evolution of the dislocation density and the stress-strain curve.Strong dislocation strengthening and remarkable lattice distortion strengthening occur in the deformation process of FeCoCrNiCu,and improve the strength.Therefore,the origins of high strength and high toughness in FeCoCrNiCu HEAs come from lattice distortion strengthening and the more activable slip systems compared with Cu.These results accelerate the discovery of HEAs with excellent mechanical properties,and provide a valuable reference for the industrial application of HEAs.

Effects of grain boundary on irradiation-induced zero-dimensional defects in an irradiated copper

Grain boundaries(GBs)can serve as effective sinks for radiation-induced defects,thus notably influencing the service performance of materials.However,the effect of GB structures on the zero-dimensional defects induced by irradiation has not been fully elucidated.Here,the evolution of cascade collision in the single-crystal(SC),bicrystalline(BC),and twinned crystalline(TC)copper is studied by atomic simulations during irradiation.The spatial distributions of vacancies and interstitials are closely related to the GB at a certain primary knock-on atom(PKA)energy.Compared with the TC,the BC displays a more obvious segregation of the interstitial atoms near GB,due to the characteristic of the greater interstitial binding energy.The evolution of Frenkel pairs is more sensitive to the change of the GB position in the BC.A more prominent defect annihilation rate is caused by the effect of the GB than that of the twin boundary(TB).The marked secondary emission phenomenon has been observed in the BC,which promotes the formation of an inverted pagoda-like defect distribution.There are similar sub-conical defect distributions and microstructures induced by cascade collision in the TC and the SC.It has been found that the influence range of the GB is wider in the BC.Meanwhile,the average flow stress of the irradiated copper is quantitatively calculated by establishing a physical strengthening model.The contribution of vacancy to the average flow stress in the irradiated BC and TC is obvious than that in the SC,due to the formation of many vacancies.This study provides a theoretical basis for further understanding and customization of the metal-based equipment with good radiation resistance.

Strengthening and softening in gradient nanotwinned FCC metallic multilayers

Plastic-deformation behaviors of gradient nanotwinned(GNT)metallic multilayers are investigated in nanoscale via molecular dynamics simulation.The evolution law of deformation behaviors of GNT metallic multilayers with different stacking fault energies(SFEs)during nanoindentation is revealed.The deformation behavior transforms from the dislocation dynamics to the twinning/detwinning in the GNT Ag,Cu,to Al with SFE increasing.In addition,it is found that the GNT Ag and GNT Cu strengthen in the case of a larger twin gradient based on more significant twin boundary(TB)strengthening and dislocation strengthening,while the GNT Al softens due to more TB migration and dislocation nucleation from TB at a larger twin gradient.The softening mechanism is further analyzed theoretically.These results not only provide an atomic insight into the plastic-deformation behaviors of certain GNT metallic multilayers with different SFEs,but also give a guideline to design the GNT metallic multilayers with required mechanical properties.

Chemical short-range-order induced multiscale strengthening in refractory medium entropy alloys

High/medium entropy alloys(H/MEAs)are generally possible to exhibit chemical short-range order(SRO).However,the complex role of SRO on mechanical properties from nano-scale to meso-scale is still challenging so far.Here,we study the strengthening mechanism and deformation behavior in a model body-centered-cubic HfNbTa MEA by using atomic-scale molecular dynamics,micro-scale dislocation dynamics,and meso-scale crystal plasticity finite element.The SRO inhibits dislocation nucleation at the atomic scale,improving the flow stress.The SRO-induced ultrastrong local stress fluctuation greatly improves the micro-scale dislocation-based strength by the significant dislocation forest strengthening.Moreover,the Ta-rich locally ordered structure leads to an obvious heterogeneous strain and stress partitioning,which forms a strong strain gradient in the adjacent grain interiors and contributes to the strong back-stress-induced strain hardening.

A novel cobalt-free oxide dispersion strengthened medium-entropy alloy with outstanding mechanical properties and irradiation resistance

A novel cobalt-free oxide dispersion strengthened(ODS)equiatomic FeCrNi medium entropy alloy(MEA)was successfully fabricated through mechanical alloying and hot extrusion(HE).The ODS FeCrNi MEA is composed of a single face-centered cubic(FCC)matrix,in which highly dispersed oxide nanoparticles,including Y_(2)Ti_(2)O_(7),Y_(2)TiO_(5) and Y_(2)O_(3),are uniformly distributed.Compared with the FeCrNi MEA,the ODS FeCrNi MEA exhibits the improved yield strength(1120 MPa)and ultimate tensile strength(1274 MPa)with adequate ductility retention(12.1%).Theoretical analysis of the strengthening mechanism indicates that the high strength is mainly attributed to the grain-boundary strengthening caused by fine grains and the precipitation strengthening resulted from the oxide nanoparticles.Meanwhile,the matrix that easily activates mechanical twinning during the deformation process is the main reason to ensure moderate ductility.In addition,the introduction of high-density oxide nanoparticles can disperse the defect distri-bution and suppress the defect growth and irradiation-induced segregation,leading to the excellent irra-diation resistance.These findings provide innovative guidance for the development of high-performance structural materials for future nuclear energy applications with balanced strength and ductility.

相溶解协同提升高熵合金的强度和塑性

热处理是金属材料热机械加工的常用手段.随着热处理温度的升高而湮灭的缺陷通常导致材料的塑性提升而强度降低.本研究中,我们通过提高热处理温度促进相溶解而协同提升了TiZrNbTa高熵合金的强度和塑性.当热处理温度从800提升至1250°C,合金的拉伸屈服强度提高了40%,达到1003±16 MPa.同时,合金的伸长率增加了近一倍,达到16.79%±1.03%.热处理温度提升引起的相溶解加剧了晶格畸变,从而增强了晶格摩擦应力并提升了屈服强度.相溶解也降低了界面失配并缓解了应力集中.此外,1250°C热处理合金中的局部化学有序结构促进了位错共平面滑移和位错增殖.两种机制共同提升了合金的塑性.该研究不仅扩展了关于金属材料中热处理和相溶解的理解,而且也为合金的强韧化设计提供了思路.

Formation process and mechanical properties in selective laser melted multi-principal-element alloys

Additive manufacturing is believed to open up a new era in precise microfabrication,and the dynamic microstructure evolution during the process as well as the experiment-simulation correlated study is conducted on a prototype multi-principal-element alloys FeCrNi fabricated using selective laser melting(SLM).Experimental results reveal that columnar crystals grow across the cladding layers and the dense cellular structures develop in the filled crystal.At the micron scale,all constituent elements are evenly distributed,while at the near-atomic scale,Cr element is obviously segregated.Simulation results at the atomic scale illustrate that i)the solid-liquid interface during the grain growth changes from horizontal to arc due to the radial temperature gradient;ii)the precipitates,microscale voids,and stacking faults also form dynamically as a result of the thermal gradient,leading to the residual stress in the SLMed structure.In addition,we established a microstructure-based physical model based on atomic simulation,which indicates that strong interface strengthening exists in the tensile deformation.The present work provides an atomic-scale understanding of the microstructural evolution in the SLM process through the combination of experiment and simulation.

混凝土构件中钢筋非均匀锈胀力研究

研究钢筋锈蚀引起混凝土保护层开裂的机理,对于评估受锈蚀影响的混凝土结构剩余寿命具有重要意义.论文提出了一种求解钢筋非均匀锈蚀应力场的解析方法.基于Muskhelishvili的复变函数法和Verruijt共形映射公式,利用基于冯·米塞斯分布的钢筋非均匀锈蚀模型作为位移边界条件,得到了钢筋非均匀锈胀应力的解析解,所得结果与ANSYS有限元模拟结果一致.研究表明:非均匀锈蚀引起的最大锈胀力远大于均匀锈蚀引起的最大锈胀力;钢筋直径越大,锈胀力越小,混凝土保护层越容易开裂;混凝土保护层厚度越大,锈胀力越大;混凝土强度等级越高,钢筋锈胀力亦越大.

Microstructure evolution and deformation mechanism of amorphous/crystalline high-entropy-alloy composites

High-entropy amorphous alloys present high hardness,but low tensile ductility.Here,deformation behavior of the amorphous/crystalline Fe Co Cr Ni high-entropy alloy(HEA)composite prepared by the previous experiment is investigated using atomic simulations.The result shows the partial dislocations in the crystal HEA layer,and the formation of shear bands in the amorphous HEA layer occurs after yielding.The strength of the amorphous/crystalline HEA composite reduces with increasing the thickness of the amorphous layer,agreeing with the previous experiments.The coupled interaction between the crystal plasticity and amorphous plasticity in amorphous/crystalline HEA composites results in a more homogeneous redistribution of plastic deformation to cause interface hardening,due to the complex stress field in the amorphous layer.The current findings provide the insight into the deformation behavior of the amorphous/crystalline HEA composite at the nanoscale,which are useful for optimizing the structure of the HEA composite with high strength and good plasticity.

Effect of lattice distortion on solid solution strengthening of BCC high-entropy alloys

An analytical model is established to study the influence of lattice distortion and fraction of Hf on the yield strength of the BCC TiNbTaZrHfx multi-component high entropy alloys （HEAs）. Meanwhile, the mechanism of solid solution strengthening caused by lattice distortion is also discussed in the HEA. The distorted unit cell is introduced to indicate the lattice distortion effects induced by the differences of the atomic size and shear modulus by doping other elements in Ti-based metal. The results show that the calculated values of the alloying yield strength considering the path of least resistance are obtained with regard to various grain sizes for the equiatomic TiNbTaZrHf HEA, which is well in line with the experimental results. Furthermore, it is predicted that the alloying yield strength is the largest value in the case of the same grain size for the Hf atomic fraction of 0.122. The meaningful modeling could provide a theoretical method to investigate the yield strength and alloying design of other BCC HEAs in the future.

High-throughput simulation combined machine learning search for optimum elemental composition in medium entropy alloy

In medium/high entropy alloys, their mechanical properties are strongly dependent on the chemicalelemental composition. Thus, searching for optimum elemental composition remains a critical issue to maximize the mechanical performance. However, this issue solved by traditional optimization process via "trial and error" or experiences of domain experts is extremely difficult. Here we propose an approach based on high-throughput simulation combined machine learning to obtain medium entropy alloys with high strength and low cost. This method not only obtains a large amount of data quickly and accurately,but also helps us to determine the relationship between the composition and mechanical properties.The results reveal a vital importance of high-throughput simulation combined machine learning to find best mechanical properties in a wide range of elemental compositions for development of alloys with expected performance.

Probing deformation mechanisms of gradient nanostructured CrCoNi medium entropy alloy

The gradient nanostructured medium entropy alloys(MEAs) exhibit a good yielding strength and great plasticity. Here, the mechanical properties, microstructure, and strain gradient in the gradient nanostructured MEA CrCoNi are studied by atomic simulations. The strong gradient stress and strain always occur in the deformed gradient nanograined MEA CrCoNi. The origin of improving strength is attributed to the formation of the 9 R phase, deformation twinning, as well as the fcc to hcp phase transformation, which prevent strain localization. A microstructure-based predictive model reveals that the lattice distortion dependent solid-solution strengthening and grain-boundary strengthening dominate the yield strength,and the dislocation strengthening governs the strain hardening. The present result provides a fundamental understanding of the gradient nanograined structure and plastic deformation in the gradient nanograined MEA, which gives insights for the design of MEAs with higher strengths.

Chemical-element-distribution-mediated deformation partitioning and its control mechanical behavior in high-entropy alloys

The chemical element distributions always strongly affect the deformation mechanisms and mechanical properties of alloying materials.However,the detailed atomic origin still remains unknown in highentropy alloys(HEAs)with a stable random solid solution.Here,considering the effect of elemental fluctuation distribution,the deformation behavior and mechanical response of the widely-studied equimolar random Co Cr Fe Mn Ni HEA are investigated by atomic simulations combined with machine learning and micro-pillar compression experiments.The elemental anisotropy factor is proposed,and then used to evaluate the chemical element distribution.The experimental and simulation results show that the local variations of chemical compositions exist and play a critical role in the deformation partitioning and mechanical properties.The high strength and good plasticity of HEAs are obtained via tuning the chemical element distributions,and the optimal elemental anisotropy factor ranges from 2.9 to 3 using machine learning.This trend can be attributed to the cooperative mechanisms depending on the local variational composition:massive partial dislocation multiplication at an initial stage of plastic deformation,and the inhibition of localized shear banding via the nucleation of deformation twinning at a later stage.Using the new insights gained here,it would be possible to create new metallic alloys with superior properties through thermal-mechanical treatment to tailoring the chemical element distribution.

A WEDGE DISCLINATION DIPOLE INTERACTING WITH A COATED CYLINDRICAL INHOMOGENEITY

A three-phase composite cylinder model is utilized to study the interaction of a wedge disclination dipole with a coated cylindrical inhomogeneity. The explicit expression of the force acting on the wedge disclination dipole is calculated. The motilities and the equilibrium po- sitions of the disclination dipole near the coated inhomogeneity are discussed for various material combinations, relative thicknesses of the coating layer and the features of the disclination dipole. The results show that the material properties of the coating layer have a major part to play in alteringi the strengthening effect or toughening effect produced by the coated inhomogeneity.