The low damage resistance and fracture toughness hinder the widespread application of ultrahighstrength dual phase(DP)steels.In this work,we propose a novel strategy to improve the fracture toughness of ultrahigh-stre...The low damage resistance and fracture toughness hinder the widespread application of ultrahighstrength dual phase(DP)steels.In this work,we propose a novel strategy to improve the fracture toughness of ultrahigh-strength DP steels by an order of magnitude without sacrificing the tensile strength.Six ultrahigh-strength DP steels with varying microstructure but comparable tensile strength(>1400 MPa)were prepared via tailoring the heat treatment process after cold rolling.Additionally,finite element(FE)method incorporated with Gurson-Tvergaad-Needleman(GTN)model and cohesive zone model(CZM)is established to simulate the fracture behavior of DP steel.Twelve model DP steels with different ferrite sizes and F/M strength differences are constructed.The combined experiment and simulation results demonstrate that(i)ferrite/martensite(F/M)interface decohesion prevails in all steels,(ii)the ferrite morphology has a strong influence on the fracture toughness of ultrahigh-strength DP steels,(iii)the effects of matrix type,ferrite size,and F/M hardness difference on the fracture toughness are relatively weak,(iv)the exceptional high fracture toughness of plate-like DP steel can be attributed to the crack deflection,crack divider and crack arrester mechanisms induced by F/M interface decohesion.展开更多
This work reveals the significant effects of cobalt(Co)on the microstructure and impact toughness of as-quenched highstrength steels by experimental characterizations and thermo-kinetic analyses.The results show that ...This work reveals the significant effects of cobalt(Co)on the microstructure and impact toughness of as-quenched highstrength steels by experimental characterizations and thermo-kinetic analyses.The results show that the Co-bearing steel exhibits finer blocks and a lower ductile-brittle transition temperature than the steel without Co.Moreover,the Co-bearing steel reveals higher transformation rates at the intermediate stage with bainite volume fraction ranging from around 0.1 to 0.6.The improved impact toughness of the Co-bearing steel results from the higher dense block boundaries dominated by the V1/V2 variant pair.Furthermore,the addition of Co induces a larger transformation driving force and a lower bainite start temperature(BS),thereby contributing to the refinement of blocks and the increase of the V1/V2 variant pair.These findings would be instructive for the composition,microstructure design,and property optimization of high-strength steels.展开更多
As a typical steel,the fatigue of marine high-strength steels has been emphasized by scholars.In this paper,the fatigue performance and crack growth mechanism of a high-strength steel for ships are investigated by exp...As a typical steel,the fatigue of marine high-strength steels has been emphasized by scholars.In this paper,the fatigue performance and crack growth mechanism of a high-strength steel for ships are investigated by experimental methods.First,the fatigue threshold test and fatigue crack growth rate test of this high-strength steel under different stress ratios were carried out.The influence of stress ratio on the fatigue properties of this steel was analyzed.Secondly,scanning electron microscope was used to analyze the crack growth specimen section of this steel.The crack growth and failure mechanism of this steel were revealed.Finally,based on the above research results,the stress ratio effect of high-strength steel was investigated from the perspectives of crack closure and driving force.Considering the fatigue behavior in the near-threshold stage and the destabilization stage,a fatigue crack growth behavior prediction model of highstrength steel was established.The accuracy of the model was verified by test data.Moreover,the applicability of the modified model to various materials and its excellent predictive ability were verified through comparison with literature data and existing models.展开更多
The optimization of deposited metal properties through the addition of rare earth elements to welding materials was explored.Utilizing optical microscope,scanning electron microscope,energy dispersive spectroscope,and...The optimization of deposited metal properties through the addition of rare earth elements to welding materials was explored.Utilizing optical microscope,scanning electron microscope,energy dispersive spectroscope,and X-ray diffractometer,combined with software tools like Matlab,Image-Pro Plus,and CHANNEL5,the influence mechanism of rare earth element addition on the strength,toughness,and inclusions of deposited metal in 1000 MPa grade high-strength steel was investigated.The results indicate that the incorporation of rare earth elements enhances the weldability of the welding materials.With the addition of rare earth elements,the tensile strength of the deposited metal increases from 935 MPa to 960 MPa.However,further addition leads to a decrease in tensile strength,while the yield strength continuously increases by 8.5%-17.2%.The addition of appropriate amounts of rare earth elements results in an increase in acicular ferrite and retained austenite content,as well as grain refinement in the deposited metal,leading to 8.5%-24.3% and 15.6%-42.2% enhancement in impact energy at−40℃ and−60℃,respectively.Additionally,the proper addition of rare earth elements modifies the inclusions and generates fine and dispersed composite inclusions that bond better with the matrix,thereby optimizing the properties of the deposited metal through various mechanisms.Adding an appropriate amount of rare earth elements can significantly enhance the properties of the deposited metal in 1000 MPa grade high-strength steel,and improve the match between high strength and toughness,meeting the demands for high-strength steel used in hydropower applications.展开更多
The influence mechanism of trace Nb on the corrosion resistance of surface corrosion products of high-strength anti-seismic rebar in the simulated marine environment was studied by combining first-principles calculati...The influence mechanism of trace Nb on the corrosion resistance of surface corrosion products of high-strength anti-seismic rebar in the simulated marine environment was studied by combining first-principles calculations with corrosion mass loss method,surface analysis,cross-sectional analysis,quantitative analysis,and electrochemical test.The results demonstrated that the addition of trace Nb effectively improved the compactness and stability of surface corrosion layer of rebar,and the corrosion resistance of corrosion layer increased with the increase in Nb content.The beneficial effect of Nb content on the corrosion layer summarized two important key points.Firstly,the addition of Nb was beneficial to promoting the improvement in the structural stability of α-FeOOH,and α-FeOOH structure of solid solution Nb atoms was beneficial to strengthening the fixation of Cl atoms,thus increasing α/(β+γ)ratio,total impedance value,and corrosion potential.Secondly,the formation of Nb oxides can not only repair the corrosion layer,but also play a role in the fixation Cl atoms,resulting in the improvement in corrosion resistance of corrosion layer.展开更多
Corrosion is an essential issue limiting the application of high-strength low-carbon steel in seawater environment. The impact of retained austenite on its corrosion behavior with immersion experiments and related cor...Corrosion is an essential issue limiting the application of high-strength low-carbon steel in seawater environment. The impact of retained austenite on its corrosion behavior with immersion experiments and related corrosion sensor technology was explored. A model that clarifies the micro-galvanic effect and the heat-induced changes to the shape and composition of retained austenite was used to discuss the findings. The results indicated that retained austenite was generated following an intercritical process and demonstrated approximately 48 mV higher Volta potential than the matrix. The retained austenite content first increased and then decreased with increasing intercritical temperatures, while reaching the maximum value of 8.5% at 660℃. With the increase in retained austenite content, the corrosion rate was increased by up to 32.8% compared to “quenching + tempering” (QT) specimen. The interfaces between the retained austenite and matrix were the priority nucleation sites for corrosion. Moreover, the retained austenite reduced the corrosion resistance of the steel by increasing the micro-galvanic effect and reducing rust layer compactness.展开更多
This study deals with the development of a 780-MPa-class hot-rolled advanced high-strength steel(AHSS)with an ultrahigh elongation at break of approximately 30%and strength-ductility product exceeding 24 GPa·%,in...This study deals with the development of a 780-MPa-class hot-rolled advanced high-strength steel(AHSS)with an ultrahigh elongation at break of approximately 30%and strength-ductility product exceeding 24 GPa·%,indicating the excellent formability of the newly developed AHSS.The microstructure of the newly developed 780-MPa-class AHSS consists mainly of the triplex phase of ferrite,bainite,and retained austenite with a volume fraction of 10%±2%.The stability of the retained austenite in the newly developed AHSS is much higher than that of conventional transformation-induced plasticity steels,in which the retained austenite is prone to transformation into martensite under deformation.At a pre-strain lower than 1.2%,the volume fraction of the retained austenite and the elongation at break of the present 780-MPa-class AHSS remain almost unchanged,showing a high tolerance in the process window during leveling or straightening.Therefore,the present 780-MPa-class AHSS is particularly suitable for the production of components with complex shapes.展开更多
Aluminum alloys that are additively manufactured using the laser powder bed fusion(LPBF)suffer from relatively poor high cycle fatigue(HCF)resistance.In an effort to alleviate this,a high-strength Al alloy,Al-Mn-Mg-Sc...Aluminum alloys that are additively manufactured using the laser powder bed fusion(LPBF)suffer from relatively poor high cycle fatigue(HCF)resistance.In an effort to alleviate this,a high-strength Al alloy,Al-Mn-Mg-Sc-Zr,with columnar,equiaxed,and bi-modal microstructures was produced by varying the scanning velocity and the substrate temperature during the LPBF process.The tensile strength of LPBF Al-Mn-Mg-Sc-Zr alloy is 475±5–516±6 MPa with favorable elongation of approximately 11%,higher than that of most of the other Al alloys,including conventional high-strength rolled/ECAP Al alloys and AM Al-Mg-Sc-Zr alloys.Specimens with bimodal microstructure and specimens with fully equiaxed microstructure both show a fatigue strength of 230 MPa(at 107 loading cycles),which is the highest among those reported for the LPBF Al alloys.The deformation synergy in the bimodal microstructure also improves the fatigue resistance in the strain-controlled low cycle fatigue(LCF)regime.The equiaxed microstructure restricts the to-and-fro dislocation motion during cyclic loading,which,in turn,minimizes the strain localization.At the later stages of strain accumulation,microcracks form at the grain boundaries,limiting the further improvement of the alloy's fatigue strength.This study demonstrates microstructural tailoring through AM enables improvement of the fatigue resistance of aluminum alloys.展开更多
High-order asymmetric flatness defects resulting from the abnormal state of roll system are the main issue of precision rolling mill in the manufacturing process of high-strength thin strip.Due to the difficulty of mo...High-order asymmetric flatness defects resulting from the abnormal state of roll system are the main issue of precision rolling mill in the manufacturing process of high-strength thin strip.Due to the difficulty of monitoring and adjusting the abnormal state,the spatial state of roll system cannot be controlled by traditional methods.It is difficult to fundamentally improve these high-order asymmetric flatness defects.Therefore,a digital twin model of flatness control process for S6-high rolling mill was established,which could be used to analyze the influence of the abnormal state on the flatness control characteristic and propose improvement strategies.The internal relationship between the force state of side support roll system and the abnormal state of roll system was proposed.The XGBoost algorithm model was established to analyze the contribution degree of the side support roll system force to the flatness characteristic quantity.The abnormal state of roll system in the S6-high rolling mill can be diagnosed by analyzing the flatness characteristic difference between flatness value of the rolled strip and calculated characteristic value of finite element simulation.The flatness optimization model of the gray wolf optimization–long short-term memory non-dominated sorting whale optimization algorithm(GWO-LSTM-NSWOA)was established,and the decision-making selection was made from the Pareto frontier based on the flatness requirements of cold rolling to regulate the abnormal state of the roll system.The results indicate that the contribution degree of the force of the side support roll system to the flatness characteristics is more than 25%,which is the main influence of high-order asymmetric flatness defect.The performance of the GWO-LSTM flatness feature prediction model has clear advantages over back propagation and LSTM.The practical applications show that optimizing the force of side support roll system can reduce the high point of high-strength strip flatness from 13.2 to 6 IU and decrease the percentage of low-strength strip flatness defects from 1.6%to 1.2%.This optimization greatly reduced the proportion of flatness defects,improved the accuracy level of flatness control of precision rolling mill,and provided a guarantee for the stable production of thin strip.展开更多
The dilatometric curves of Q690 steel at different cooling rates were tested using Gleeble 3800 to study the continuous cooling transformation behavior of high-strength steel with low welding crack sensitivity.The con...The dilatometric curves of Q690 steel at different cooling rates were tested using Gleeble 3800 to study the continuous cooling transformation behavior of high-strength steel with low welding crack sensitivity.The continuous cooling transformation curves of Q690 steel were constructed according to the transformation tem-peratures determined using dilatometric curves and corresponding microstructures at different cooling rates.The results show that the microstructure of Q690 steel remarkably changed with the increase in the cooling rate.The matrix of the steel was ferrite and pearlite when the cooling rate was between 0.1 K/s and 0.5 K/s.With increasing cooling rate, the microstructure of the steel was considerably refined, and the Vickers hardness of the steel increased.When the cooling rate reached 1 K/s, the microstructure of the steel was further refined, and bainite transformation occurred.Polygonal ferrite almost disappeared from the steel when the cooling rate reached 10 K/s, and the matrix was mainly composed of bainite and a small amount of lath martensite.With the increase in the cooling rate, the ferrite decreased and martensite increased in the steel, resulting in higher Vickers hardness.When the cooling rate reached 30 K/s, the microstructure of the tested steel was full martensite matrix.展开更多
A secondary-cooling-segment electromagnetic stirring(S-EMS)experiment was performed at 150 A and 4 Hz to evaluate the effect of S-EMS on solidification characterization near the white band.The upper and lower parts of...A secondary-cooling-segment electromagnetic stirring(S-EMS)experiment was performed at 150 A and 4 Hz to evaluate the effect of S-EMS on solidification characterization near the white band.The upper and lower parts of the white band exhibited average secondary dendritic arm spacing of 205.4 and 214.4μm,respectively.The S-EMS operation resulted in large Lorentz forces and cooling intensity,which could produce additional dendritic arms with low carbon concentrations,leading to local negative segregation.Moreover,a 3D flow-temperature-magnetic coupling numerical model was established.The results revealed that the magnetic induction intensity and Lorentz force were symmetrically distributed along rollers S1 and S2.The average velocity magnitude increased by approximately 42.52%,58.69%,and 64.11%for liquid fractions of 0.7,0.8,and 0.9,respectively.During the S-EMS operation,the Lorentz force may alter the velocity of the solidification front and promote the dissipation of superheat.Additionally,the influence of S-EMS on grain nucleation and growth was investigated using Gibbs free energy theory and component undercooling.Furthermore,a formation model for the white band was established,and the mechanism of white band formation was elucidated according to the changes in the solute-enriched layer,solute precipitation,and diffusion.展开更多
Zwitterionic polymers are polymers containing a pair of oppositely charged groups in their repeating units,which facilitate the formation of a hydration layer on the surface through ionic solvation.This strong hydrati...Zwitterionic polymers are polymers containing a pair of oppositely charged groups in their repeating units,which facilitate the formation of a hydration layer on the surface through ionic solvation.This strong hydration results in the remarkable properties of zwitterionic polymer hydrogels,including antifouling,lubricating,and anti-freezing capabilities.Owing to these properties,zwitterionic polymer hydrogels have attracted notable attention in biomedical and engineering fields.However,the superhydrophilicity of zwitterionic polymer hydrogels renders them brittle and weak,considerably limiting their use in load-bearing applications.Thus,there is an urgent need to improve the mechanical properties of zwitterionic hydrogels.In this work,we systematically review mechanical enhancement strategies for zwitterionic polymer hydrogels.We cover strate-gies applicable to hybrid and pure high-strength zwitterionic polymer hydrogels.Additionally,we discuss the advantages and limitations of various strength enhancement strategies.展开更多
The effects of heat treatment on microstructure and creep properties of β high-strength titanium alloy,Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe,were studied.After solution treatment at 790℃ and aging treatment(HT1),the microst...The effects of heat treatment on microstructure and creep properties of β high-strength titanium alloy,Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe,were studied.After solution treatment at 790℃ and aging treatment(HT1),the microstructure is composed of equiaxedαp phase,β phase,α_(p) phase,and becomes β phase and α_(s) phases after solution treatment at 840℃ and aging treatment(HT2).The creep behavior at 400℃ was analyzed.The stress exponents of both alloys are between 1 and 2,indicating that the diffusional creep mechanism is one of the dominant creep mechanisms.The alloy after HT2 treatment has better creep resistance and a subsequent creep test on this alloy was performed at 450℃ under 400 MPa.The creep fracture has the mixed ductile-brittle characteristics.The phase interfaces can hinder the dislocation movement,and theαs phase can coordinate with the matrix to deform,thereby reducing the occurrence of intragranular cracks.展开更多
Heavy components of low-alloy high-strength(LAHS) steels are generally formed by multi-pass forging. It is necessary to explore the flow characteristics and hot workability of LAHS steels during the multi-pass forging...Heavy components of low-alloy high-strength(LAHS) steels are generally formed by multi-pass forging. It is necessary to explore the flow characteristics and hot workability of LAHS steels during the multi-pass forging process, which is beneficial to the formulation of actual processing parameters. In the study, the multi-pass hot compression experiments of a typical LAHS steel are carried out at a wide range of deformation temperatures and strain rates. It is found that the work hardening rate of the experimental material depends on deformation parameters and deformation passes, which is ascribed to the impacts of static and dynamic softening behaviors. A new model is established to describe the flow characteristics at various deformation passes. Compared to the classical Arrhenius model and modified Zerilli and Armstrong model, the newly proposed model shows higher prediction accuracy with a confidence level of 0.98565. Furthermore, the connection between power dissipation efficiency(PDE) and deformation parameters is revealed by analyzing the microstructures. The PDE cannot be utilized to reflect the efficiency of energy dissipation for microstructure evolution during the entire deformation process, but only to assess the efficiency of energy dissipation for microstructure evolution in a specific deformation parameter state.As a result, an integrated processing map is proposed to better study the hot workability of the LAHS steel, which considers the effects of instability factor(IF), PDE, and distribution and size of grains. The optimized processing parameters for the multi-pass deformation process are the deformation parameters of 1223–1318 K and 0.01–0.08 s^(-1). Complete dynamic recrystallization occurs within the optimized processing parameters with an average grain size of 18.36–42.3 μm. This study will guide the optimization of the forging process of heavy components.展开更多
High-strength steels are mainly composed of medium-or low-temperature microstructures,such as bainite or martensite,with coherent transformation characteristics.This type of microstructure has a high density of disloc...High-strength steels are mainly composed of medium-or low-temperature microstructures,such as bainite or martensite,with coherent transformation characteristics.This type of microstructure has a high density of dislocations and fine crystallographic structural units,which ease the coordinated matching of high strength,toughness,and plasticity.Meanwhile,given its excellent welding perform-ance,high-strength steel has been widely used in major engineering constructions,such as pipelines,ships,and bridges.However,visual-ization and digitization of the effective units of these coherent transformation structures using traditional methods(optical microscopy and scanning electron microscopy)is difficult due to their complex morphology.Moreover,the establishment of quantitative relationships with macroscopic mechanical properties and key process parameters presents additional difficulty.This article reviews the latest progress in microstructural visualization and digitization of high-strength steel,with a focus on the application of crystallographic methods in the development of high-strength steel plates and welding.We obtained the crystallographic data(Euler angle)of the transformed microstruc-tures through electron back-scattering diffraction and combined them with the calculation of inverse transformation from bainite or martensite to austenite to determine the reconstruction of high-temperature parent austenite and orientation relationship(OR)during con-tinuous cooling transformation.Furthermore,visualization of crystallographic packets,blocks,and variants based on actual OR and digit-ization of various grain boundaries can be effectively completed to establish quantitative relationships with alloy composition and key process parameters,thereby providing reverse design guidance for the development of high-strength steel.展开更多
The key role of oxide inclusions on the microstructure and mechanical property of a high-strength low-alloy steel was investigated.The field emission scanning electron microscope equipped with energy-dispersive spectr...The key role of oxide inclusions on the microstructure and mechanical property of a high-strength low-alloy steel was investigated.The field emission scanning electron microscope equipped with energy-dispersive spectrometry was used to characterize MnS precipitates.Oxide inclusions play an important role in the shape control of MnS precipitates.More oxides fovored to decrease the size and the aspect ratio of MnS precipitates.With less oxide inclusions in the steel,approximately over 16.7%MnS precipitates were with aspect ratio a>5 and pure MnS precipitates accounted for 75.9%in number.However,with more oxide inclusions in the steel,only 7.4%MnS precipitates were with a>5 and pure MnS precipitates accounted for 60.1%in number.Refinement of MnS by oxide inclusions improved the strength and inhibited the anisotropy.More oxide inclusions in the steel increased the yield strength and tensile strength of the steel in both longitudinal and transverse directions,and lowered the anisotropy of the mechanical property.展开更多
Strain-controlled cyclic deformation behavior of a high-strength low-alloy(HSLA)Mg-1.2Zn-0.1Ca alloy fabricated via low-temperature extrusion at 150℃ was investigated at different strain amplitudes.Due to the partial...Strain-controlled cyclic deformation behavior of a high-strength low-alloy(HSLA)Mg-1.2Zn-0.1Ca alloy fabricated via low-temperature extrusion at 150℃ was investigated at different strain amplitudes.Due to the partial dynamic recrystallization(DRX)during extrusion,the extruded HSLA magnesium alloy consisted of a unique heterostructure containing coarse unDRX grains and ultra-fine DRX grains of0.8μm,leading to a high tensile yield strength of 374 MPa and an elongation of 14%.The HSLA magnesium alloy exhibited cyclic stabilization at strain amplitudes of≤0.4%,while cyclic hardening occurred at strain amplitudes of≥0.6%.In contrast,the homogenized alloy with a uniform coarse-grained microstructure showed a strong cyclic hardening characteristic.Compared with the homogenized alloy,the HSLA magnesium alloy had a significantly higher cyclic stress level at all strain amplitudes,along with a longer fatigue life at lower and intermediate strain amplitudes owing to its higher monotonic strength.However,the homogenized alloy showed a longer fatigue life at a high strain amplitude of 0.8%due to its better ductility and stronger capacity of storing deformation.While{10-12}<10-11>extension twinning occurred in both the homogenized and HSLA samples at high strain amplitudes,twins were primarily formed in the coarse un DRX grains in the compressive phase during cyclic deformation due to the c-axes of un DRX grains perpendicular to the loading direction,with twinning in the ultra-fine DRX grains being suppressed.The low-cycle fatigue life of both the homogenized and HSLA samples can be well predicted through an accumulative damage model based on the strain-energy density calculation and intrinsic fatigue toughness concept.展开更多
The evolution of the microstructure and toughness of APL5L X80 pipeline steel after thermal welding simulation was investigated by X-ray diffraction,electron backscatter diffraction,and transmission electron microscop...The evolution of the microstructure and toughness of APL5L X80 pipeline steel after thermal welding simulation was investigated by X-ray diffraction,electron backscatter diffraction,and transmission electron microscopy.The results indicated that primary heat-affected zones can be divided into weld,coarse-grained,fine-grained,intercritical,and sub-critical zones.The microstructure of the weld zone is mainly composed of bainitic ferrite and a small amount of granular bainite;however,the original austenite grains are distributed in the columnar grains.The structure of the coarse-grained zone is similar to that of the weld zone,but the original austenite grains are equiaxed.In contrast,the microstructure in the fine-grained zone is dominated by fine granular bainite,and the effective grain size is only 8.15μm,thus providing the highest toughness in the entire heat-affected zone.The intercritical and subcritical zones were brittle valley regions,and the microstructure was dominated by granular bainite.However,the martensite-austenite(M/A)constituents are present in island chains along the grain boundaries,and the coarse size of the M/A constituents seriously reduces the toughness.The results of the crack propagation analyzes revealed that high-angle grain boundaries can significantly slow down crack growth and change the crack direction,thereby increasing the material toughness.The impact toughness of the low-temperature tempering zone was equivalent to that of the columnar grain zone,and the impact toughness was between those of the critical and fine-grained zones.展开更多
A 2D axisymmetric numerical model was established to investigate the variations of molten pool with different melt rates during the vacuum arc remelting of 8Cr4Mo4V high-strength steel,and the ingot growth was simulat...A 2D axisymmetric numerical model was established to investigate the variations of molten pool with different melt rates during the vacuum arc remelting of 8Cr4Mo4V high-strength steel,and the ingot growth was simulated by dynamic mesh techniques.The results show that as the ingot grows,the molten pool profile changes from shallow and flat to V-shaped,and both the molten pool depth and the mushy width increase.Meanwhile,the variation of both the molten pool shape and the mushy width melt rate is clarified by the thermal equilibrium analysis.As melt rate increases,both the molten pool depth and the mushy width increase.It is caused by the increment in sensible heat stored in the ingot due to the limitation of the cooling capacity of the mold.The nonlinear increment in sensible heat leads to a nonlinear increase in the mushy width.In addition,as melt rate increases,the local solidification time(LST)of ingot decreases obviously at first and then increases.When melt rate is controlled in a suitable range,LST is the lowest and the secondary dendrite arm spacing of the ingot is the smallest,which can effectively improve the compactness degree of 8Cr4Mo4V high-strength steel.展开更多
Fretting damage is common in the high-strength titanium alloy fastener widely used in the aeronautic industry,leading to the failure of fastening fit or the initiation of crack.The titanium alloy fasteners often have ...Fretting damage is common in the high-strength titanium alloy fastener widely used in the aeronautic industry,leading to the failure of fastening fit or the initiation of crack.The titanium alloy fasteners often have typical preferred orientation characteristics(i.e.,texture),and this is one of the important factors affecting its performance.However,the investigations on the mechanism ofβrolling-texture intensity on fretting damage resistance and subsurface deformation are less addressed.Hence,fretting wear tests were carried out on samples with different rolling texture intensities.The results demonstrate that the samples with quite low(A-10%sample)and quite high(D-70%sample)rolling-texture intensity both exhibit excellent fretting wear resistance,but their mechanisms are completely different.Uniformly dispersed grain orientation renders the A-10%sample with good recovery ability and a positive friction effect during wear.Low stress only concentrating at grain boundaries(GBs)weakens cracks’initiation and propagation.The unique orientation-layered structure(OLS)leads to excellent recovery ability and a positive friction effect.Crack propagation is inhibited and only propagates along the OLS boundary without a connected trend.However,samples with moderate rolling texture intensity exhibit severe wear.Dislocations are restricted in local areas,so the poor recovery ability makes them have a negative friction effect.Crack propagation driving force continuously increases.Appropriate rolling texture intensity can reduce wear by three times.This study can provide information on the principle for designing fretting damage-resistant alloys.展开更多
基金financially supported by the National Key R&D program(no.2022YFB3707501)GDAS’Project of Sci-ence and Technology(no.2021GDASYL-20210102002)+1 种基金Guangdong Provincial Project(nos.2022A0505050053,2021B1515120071,and 2020B1515130007)National Natural Science Foundation of China(no.52130102).
文摘The low damage resistance and fracture toughness hinder the widespread application of ultrahighstrength dual phase(DP)steels.In this work,we propose a novel strategy to improve the fracture toughness of ultrahigh-strength DP steels by an order of magnitude without sacrificing the tensile strength.Six ultrahigh-strength DP steels with varying microstructure but comparable tensile strength(>1400 MPa)were prepared via tailoring the heat treatment process after cold rolling.Additionally,finite element(FE)method incorporated with Gurson-Tvergaad-Needleman(GTN)model and cohesive zone model(CZM)is established to simulate the fracture behavior of DP steel.Twelve model DP steels with different ferrite sizes and F/M strength differences are constructed.The combined experiment and simulation results demonstrate that(i)ferrite/martensite(F/M)interface decohesion prevails in all steels,(ii)the ferrite morphology has a strong influence on the fracture toughness of ultrahigh-strength DP steels,(iii)the effects of matrix type,ferrite size,and F/M hardness difference on the fracture toughness are relatively weak,(iv)the exceptional high fracture toughness of plate-like DP steel can be attributed to the crack deflection,crack divider and crack arrester mechanisms induced by F/M interface decohesion.
基金supported by the National Natural Science Foundation of China(No.52271089)the financial support from the C hina Postdoctoral Science Foundation(No.2023M732192)。
文摘This work reveals the significant effects of cobalt(Co)on the microstructure and impact toughness of as-quenched highstrength steels by experimental characterizations and thermo-kinetic analyses.The results show that the Co-bearing steel exhibits finer blocks and a lower ductile-brittle transition temperature than the steel without Co.Moreover,the Co-bearing steel reveals higher transformation rates at the intermediate stage with bainite volume fraction ranging from around 0.1 to 0.6.The improved impact toughness of the Co-bearing steel results from the higher dense block boundaries dominated by the V1/V2 variant pair.Furthermore,the addition of Co induces a larger transformation driving force and a lower bainite start temperature(BS),thereby contributing to the refinement of blocks and the increase of the V1/V2 variant pair.These findings would be instructive for the composition,microstructure design,and property optimization of high-strength steels.
文摘As a typical steel,the fatigue of marine high-strength steels has been emphasized by scholars.In this paper,the fatigue performance and crack growth mechanism of a high-strength steel for ships are investigated by experimental methods.First,the fatigue threshold test and fatigue crack growth rate test of this high-strength steel under different stress ratios were carried out.The influence of stress ratio on the fatigue properties of this steel was analyzed.Secondly,scanning electron microscope was used to analyze the crack growth specimen section of this steel.The crack growth and failure mechanism of this steel were revealed.Finally,based on the above research results,the stress ratio effect of high-strength steel was investigated from the perspectives of crack closure and driving force.Considering the fatigue behavior in the near-threshold stage and the destabilization stage,a fatigue crack growth behavior prediction model of highstrength steel was established.The accuracy of the model was verified by test data.Moreover,the applicability of the modified model to various materials and its excellent predictive ability were verified through comparison with literature data and existing models.
基金Provincial Key Research and Development Plan of Heilongjiang(2022ZX04A01)。
文摘The optimization of deposited metal properties through the addition of rare earth elements to welding materials was explored.Utilizing optical microscope,scanning electron microscope,energy dispersive spectroscope,and X-ray diffractometer,combined with software tools like Matlab,Image-Pro Plus,and CHANNEL5,the influence mechanism of rare earth element addition on the strength,toughness,and inclusions of deposited metal in 1000 MPa grade high-strength steel was investigated.The results indicate that the incorporation of rare earth elements enhances the weldability of the welding materials.With the addition of rare earth elements,the tensile strength of the deposited metal increases from 935 MPa to 960 MPa.However,further addition leads to a decrease in tensile strength,while the yield strength continuously increases by 8.5%-17.2%.The addition of appropriate amounts of rare earth elements results in an increase in acicular ferrite and retained austenite content,as well as grain refinement in the deposited metal,leading to 8.5%-24.3% and 15.6%-42.2% enhancement in impact energy at−40℃ and−60℃,respectively.Additionally,the proper addition of rare earth elements modifies the inclusions and generates fine and dispersed composite inclusions that bond better with the matrix,thereby optimizing the properties of the deposited metal through various mechanisms.Adding an appropriate amount of rare earth elements can significantly enhance the properties of the deposited metal in 1000 MPa grade high-strength steel,and improve the match between high strength and toughness,meeting the demands for high-strength steel used in hydropower applications.
基金supported by National Natural Science Foundation of China(Grant No.52074095)Guizhou Provincial Basic Research Program(Natural Science)(Grant No.QKHJC-ZK[2023]YB072)+2 种基金Guizhou Provincial Key Technology R&D Program(Grant No.QKHZC[2023]YB404)Guizhou Provincial Key Technology R&D Program(Grant No.QKHZC[2022]YB053)The numerical calculation of the first-principles was supported and assisted by the High-Performance Computing Center of Guizhou University.
文摘The influence mechanism of trace Nb on the corrosion resistance of surface corrosion products of high-strength anti-seismic rebar in the simulated marine environment was studied by combining first-principles calculations with corrosion mass loss method,surface analysis,cross-sectional analysis,quantitative analysis,and electrochemical test.The results demonstrated that the addition of trace Nb effectively improved the compactness and stability of surface corrosion layer of rebar,and the corrosion resistance of corrosion layer increased with the increase in Nb content.The beneficial effect of Nb content on the corrosion layer summarized two important key points.Firstly,the addition of Nb was beneficial to promoting the improvement in the structural stability of α-FeOOH,and α-FeOOH structure of solid solution Nb atoms was beneficial to strengthening the fixation of Cl atoms,thus increasing α/(β+γ)ratio,total impedance value,and corrosion potential.Secondly,the formation of Nb oxides can not only repair the corrosion layer,but also play a role in the fixation Cl atoms,resulting in the improvement in corrosion resistance of corrosion layer.
文摘Corrosion is an essential issue limiting the application of high-strength low-carbon steel in seawater environment. The impact of retained austenite on its corrosion behavior with immersion experiments and related corrosion sensor technology was explored. A model that clarifies the micro-galvanic effect and the heat-induced changes to the shape and composition of retained austenite was used to discuss the findings. The results indicated that retained austenite was generated following an intercritical process and demonstrated approximately 48 mV higher Volta potential than the matrix. The retained austenite content first increased and then decreased with increasing intercritical temperatures, while reaching the maximum value of 8.5% at 660℃. With the increase in retained austenite content, the corrosion rate was increased by up to 32.8% compared to “quenching + tempering” (QT) specimen. The interfaces between the retained austenite and matrix were the priority nucleation sites for corrosion. Moreover, the retained austenite reduced the corrosion resistance of the steel by increasing the micro-galvanic effect and reducing rust layer compactness.
文摘This study deals with the development of a 780-MPa-class hot-rolled advanced high-strength steel(AHSS)with an ultrahigh elongation at break of approximately 30%and strength-ductility product exceeding 24 GPa·%,indicating the excellent formability of the newly developed AHSS.The microstructure of the newly developed 780-MPa-class AHSS consists mainly of the triplex phase of ferrite,bainite,and retained austenite with a volume fraction of 10%±2%.The stability of the retained austenite in the newly developed AHSS is much higher than that of conventional transformation-induced plasticity steels,in which the retained austenite is prone to transformation into martensite under deformation.At a pre-strain lower than 1.2%,the volume fraction of the retained austenite and the elongation at break of the present 780-MPa-class AHSS remain almost unchanged,showing a high tolerance in the process window during leveling or straightening.Therefore,the present 780-MPa-class AHSS is particularly suitable for the production of components with complex shapes.
基金the National Natural Science Foundation of China(No.52171026)the Equipment Pre-Research Field Foundation(No.80923010304).
文摘Aluminum alloys that are additively manufactured using the laser powder bed fusion(LPBF)suffer from relatively poor high cycle fatigue(HCF)resistance.In an effort to alleviate this,a high-strength Al alloy,Al-Mn-Mg-Sc-Zr,with columnar,equiaxed,and bi-modal microstructures was produced by varying the scanning velocity and the substrate temperature during the LPBF process.The tensile strength of LPBF Al-Mn-Mg-Sc-Zr alloy is 475±5–516±6 MPa with favorable elongation of approximately 11%,higher than that of most of the other Al alloys,including conventional high-strength rolled/ECAP Al alloys and AM Al-Mg-Sc-Zr alloys.Specimens with bimodal microstructure and specimens with fully equiaxed microstructure both show a fatigue strength of 230 MPa(at 107 loading cycles),which is the highest among those reported for the LPBF Al alloys.The deformation synergy in the bimodal microstructure also improves the fatigue resistance in the strain-controlled low cycle fatigue(LCF)regime.The equiaxed microstructure restricts the to-and-fro dislocation motion during cyclic loading,which,in turn,minimizes the strain localization.At the later stages of strain accumulation,microcracks form at the grain boundaries,limiting the further improvement of the alloy's fatigue strength.This study demonstrates microstructural tailoring through AM enables improvement of the fatigue resistance of aluminum alloys.
基金financially supported by the National Key Research and Development Program of China(Grant No.2023YFB3812602).
文摘High-order asymmetric flatness defects resulting from the abnormal state of roll system are the main issue of precision rolling mill in the manufacturing process of high-strength thin strip.Due to the difficulty of monitoring and adjusting the abnormal state,the spatial state of roll system cannot be controlled by traditional methods.It is difficult to fundamentally improve these high-order asymmetric flatness defects.Therefore,a digital twin model of flatness control process for S6-high rolling mill was established,which could be used to analyze the influence of the abnormal state on the flatness control characteristic and propose improvement strategies.The internal relationship between the force state of side support roll system and the abnormal state of roll system was proposed.The XGBoost algorithm model was established to analyze the contribution degree of the side support roll system force to the flatness characteristic quantity.The abnormal state of roll system in the S6-high rolling mill can be diagnosed by analyzing the flatness characteristic difference between flatness value of the rolled strip and calculated characteristic value of finite element simulation.The flatness optimization model of the gray wolf optimization–long short-term memory non-dominated sorting whale optimization algorithm(GWO-LSTM-NSWOA)was established,and the decision-making selection was made from the Pareto frontier based on the flatness requirements of cold rolling to regulate the abnormal state of the roll system.The results indicate that the contribution degree of the force of the side support roll system to the flatness characteristics is more than 25%,which is the main influence of high-order asymmetric flatness defect.The performance of the GWO-LSTM flatness feature prediction model has clear advantages over back propagation and LSTM.The practical applications show that optimizing the force of side support roll system can reduce the high point of high-strength strip flatness from 13.2 to 6 IU and decrease the percentage of low-strength strip flatness defects from 1.6%to 1.2%.This optimization greatly reduced the proportion of flatness defects,improved the accuracy level of flatness control of precision rolling mill,and provided a guarantee for the stable production of thin strip.
文摘The dilatometric curves of Q690 steel at different cooling rates were tested using Gleeble 3800 to study the continuous cooling transformation behavior of high-strength steel with low welding crack sensitivity.The continuous cooling transformation curves of Q690 steel were constructed according to the transformation tem-peratures determined using dilatometric curves and corresponding microstructures at different cooling rates.The results show that the microstructure of Q690 steel remarkably changed with the increase in the cooling rate.The matrix of the steel was ferrite and pearlite when the cooling rate was between 0.1 K/s and 0.5 K/s.With increasing cooling rate, the microstructure of the steel was considerably refined, and the Vickers hardness of the steel increased.When the cooling rate reached 1 K/s, the microstructure of the steel was further refined, and bainite transformation occurred.Polygonal ferrite almost disappeared from the steel when the cooling rate reached 10 K/s, and the matrix was mainly composed of bainite and a small amount of lath martensite.With the increase in the cooling rate, the ferrite decreased and martensite increased in the steel, resulting in higher Vickers hardness.When the cooling rate reached 30 K/s, the microstructure of the tested steel was full martensite matrix.
基金supported by the National Natural Science Foundation of China(No.51774031)the Project funded by China Postdoctoral Science Foundation(No.2023M730230).
文摘A secondary-cooling-segment electromagnetic stirring(S-EMS)experiment was performed at 150 A and 4 Hz to evaluate the effect of S-EMS on solidification characterization near the white band.The upper and lower parts of the white band exhibited average secondary dendritic arm spacing of 205.4 and 214.4μm,respectively.The S-EMS operation resulted in large Lorentz forces and cooling intensity,which could produce additional dendritic arms with low carbon concentrations,leading to local negative segregation.Moreover,a 3D flow-temperature-magnetic coupling numerical model was established.The results revealed that the magnetic induction intensity and Lorentz force were symmetrically distributed along rollers S1 and S2.The average velocity magnitude increased by approximately 42.52%,58.69%,and 64.11%for liquid fractions of 0.7,0.8,and 0.9,respectively.During the S-EMS operation,the Lorentz force may alter the velocity of the solidification front and promote the dissipation of superheat.Additionally,the influence of S-EMS on grain nucleation and growth was investigated using Gibbs free energy theory and component undercooling.Furthermore,a formation model for the white band was established,and the mechanism of white band formation was elucidated according to the changes in the solute-enriched layer,solute precipitation,and diffusion.
基金supported by the National Natural Science Foundation of China(Nos.T2222013,52233008 and 52073203).
文摘Zwitterionic polymers are polymers containing a pair of oppositely charged groups in their repeating units,which facilitate the formation of a hydration layer on the surface through ionic solvation.This strong hydration results in the remarkable properties of zwitterionic polymer hydrogels,including antifouling,lubricating,and anti-freezing capabilities.Owing to these properties,zwitterionic polymer hydrogels have attracted notable attention in biomedical and engineering fields.However,the superhydrophilicity of zwitterionic polymer hydrogels renders them brittle and weak,considerably limiting their use in load-bearing applications.Thus,there is an urgent need to improve the mechanical properties of zwitterionic hydrogels.In this work,we systematically review mechanical enhancement strategies for zwitterionic polymer hydrogels.We cover strate-gies applicable to hybrid and pure high-strength zwitterionic polymer hydrogels.Additionally,we discuss the advantages and limitations of various strength enhancement strategies.
基金supported by the National Natural Science Foundation of China(No.51975147)。
文摘The effects of heat treatment on microstructure and creep properties of β high-strength titanium alloy,Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe,were studied.After solution treatment at 790℃ and aging treatment(HT1),the microstructure is composed of equiaxedαp phase,β phase,α_(p) phase,and becomes β phase and α_(s) phases after solution treatment at 840℃ and aging treatment(HT2).The creep behavior at 400℃ was analyzed.The stress exponents of both alloys are between 1 and 2,indicating that the diffusional creep mechanism is one of the dominant creep mechanisms.The alloy after HT2 treatment has better creep resistance and a subsequent creep test on this alloy was performed at 450℃ under 400 MPa.The creep fracture has the mixed ductile-brittle characteristics.The phase interfaces can hinder the dislocation movement,and theαs phase can coordinate with the matrix to deform,thereby reducing the occurrence of intragranular cracks.
基金National Natural Science Foundation of China(No.52305373)Jiangxi Provincial Natural Science Foundation(No.20232BAB214053)+2 种基金Science and Technology Major Project of Jiangxi,China(No.20194ABC28001)Fund of Jiangxi Key Laboratory of Forming and Joining Technology for Aerospace Components,Nanchang Hangkong University(No.EL202303299)PhD Starting Foundation of Nanchang Hangkong University(No,EA202303235).
文摘Heavy components of low-alloy high-strength(LAHS) steels are generally formed by multi-pass forging. It is necessary to explore the flow characteristics and hot workability of LAHS steels during the multi-pass forging process, which is beneficial to the formulation of actual processing parameters. In the study, the multi-pass hot compression experiments of a typical LAHS steel are carried out at a wide range of deformation temperatures and strain rates. It is found that the work hardening rate of the experimental material depends on deformation parameters and deformation passes, which is ascribed to the impacts of static and dynamic softening behaviors. A new model is established to describe the flow characteristics at various deformation passes. Compared to the classical Arrhenius model and modified Zerilli and Armstrong model, the newly proposed model shows higher prediction accuracy with a confidence level of 0.98565. Furthermore, the connection between power dissipation efficiency(PDE) and deformation parameters is revealed by analyzing the microstructures. The PDE cannot be utilized to reflect the efficiency of energy dissipation for microstructure evolution during the entire deformation process, but only to assess the efficiency of energy dissipation for microstructure evolution in a specific deformation parameter state.As a result, an integrated processing map is proposed to better study the hot workability of the LAHS steel, which considers the effects of instability factor(IF), PDE, and distribution and size of grains. The optimized processing parameters for the multi-pass deformation process are the deformation parameters of 1223–1318 K and 0.01–0.08 s^(-1). Complete dynamic recrystallization occurs within the optimized processing parameters with an average grain size of 18.36–42.3 μm. This study will guide the optimization of the forging process of heavy components.
基金supported by the National Key Research and Development Project of China(Nos.2022YFB3708200 and 2021YFB3703500)the National Natural Science Foundation of China(Nos.52271089 and 52001023).
文摘High-strength steels are mainly composed of medium-or low-temperature microstructures,such as bainite or martensite,with coherent transformation characteristics.This type of microstructure has a high density of dislocations and fine crystallographic structural units,which ease the coordinated matching of high strength,toughness,and plasticity.Meanwhile,given its excellent welding perform-ance,high-strength steel has been widely used in major engineering constructions,such as pipelines,ships,and bridges.However,visual-ization and digitization of the effective units of these coherent transformation structures using traditional methods(optical microscopy and scanning electron microscopy)is difficult due to their complex morphology.Moreover,the establishment of quantitative relationships with macroscopic mechanical properties and key process parameters presents additional difficulty.This article reviews the latest progress in microstructural visualization and digitization of high-strength steel,with a focus on the application of crystallographic methods in the development of high-strength steel plates and welding.We obtained the crystallographic data(Euler angle)of the transformed microstruc-tures through electron back-scattering diffraction and combined them with the calculation of inverse transformation from bainite or martensite to austenite to determine the reconstruction of high-temperature parent austenite and orientation relationship(OR)during con-tinuous cooling transformation.Furthermore,visualization of crystallographic packets,blocks,and variants based on actual OR and digit-ization of various grain boundaries can be effectively completed to establish quantitative relationships with alloy composition and key process parameters,thereby providing reverse design guidance for the development of high-strength steel.
基金the National Natural Science Foundation of China(Grant Nos.52274398 and U22A20171)S&T Program of Hebei(Grant Nos.20311005D and 20591001D)the High Steel Center(HSC)at North China University of Technology and Yanshan University,China.
文摘The key role of oxide inclusions on the microstructure and mechanical property of a high-strength low-alloy steel was investigated.The field emission scanning electron microscope equipped with energy-dispersive spectrometry was used to characterize MnS precipitates.Oxide inclusions play an important role in the shape control of MnS precipitates.More oxides fovored to decrease the size and the aspect ratio of MnS precipitates.With less oxide inclusions in the steel,approximately over 16.7%MnS precipitates were with aspect ratio a>5 and pure MnS precipitates accounted for 75.9%in number.However,with more oxide inclusions in the steel,only 7.4%MnS precipitates were with a>5 and pure MnS precipitates accounted for 60.1%in number.Refinement of MnS by oxide inclusions improved the strength and inhibited the anisotropy.More oxide inclusions in the steel increased the yield strength and tensile strength of the steel in both longitudinal and transverse directions,and lowered the anisotropy of the mechanical property.
基金Key-Area Research and Development Program of Guangdong Province(No.2020B010186002)Natural Science Foundation of Guangdong for Research Team(No.2015A030312003)+1 种基金financial support by Natural Sciences and Engineering Research Council of Canada(NSERC)Study Abroad Fund for supporting his study at Toronto Metropolitan University。
文摘Strain-controlled cyclic deformation behavior of a high-strength low-alloy(HSLA)Mg-1.2Zn-0.1Ca alloy fabricated via low-temperature extrusion at 150℃ was investigated at different strain amplitudes.Due to the partial dynamic recrystallization(DRX)during extrusion,the extruded HSLA magnesium alloy consisted of a unique heterostructure containing coarse unDRX grains and ultra-fine DRX grains of0.8μm,leading to a high tensile yield strength of 374 MPa and an elongation of 14%.The HSLA magnesium alloy exhibited cyclic stabilization at strain amplitudes of≤0.4%,while cyclic hardening occurred at strain amplitudes of≥0.6%.In contrast,the homogenized alloy with a uniform coarse-grained microstructure showed a strong cyclic hardening characteristic.Compared with the homogenized alloy,the HSLA magnesium alloy had a significantly higher cyclic stress level at all strain amplitudes,along with a longer fatigue life at lower and intermediate strain amplitudes owing to its higher monotonic strength.However,the homogenized alloy showed a longer fatigue life at a high strain amplitude of 0.8%due to its better ductility and stronger capacity of storing deformation.While{10-12}<10-11>extension twinning occurred in both the homogenized and HSLA samples at high strain amplitudes,twins were primarily formed in the coarse un DRX grains in the compressive phase during cyclic deformation due to the c-axes of un DRX grains perpendicular to the loading direction,with twinning in the ultra-fine DRX grains being suppressed.The low-cycle fatigue life of both the homogenized and HSLA samples can be well predicted through an accumulative damage model based on the strain-energy density calculation and intrinsic fatigue toughness concept.
基金The authors appreciate the financial support from National Key Research and Development Program of China(2017YFBO304900).
文摘The evolution of the microstructure and toughness of APL5L X80 pipeline steel after thermal welding simulation was investigated by X-ray diffraction,electron backscatter diffraction,and transmission electron microscopy.The results indicated that primary heat-affected zones can be divided into weld,coarse-grained,fine-grained,intercritical,and sub-critical zones.The microstructure of the weld zone is mainly composed of bainitic ferrite and a small amount of granular bainite;however,the original austenite grains are distributed in the columnar grains.The structure of the coarse-grained zone is similar to that of the weld zone,but the original austenite grains are equiaxed.In contrast,the microstructure in the fine-grained zone is dominated by fine granular bainite,and the effective grain size is only 8.15μm,thus providing the highest toughness in the entire heat-affected zone.The intercritical and subcritical zones were brittle valley regions,and the microstructure was dominated by granular bainite.However,the martensite-austenite(M/A)constituents are present in island chains along the grain boundaries,and the coarse size of the M/A constituents seriously reduces the toughness.The results of the crack propagation analyzes revealed that high-angle grain boundaries can significantly slow down crack growth and change the crack direction,thereby increasing the material toughness.The impact toughness of the low-temperature tempering zone was equivalent to that of the columnar grain zone,and the impact toughness was between those of the critical and fine-grained zones.
基金financially supported by National Natural Science Foundation of China(Nos.U1908223 and U1960203)Fundamental Research Funds for the Central Universities(Grant No.N2125017)Talent Project of Revitalizing Liaoning(Grant No.XLYC1902046).
文摘A 2D axisymmetric numerical model was established to investigate the variations of molten pool with different melt rates during the vacuum arc remelting of 8Cr4Mo4V high-strength steel,and the ingot growth was simulated by dynamic mesh techniques.The results show that as the ingot grows,the molten pool profile changes from shallow and flat to V-shaped,and both the molten pool depth and the mushy width increase.Meanwhile,the variation of both the molten pool shape and the mushy width melt rate is clarified by the thermal equilibrium analysis.As melt rate increases,both the molten pool depth and the mushy width increase.It is caused by the increment in sensible heat stored in the ingot due to the limitation of the cooling capacity of the mold.The nonlinear increment in sensible heat leads to a nonlinear increase in the mushy width.In addition,as melt rate increases,the local solidification time(LST)of ingot decreases obviously at first and then increases.When melt rate is controlled in a suitable range,LST is the lowest and the secondary dendrite arm spacing of the ingot is the smallest,which can effectively improve the compactness degree of 8Cr4Mo4V high-strength steel.
基金supported by the National Natural Science Foun-dation of China(No.52105211)the Research Fund of the State Key Laboratory of Solidification Processing(NPU),China(No.2023-TS-04)the Fundamental Research Funds for the Central Universi-ties of China(No.3102019JC001).
文摘Fretting damage is common in the high-strength titanium alloy fastener widely used in the aeronautic industry,leading to the failure of fastening fit or the initiation of crack.The titanium alloy fasteners often have typical preferred orientation characteristics(i.e.,texture),and this is one of the important factors affecting its performance.However,the investigations on the mechanism ofβrolling-texture intensity on fretting damage resistance and subsurface deformation are less addressed.Hence,fretting wear tests were carried out on samples with different rolling texture intensities.The results demonstrate that the samples with quite low(A-10%sample)and quite high(D-70%sample)rolling-texture intensity both exhibit excellent fretting wear resistance,but their mechanisms are completely different.Uniformly dispersed grain orientation renders the A-10%sample with good recovery ability and a positive friction effect during wear.Low stress only concentrating at grain boundaries(GBs)weakens cracks’initiation and propagation.The unique orientation-layered structure(OLS)leads to excellent recovery ability and a positive friction effect.Crack propagation is inhibited and only propagates along the OLS boundary without a connected trend.However,samples with moderate rolling texture intensity exhibit severe wear.Dislocations are restricted in local areas,so the poor recovery ability makes them have a negative friction effect.Crack propagation driving force continuously increases.Appropriate rolling texture intensity can reduce wear by three times.This study can provide information on the principle for designing fretting damage-resistant alloys.
