We present a numerical framework for simulating viscous compressible flows in the presence of solid particles with large size ratios.The volume-filtered Navier-Stokes equations are discretized using a class of high-or...We present a numerical framework for simulating viscous compressible flows in the presence of solid particles with large size ratios.The volume-filtered Navier-Stokes equations are discretized using a class of high-order low-dissipative finite difference operators with energy-preserving properties.No-slip,adiabatic boundary conditions are enforced at the surface of large particles(with diameters significantly larger than the local grid spacing)using a ghost-point immersed boundary method.Two-way coupling between the gas phase and small particles(with diameters proportional to the grid spacing)is accounted for through volumetric source terms for interphase momentum and energy exchange.A simple and efficient approach for collision detection between small and large particles is proposed.The framework is applied to simulations of planar shocks interacting with bidisperse distributions of particles with size ratios of approximately thirty.Particle dispersion and size segregation are reported and a simple analytical model for size segregation is proposed.展开更多
Rock-ice avalanches in cold high-mountain regions pose severe hazards due to their high mobility,yet the quantitative controls of particle-size ratio and ice content remain insufficiently constrained.This study invest...Rock-ice avalanches in cold high-mountain regions pose severe hazards due to their high mobility,yet the quantitative controls of particle-size ratio and ice content remain insufficiently constrained.This study investigates their coupled effects using inclinedflume experiments and Discrete Element Method(DEM)simulations,covering three gravel sizes(2-5 mm,5-7 mm,7-10 mm)and four ice-content levels(0%,20%,40%,60%).Run-out distance,velocity,energy components,flow regime(Savage number),and segregation indexαwere quantified.Increasing ice content significantly enhances mobility,but with diminishing marginal effectiveness.From 0%to 40%ice content,run-out distance increases by 41%-86%,whereas the additional increase from 40%to 60%contributes only 12%-23%.Particle-size ratio strongly governs segregation intensity.Fine-gravel groups reach segregation indices ofα=0.92-0.98,indicating nearly complete upward migration of ice,whereas medium-gravel and coarse-gravel groups exhibit much weaker segregation,stabilizing atα=0.68-0.74 and 0.60-0.69.Savage number analyses reveal marked flow-regime transitions.At 0%ice content,Savage numbers reach 1.0-1.5,indicating a collisional regime.Increasing ice content suppresses collisionality,with Savage numbers decreasing to 0.03-0.07 at 60%ice content,consistent with dense-regime flow.DEM energy analyses confirm this regime shift:for finegravel mixtures,collision energy decreases by 14%,while sliding-friction energy increases by 33%as ice content increases from 0%to 60%,reflecting enhanced overburden effects imposed by upward-segregated ice layers.Medium and coarse mixtures exhibit weaker or opposite energy-shift patterns,demonstrating strong size dependence.Mechanistically,large particle-size contrasts promote strong segregation and form dense basal rock layers that increase basal friction and reduce mobility.When particle sizes are similar or ice content is high,segregation remains limited,allowing ice to mix into the basal layer,thereby reducing basal friction and enhancing mobility.This research quantitatively demonstrates how composition controls particle spatial distribution,flow regime,and energy dissipation,offering new mechanistic insights into the propagation and deposition behaviors of rock-ice avalanches and improving hazard assessment in vulnerable high-mountain regions.展开更多
The high-alloyed wrought superalloy GH4975 tends to form coarse MC carbides and eutectic(γ+γ′)phases,which adversely affect the cogging and homogenization process.To provide theoretical guidance for control of MC c...The high-alloyed wrought superalloy GH4975 tends to form coarse MC carbides and eutectic(γ+γ′)phases,which adversely affect the cogging and homogenization process.To provide theoretical guidance for control of MC carbides and eutectic(γ+γ′)formation,differential thermal analysis(DTA)was utilized to investigate the effect of cooing rate(10-90℃·min^(-1))on solidification behavior and micro-segregation of GH4975 alloy.According to the thermodynamic calculation and distribution characteristics of precipitates,the MC carbides can act as nucleation sites forγdendrites,but the nucleation ofγdendrites becomes less dependent on the MC carbide primers at higher cooling rates.As theγdendrites grow,the elements including Ti and Nb gradually accumulate in the residual liquid and leads to the formation of more MC carbides near the interdendritic region.Finally,the solidification is terminated with the formation of eutectic(γ+γ′).With an increase in cooling rate,the liquidus temperature rises,but the solidus temperature decreases,and thus the solidification range is obviously enlarged.The dendritic structure is significantly refined by the increase of cooling rate.The secondary dendrite arm spacing,λ_(2),as a function of cooling rate,T,can be expressed asλ_(2)=216.78T^(-0.42).Moreover,the increasing cooling rate weakens the back diffusion of Al,Ti,and Nb,increases the undercooling,and limits the growth of precipitates.Consequently,the sizes of MC carbides,eutectic(γ+γ′),and primaryγ′significantly decrease,but the area fraction of eutectic(γ+γ′)linerly increases as the cooling rate rises.Thus moderate cooling rate(such as 30℃·min^(-1))should be selected during the solidification process of GH4975 alloy.展开更多
In response to the urgent demand for lightweight,magnesium(Mg)alloys have garnered considerable attention owing to their low density.Nonetheless,the intrinsic poor room-temperature formability of Mg alloys remains a m...In response to the urgent demand for lightweight,magnesium(Mg)alloys have garnered considerable attention owing to their low density.Nonetheless,the intrinsic poor room-temperature formability of Mg alloys remains a major obstacle in shaping precise complex components,necessitating the development of superplastic Mg alloys.Excellent superplasticity is usually acquired in high-alloyed Mg alloys with enhanced microstructural thermal stability facilitated by abundant optimized second-phase particles.While for cost-effective low-alloyed Mg alloys lacking particles,regulating solute segregation has emerged as a promising approach to achieve superplasticity recently.Moreover,the potential of bimodal-grained Mg alloys for superplastic deformation has been revealed,expanding the options for designing superplastic materials beyond the conventional approach of fine-grained microstructures.This study reviews significant developments in superplastic Mg alloys from the view of alloying strategies,grain structure control and deformation mechanisms,with potential implications for future research and industrial applications of superplastic Mg alloys.展开更多
Asymmetric tilt boundaries on conventional twin boundaries(TBs)are significant for understanding the role of twins on coordinating plastic deformation in many metallic alloys.However,the formation modes of many asymme...Asymmetric tilt boundaries on conventional twin boundaries(TBs)are significant for understanding the role of twins on coordinating plastic deformation in many metallic alloys.However,the formation modes of many asymmetric tilt boundaries are hard to be accounted for based on traditional theoretical models,and the corresponding solute segregation is complex.Herein,atomic structures of a specific asymmetric boundary on{1012}TBs were reveled using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM),molecular dynamics(MD)and density functional theory(DFT)simulations.Reaction between<a60>M dislocations and the{1012}TB can generate a~61°/25°asymmetric tilt boundary.The segregation of Gd and Zn atoms is closely related to the aggregateddislocations and the interfacial interstices of the asymmetric tilt boundary,which is energetically favorable in reducing the total system energy.展开更多
The limited creep resistance of wrought Mg-Al alloys restricts their lightweight applications at intermediate temperatures due to the softening effect of discontinuous precipitation(DP)on the dislocation-controlled cr...The limited creep resistance of wrought Mg-Al alloys restricts their lightweight applications at intermediate temperatures due to the softening effect of discontinuous precipitation(DP)on the dislocation-controlled creep.Here,we developed a creep-resistant wrought Mg-Al alloy through microalloying of Y and Ca.The resulting alloy exhibited an order of magnitude enhancement in the creep resistance at 125℃/50±100 MPa.In contrast to the grain boundary instabilities by DP in the previously reported wrought Mg-Al alloys,we show that the addition of 0.21Y+0.15Ca wt%produces a(Zn+Ca)co-segregation at the grain boundaries as a result of their segregation energy and the activation energy of grain boundary migration,thereby stabilizing the grain boundaries.The(Zn+Ca)co-segregation inhibits the dynamic DP and promotes the formation of intragranular Al-enriched clusters,which favorthe formation of Al_(2)Y,Mg_(17)Al_(12)nano precipitates,thereby impeding intragranular dislocation motion during creep.Furthermore,the addition of 0.21Y+0.15Ca wt%facilitates the formation of a fine and uniform recrystallization structure in the microalloyed alloys compared to AZ80 due to the high activation energy of mobility for the(Zn+Ca)segregated grain boundary.Therefore,the microalloyed alloys exhibit good tensile properties with 380 MPa tensile strength and 18%elongation.Our constitutive analysis revealed that the(Y+Ca)microalloying decreased the creep stress exponent by 29%and increased the creep resistance in the medium to high-stress range.Microalloying provides a promising way to develop low-cost creep-resistant wrought Mg-Al alloys.展开更多
Four powder metallurgy(PM)Ni-based superalloys with different Hf and Ta contents were creep-tested at 650℃ and 970 MPa,700℃ and 770 MPa,and 750℃ and 580 MPa,respectively.The effect of Hf and Ta on creep deformation...Four powder metallurgy(PM)Ni-based superalloys with different Hf and Ta contents were creep-tested at 650℃ and 970 MPa,700℃ and 770 MPa,and 750℃ and 580 MPa,respectively.The effect of Hf and Ta on creep deformation behaviors of the superalloys was studied from multiple scales by SEM,electron backscatter diffraction(EBSD),and aberration-corrected scanning transmission electron microscope(AC-STEM).The results showed that Hf and Ta suppressed the intergranular fracture and initiation of cracks during the acceleration creep stage,which prolonged the creep rupture time.Hf and Ta inhibited the stacking faults extending and the dislocation climbing and promoted the Suzuki segregation of W during the steady-state creep stage,which reduced the minimum creep rate and delayed the start time of the acceleration creep stage.The Suzuki segregation of Co,Cr,Mo,Ti,Nb,W,and Ta along stacking faults was observed after Hf and Ta addition,leading to the localized phase transformation in the γ′phase,and the stacking fault phase was chemically disordered.This study provided ideas for the composition design of novel PM Ni-based superalloys and theoretical foundations for the combined addition of Hf and Ta.展开更多
The impact of heavy reduction on dendritic morphology was explored by combining experimental research and numerical simulation in metallurgy,including a detailed three-dimensional(3D)analysis and reconstruction of den...The impact of heavy reduction on dendritic morphology was explored by combining experimental research and numerical simulation in metallurgy,including a detailed three-dimensional(3D)analysis and reconstruction of dendritic solidification structures.Combining scanning electron microscopy and energy-dispersive scanning analysis and ANSYS simulation,the high-precision image processing software Mimics Research was utilized to conduct the extraction of dendritic morphologies.Reverse engineering software NX Imageware was employed for the 3D reconstruction of two-dimensional dendritic morphologies,restoring the dendritic characteristics in three-dimensional space.The results demonstrate that in a two-dimensional plane,dendrites connect with each other to form irregularly shaped“ring-like”structures.These dendrites have a thickness greater than 0.1 mm along the Z-axis direction,leading to the envelopment of molten steel by dendrites in a 3D space of at least 0.1 mm.This results in obstructed flow,confirming the“bridging”of dendrites in three-dimensional space,resulting in a tendency for central segregation.Dense and dispersed tiny dendrites,under the influence of heat flow direction,interconnect and continuously grow,gradually forming primary and secondary dendrites in three-dimensional space.After the completion of dendritic solidification and growth,these microdendrites appear dense and dispersed on the two-dimensional plane,providing the nuclei for the formation of new dendrites.When reduction occurs at a solid fraction of 0.46,there is a noticeable decrease in dendritic spacing,resulting in improved central segregation.展开更多
Herein,the effect of direct current(DC)attached the mold on refining the microstructure and alleviating the central segregation of a tin–bismuth(Sn–10 wt.%Bi)alloy ingot during the solidification process has been in...Herein,the effect of direct current(DC)attached the mold on refining the microstructure and alleviating the central segregation of a tin–bismuth(Sn–10 wt.%Bi)alloy ingot during the solidification process has been investigated.The experiment used a self-made device,which can achieve the effect of refining the solidified structure and alleviate the segregation of the metal casting.Numerical simulations were performed to calculate the Lorentz force,Joule heating and induced melt vortex flow for the magneto-hydrodynamic case.Our results show that the maximum velocity of the global electro-vortex reached 0.017 m s^(–1).The DC-induced electro-vortex was found to be the primary reason of refining the equiaxed grain and alleviating the segregation of theβ-Sn crystal boundary.The grain refining effect observed in these experiments can be solely attributed to the forced melt flow driven by the Lorentz force.DC field attached the mold can lead to grain refinement and alleviate the segregation of the ingot via a global vortex.The technology can be applied not only to opened molds,but also toward improving the quality in closed molds.展开更多
Cation segregation on cathode surfaces plays a key role in determining the activity and operational stability of solid oxide fuel cells(SOFCs).The double perovskite oxide PrBa_(0.8)Ca_(0.2)Co_(2)O_(5+δ)(PBCC)has been...Cation segregation on cathode surfaces plays a key role in determining the activity and operational stability of solid oxide fuel cells(SOFCs).The double perovskite oxide PrBa_(0.8)Ca_(0.2)Co_(2)O_(5+δ)(PBCC)has been widely studied as an active cathode but still suffer from serious detrimental segregations.To enhance the cathode stability,a PBCC derived A-site medium-entropy Pr_(0.6)La_(0.1)Nd_(0.1)Sm_(0.1)Gd_(0.1)Ba_(0.8)Ca_(0.2)Co_(2)O_(5+δ)(ME-PBCC)oxide was prepared and its segregation behaviors were investigated under different conditions.Compared with initial PBCC oxide,the segregations of BaO and Co_(3)O_(4)on the surface of ME-PBCC material are significantly suppressed,especially for Co_(3)O_(4),which is attributed to its higher configuration entropy.Our results also confirm the improved electrochemical performance and structural stability of ME-PBCC material,enabling it as a promising cathode for SOFCs.展开更多
A newly developed P-doped CrCoNi medium-entropy alloy(MEA)provides both higher yield strength and larger uniform elongation than the conventional CrCoNi MEA,even superior tensile ductility to the other-element-doped C...A newly developed P-doped CrCoNi medium-entropy alloy(MEA)provides both higher yield strength and larger uniform elongation than the conventional CrCoNi MEA,even superior tensile ductility to the other-element-doped CrCoNi MEAs at similar yield strength levels.P segregation at grain boundaries(GBs)and dissolution inside grain interiors,together with the related lower stacking fault energy(SFE)are found in the P-doped CrCoNi MEA.Higher hetero-deformation-induced(HDI)hardening rate is observed in the P-doped CrCoNi MEA due to the grain-to-grain plastic deformation and the dynamic structural refinement by high-density stacking fault-walls(SFWs).The enhanced yield strength in the P-doped CoCrNi MEA can be attributed to the strong substitutional solid-solution strengthening by severer lattice distortion and the GB strengthening by phosphorus segregation at GBs.During the tensile deformation,the multiple SFW frames inundated with massive multi-orientational tiny planar stacking faults(SFs)between them,rather than deformation twins,are observed to induce dynamic structural refinement for forming par-allelepiped domains in the P-doped CoCrNi MEA,due to the lower SFE and even lower atomically-local SFE.These nano-sized domains with domain boundary spacing at tens of nanometers can block disloca-tion movement for strengthening on one hand,and can accumulate defects in the interiors of domains for exceptionally high hardening rate on the other hand.展开更多
Solute atoms and precipitates significantly influence the mechanical properties of Mg alloys.Previous studies have mainly focused on the segregation behaviors of Mg alloys after annealing.In this study,we investigated...Solute atoms and precipitates significantly influence the mechanical properties of Mg alloys.Previous studies have mainly focused on the segregation behaviors of Mg alloys after annealing.In this study,we investigated the segregation behaviors of an Mg-RE alloy under deformation.We found that the enrichment of solute atoms occurred in{101^(-)1}compressive twin boundaries under compression at 298 K without any annealing in an Mg-RE alloy by scanning transmission electron microscopy and energy-dispersive X-ray analysis.The segregated solutes and precipitates impeded the twin growth,partially contributing to the formation of small-sized{101^(-)1}compressive twins.This research indicates the twin boundaries can be strengthened by segregated solutes and precipitates formed under deformation at room temperature.展开更多
This study aims to clarify the mechanisms for the grain boundary(GB)segregation through investigating the absorption of excess solute atoms at GBs in Al−Cu alloys by using the hybrid molecular dynamics/Monte Carlo sim...This study aims to clarify the mechanisms for the grain boundary(GB)segregation through investigating the absorption of excess solute atoms at GBs in Al−Cu alloys by using the hybrid molecular dynamics/Monte Carlo simulations.Two segregation mechanisms,substitutional and interstitial mechanisms,are observed.The intergranular defects,including dislocations,steps and vacancies,and the intervals in structural units are conductive to the prevalence of interstitial mechanism.And substitutional mechanism is favored by the highly ordered twin GBs.Furthermore,the two mechanisms affect the GB structure differently.It is quantified that interstitial mechanism is less destructive to GB structure than substitutional one,and often leads to a segregation level being up to about 6 times higher than the latter.These findings contribute to atomic scale insights into the microscopic mechanisms about how solute atoms are absorbed by GB structures,and clarify the correlation among intergranular structures,segregation mechanisms and kinetics.展开更多
First-principles theory calculations were used to investigate the segregation behavior of P and Mg as well as the interactions between Mg and P atα-FeΣ3(111)[11¯0]symmetrical tilt grain boundary(GB).Results dem...First-principles theory calculations were used to investigate the segregation behavior of P and Mg as well as the interactions between Mg and P atα-FeΣ3(111)[11¯0]symmetrical tilt grain boundary(GB).Results demonstrate that both P and Mg are segregated at GB,and P has a stronger segregation potency.Mg prefers to substitute at grain boundary plane with the largest absorbable vacancy,whereas P inclines to substitute at the sites near Fe atoms to form strong covalent Fe-P bonds.When Mg exists at GB,the segregation behavior of P may be greatly inhibited by the decrease in possible solution sites and the increase in segregation energy.P has stronger interactions with Mg at GB,forming a lower energy hybridization peak.These results can be used to explain why the addition of a small amount of Mg can ameliorate the temper embrittlement phenomenon.展开更多
Grain boundary segregation(GBS)of solutes influences the grain size,texture,and strength of Mg wrought alloys.So far,solutes'GBS in Mg has mostly been investigated by qualitative experimental observations.In this ...Grain boundary segregation(GBS)of solutes influences the grain size,texture,and strength of Mg wrought alloys.So far,solutes'GBS in Mg has mostly been investigated by qualitative experimental observations.In this work,we develop a quantitative model to compute the grain boundary segregation energy(ΔE_(seg))in binary Mg based alloys that takes the relative atomic density of GB into account.The model is utilized to computeΔE_(seg)of Al,Zn,Ca,Sn,Y,Gd,and Nd solutes in Mg.The result suggests that rare earth elements and Ca are more prone to GBS than Al,Zn,and Sn.Segregation of Gd solutes can explain the smaller grain size and slower grain growth in Mg-Gd extruded alloys than Mg-Al and Mg-Zn counterparts.It also provides an explanation for the weak extrusion texture in Mg-Gd.展开更多
Upon ageing of a deformed metal,compositional segregation to dislocations and stacking faults is well known to elevate strength.However,Suzuki segregation effects typically result in a modest strength in-crease on the...Upon ageing of a deformed metal,compositional segregation to dislocations and stacking faults is well known to elevate strength.However,Suzuki segregation effects typically result in a modest strength in-crease on the order of 10 MPa for many substitutional face-centered-cubic solid solutions.Severe pre-deformation can lead to significant hardening but often at the cost of substantial tensile ductility af-ter subsequent aging.Here we propose a novel strategy to improve the Suzuki hardening effect in a single-phase CoCrNi alloy by meticulously controlling repetitive straining and annealing conditions with-out compromising ductility.Our findings revealed that multiple stages of annealing along the way of pre-straining significantly increase the fraction of dislocations that trap partitioning species(i.e.Cr),far exceeding the levels achievable through single-shot annealing after straight pre-deformation to the same accumulative strain(40%).Thermodynamically,the segregation of Cr into stacking faults is driven by re-duced local stacking fault energy(SEF)and system energy.The decreased SFE inhibits dislocation cross-slip,promotes partial dislocation nucleation,and facilitates dislocation intersection,leading to a high den-sity of extended stacking fault ribbons in the multi-pass strained and annealed samples.As a result,the yield strength increments of multi-pass treated samples(75±10 MPa)are four times higher than those of single-pass treated samples(18±8 MPa),while retaining an adequate strain hardening rate,thus pre-serving tensile ductility despite of plastic flow at higher stresses.Our strategy shows promise for broader applications,particularly in scenarios where conventional thermomechanical treatments fail to yield sat-isfactory results.展开更多
In this work,a good balance of strength and ductility(a yield strength of 185 MPa and a uniform elongation of 20%)has been obtained in a dilute Mg-1.8Zn-0.3Y-0.3Ca-0.3Zr(wt.%)alloy using hard plate rolling(HPR)followe...In this work,a good balance of strength and ductility(a yield strength of 185 MPa and a uniform elongation of 20%)has been obtained in a dilute Mg-1.8Zn-0.3Y-0.3Ca-0.3Zr(wt.%)alloy using hard plate rolling(HPR)followed by annealing,with a low anisotropy in mechanical properties.More importantly,the HPR-annealed alloy shows an excellent formability at the same time,i.e.,the index Erichsen(I.E.)value reaches 7.9 mm(the Erichsen cupping test)at room temperature,which is higher compared with the Mg-1.8Zn-0.3Y-0.3Ca0.3Zr alloy produced by conventional multi-pass rolling(CR)followed by annealing.The excellent synergy of strength and formability of the HPR-annealed alloy is mainly attributed to a weak elliptical ring texture,as well as finer and denser Zn_(2)Zr_(3)precipitates.The formation of weak elliptical ring texture is related to the preferential co-segregation of Zn and Ca elements at boundaries of basal grains with smal misorientation angles during annealing,which inhibits the growth of basal grains and promotes the preferential growth of non-basal grains At the same time,in comparison with the CR-annealed alloy,the HPR-annealed alloy contains finer and denser Zn_(2)Zr_(3)precipitates that ar less likely to become sources of cracks,leading to the higher strength and formability of the HPR-annealed alloy.The results in this work can provide reference for the development of high strength Mg alloy sheets with excellent room temperature formability,which also shed light on mitigating planar anisotropy in mechanical properties for Mg alloy sheets.展开更多
Elements(As,Bi)and(Cu,Fe)exhibiting two typical segregation behavior in liquid Sb alloys were selected as solute atoms for analysis.Ab initio molecular dynamics(AIMD)simulations were employed to study the molten Sb al...Elements(As,Bi)and(Cu,Fe)exhibiting two typical segregation behavior in liquid Sb alloys were selected as solute atoms for analysis.Ab initio molecular dynamics(AIMD)simulations were employed to study the molten Sb alloy at different temperatures.By analyzing its pair correlation function(PCF),bond pairs,bond angle distribution function(BADF),and Voronoi polyhedron(VP),the short-range order(SRO)of the alloy was investigated.In the Sb melt,the solute atoms Cu and Fe,which have smaller distribution coefficients,exhibit a stronger affinity for Sb than the solute atoms As and Bi,which have larger distribution coefficients.The BADF of As and Bi with larger distribution coefficients shows a lower probability of small-angle peaks compared to large-angle peaks,whereas the BADF of Cu and Fe with smaller distribution coefficients exhibits the opposite trend.The BADF reveals that Sb-As and Sb-Bi approach pure Sb melt,while Sb-Cu and Sb-Fe deviate significantly.Compared to Sb-Cu and Sb-Fe,the Sb-As and Sb-Bi systems exhibit more low-index bonds,suggesting weaker interactions and more disorder.The VP fractions around As and Bi atoms are lower than those around Cu and Fe,and the VP face distributions around As and Bi are more complex.There are differences in the VP around different solute atoms,primarily due to the varying bond pair fractions associated with each solute atom.Fe has the smallest diffusion coefficient,primarily due to its compact local structure.展开更多
Through thermodynamic calculations and microstructural characterization,the effect of niobium(Nb)content on the solidifica-tion characteristics of Alloy 625 Plus was systematically investigated.Subsequently,the effect...Through thermodynamic calculations and microstructural characterization,the effect of niobium(Nb)content on the solidifica-tion characteristics of Alloy 625 Plus was systematically investigated.Subsequently,the effect of Nb content on hot deformation behaviorwas examined through hot compression experiments.The results indicated that increasing the Nb content lowers the liquidus temperatureof the alloy by 51℃,producing a denser solidification microstructure.The secondary dendrite arm spacing(SDAS)of the alloy decreasesfrom 39.09 to 22.61μm.Increasing the Nb content alleviates element segregation but increases interdendritic precipitates,increasing theirarea fraction from 0.15% to 5.82%.These precipitates are primarily composed of large Laves,δ,η,and γ″phases,and trace amounts of Nb C.The shapes of these precipitates change from small chunks to large elongated forms.No significant change in the type or amount ofinclusions within the alloy is detected.The inclusions are predominantly individual Al_(2)O_(3) and TiN,as well as Al_(2)O_(3)/Ti N composite inclu-sions.Samples with varying Nb contents underwent hot compression deformation at a true strain of 0.69,a strain rate of 0.5 s^(-1),and a de-formation temperature of 1150℃.Increasing the Nb content also elevates the peak stress observed in the flow curves.However,alloyswith higher Nb content exhibit more pronounced recrystallization softening effects.The Laves phase precipitates do not completely redis-solve during hot deformation and are stretched to elongated shapes.The high-strain energy storage increases the recrystallization fractionfrom 32.4% to 95.5%,significantly enhancing the degree of recrystallization and producing a more uniform deformation microstructure.This effect is primarily attributed to the addition of Nb,which refines the initial grains of the alloy,enhances the solid solution strengthen-ing of the matrix,and improves the induction of particle-stimulated nucleation.展开更多
基金This work used Expanse systems at UCSD through an allocation[PHY240089]from the Advanced Cyberinfrastructure Coordination Ecosystem:Services&Support(ACCESS)programsupported by U.S.National Science Foundation(Grant Nos.2138259,2138286,2138307,2137603 and 2138296).
文摘We present a numerical framework for simulating viscous compressible flows in the presence of solid particles with large size ratios.The volume-filtered Navier-Stokes equations are discretized using a class of high-order low-dissipative finite difference operators with energy-preserving properties.No-slip,adiabatic boundary conditions are enforced at the surface of large particles(with diameters significantly larger than the local grid spacing)using a ghost-point immersed boundary method.Two-way coupling between the gas phase and small particles(with diameters proportional to the grid spacing)is accounted for through volumetric source terms for interphase momentum and energy exchange.A simple and efficient approach for collision detection between small and large particles is proposed.The framework is applied to simulations of planar shocks interacting with bidisperse distributions of particles with size ratios of approximately thirty.Particle dispersion and size segregation are reported and a simple analytical model for size segregation is proposed.
基金funded by the Natural Science Foundation of China(Grants No 42277127)。
文摘Rock-ice avalanches in cold high-mountain regions pose severe hazards due to their high mobility,yet the quantitative controls of particle-size ratio and ice content remain insufficiently constrained.This study investigates their coupled effects using inclinedflume experiments and Discrete Element Method(DEM)simulations,covering three gravel sizes(2-5 mm,5-7 mm,7-10 mm)and four ice-content levels(0%,20%,40%,60%).Run-out distance,velocity,energy components,flow regime(Savage number),and segregation indexαwere quantified.Increasing ice content significantly enhances mobility,but with diminishing marginal effectiveness.From 0%to 40%ice content,run-out distance increases by 41%-86%,whereas the additional increase from 40%to 60%contributes only 12%-23%.Particle-size ratio strongly governs segregation intensity.Fine-gravel groups reach segregation indices ofα=0.92-0.98,indicating nearly complete upward migration of ice,whereas medium-gravel and coarse-gravel groups exhibit much weaker segregation,stabilizing atα=0.68-0.74 and 0.60-0.69.Savage number analyses reveal marked flow-regime transitions.At 0%ice content,Savage numbers reach 1.0-1.5,indicating a collisional regime.Increasing ice content suppresses collisionality,with Savage numbers decreasing to 0.03-0.07 at 60%ice content,consistent with dense-regime flow.DEM energy analyses confirm this regime shift:for finegravel mixtures,collision energy decreases by 14%,while sliding-friction energy increases by 33%as ice content increases from 0%to 60%,reflecting enhanced overburden effects imposed by upward-segregated ice layers.Medium and coarse mixtures exhibit weaker or opposite energy-shift patterns,demonstrating strong size dependence.Mechanistically,large particle-size contrasts promote strong segregation and form dense basal rock layers that increase basal friction and reduce mobility.When particle sizes are similar or ice content is high,segregation remains limited,allowing ice to mix into the basal layer,thereby reducing basal friction and enhancing mobility.This research quantitatively demonstrates how composition controls particle spatial distribution,flow regime,and energy dissipation,offering new mechanistic insights into the propagation and deposition behaviors of rock-ice avalanches and improving hazard assessment in vulnerable high-mountain regions.
基金financially supported by the National Natural Science Foundation of China(Grant Nos.52474362,52174317 and 51904146)the General Project Funded by Liaoning Province Education Department(Grant No.JYTMS20230943)。
文摘The high-alloyed wrought superalloy GH4975 tends to form coarse MC carbides and eutectic(γ+γ′)phases,which adversely affect the cogging and homogenization process.To provide theoretical guidance for control of MC carbides and eutectic(γ+γ′)formation,differential thermal analysis(DTA)was utilized to investigate the effect of cooing rate(10-90℃·min^(-1))on solidification behavior and micro-segregation of GH4975 alloy.According to the thermodynamic calculation and distribution characteristics of precipitates,the MC carbides can act as nucleation sites forγdendrites,but the nucleation ofγdendrites becomes less dependent on the MC carbide primers at higher cooling rates.As theγdendrites grow,the elements including Ti and Nb gradually accumulate in the residual liquid and leads to the formation of more MC carbides near the interdendritic region.Finally,the solidification is terminated with the formation of eutectic(γ+γ′).With an increase in cooling rate,the liquidus temperature rises,but the solidus temperature decreases,and thus the solidification range is obviously enlarged.The dendritic structure is significantly refined by the increase of cooling rate.The secondary dendrite arm spacing,λ_(2),as a function of cooling rate,T,can be expressed asλ_(2)=216.78T^(-0.42).Moreover,the increasing cooling rate weakens the back diffusion of Al,Ti,and Nb,increases the undercooling,and limits the growth of precipitates.Consequently,the sizes of MC carbides,eutectic(γ+γ′),and primaryγ′significantly decrease,but the area fraction of eutectic(γ+γ′)linerly increases as the cooling rate rises.Thus moderate cooling rate(such as 30℃·min^(-1))should be selected during the solidification process of GH4975 alloy.
基金primarily supported by The National Natural Science Foundation of China(under Nos.52234009 and 52271103)Partial financial support came from the Program for the Central University Youth Innovation Team(No.419021423505)the Fundamental Research Funds for the Central Universities,JLU.
文摘In response to the urgent demand for lightweight,magnesium(Mg)alloys have garnered considerable attention owing to their low density.Nonetheless,the intrinsic poor room-temperature formability of Mg alloys remains a major obstacle in shaping precise complex components,necessitating the development of superplastic Mg alloys.Excellent superplasticity is usually acquired in high-alloyed Mg alloys with enhanced microstructural thermal stability facilitated by abundant optimized second-phase particles.While for cost-effective low-alloyed Mg alloys lacking particles,regulating solute segregation has emerged as a promising approach to achieve superplasticity recently.Moreover,the potential of bimodal-grained Mg alloys for superplastic deformation has been revealed,expanding the options for designing superplastic materials beyond the conventional approach of fine-grained microstructures.This study reviews significant developments in superplastic Mg alloys from the view of alloying strategies,grain structure control and deformation mechanisms,with potential implications for future research and industrial applications of superplastic Mg alloys.
基金supported by the Scientific and Technological Developing Scheme of Jilin Province under grants no.YDZJ202301ZYTS538the Chinese Academy of Sciences Youth Innovation Promotion Association under grants number 2023234+3 种基金the National Natural Science Foundation of China under grants number U21A20323the Scientific and Technological Developing Scheme of Jilin Province under grants no.SKL202302038the Major Scientific and Technological Projects of Hebei Province under grants No.23291001Zthe Scientific and Technology Project of Hanjiang District.
文摘Asymmetric tilt boundaries on conventional twin boundaries(TBs)are significant for understanding the role of twins on coordinating plastic deformation in many metallic alloys.However,the formation modes of many asymmetric tilt boundaries are hard to be accounted for based on traditional theoretical models,and the corresponding solute segregation is complex.Herein,atomic structures of a specific asymmetric boundary on{1012}TBs were reveled using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM),molecular dynamics(MD)and density functional theory(DFT)simulations.Reaction between<a60>M dislocations and the{1012}TB can generate a~61°/25°asymmetric tilt boundary.The segregation of Gd and Zn atoms is closely related to the aggregateddislocations and the interfacial interstices of the asymmetric tilt boundary,which is energetically favorable in reducing the total system energy.
基金financial support from the National Natural Science Foundation of China(No:52061040,No:52371121,No:52031011)China Postdoctoral Science Foundation(No:2021M692512)the 2020 open projects(No:KLATM202003)of Key Laboratory of Advanced Technologies of Materials,Ministry of Education China,Southwest Jiaotong University。
文摘The limited creep resistance of wrought Mg-Al alloys restricts their lightweight applications at intermediate temperatures due to the softening effect of discontinuous precipitation(DP)on the dislocation-controlled creep.Here,we developed a creep-resistant wrought Mg-Al alloy through microalloying of Y and Ca.The resulting alloy exhibited an order of magnitude enhancement in the creep resistance at 125℃/50±100 MPa.In contrast to the grain boundary instabilities by DP in the previously reported wrought Mg-Al alloys,we show that the addition of 0.21Y+0.15Ca wt%produces a(Zn+Ca)co-segregation at the grain boundaries as a result of their segregation energy and the activation energy of grain boundary migration,thereby stabilizing the grain boundaries.The(Zn+Ca)co-segregation inhibits the dynamic DP and promotes the formation of intragranular Al-enriched clusters,which favorthe formation of Al_(2)Y,Mg_(17)Al_(12)nano precipitates,thereby impeding intragranular dislocation motion during creep.Furthermore,the addition of 0.21Y+0.15Ca wt%facilitates the formation of a fine and uniform recrystallization structure in the microalloyed alloys compared to AZ80 due to the high activation energy of mobility for the(Zn+Ca)segregated grain boundary.Therefore,the microalloyed alloys exhibit good tensile properties with 380 MPa tensile strength and 18%elongation.Our constitutive analysis revealed that the(Y+Ca)microalloying decreased the creep stress exponent by 29%and increased the creep resistance in the medium to high-stress range.Microalloying provides a promising way to develop low-cost creep-resistant wrought Mg-Al alloys.
基金financially supported by the National Science and Technology Major Project of China(No.2017-Ⅵ-0008-0078)。
文摘Four powder metallurgy(PM)Ni-based superalloys with different Hf and Ta contents were creep-tested at 650℃ and 970 MPa,700℃ and 770 MPa,and 750℃ and 580 MPa,respectively.The effect of Hf and Ta on creep deformation behaviors of the superalloys was studied from multiple scales by SEM,electron backscatter diffraction(EBSD),and aberration-corrected scanning transmission electron microscope(AC-STEM).The results showed that Hf and Ta suppressed the intergranular fracture and initiation of cracks during the acceleration creep stage,which prolonged the creep rupture time.Hf and Ta inhibited the stacking faults extending and the dislocation climbing and promoted the Suzuki segregation of W during the steady-state creep stage,which reduced the minimum creep rate and delayed the start time of the acceleration creep stage.The Suzuki segregation of Co,Cr,Mo,Ti,Nb,W,and Ta along stacking faults was observed after Hf and Ta addition,leading to the localized phase transformation in the γ′phase,and the stacking fault phase was chemically disordered.This study provided ideas for the composition design of novel PM Ni-based superalloys and theoretical foundations for the combined addition of Hf and Ta.
基金supported by Open Foundation of the State Key Laboratory of Refractories and Metallurgy(No.G201711)the National Natural Science Foundation of China(Nos.52104317 and 51874001).
文摘The impact of heavy reduction on dendritic morphology was explored by combining experimental research and numerical simulation in metallurgy,including a detailed three-dimensional(3D)analysis and reconstruction of dendritic solidification structures.Combining scanning electron microscopy and energy-dispersive scanning analysis and ANSYS simulation,the high-precision image processing software Mimics Research was utilized to conduct the extraction of dendritic morphologies.Reverse engineering software NX Imageware was employed for the 3D reconstruction of two-dimensional dendritic morphologies,restoring the dendritic characteristics in three-dimensional space.The results demonstrate that in a two-dimensional plane,dendrites connect with each other to form irregularly shaped“ring-like”structures.These dendrites have a thickness greater than 0.1 mm along the Z-axis direction,leading to the envelopment of molten steel by dendrites in a 3D space of at least 0.1 mm.This results in obstructed flow,confirming the“bridging”of dendrites in three-dimensional space,resulting in a tendency for central segregation.Dense and dispersed tiny dendrites,under the influence of heat flow direction,interconnect and continuously grow,gradually forming primary and secondary dendrites in three-dimensional space.After the completion of dendritic solidification and growth,these microdendrites appear dense and dispersed on the two-dimensional plane,providing the nuclei for the formation of new dendrites.When reduction occurs at a solid fraction of 0.46,there is a noticeable decrease in dendritic spacing,resulting in improved central segregation.
基金the National Natural Science Foundation of China(51974155)the Outstanding Young Scientific and Technological Talents Project of University of Science and Technology Liaoning(2023YQ07)+4 种基金the University of Science and Technology Liaoning Young Foundation(2018QN06)National Natural Science Foundation of China(51774178)the National Key Research and Development Program(2021YFB3702005)the Central Government Guides Local Science and Technology Development Fund Projects(2023JH6/100100046)the Liaoning Provincial Department of Education Basic Research Projects for Universities(JYTQN2023242).
文摘Herein,the effect of direct current(DC)attached the mold on refining the microstructure and alleviating the central segregation of a tin–bismuth(Sn–10 wt.%Bi)alloy ingot during the solidification process has been investigated.The experiment used a self-made device,which can achieve the effect of refining the solidified structure and alleviate the segregation of the metal casting.Numerical simulations were performed to calculate the Lorentz force,Joule heating and induced melt vortex flow for the magneto-hydrodynamic case.Our results show that the maximum velocity of the global electro-vortex reached 0.017 m s^(–1).The DC-induced electro-vortex was found to be the primary reason of refining the equiaxed grain and alleviating the segregation of theβ-Sn crystal boundary.The grain refining effect observed in these experiments can be solely attributed to the forced melt flow driven by the Lorentz force.DC field attached the mold can lead to grain refinement and alleviate the segregation of the ingot via a global vortex.The technology can be applied not only to opened molds,but also toward improving the quality in closed molds.
基金Project supported by the National Natural Science Foundation of China(22279025,21773048,52302119)the Fundamental Research Funds for the Central Universities(2023FRFK06005,HIT.NSRIF202204)。
文摘Cation segregation on cathode surfaces plays a key role in determining the activity and operational stability of solid oxide fuel cells(SOFCs).The double perovskite oxide PrBa_(0.8)Ca_(0.2)Co_(2)O_(5+δ)(PBCC)has been widely studied as an active cathode but still suffer from serious detrimental segregations.To enhance the cathode stability,a PBCC derived A-site medium-entropy Pr_(0.6)La_(0.1)Nd_(0.1)Sm_(0.1)Gd_(0.1)Ba_(0.8)Ca_(0.2)Co_(2)O_(5+δ)(ME-PBCC)oxide was prepared and its segregation behaviors were investigated under different conditions.Compared with initial PBCC oxide,the segregations of BaO and Co_(3)O_(4)on the surface of ME-PBCC material are significantly suppressed,especially for Co_(3)O_(4),which is attributed to its higher configuration entropy.Our results also confirm the improved electrochemical performance and structural stability of ME-PBCC material,enabling it as a promising cathode for SOFCs.
基金supported by the National Key R&D Program of China(No.2019YFA0209902)the Natural Science Foundation of China(Nos.52071326,52192593,51601204)+1 种基金the NSFC Basic Science Center Program for Multiscale Problems in Nonlinear Mechanics(No.11988102)the Strategic Priority Research Program of the Chinese Academy of Sciences(No.XDB22040503).
文摘A newly developed P-doped CrCoNi medium-entropy alloy(MEA)provides both higher yield strength and larger uniform elongation than the conventional CrCoNi MEA,even superior tensile ductility to the other-element-doped CrCoNi MEAs at similar yield strength levels.P segregation at grain boundaries(GBs)and dissolution inside grain interiors,together with the related lower stacking fault energy(SFE)are found in the P-doped CrCoNi MEA.Higher hetero-deformation-induced(HDI)hardening rate is observed in the P-doped CrCoNi MEA due to the grain-to-grain plastic deformation and the dynamic structural refinement by high-density stacking fault-walls(SFWs).The enhanced yield strength in the P-doped CoCrNi MEA can be attributed to the strong substitutional solid-solution strengthening by severer lattice distortion and the GB strengthening by phosphorus segregation at GBs.During the tensile deformation,the multiple SFW frames inundated with massive multi-orientational tiny planar stacking faults(SFs)between them,rather than deformation twins,are observed to induce dynamic structural refinement for forming par-allelepiped domains in the P-doped CoCrNi MEA,due to the lower SFE and even lower atomically-local SFE.These nano-sized domains with domain boundary spacing at tens of nanometers can block disloca-tion movement for strengthening on one hand,and can accumulate defects in the interiors of domains for exceptionally high hardening rate on the other hand.
基金support from Interdisciplinary Research Project for Young Teachers of USTB Fundamental Research Funds for the Central Universities(Grant no.FRF-IDRY-23-030).
文摘Solute atoms and precipitates significantly influence the mechanical properties of Mg alloys.Previous studies have mainly focused on the segregation behaviors of Mg alloys after annealing.In this study,we investigated the segregation behaviors of an Mg-RE alloy under deformation.We found that the enrichment of solute atoms occurred in{101^(-)1}compressive twin boundaries under compression at 298 K without any annealing in an Mg-RE alloy by scanning transmission electron microscopy and energy-dispersive X-ray analysis.The segregated solutes and precipitates impeded the twin growth,partially contributing to the formation of small-sized{101^(-)1}compressive twins.This research indicates the twin boundaries can be strengthened by segregated solutes and precipitates formed under deformation at room temperature.
基金supported by grants from the National Natural Science Foundation of China(Nos.52031017,51801237)the National Key Laboratory of Science and Technology on High-strength Structural Materials in Central South University,China(No.6142912200106).
文摘This study aims to clarify the mechanisms for the grain boundary(GB)segregation through investigating the absorption of excess solute atoms at GBs in Al−Cu alloys by using the hybrid molecular dynamics/Monte Carlo simulations.Two segregation mechanisms,substitutional and interstitial mechanisms,are observed.The intergranular defects,including dislocations,steps and vacancies,and the intervals in structural units are conductive to the prevalence of interstitial mechanism.And substitutional mechanism is favored by the highly ordered twin GBs.Furthermore,the two mechanisms affect the GB structure differently.It is quantified that interstitial mechanism is less destructive to GB structure than substitutional one,and often leads to a segregation level being up to about 6 times higher than the latter.These findings contribute to atomic scale insights into the microscopic mechanisms about how solute atoms are absorbed by GB structures,and clarify the correlation among intergranular structures,segregation mechanisms and kinetics.
文摘First-principles theory calculations were used to investigate the segregation behavior of P and Mg as well as the interactions between Mg and P atα-FeΣ3(111)[11¯0]symmetrical tilt grain boundary(GB).Results demonstrate that both P and Mg are segregated at GB,and P has a stronger segregation potency.Mg prefers to substitute at grain boundary plane with the largest absorbable vacancy,whereas P inclines to substitute at the sites near Fe atoms to form strong covalent Fe-P bonds.When Mg exists at GB,the segregation behavior of P may be greatly inhibited by the decrease in possible solution sites and the increase in segregation energy.P has stronger interactions with Mg at GB,forming a lower energy hybridization peak.These results can be used to explain why the addition of a small amount of Mg can ameliorate the temper embrittlement phenomenon.
基金supported by the National Key Research and Development Program of China(No.2021YFB3702602)the National Natural Science Foundation of China(Nos.51825101,52425101)。
文摘Grain boundary segregation(GBS)of solutes influences the grain size,texture,and strength of Mg wrought alloys.So far,solutes'GBS in Mg has mostly been investigated by qualitative experimental observations.In this work,we develop a quantitative model to compute the grain boundary segregation energy(ΔE_(seg))in binary Mg based alloys that takes the relative atomic density of GB into account.The model is utilized to computeΔE_(seg)of Al,Zn,Ca,Sn,Y,Gd,and Nd solutes in Mg.The result suggests that rare earth elements and Ca are more prone to GBS than Al,Zn,and Sn.Segregation of Gd solutes can explain the smaller grain size and slower grain growth in Mg-Gd extruded alloys than Mg-Al and Mg-Zn counterparts.It also provides an explanation for the weak extrusion texture in Mg-Gd.
基金sponsored by the National Key Research and Development Program,Major Scientific Instrument Special Pro-gram for Basic Research-Development and Application of All-domestic Three-dimensional Atom Probe Precision Measurement Instrument Project(No.2023YFF0716200)the State Key Lab-oratory of Powder Metallurgy,Central South University,Changsha,China.Q.Cheng was supported by the China Postdoctoral Science Foundation(No.2023M741111,No.GZC20230752)+2 种基金E.Ma acknowl-edges the National Natural Science Foundation of China(Grant No.52231001)J.Ding was supported by the Natural Science Founda-tion of China(Grant No.12004294)the National Youth Talents Program and the HPC platform of Xi’an Jiaotong University.M.W.Chen was supported by the U.S.National Science Foundation grant DMR-1804320.J.H.Chen acknowledges the financial support from the National Natural Science Foundation of China(No.51831004)。
文摘Upon ageing of a deformed metal,compositional segregation to dislocations and stacking faults is well known to elevate strength.However,Suzuki segregation effects typically result in a modest strength in-crease on the order of 10 MPa for many substitutional face-centered-cubic solid solutions.Severe pre-deformation can lead to significant hardening but often at the cost of substantial tensile ductility af-ter subsequent aging.Here we propose a novel strategy to improve the Suzuki hardening effect in a single-phase CoCrNi alloy by meticulously controlling repetitive straining and annealing conditions with-out compromising ductility.Our findings revealed that multiple stages of annealing along the way of pre-straining significantly increase the fraction of dislocations that trap partitioning species(i.e.Cr),far exceeding the levels achievable through single-shot annealing after straight pre-deformation to the same accumulative strain(40%).Thermodynamically,the segregation of Cr into stacking faults is driven by re-duced local stacking fault energy(SEF)and system energy.The decreased SFE inhibits dislocation cross-slip,promotes partial dislocation nucleation,and facilitates dislocation intersection,leading to a high den-sity of extended stacking fault ribbons in the multi-pass strained and annealed samples.As a result,the yield strength increments of multi-pass treated samples(75±10 MPa)are four times higher than those of single-pass treated samples(18±8 MPa),while retaining an adequate strain hardening rate,thus pre-serving tensile ductility despite of plastic flow at higher stresses.Our strategy shows promise for broader applications,particularly in scenarios where conventional thermomechanical treatments fail to yield sat-isfactory results.
基金Tral Science Foundation of China(Nos.52271103,52334010and 52271031)Partial financial support came from JilinScientific and Technological Development Program(No.20220301026GX)Program for the Central UniversityYouth Innovation Team。
文摘In this work,a good balance of strength and ductility(a yield strength of 185 MPa and a uniform elongation of 20%)has been obtained in a dilute Mg-1.8Zn-0.3Y-0.3Ca-0.3Zr(wt.%)alloy using hard plate rolling(HPR)followed by annealing,with a low anisotropy in mechanical properties.More importantly,the HPR-annealed alloy shows an excellent formability at the same time,i.e.,the index Erichsen(I.E.)value reaches 7.9 mm(the Erichsen cupping test)at room temperature,which is higher compared with the Mg-1.8Zn-0.3Y-0.3Ca0.3Zr alloy produced by conventional multi-pass rolling(CR)followed by annealing.The excellent synergy of strength and formability of the HPR-annealed alloy is mainly attributed to a weak elliptical ring texture,as well as finer and denser Zn_(2)Zr_(3)precipitates.The formation of weak elliptical ring texture is related to the preferential co-segregation of Zn and Ca elements at boundaries of basal grains with smal misorientation angles during annealing,which inhibits the growth of basal grains and promotes the preferential growth of non-basal grains At the same time,in comparison with the CR-annealed alloy,the HPR-annealed alloy contains finer and denser Zn_(2)Zr_(3)precipitates that ar less likely to become sources of cracks,leading to the higher strength and formability of the HPR-annealed alloy.The results in this work can provide reference for the development of high strength Mg alloy sheets with excellent room temperature formability,which also shed light on mitigating planar anisotropy in mechanical properties for Mg alloy sheets.
文摘Elements(As,Bi)and(Cu,Fe)exhibiting two typical segregation behavior in liquid Sb alloys were selected as solute atoms for analysis.Ab initio molecular dynamics(AIMD)simulations were employed to study the molten Sb alloy at different temperatures.By analyzing its pair correlation function(PCF),bond pairs,bond angle distribution function(BADF),and Voronoi polyhedron(VP),the short-range order(SRO)of the alloy was investigated.In the Sb melt,the solute atoms Cu and Fe,which have smaller distribution coefficients,exhibit a stronger affinity for Sb than the solute atoms As and Bi,which have larger distribution coefficients.The BADF of As and Bi with larger distribution coefficients shows a lower probability of small-angle peaks compared to large-angle peaks,whereas the BADF of Cu and Fe with smaller distribution coefficients exhibits the opposite trend.The BADF reveals that Sb-As and Sb-Bi approach pure Sb melt,while Sb-Cu and Sb-Fe deviate significantly.Compared to Sb-Cu and Sb-Fe,the Sb-As and Sb-Bi systems exhibit more low-index bonds,suggesting weaker interactions and more disorder.The VP fractions around As and Bi atoms are lower than those around Cu and Fe,and the VP face distributions around As and Bi are more complex.There are differences in the VP around different solute atoms,primarily due to the varying bond pair fractions associated with each solute atom.Fe has the smallest diffusion coefficient,primarily due to its compact local structure.
基金the financial support from the National Natural Science Foundation of China(No.52174303)the Program of Introducing Talents of Disciplineto Universities,China(No.B21001)the Joint Program of Science and Technology Plans in Liaoning Province,China(No.2023JH2/101700302t)。
文摘Through thermodynamic calculations and microstructural characterization,the effect of niobium(Nb)content on the solidifica-tion characteristics of Alloy 625 Plus was systematically investigated.Subsequently,the effect of Nb content on hot deformation behaviorwas examined through hot compression experiments.The results indicated that increasing the Nb content lowers the liquidus temperatureof the alloy by 51℃,producing a denser solidification microstructure.The secondary dendrite arm spacing(SDAS)of the alloy decreasesfrom 39.09 to 22.61μm.Increasing the Nb content alleviates element segregation but increases interdendritic precipitates,increasing theirarea fraction from 0.15% to 5.82%.These precipitates are primarily composed of large Laves,δ,η,and γ″phases,and trace amounts of Nb C.The shapes of these precipitates change from small chunks to large elongated forms.No significant change in the type or amount ofinclusions within the alloy is detected.The inclusions are predominantly individual Al_(2)O_(3) and TiN,as well as Al_(2)O_(3)/Ti N composite inclu-sions.Samples with varying Nb contents underwent hot compression deformation at a true strain of 0.69,a strain rate of 0.5 s^(-1),and a de-formation temperature of 1150℃.Increasing the Nb content also elevates the peak stress observed in the flow curves.However,alloyswith higher Nb content exhibit more pronounced recrystallization softening effects.The Laves phase precipitates do not completely redis-solve during hot deformation and are stretched to elongated shapes.The high-strain energy storage increases the recrystallization fractionfrom 32.4% to 95.5%,significantly enhancing the degree of recrystallization and producing a more uniform deformation microstructure.This effect is primarily attributed to the addition of Nb,which refines the initial grains of the alloy,enhances the solid solution strengthen-ing of the matrix,and improves the induction of particle-stimulated nucleation.
