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
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
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
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
Nd_9Fe_（85–x）Ti_4C_2B_x（x=10–15） magnetic alloys were investigated by differential thermal analysis and X-ray diffraction analysis. The results showed that with the B content increasing from 10 at.% to 15 at.%, ...Nd_9Fe_（85–x）Ti_4C_2B_x（x=10–15） magnetic alloys were investigated by differential thermal analysis and X-ray diffraction analysis. The results showed that with the B content increasing from 10 at.% to 15 at.%, the liquidus temperatures TL of the alloys decreased from 1498.5 to 1472.5 K; the solidus temperatures TS of them increased from 1353.2 to 1358.3 K; and the nucleation undercooling of the alloy melts cooled at the rate of 40 K/min decreased from 122.8 to 95.9 K, resulting in the solidification structures consisting of Nd_2Fe_（14）B, Fe_3B, α-Fe, Nd1.1Fe4B4 and TiC nanocrystallines. Furthermore, the Nd_9Fe_（85–x）Ti_4C_2B_x（x=11, 13, 15） bulk alloys in sheet form with the thickness of 0.7 mm were prepared by copper mold suction casting and their solidification characteristics and solidification structures under sub-rapidly cooling rate were investigated. The results showed that partially amorphous structures were obtained in the as-cast bulk alloys and the amount of amorphous decreased with the increase of the B content. By annealing the as-cast bulk alloys at 923 K for 10 min, the nanocomposite microstructures composed with Nd_2Fe_（14）B, Fe_3B and α-Fe nanocrystallines, which showed a single-phase hard magnetic behavior and enhanced magnetic properties, were achieved.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
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
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 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 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
The solidification of Sn-Ni peritectic alloys in which both the primary Ni_(3)Sn_(2)and peritectic Ni_(3)Sn_(4)phases were intermetallic compound phases(IMCs)with narrow solubility ranges was investigated through conf...The solidification of Sn-Ni peritectic alloys in which both the primary Ni_(3)Sn_(2)and peritectic Ni_(3)Sn_(4)phases were intermetallic compound phases(IMCs)with narrow solubility ranges was investigated through confocal laser scanning microscope.Analysis on the interface migration at different cooling rates shows that the rate of peritectic reaction is much smaller than previous reports,and the growth of peritectic phase is mainly attributed to direct precipitation from the melt in Sn-Ni alloy after peritectic reaction.In addition,different from other peritectic alloys where the solidified phases are solid solution phases,the"step"growth of both Ni_(3)Sn_(2)and Ni_(3)Sn_(4)phases was observed.The dependences of the step thickness on both the cooling rate and solidification time were measured,which shows that the step thicknesses of both phases gradually decrease as solidification proceeds.This was confirmed to be attributed to the difference between the actual and equilibrium melt concentrations during solidification.In addition,the increase of the normal growth velocity of Ni_(3)Sn_(4)phase with increasing cooling rate was also proved through both the experimental observation and quantitative prediction.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
We simulate the evolution of hydrogen concentration and gas pore formation as equiaxed dendrites grow during solidification of a hypoeutectic aluminum-silicon(Al-Si)alloy.The applied lattice Boltzmann-cellular automat...We simulate the evolution of hydrogen concentration and gas pore formation as equiaxed dendrites grow during solidification of a hypoeutectic aluminum-silicon(Al-Si)alloy.The applied lattice Boltzmann-cellular automaton-finite difference model incorporates the physical mechanisms of solute and hydrogen partitioning on the solid/liquid interface,as well as the transports of solute and hydrogen.After the quantitative validation by the simulation of capillary intrusion,the model is utilized to investigate the growth of the equiaxed dendrites and hydrogen porosity formation for an Al-(5 wt.%)Si alloy under different solidification conditions.The simulation data reveal that the gas pores favorably nucleate in the corners surrounded by the nearby dendrite arms.Then,the gas pores grow in a competitive mode.With the cooling rate increasing,the competition among different growing gas pores is found to be hindered,which accordingly increases the pore number density in the final solidification microstructure.In the late solidification stage,even though the solid fraction is increasing,the mean concentration of hydrogen in the residue melt tends to be constant,corresponding to a dynamic equilibrium state of hydrogen concentration in liquid.As the cooling rate increases or the initial hydrogen concentration decreases,the temperature of gas pore nucleation,the porosity fraction,and the mean porosity size decrease,whilst the mean hydrogen concentration in liquid increases in the late solidification stage.The simulated data present identical trends with the experimental results reported in literature.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
In this paper,the diversity of complicated dendrite microstructure and its evolution behavior during solidification in different magnesium alloys under various processing conditions were illustrated using synchrotron ...In this paper,the diversity of complicated dendrite microstructure and its evolution behavior during solidification in different magnesium alloys under various processing conditions were illustrated using synchrotron X-ray imaging technique.A variety of dendritic morphologies and branching structures were revealed,i.e.,sixfold plate-like symmetric structure in Mg-Al-based structure,12-branch structure in Mg-Zn-based alloys and 18-branch structure in Mg-Sn-and Mg-Ca-based alloys as well as seaweed like hyper-branched structure in Mg-38wt%Zn alloy.In addition,a dendrite morphology and orientation transition with increasing addition of Zn content were also observed in Mg-Zn alloy,with dendrite growth pattern transform from anisotropy(low Zn addition)with sixfold symmetric snow-flake structure to relative isotropy(intermediate Zn addition)where seaweed morphology presented and then back to anisotropy(high Zn addition)when only 12 branches with preferred<11 2 1>orientations were observed.The phase-field model representing the typical dendritic morphologies and branching structures under various conditions was also depicted and discussed.Further,the two-dimensional(2D)real-time dendrite growth dynamics in different Mg-based alloys captured using synchrotron X-ray radiography for unveiling the originate of theα-Mg dendrite was reviewed.Following this,the four-dimensional(3D+time)synchrotron X-ray tomographic in situ observation of dendritic morphology evolution indicating the formation mechanism of the diverse dendritic morphology during Mg-Sn-and Mg-Zn-based alloys was also summarized.Finally,the future study on exploring the complicated dendritic morphologies and their origination during solidification of Mg-based alloys is prospected.展开更多
The microstructures of austenitic stainless steel strip were studied using color metallographic method and electron probe micro analysis （EPMA）. In the cast strips, there are three kinds of solidification structures...The microstructures of austenitic stainless steel strip were studied using color metallographic method and electron probe micro analysis （EPMA）. In the cast strips, there are three kinds of solidification structures： fine cel- lular dendrite in the surface layer, equiaxed grains in the center and fine dendrite between them. The solidification mode in the surface layer is the primary austenite AF mode because of extremely high cooling rate, with the retained ferrite located around the primary cellular austenite. In the fine dendrite zone, the solidification mode of molten stainless steel changes to FA mode and the residual ferrite with fish-bone morphology is located at the core of the dendrite. The retained ferrite of equiaxed grains in the center is located in the center of broken primary ferrite dendrite with vermicular morphology.展开更多
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.
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(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 supported by National Natural Science Foundation of China(51174121,51274125)Zhejiang Province Science and Technology Innovation Team of Key Projects(2010R50016-30)
文摘Nd_9Fe_（85–x）Ti_4C_2B_x（x=10–15） magnetic alloys were investigated by differential thermal analysis and X-ray diffraction analysis. The results showed that with the B content increasing from 10 at.% to 15 at.%, the liquidus temperatures TL of the alloys decreased from 1498.5 to 1472.5 K; the solidus temperatures TS of them increased from 1353.2 to 1358.3 K; and the nucleation undercooling of the alloy melts cooled at the rate of 40 K/min decreased from 122.8 to 95.9 K, resulting in the solidification structures consisting of Nd_2Fe_（14）B, Fe_3B, α-Fe, Nd1.1Fe4B4 and TiC nanocrystallines. Furthermore, the Nd_9Fe_（85–x）Ti_4C_2B_x（x=11, 13, 15） bulk alloys in sheet form with the thickness of 0.7 mm were prepared by copper mold suction casting and their solidification characteristics and solidification structures under sub-rapidly cooling rate were investigated. The results showed that partially amorphous structures were obtained in the as-cast bulk alloys and the amount of amorphous decreased with the increase of the B content. By annealing the as-cast bulk alloys at 923 K for 10 min, the nanocomposite microstructures composed with Nd_2Fe_（14）B, Fe_3B and α-Fe nanocrystallines, which showed a single-phase hard magnetic behavior and enhanced magnetic properties, were achieved.
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.
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.
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.51871118)the Fundamental Research Funds for the Central Universities(No.lzujbky-2019-sp03)the fund of Science and Technology Project of Lanzhou(No.2019-1-30)。
文摘The solidification of Sn-Ni peritectic alloys in which both the primary Ni_(3)Sn_(2)and peritectic Ni_(3)Sn_(4)phases were intermetallic compound phases(IMCs)with narrow solubility ranges was investigated through confocal laser scanning microscope.Analysis on the interface migration at different cooling rates shows that the rate of peritectic reaction is much smaller than previous reports,and the growth of peritectic phase is mainly attributed to direct precipitation from the melt in Sn-Ni alloy after peritectic reaction.In addition,different from other peritectic alloys where the solidified phases are solid solution phases,the"step"growth of both Ni_(3)Sn_(2)and Ni_(3)Sn_(4)phases was observed.The dependences of the step thickness on both the cooling rate and solidification time were measured,which shows that the step thicknesses of both phases gradually decrease as solidification proceeds.This was confirmed to be attributed to the difference between the actual and equilibrium melt concentrations during solidification.In addition,the increase of the normal growth velocity of Ni_(3)Sn_(4)phase with increasing cooling rate was also proved through both the experimental observation and quantitative prediction.
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Project supported by the National Natural Science Foundation of China(Grant No.51901148)the Fund of the State Key Laboratory of Solidification Processing(Northwestern Polytechnical University),China(Grant No.SKLSP202006)the State Key Lab of Advanced Metals and Materials(University of Science and Technology Beijing),China(Grant No.2019-Z15).
文摘We simulate the evolution of hydrogen concentration and gas pore formation as equiaxed dendrites grow during solidification of a hypoeutectic aluminum-silicon(Al-Si)alloy.The applied lattice Boltzmann-cellular automaton-finite difference model incorporates the physical mechanisms of solute and hydrogen partitioning on the solid/liquid interface,as well as the transports of solute and hydrogen.After the quantitative validation by the simulation of capillary intrusion,the model is utilized to investigate the growth of the equiaxed dendrites and hydrogen porosity formation for an Al-(5 wt.%)Si alloy under different solidification conditions.The simulation data reveal that the gas pores favorably nucleate in the corners surrounded by the nearby dendrite arms.Then,the gas pores grow in a competitive mode.With the cooling rate increasing,the competition among different growing gas pores is found to be hindered,which accordingly increases the pore number density in the final solidification microstructure.In the late solidification stage,even though the solid fraction is increasing,the mean concentration of hydrogen in the residue melt tends to be constant,corresponding to a dynamic equilibrium state of hydrogen concentration in liquid.As the cooling rate increases or the initial hydrogen concentration decreases,the temperature of gas pore nucleation,the porosity fraction,and the mean porosity size decrease,whilst the mean hydrogen concentration in liquid increases in the late solidification stage.The simulated data present identical trends with the experimental results reported in literature.
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.
基金supported by National Nature Science Foundation of China(No.51701112 and No.51690162)National Key Research and Development Program of China(No.2019YFA0705300)+1 种基金Shanghai Rising-Star Program(20QA1403800 and 21QC1401500)open fund of State Key Laboratory of Solidifi cation Processing in NWPU(Grant No.SKLSP202107)。
文摘In this paper,the diversity of complicated dendrite microstructure and its evolution behavior during solidification in different magnesium alloys under various processing conditions were illustrated using synchrotron X-ray imaging technique.A variety of dendritic morphologies and branching structures were revealed,i.e.,sixfold plate-like symmetric structure in Mg-Al-based structure,12-branch structure in Mg-Zn-based alloys and 18-branch structure in Mg-Sn-and Mg-Ca-based alloys as well as seaweed like hyper-branched structure in Mg-38wt%Zn alloy.In addition,a dendrite morphology and orientation transition with increasing addition of Zn content were also observed in Mg-Zn alloy,with dendrite growth pattern transform from anisotropy(low Zn addition)with sixfold symmetric snow-flake structure to relative isotropy(intermediate Zn addition)where seaweed morphology presented and then back to anisotropy(high Zn addition)when only 12 branches with preferred<11 2 1>orientations were observed.The phase-field model representing the typical dendritic morphologies and branching structures under various conditions was also depicted and discussed.Further,the two-dimensional(2D)real-time dendrite growth dynamics in different Mg-based alloys captured using synchrotron X-ray radiography for unveiling the originate of theα-Mg dendrite was reviewed.Following this,the four-dimensional(3D+time)synchrotron X-ray tomographic in situ observation of dendritic morphology evolution indicating the formation mechanism of the diverse dendritic morphology during Mg-Sn-and Mg-Zn-based alloys was also summarized.Finally,the future study on exploring the complicated dendritic morphologies and their origination during solidification of Mg-based alloys is prospected.
基金Sponsored by National High Technology and Development Program of China (50434040)
文摘The microstructures of austenitic stainless steel strip were studied using color metallographic method and electron probe micro analysis （EPMA）. In the cast strips, there are three kinds of solidification structures： fine cel- lular dendrite in the surface layer, equiaxed grains in the center and fine dendrite between them. The solidification mode in the surface layer is the primary austenite AF mode because of extremely high cooling rate, with the retained ferrite located around the primary cellular austenite. In the fine dendrite zone, the solidification mode of molten stainless steel changes to FA mode and the residual ferrite with fish-bone morphology is located at the core of the dendrite. The retained ferrite of equiaxed grains in the center is located in the center of broken primary ferrite dendrite with vermicular morphology.
