Tungsten heavy alloys(WHAs)prepared using laser additive manufacturing(AM)exhibit intricate ge-ometries,albeit with limited mechanical properties.Here we designed a high-strength WHA featuring a FeCrCoNi high entropy ...Tungsten heavy alloys(WHAs)prepared using laser additive manufacturing(AM)exhibit intricate ge-ometries,albeit with limited mechanical properties.Here we designed a high-strength WHA featuring a FeCrCoNi high entropy alloy(HEA)binder via the laser metal deposition(LMD)technique.Due to the distinctive thermal cycle and rapid cooling rate,the as-deposited alloys exhibit microstructures with hy-poeutectic,eutectic-like,and spot-like characteristics.To elucidate this phenomenon,the solidification paths were delineated and analyzed by combining microstructural characterization and phase equilib-rium simulation.Theμphase precipitated out from the supersaturated solid solution,thereby nucleating massive dislocations on the FeCrCoNi matrix to increase the work hardening rate.Furthermore,theμphase formed an ultrafine intermetallic compound(IMC)layer around the W grain,reducing the hole or crack between the W grain and FeCrCoNi matrix.Attributed to the precipitation strengthening,the solid solution of the FeCrCoNi binder,along with the load-bearing strength of W,the developed alloy achieved ultrahigh compressive stress and strain of 2047 MPa and 32%respectively at room temperature.These findings contribute valuable insights to the advancement of additive manufacturing for tungsten alloys,leveraging their excellent properties.展开更多
The effect of melt superheating treatment on the solidification microstructure and mechanical properties of theγ'phase precipitation-strengthened K424 superalloy was investigated.Differential scanning calorimetry...The effect of melt superheating treatment on the solidification microstructure and mechanical properties of theγ'phase precipitation-strengthened K424 superalloy was investigated.Differential scanning calorimetry(DSC)experiments were conducted to explore the influence of melt treatment temperature on the undercooling of the superalloy.Additionally,pouring experiments were carried out to assess how alterations in both the temperature and duration of melt treatment impacted the grain size,secondary dendrite arm spacing(SDAS),elemental segregation,and mechanical properties of the alloy.Metallographic analysis,scanning electron microscopy,energy dispersive spectroscopy(EDS)and Thermo-Calc software were employed for microstructure characterization.The test specimens were subjected to tensile testing at room temperature and stress rupture testing at 975℃ under 196 MPa.The findings reveal that appropriate melt treatment conditions result in decreased grain size,refined SDAS,minimized elemental segregation,and significant improvements in mechanical properties.Specifically,the study demonstrates that a melt treatment at 1,650℃ for 5 min results in the smallest average grain size of 949μm and the smallest SDAS of 25.38μm.Furthermore,the room temperature tensile properties and creep resistance are notably affected by the melt treatment parameters.It is shown that specific melt treatment conditions,such as holding at 1,650℃ for 5 min,result in superior room temperature strength and extended stress rupture life of the K424 superalloy,while a balance between strength and stability is achieved at 1,600℃ with a holding time of 10 min.These findings offer guidance for optimizing the melt treatment parameters for the K424 superalloy,laying a foundation for further investigations.展开更多
Understanding the behaviors of heat transfer and fluid flow in weld pool and their effects on the solidification microstructure are significant for performance improvement of laser welds.This paper develops a three-di...Understanding the behaviors of heat transfer and fluid flow in weld pool and their effects on the solidification microstructure are significant for performance improvement of laser welds.This paper develops a three-dimensional numerical model to understand the multi-physical processes such as heat transfer,melt convection and solidification behavior in full-penetration laser welding of thin 5083 aluminum sheet.Solidification parameters including temperature gradient G and solidification rate R,and their combined forms are evaluated to interpret solidification microstructure.The predicted weld dimensions and the microstructure morphology and scale agree well with experiments.Results indicate that heat conduction is the dominant mechanism of heat transfer in weld pool,and melt convection plays a critical role in microstructure scale.The mushy zone shape/size and solidification parameters can be modulated by changing process parameters.Dendritic structures form because of the low G/R value.The scale of dendritic structures can be reduced by increasing GR via decreasing heat input.The columnar to equiaxed transition is predicted quantitatively via the process related G^3/R.These findings illustrate how heat transfer and fluid flow affect the solidification parameters and hence the microstructure,and show how to improve microstructure by optimizing the process.展开更多
IN617B nickel-base superalloy is considered as a good candidate material in 700℃advanced ultrasupercritical coal-fired power plants.The effect of Ta addition on solidification microstructure and element segregation o...IN617B nickel-base superalloy is considered as a good candidate material in 700℃advanced ultrasupercritical coal-fired power plants.The effect of Ta addition on solidification microstructure and element segregation of IN617B alloy was investigated by OM,SEM,TEM,EDS,EPMA and thermodynamic calculation.The results showed that the solidification microstructure exhibited a dendritic segregation pattern with many primary carbides distributed in interdendritic regions,such as network M_(6)C,lath M_(23)C_(6) and granular Ti(C,N).The addition of Ta promoted the precipitation of Ta-rich MC significantly inhibiting the precipitation of M_(6)C and M_(23)C_(6),and reduced the segregation degree of Al,Mo and Ti alloying elements.The addition of Ta decreased the melting temperature of MC carbide,but did not impact the solidification path,that was,L→γmatrix→MC or Ti(C,N)→M_(6)C→M_(23)C_(6),where MC and Ti(C,N)tended to form symbiotic microstructure with M_(6)C.This study will provide theoretical basis and data support for the alloy optimization and casting structure control of IN617B nickel-based superalloy.展开更多
The solidification microstructure of Mg-Gd-YZr alloy was investigated via an experimental study and cellular automaton(CA)simulation.In this study,stepshaped castings were produced,and the temperature variation inside...The solidification microstructure of Mg-Gd-YZr alloy was investigated via an experimental study and cellular automaton(CA)simulation.In this study,stepshaped castings were produced,and the temperature variation inside the casting was recorded using thermocouples during the solidification process.The effects of the cooling rate and Zr content on the grain size of the Mg-Gd-Y-Zr alloy were studied.The results showed that the grain size decreased with an increase in the cooling rate and Zr content.Based on the experimental data,a quantitative model for calculating the heterogeneous nucleation rate was developed,and the model parameters were determined.The evolution of the solidification microstructure was simulated using the CA method,where the quantitative nucleation model was used and a solute partition ceoefficient was introduced to deal with the solute trapping in front of the solid-liquid(S/L)interface.The simulation results of the grain size were in good agreement with the experimental data.The simulation also showed that the fraction of the eutectics decreased with an increasing cooling rate in the range of 2.6-11.0℃·s^(-1),which was verified indirectly by the experimental data.展开更多
Beta-solidifying TiAl alloy has great potential in the field of aero-industry as a cast alloy.In the present work,the influence of cooling rate during mushy zone on solidification behavior of Ti-44Al-4Nb-2Cr-0.1B allo...Beta-solidifying TiAl alloy has great potential in the field of aero-industry as a cast alloy.In the present work,the influence of cooling rate during mushy zone on solidification behavior of Ti-44Al-4Nb-2Cr-0.1B alloy was investigated.A vacuum induction heating device combining with temperature control system was used.The Ti-44Al-4Nb-2Cr-0.1B alloy solidified from superheated was melted to β phase with the cooling rates of 10,50,100,200,400 and 700 K·min^（-1）,respectively.Results show that with the increase in cooling rate from 10 to 700 K·min^（-1）,the colony size of α_2/γ lamella decreases from 1513 to48 urn and the solidification segregation significantly decreases.Also the content of residual B2 phase within α_2/γlamellar colony decreases with the increase in cooling rate.In addition,the alloy in local interdendritic regions would solidify in a hypo-peritectic way,which can be attributed to the solute redistribution and enrichment of Al element in solidification.展开更多
To study the effects of rare earth(RE)and Ti on the solidification micros true ture of high borated stainless steels,1.6 wt%B stainless steel doped with RE and2.1 wt%B stainless steel doped with Ti were prepared by in...To study the effects of rare earth(RE)and Ti on the solidification micros true ture of high borated stainless steels,1.6 wt%B stainless steel doped with RE and2.1 wt%B stainless steel doped with Ti were prepared by ingot casting,respectively.The solidification microstructure of researched steels was characterized in detail.The modification mechanism was clarified based on the heterogeneous nucleation theory and the thermodynamic calculation.The solidification microstructure of 1.6 wt%B and 2.1 wt%B stainless steels was characterized by the continuous and network-like eutectic borides around the matrix grains.It was found that the fine RE compounds could act as the heterogeneous nuclei for both borides and austenite during solidification.Thus,the eutectic borides were more dispersed in the modified steel.Moreover,lots of fine‘eutectic cells’were formed in the matrix regions.As a result of the preferential formation of TiB2 during solidification,the amount of the eutectic borides in the steel modified with Ti was significantly decreased.Besides,the continuity of the eutectic borides network was weakened.In a word,the present work provides a promising method to modify the solidification microstructure for high borated stainless steels.展开更多
Solidification microstructure and mechanical property are explored.Furthermore,tensile fracture and microstructure are analyzed by using SEM and JXA 840A electron probe.The results indicate that SiC particles in SiC...Solidification microstructure and mechanical property are explored.Furthermore,tensile fracture and microstructure are analyzed by using SEM and JXA 840A electron probe.The results indicate that SiC particles in SiCp/ZA27 composite are mainly distributed on interfaces or between dendrites and surrounded by primary α phase.The dendrite of α phase is fined by SiCp.The tensile strength at room temperature decreases with the increasing of SiCp addition.The tensile strength at elevated temperature increases with the addition of SiCp.The fracture of SiCp/ZA27 composites is the mixture of tough and brittle fracture.The carck is prone to extend along the interface and the region of dispersed shrinkage.展开更多
The solidification microstructures and solute segregation of a newly developed hot corrosion resistant single-crystal Ni-base superalloy were investigated with a zone-melting and ultra-high thermal gradient unidirecti...The solidification microstructures and solute segregation of a newly developed hot corrosion resistant single-crystal Ni-base superalloy were investigated with a zone-melting and ultra-high thermal gradient unidirectional solidification apparatus.Compared with the microstructures solidified at conventional low thermal gradient conditions,the dendrite arm spacings,the interdendritic microporosity and γ/γ' eutectic,and the severity of solute segregation of the single-crystal superalloy solidified at ultra-high thermal gradient conditions were considerably reduced.It was shown that the microstructure solidified under ultra-high thermal gradient condition is ideal for the full exploitation of the excellent property potentials of single-crystal superalloys.展开更多
A two-dimensional(2-D)multi-component and multi-phase cellular automaton(CA)model coupled with the Calphad method and finite difference method(FDM)is proposed to simulate the gas pore formation and microstructures in ...A two-dimensional(2-D)multi-component and multi-phase cellular automaton(CA)model coupled with the Calphad method and finite difference method(FDM)is proposed to simulate the gas pore formation and microstructures in solidification process of hypoeutectic Al-Si-Mg alloys.In this model,the pore growth,and dendritic and eutectic solidification are simulated using a CA technique.To achieve the equilibrium among multiple phases during ternary Al-based alloy solidification,the phase transition thermodynamics and kinetics are evaluated by adopting the Calphad method.The diffusion equations of hydrogen and two solutes are solved by FDM.The developed CA-FDM coupled model can be used for simulating the evolution of gas microporosity and microstructures,involving dendrites and irregular binary and ternary eutectics,of ternary hypoeutectic Al-Si-Mg alloys.It has the capability of reproducing the interactions between the hydrogen microporosity formation and the growth of dendrites and eutectics,the competitive growth among the growing gas pores of different sizes,together with the time-evolving concentration fields of hydrogen and solutes.The simulated morphology of gas pore and microstructure has a good agreement with the experimental observation.The influences of the initial hydrogen concentration and cooling rate on the microporosity formation are investigated.It is found that the main portion of porosity formation occurs in the eutectic solidification stage through analyzing the profiles of porosity percentage and solid fraction varying with solidification time.The varying features of simulated porosity percentage,the maximum and average pores radii indicate that increasing initial hydrogen concentration promotes the formation of higher final porosity percentage and larger pores,while the size of gas pores will significantly reduce with increasing cooling rate,leading to a lower final porosity percentage.展开更多
The Cu?Al composite casts were prepared by the method of pouring molten aluminum. The solidification process and themicrostructure of the transition layer were investigated during the recombination process of the liqu...The Cu?Al composite casts were prepared by the method of pouring molten aluminum. The solidification process and themicrostructure of the transition layer were investigated during the recombination process of the liquid Al and the solid Cu. The results reveal that the microstructure of the transition layer in the Cu?Al composite cast consists of α(Al)+α(Al)?CuAl2 eutectic,α(Al)?CuAl2 eutectic, CuAl2+α(Al)?CuAl2 eutectic and Cu9Al4. Additionally, the pouring temperature, cooling mode of the Cu platesurface and start time of the forced cooling after pouring have no effect on the microstructure species. But the proportion of thevarious microstructures in the transition layer changes with the process parameters. The pure Al at the top of the transition layer startsto solidify first and then the α(Al) phase grows in a dendritic way, while the CuAl2 phase exhibits plane or cellular crystal growth from the two sides of the transition layer towards its interior. The stronger the cooling intensity of the Cu plate outer surface, the more developed the dendrite, and the easier it is for the CuAl2 phase to grow into a plane crystal.展开更多
The effect of different scales thermoelectric magnetic convection(TEMC)on the radial solidification microstructure of hypereutectic Al alloy has been investigated under transverse static magnetic field during directio...The effect of different scales thermoelectric magnetic convection(TEMC)on the radial solidification microstructure of hypereutectic Al alloy has been investigated under transverse static magnetic field during directional solidification,focusing on the formation of freckle.Our experimental and numerical simulation results indicate that the TEMC circulation at sample scale under transverse static magnetic field leads to the enrichment of solute Al on one side of the sample.The TEMC and the solute enrichment degree increase with the increase of magnetic field when the magnetic field increases to 0.5 T.The enrichment degree of solute elements under magnetic field is affected by temperature gradient and growth rate.The non-uniform distribution of solute Al in the radial direction of the sample results in the non-uniform distribution of primary dendrite arm spacing(PDAS).Moreover,the applied magnetic field can lead to freckle formation and its number increases with the increase of magnetic field.The change of freckle is consistent with the anisotropy TEMC caused by the anisotropy of primary dendrite or primary dendrite network under magnetic field.Finally,the mechanism of synergism effect of the anisotropy TEMC,the distribution of solute Al and the PDAS on freckle formation and evolution is studied during directional solidification under magnetic field.展开更多
The liquid quenching method was adopted to study the solidification morphology and microstructure of AZglD Mg alloy in semisolid. The results indicate that cooling rate has important effects upon the solidification st...The liquid quenching method was adopted to study the solidification morphology and microstructure of AZglD Mg alloy in semisolid. The results indicate that cooling rate has important effects upon the solidification structures. Under the cooling rate of liquid quenching, primary α-phase grows first by attaching on the original α grains, or independent nucleation and growth. The high cooling rate makes primary α-phase grow in ＂rags＂ or dendrite shape. Eutectic solidification is carried out in terms of both dissociated growth and symbiotic growth. The dissociated growth forms rough and large β-phase at grain boundaries, while symbiotic growth forms eutectic of laminar structure. The small liquid pool inside the original α-phase solidifies basically in the same way as that of intergranular liquid, but owing to less amount of liquid phase, the eutectic solidification is mainly carried out in the dissociated pattern.展开更多
The solidification microstructure of Al-Si alloy was observed in the experiment,the second dendrite arm spacing(SDAS)was measured,and the effect of temperature on the microstructure was analyzed.Phase-field(PF)model i...The solidification microstructure of Al-Si alloy was observed in the experiment,the second dendrite arm spacing(SDAS)was measured,and the effect of temperature on the microstructure was analyzed.Phase-field(PF)model incorporating natural convection caused by gravity was employed to simulate the microstructure evolution of Al-Si alloy under the experimental conditions.Good agreements between the experimental and simulation results verified the reliability of the simulation approach proposed in this study.Based on the proposed model,a series of simulation cases(2D and 3D)were performed to investigate the evolution of columnar and equiaxed dendritic structures.It was found that the solute content of the alloy had little impact on the microstructure evolution,while the solute expansion coefficient had obvious effect on the dendrite tip velocities.Significant improvement of computational efficiency was achieved via novel algorithms,making it possible to perform massive simulation for studying the evolution of solidification microstructures,which is hard to be directly observed in experiments via synchrotron radiation for Al-Si alloy.展开更多
The effect of high pressure during solidification on the microstructure and mechanical property of Mg-6Zn-1Y and Mg-6Zn-3Y was investigated using optical microscopy, scanning electronic microscopy, X-ray diffraction（...The effect of high pressure during solidification on the microstructure and mechanical property of Mg-6Zn-1Y and Mg-6Zn-3Y was investigated using optical microscopy, scanning electronic microscopy, X-ray diffraction（XRD） and Vickers-hardness testing. Under atmospheric-pressure solidification, Mg-6Zn-1Y consisted of α-Mg, Mg7Zn3 and Mg_3YZn_6; whilst Mg-6Zn-3Y consisted of α-Mg, Mg_3Y_2Zn_3 and Mg_3YZn_6. Under 6 GPa high-pressure solidification, both alloy consisted of α-Mg, MgZ n and Mg12 YZn. The shape of the main second phase changed from a lamellar structure formed for atmospheric-pressure solidification to small particles formed for solidification at 6 GPa pressure. The dendrite microstructure was refined and was more regular, and the length of the primary dendrite arm increased under 6 GPa high-pressure solidification, which was attributed to increasing thermal undercooling, compositional undercooling and kinetics undercooling. After solidification at 6 GPa pressure, the solid solubility of Y in the second phase and the Vickers-hardness increased from 15 wt.% and 69 MPa for Mg-6Zn-1Y to 49 wt.% and 97 MPa; and from 19 wt.% and 71 MPa for Mg-6Zn-3Y alloy to 20 wt.% and 92 MPa, respectively.展开更多
The present work focused on the Ni_3Al-based alloy with a high melting point. The aim of the research is to study the effect of withdrawal rate on the microstructures and mechanical properties of directionally solidif...The present work focused on the Ni_3Al-based alloy with a high melting point. The aim of the research is to study the effect of withdrawal rate on the microstructures and mechanical properties of directionally solidified Ni-25 Al alloy. Ni_3 Al intermetallics were prepared at different withdrawal rates by directional solidification(DS) in an electromagnetic cold crucible directional solidification furnace. The DS samples contain Ni_3 Al and Ni Al phases. The primary dendritic spacing(λ) decreases with the increasing of withdrawal rate(V), and the volume fraction of Ni Al phase increases as the withdrawal rate increases. Results of tensile tests show that ductility of DS samples is enhanced with a decrease in the withdrawal rate.展开更多
The Ru-free and Ru-containing single crystal superalloys were cast in the directionally solidified furnace,while other alloying element contents were basically kept unchanged.The effects of Ru on the solidification ch...The Ru-free and Ru-containing single crystal superalloys were cast in the directionally solidified furnace,while other alloying element contents were basically kept unchanged.The effects of Ru on the solidification characteristic and microstructures of single crystal superalloy were investigated with differential scanning calorimetry,electron probe micro analyzer,energy-dispersive X-ray spectroscope,scanning electron microscope and transmission electron microscope.The results show that the liquidus temperature of the single crystal superalloy decreases with Ru addition.The primary dendrite arm spacing and volume fraction of γ/γ＇ eutectic both decrease with Ru addition.The sizes of γ＇ phase of dendritic core and interdendritic region have no obvious change with Ru addition.Ru tends to segregate slightly in the dendritic core.The extent of elements segregation decreases with the Ru addition.Ru tends to partition preferentially into the γ matrix.The addition of Ru decreases the partition ratio（the ratio of the γ phases composition over the γ＇ phases composition） of elements Re,W and Mo.展开更多
The microstructure and microsegregation of atomized powder,which depend on their sizes,are of great importance to the mechanical properties of the consolidated bulk materials.Therefore,it is necessary to reveal the re...The microstructure and microsegregation of atomized powder,which depend on their sizes,are of great importance to the mechanical properties of the consolidated bulk materials.Therefore,it is necessary to reveal the relationship between particle size and powder attributes.The effects of particle size on the so-lidification characterization of the atomized Ni-based superalloy powders were studied via finite element simulation.Based on the simulations,a model was developed to predict the microsegregation and mi-crostructure of atomized powders with different sizes and study the influence of thermal history on the powder attributes during the atomization processes.The radiation heat transfer and temperature gradi-ent within the rapid solidification alloy powders were taken into account in this model.For validating the accuracy of the model,the predictions of the present model were compared with the microsegregation and microstructure of the specific size powder close to the screen mesh size.The results showed that mi-crostructure depended primarily on the temperature gradient within the powder,while the solidification rate had a more significant effect on the microsegregation.The model predicted microstructure features in agreement with the experiment,and for microsegregation,the deviations of prediction for most ele-ments were less than 10%.This work provides a new model to precisely predict the microsegregation and microstructure of the atomized alloy powders and sets a foundation to control the powder features for various engineering applications.展开更多
Understanding the effects of various elements on solidification behavior is crucial for designing the composition ofγ’-strengthened Co-based superalloys and is fundamental for controlling the as-cast structure and f...Understanding the effects of various elements on solidification behavior is crucial for designing the composition ofγ’-strengthened Co-based superalloys and is fundamental for controlling the as-cast structure and formulating subsequent heat treatment processes.This research investigated the effects of replacing 1 at.%W with 1 at.%Nb or Hf elements on the solidification behavior of Co-Ni-Al-W-based superalloys.The findings revealed that substituting W with Nb and Hf resulted in a notable decrease in both the solidus temperature(TS)and liquidus temperature(TL).Specifically,the substitution of W with Nb lowered TS from 1353℃ to 1332℃ and TL from 1383℃ to 1368℃,while replacing W with Hf decreased TS from 1353℃ to 1330℃ and TL from 1383℃ to 1366℃.Moreover,both Nb and Hf element are positive segregation element,while Nb decreases and Hf increases W segregation,respectively.During the final solidification stage,the substitution of W with Nb resulted in the formation of eutectic(γ+γ’),Co_(3)Ta,and a small amount ofμ-Co7Nb6 phase,while replacing W with Hf resulted in the formation of the Laves phase andβ-CoAl phase.The solidification paths of the three alloys were confirmed based on the result of differential scanning calorimetry,isothermal solidification experiment and Thermo-calc simulation.These results offer a theoretical basis for the composition design and optimization of heat treatment processes for Co-Ni-Al-W-based superalloys.展开更多
This paper briefly reviews the recent research on the near rapid directional solidification and microstructure superfining. The morphology transitions and the corresponding mechanical properties are presented. The cri...This paper briefly reviews the recent research on the near rapid directional solidification and microstructure superfining. The morphology transitions and the corresponding mechanical properties are presented. The critical velocities relevant to the morphology transitions are comprehensively given. Meanwhile the solidification characteristics near absolute stability limit are studied.It can be clearly seen that the superfine microstructures possess extremely better properties compared with the conventional microstructures.展开更多
基金financially suppoted by the National Natural Sci-ence Foundation of China(No.52371041).
文摘Tungsten heavy alloys(WHAs)prepared using laser additive manufacturing(AM)exhibit intricate ge-ometries,albeit with limited mechanical properties.Here we designed a high-strength WHA featuring a FeCrCoNi high entropy alloy(HEA)binder via the laser metal deposition(LMD)technique.Due to the distinctive thermal cycle and rapid cooling rate,the as-deposited alloys exhibit microstructures with hy-poeutectic,eutectic-like,and spot-like characteristics.To elucidate this phenomenon,the solidification paths were delineated and analyzed by combining microstructural characterization and phase equilib-rium simulation.Theμphase precipitated out from the supersaturated solid solution,thereby nucleating massive dislocations on the FeCrCoNi matrix to increase the work hardening rate.Furthermore,theμphase formed an ultrafine intermetallic compound(IMC)layer around the W grain,reducing the hole or crack between the W grain and FeCrCoNi matrix.Attributed to the precipitation strengthening,the solid solution of the FeCrCoNi binder,along with the load-bearing strength of W,the developed alloy achieved ultrahigh compressive stress and strain of 2047 MPa and 32%respectively at room temperature.These findings contribute valuable insights to the advancement of additive manufacturing for tungsten alloys,leveraging their excellent properties.
基金financially supported by the Natural Science Foundation Joint Fund of Liaoning Province,China(No.2023-MSLH-342).
文摘The effect of melt superheating treatment on the solidification microstructure and mechanical properties of theγ'phase precipitation-strengthened K424 superalloy was investigated.Differential scanning calorimetry(DSC)experiments were conducted to explore the influence of melt treatment temperature on the undercooling of the superalloy.Additionally,pouring experiments were carried out to assess how alterations in both the temperature and duration of melt treatment impacted the grain size,secondary dendrite arm spacing(SDAS),elemental segregation,and mechanical properties of the alloy.Metallographic analysis,scanning electron microscopy,energy dispersive spectroscopy(EDS)and Thermo-Calc software were employed for microstructure characterization.The test specimens were subjected to tensile testing at room temperature and stress rupture testing at 975℃ under 196 MPa.The findings reveal that appropriate melt treatment conditions result in decreased grain size,refined SDAS,minimized elemental segregation,and significant improvements in mechanical properties.Specifically,the study demonstrates that a melt treatment at 1,650℃ for 5 min results in the smallest average grain size of 949μm and the smallest SDAS of 25.38μm.Furthermore,the room temperature tensile properties and creep resistance are notably affected by the melt treatment parameters.It is shown that specific melt treatment conditions,such as holding at 1,650℃ for 5 min,result in superior room temperature strength and extended stress rupture life of the K424 superalloy,while a balance between strength and stability is achieved at 1,600℃ with a holding time of 10 min.These findings offer guidance for optimizing the melt treatment parameters for the K424 superalloy,laying a foundation for further investigations.
基金the National Natural Science Foundation of China under Grant No.5181101756,51861165202 and No.51721092the Major Project of Science and Technology Innovation Special for Hubei Province under Grant No.2018AAA027+3 种基金the Fundamental Research Funds for the Central Universities,HUST:No.2018JYCXJJ034 and No.2019JYCXJJ025the Postdoctoral Science Foundation of China under Grant No.2018M632837the opening project of State Key Laboratory of Digital Manufacturing Equipment and Technology(HUST)under grant No.DMETKF2018001supported by the China Scholarship Council as a visiting scholar at the University of Virginia。
文摘Understanding the behaviors of heat transfer and fluid flow in weld pool and their effects on the solidification microstructure are significant for performance improvement of laser welds.This paper develops a three-dimensional numerical model to understand the multi-physical processes such as heat transfer,melt convection and solidification behavior in full-penetration laser welding of thin 5083 aluminum sheet.Solidification parameters including temperature gradient G and solidification rate R,and their combined forms are evaluated to interpret solidification microstructure.The predicted weld dimensions and the microstructure morphology and scale agree well with experiments.Results indicate that heat conduction is the dominant mechanism of heat transfer in weld pool,and melt convection plays a critical role in microstructure scale.The mushy zone shape/size and solidification parameters can be modulated by changing process parameters.Dendritic structures form because of the low G/R value.The scale of dendritic structures can be reduced by increasing GR via decreasing heat input.The columnar to equiaxed transition is predicted quantitatively via the process related G^3/R.These findings illustrate how heat transfer and fluid flow affect the solidification parameters and hence the microstructure,and show how to improve microstructure by optimizing the process.
文摘IN617B nickel-base superalloy is considered as a good candidate material in 700℃advanced ultrasupercritical coal-fired power plants.The effect of Ta addition on solidification microstructure and element segregation of IN617B alloy was investigated by OM,SEM,TEM,EDS,EPMA and thermodynamic calculation.The results showed that the solidification microstructure exhibited a dendritic segregation pattern with many primary carbides distributed in interdendritic regions,such as network M_(6)C,lath M_(23)C_(6) and granular Ti(C,N).The addition of Ta promoted the precipitation of Ta-rich MC significantly inhibiting the precipitation of M_(6)C and M_(23)C_(6),and reduced the segregation degree of Al,Mo and Ti alloying elements.The addition of Ta decreased the melting temperature of MC carbide,but did not impact the solidification path,that was,L→γmatrix→MC or Ti(C,N)→M_(6)C→M_(23)C_(6),where MC and Ti(C,N)tended to form symbiotic microstructure with M_(6)C.This study will provide theoretical basis and data support for the alloy optimization and casting structure control of IN617B nickel-based superalloy.
基金financially supported by the National Key Research and Development Program of China(No.2016YFB0701204)the National Science and Technology Major Project of China(No.2017ZX04006001)the National Natural Science Foundation of China(No.U1737208)。
文摘The solidification microstructure of Mg-Gd-YZr alloy was investigated via an experimental study and cellular automaton(CA)simulation.In this study,stepshaped castings were produced,and the temperature variation inside the casting was recorded using thermocouples during the solidification process.The effects of the cooling rate and Zr content on the grain size of the Mg-Gd-Y-Zr alloy were studied.The results showed that the grain size decreased with an increase in the cooling rate and Zr content.Based on the experimental data,a quantitative model for calculating the heterogeneous nucleation rate was developed,and the model parameters were determined.The evolution of the solidification microstructure was simulated using the CA method,where the quantitative nucleation model was used and a solute partition ceoefficient was introduced to deal with the solute trapping in front of the solid-liquid(S/L)interface.The simulation results of the grain size were in good agreement with the experimental data.The simulation also showed that the fraction of the eutectics decreased with an increasing cooling rate in the range of 2.6-11.0℃·s^(-1),which was verified indirectly by the experimental data.
基金financially supported by the National Natural Science Foundation of China(No.51401168)the Fundamental Research Funds for the Central Universities(No.3102014JCQ01026)
文摘Beta-solidifying TiAl alloy has great potential in the field of aero-industry as a cast alloy.In the present work,the influence of cooling rate during mushy zone on solidification behavior of Ti-44Al-4Nb-2Cr-0.1B alloy was investigated.A vacuum induction heating device combining with temperature control system was used.The Ti-44Al-4Nb-2Cr-0.1B alloy solidified from superheated was melted to β phase with the cooling rates of 10,50,100,200,400 and 700 K·min^（-1）,respectively.Results show that with the increase in cooling rate from 10 to 700 K·min^（-1）,the colony size of α_2/γ lamella decreases from 1513 to48 urn and the solidification segregation significantly decreases.Also the content of residual B2 phase within α_2/γlamellar colony decreases with the increase in cooling rate.In addition,the alloy in local interdendritic regions would solidify in a hypo-peritectic way,which can be attributed to the solute redistribution and enrichment of Al element in solidification.
基金the National Natural Science Foundation of China(Nos.51374002,51574078 and 51774081)the Fundamental Research Funds for the Central Universities(No.N160705001)+1 种基金China Postdoctoral Science Foundation(Nos.2014M560218 and 2016T90228)the Student’s Platform for Innovation and Entrepreneurship Training Program(No.160111)。
文摘To study the effects of rare earth(RE)and Ti on the solidification micros true ture of high borated stainless steels,1.6 wt%B stainless steel doped with RE and2.1 wt%B stainless steel doped with Ti were prepared by ingot casting,respectively.The solidification microstructure of researched steels was characterized in detail.The modification mechanism was clarified based on the heterogeneous nucleation theory and the thermodynamic calculation.The solidification microstructure of 1.6 wt%B and 2.1 wt%B stainless steels was characterized by the continuous and network-like eutectic borides around the matrix grains.It was found that the fine RE compounds could act as the heterogeneous nuclei for both borides and austenite during solidification.Thus,the eutectic borides were more dispersed in the modified steel.Moreover,lots of fine‘eutectic cells’were formed in the matrix regions.As a result of the preferential formation of TiB2 during solidification,the amount of the eutectic borides in the steel modified with Ti was significantly decreased.Besides,the continuity of the eutectic borides network was weakened.In a word,the present work provides a promising method to modify the solidification microstructure for high borated stainless steels.
文摘Solidification microstructure and mechanical property are explored.Furthermore,tensile fracture and microstructure are analyzed by using SEM and JXA 840A electron probe.The results indicate that SiC particles in SiCp/ZA27 composite are mainly distributed on interfaces or between dendrites and surrounded by primary α phase.The dendrite of α phase is fined by SiCp.The tensile strength at room temperature decreases with the increasing of SiCp addition.The tensile strength at elevated temperature increases with the addition of SiCp.The fracture of SiCp/ZA27 composites is the mixture of tough and brittle fracture.The carck is prone to extend along the interface and the region of dispersed shrinkage.
文摘The solidification microstructures and solute segregation of a newly developed hot corrosion resistant single-crystal Ni-base superalloy were investigated with a zone-melting and ultra-high thermal gradient unidirectional solidification apparatus.Compared with the microstructures solidified at conventional low thermal gradient conditions,the dendrite arm spacings,the interdendritic microporosity and γ/γ' eutectic,and the severity of solute segregation of the single-crystal superalloy solidified at ultra-high thermal gradient conditions were considerably reduced.It was shown that the microstructure solidified under ultra-high thermal gradient condition is ideal for the full exploitation of the excellent property potentials of single-crystal superalloys.
基金the National Natural Science Foundation of China(No.51371051)Jiangsu Key Laboratory of Advanced Metallic Materials(No.BM2007204)。
文摘A two-dimensional(2-D)multi-component and multi-phase cellular automaton(CA)model coupled with the Calphad method and finite difference method(FDM)is proposed to simulate the gas pore formation and microstructures in solidification process of hypoeutectic Al-Si-Mg alloys.In this model,the pore growth,and dendritic and eutectic solidification are simulated using a CA technique.To achieve the equilibrium among multiple phases during ternary Al-based alloy solidification,the phase transition thermodynamics and kinetics are evaluated by adopting the Calphad method.The diffusion equations of hydrogen and two solutes are solved by FDM.The developed CA-FDM coupled model can be used for simulating the evolution of gas microporosity and microstructures,involving dendrites and irregular binary and ternary eutectics,of ternary hypoeutectic Al-Si-Mg alloys.It has the capability of reproducing the interactions between the hydrogen microporosity formation and the growth of dendrites and eutectics,the competitive growth among the growing gas pores of different sizes,together with the time-evolving concentration fields of hydrogen and solutes.The simulated morphology of gas pore and microstructure has a good agreement with the experimental observation.The influences of the initial hydrogen concentration and cooling rate on the microporosity formation are investigated.It is found that the main portion of porosity formation occurs in the eutectic solidification stage through analyzing the profiles of porosity percentage and solid fraction varying with solidification time.The varying features of simulated porosity percentage,the maximum and average pores radii indicate that increasing initial hydrogen concentration promotes the formation of higher final porosity percentage and larger pores,while the size of gas pores will significantly reduce with increasing cooling rate,leading to a lower final porosity percentage.
基金Project(LJQ2014062)supported by the Outstanding Young Scholars in Colleges and Universities of Liaoning Province,China
文摘The Cu?Al composite casts were prepared by the method of pouring molten aluminum. The solidification process and themicrostructure of the transition layer were investigated during the recombination process of the liquid Al and the solid Cu. The results reveal that the microstructure of the transition layer in the Cu?Al composite cast consists of α(Al)+α(Al)?CuAl2 eutectic,α(Al)?CuAl2 eutectic, CuAl2+α(Al)?CuAl2 eutectic and Cu9Al4. Additionally, the pouring temperature, cooling mode of the Cu platesurface and start time of the forced cooling after pouring have no effect on the microstructure species. But the proportion of thevarious microstructures in the transition layer changes with the process parameters. The pure Al at the top of the transition layer startsto solidify first and then the α(Al) phase grows in a dendritic way, while the CuAl2 phase exhibits plane or cellular crystal growth from the two sides of the transition layer towards its interior. The stronger the cooling intensity of the Cu plate outer surface, the more developed the dendrite, and the easier it is for the CuAl2 phase to grow into a plane crystal.
基金the National Natural Science Foundation of China(Nos.51571056,51690164 and 51904183)the Technological Innovation Projects of Universities in Guangdong Province(Nos.2017KTSCX177 and 2020KQNCX084)+2 种基金China Postdoctoral Science Foundation(No.2020M683463)Guangdong Basic and Applied Basic Research Foundation(No.2019A1515110135)the Natural Science Foundation of Guangdong Province(No.2018A030310024)。
文摘The effect of different scales thermoelectric magnetic convection(TEMC)on the radial solidification microstructure of hypereutectic Al alloy has been investigated under transverse static magnetic field during directional solidification,focusing on the formation of freckle.Our experimental and numerical simulation results indicate that the TEMC circulation at sample scale under transverse static magnetic field leads to the enrichment of solute Al on one side of the sample.The TEMC and the solute enrichment degree increase with the increase of magnetic field when the magnetic field increases to 0.5 T.The enrichment degree of solute elements under magnetic field is affected by temperature gradient and growth rate.The non-uniform distribution of solute Al in the radial direction of the sample results in the non-uniform distribution of primary dendrite arm spacing(PDAS).Moreover,the applied magnetic field can lead to freckle formation and its number increases with the increase of magnetic field.The change of freckle is consistent with the anisotropy TEMC caused by the anisotropy of primary dendrite or primary dendrite network under magnetic field.Finally,the mechanism of synergism effect of the anisotropy TEMC,the distribution of solute Al and the PDAS on freckle formation and evolution is studied during directional solidification under magnetic field.
文摘The liquid quenching method was adopted to study the solidification morphology and microstructure of AZglD Mg alloy in semisolid. The results indicate that cooling rate has important effects upon the solidification structures. Under the cooling rate of liquid quenching, primary α-phase grows first by attaching on the original α grains, or independent nucleation and growth. The high cooling rate makes primary α-phase grow in ＂rags＂ or dendrite shape. Eutectic solidification is carried out in terms of both dissociated growth and symbiotic growth. The dissociated growth forms rough and large β-phase at grain boundaries, while symbiotic growth forms eutectic of laminar structure. The small liquid pool inside the original α-phase solidifies basically in the same way as that of intergranular liquid, but owing to less amount of liquid phase, the eutectic solidification is mainly carried out in the dissociated pattern.
基金financial supports from the National Key R&D Program of China(No.2016YFB0701201)the Fostering Project in Innovation Funds of China Academy of Engineering Physics(No.PY2019078)financial support from China Scholarship Council。
文摘The solidification microstructure of Al-Si alloy was observed in the experiment,the second dendrite arm spacing(SDAS)was measured,and the effect of temperature on the microstructure was analyzed.Phase-field(PF)model incorporating natural convection caused by gravity was employed to simulate the microstructure evolution of Al-Si alloy under the experimental conditions.Good agreements between the experimental and simulation results verified the reliability of the simulation approach proposed in this study.Based on the proposed model,a series of simulation cases(2D and 3D)were performed to investigate the evolution of columnar and equiaxed dendritic structures.It was found that the solute content of the alloy had little impact on the microstructure evolution,while the solute expansion coefficient had obvious effect on the dendrite tip velocities.Significant improvement of computational efficiency was achieved via novel algorithms,making it possible to perform massive simulation for studying the evolution of solidification microstructures,which is hard to be directly observed in experiments via synchrotron radiation for Al-Si alloy.
基金Project supported by the China Scholarship Council(2011836024)the International Science and Technology Cooperation Project of Jiangxi Province(20151BDH80006)+5 种基金the Prior Science and Technology Program led by the Returned Overseas Chinese Talents(RSTH[2015]192-GRSZ[2015]273)the Key Program of Natural Science Foundation of Jiangxi Province(20133BAB2000820144ACB20013)the Science and Technology Innovation Project of Jiangxi Academy of Sciences(2013-YYB-12013-XTPH1-192015XTTD04)
文摘The effect of high pressure during solidification on the microstructure and mechanical property of Mg-6Zn-1Y and Mg-6Zn-3Y was investigated using optical microscopy, scanning electronic microscopy, X-ray diffraction（XRD） and Vickers-hardness testing. Under atmospheric-pressure solidification, Mg-6Zn-1Y consisted of α-Mg, Mg7Zn3 and Mg_3YZn_6; whilst Mg-6Zn-3Y consisted of α-Mg, Mg_3Y_2Zn_3 and Mg_3YZn_6. Under 6 GPa high-pressure solidification, both alloy consisted of α-Mg, MgZ n and Mg12 YZn. The shape of the main second phase changed from a lamellar structure formed for atmospheric-pressure solidification to small particles formed for solidification at 6 GPa pressure. The dendrite microstructure was refined and was more regular, and the length of the primary dendrite arm increased under 6 GPa high-pressure solidification, which was attributed to increasing thermal undercooling, compositional undercooling and kinetics undercooling. After solidification at 6 GPa pressure, the solid solubility of Y in the second phase and the Vickers-hardness increased from 15 wt.% and 69 MPa for Mg-6Zn-1Y to 49 wt.% and 97 MPa; and from 19 wt.% and 71 MPa for Mg-6Zn-3Y alloy to 20 wt.% and 92 MPa, respectively.
基金financially supported by the National Natural Science Foundation of China(Grant No.51471062)
文摘The present work focused on the Ni_3Al-based alloy with a high melting point. The aim of the research is to study the effect of withdrawal rate on the microstructures and mechanical properties of directionally solidified Ni-25 Al alloy. Ni_3 Al intermetallics were prepared at different withdrawal rates by directional solidification(DS) in an electromagnetic cold crucible directional solidification furnace. The DS samples contain Ni_3 Al and Ni Al phases. The primary dendritic spacing(λ) decreases with the increasing of withdrawal rate(V), and the volume fraction of Ni Al phase increases as the withdrawal rate increases. Results of tensile tests show that ductility of DS samples is enhanced with a decrease in the withdrawal rate.
文摘The Ru-free and Ru-containing single crystal superalloys were cast in the directionally solidified furnace,while other alloying element contents were basically kept unchanged.The effects of Ru on the solidification characteristic and microstructures of single crystal superalloy were investigated with differential scanning calorimetry,electron probe micro analyzer,energy-dispersive X-ray spectroscope,scanning electron microscope and transmission electron microscope.The results show that the liquidus temperature of the single crystal superalloy decreases with Ru addition.The primary dendrite arm spacing and volume fraction of γ/γ＇ eutectic both decrease with Ru addition.The sizes of γ＇ phase of dendritic core and interdendritic region have no obvious change with Ru addition.Ru tends to segregate slightly in the dendritic core.The extent of elements segregation decreases with the Ru addition.Ru tends to partition preferentially into the γ matrix.The addition of Ru decreases the partition ratio（the ratio of the γ phases composition over the γ＇ phases composition） of elements Re,W and Mo.
基金support of this work by the National Science and Technology Major Project(No.2017-Ⅵ-0008-0078)the National Key Research and Development Program of China(No.2022YFB3803802)the National Natural Science Foundation of China(No.U1560106).
文摘The microstructure and microsegregation of atomized powder,which depend on their sizes,are of great importance to the mechanical properties of the consolidated bulk materials.Therefore,it is necessary to reveal the relationship between particle size and powder attributes.The effects of particle size on the so-lidification characterization of the atomized Ni-based superalloy powders were studied via finite element simulation.Based on the simulations,a model was developed to predict the microsegregation and mi-crostructure of atomized powders with different sizes and study the influence of thermal history on the powder attributes during the atomization processes.The radiation heat transfer and temperature gradi-ent within the rapid solidification alloy powders were taken into account in this model.For validating the accuracy of the model,the predictions of the present model were compared with the microsegregation and microstructure of the specific size powder close to the screen mesh size.The results showed that mi-crostructure depended primarily on the temperature gradient within the powder,while the solidification rate had a more significant effect on the microsegregation.The model predicted microstructure features in agreement with the experiment,and for microsegregation,the deviations of prediction for most ele-ments were less than 10%.This work provides a new model to precisely predict the microsegregation and microstructure of the atomized alloy powders and sets a foundation to control the powder features for various engineering applications.
基金supported by the National Key Research and Development Program of China(2023YFB3712003)the National Science and Technology Major Project(J2019-VI-0018-0133)+2 种基金the AECC Independent Innovation Special Fund Project(ZZCX-2022-040)the National Natural Science Foundation of China(Nos.51701212,51771191 and 51971214)the Youth Innovation Promotion Association Project,Chinese Academy of Sciences(2020198)。
文摘Understanding the effects of various elements on solidification behavior is crucial for designing the composition ofγ’-strengthened Co-based superalloys and is fundamental for controlling the as-cast structure and formulating subsequent heat treatment processes.This research investigated the effects of replacing 1 at.%W with 1 at.%Nb or Hf elements on the solidification behavior of Co-Ni-Al-W-based superalloys.The findings revealed that substituting W with Nb and Hf resulted in a notable decrease in both the solidus temperature(TS)and liquidus temperature(TL).Specifically,the substitution of W with Nb lowered TS from 1353℃ to 1332℃ and TL from 1383℃ to 1368℃,while replacing W with Hf decreased TS from 1353℃ to 1330℃ and TL from 1383℃ to 1366℃.Moreover,both Nb and Hf element are positive segregation element,while Nb decreases and Hf increases W segregation,respectively.During the final solidification stage,the substitution of W with Nb resulted in the formation of eutectic(γ+γ’),Co_(3)Ta,and a small amount ofμ-Co7Nb6 phase,while replacing W with Hf resulted in the formation of the Laves phase andβ-CoAl phase.The solidification paths of the three alloys were confirmed based on the result of differential scanning calorimetry,isothermal solidification experiment and Thermo-calc simulation.These results offer a theoretical basis for the composition design and optimization of heat treatment processes for Co-Ni-Al-W-based superalloys.
文摘This paper briefly reviews the recent research on the near rapid directional solidification and microstructure superfining. The morphology transitions and the corresponding mechanical properties are presented. The critical velocities relevant to the morphology transitions are comprehensively given. Meanwhile the solidification characteristics near absolute stability limit are studied.It can be clearly seen that the superfine microstructures possess extremely better properties compared with the conventional microstructures.
