The high-alloyed wrought superalloy GH4975 tends to form coarse MC carbides and eutectic(γ+γ′)phases,which adversely affect the cogging and homogenization process.To provide theoretical guidance for control of MC c...The high-alloyed wrought superalloy GH4975 tends to form coarse MC carbides and eutectic(γ+γ′)phases,which adversely affect the cogging and homogenization process.To provide theoretical guidance for control of MC carbides and eutectic(γ+γ′)formation,differential thermal analysis(DTA)was utilized to investigate the effect of cooing rate(10-90℃·min^(-1))on solidification behavior and micro-segregation of GH4975 alloy.According to the thermodynamic calculation and distribution characteristics of precipitates,the MC carbides can act as nucleation sites forγdendrites,but the nucleation ofγdendrites becomes less dependent on the MC carbide primers at higher cooling rates.As theγdendrites grow,the elements including Ti and Nb gradually accumulate in the residual liquid and leads to the formation of more MC carbides near the interdendritic region.Finally,the solidification is terminated with the formation of eutectic(γ+γ′).With an increase in cooling rate,the liquidus temperature rises,but the solidus temperature decreases,and thus the solidification range is obviously enlarged.The dendritic structure is significantly refined by the increase of cooling rate.The secondary dendrite arm spacing,λ_(2),as a function of cooling rate,T,can be expressed asλ_(2)=216.78T^(-0.42).Moreover,the increasing cooling rate weakens the back diffusion of Al,Ti,and Nb,increases the undercooling,and limits the growth of precipitates.Consequently,the sizes of MC carbides,eutectic(γ+γ′),and primaryγ′significantly decrease,but the area fraction of eutectic(γ+γ′)linerly increases as the cooling rate rises.Thus moderate cooling rate(such as 30℃·min^(-1))should be selected during the solidification process of GH4975 alloy.展开更多
Micro-alloying is an effective approach for improving the corrosion resistance of cast AZ91.However,the effect of micro-alloyed elements on corrosion resistance can be varied depending on the solidification rate influ...Micro-alloying is an effective approach for improving the corrosion resistance of cast AZ91.However,the effect of micro-alloyed elements on corrosion resistance can be varied depending on the solidification rate influencing the diffusion and precipitation behavior of micro-alloying elements.This study investigated the effects of the cooling rate on the microstructure and corrosion behavior of micro-Ca and-Y alloyed cast AZ91 alloy(i.e.,AZXW9100).To achieve various cooling rates,the alloys were prepared using three methods:steel mold casting(SMC),copper step mold casting(CSMC),and high-pressure die casting(HPDC).The corrosion behavior was analyzed through weight loss measurements,electrochemical impedance spectroscopy,and corrosion morphology observations.The results showed that the key microstructural factors influencing corrosion resistance differed between short-and long-term corrosion.As the cooling rate increased,the short-term corrosion rate was lowered from 0.91 mm/y(SMC)to 0.38 mm/y(HPDC),which was attributed to the decrease in the total area fractions of the eutecticαandβphases acting as galvanic corrosion sources.The long-term corrosion rate was reduced from 17.20 mm/y(SMC)to 0.71 mm/y(HPDC),which was revealed to be due to the enhanced connectivity of theβphase acting as corrosion barriers.Meanwhile,the increase in the cooling rate led to a modification of the Zn molar ratio in theβphase,reducing the Volta potential of theβphase from 101.8 m V to 66.9 m V.This reduction in the Volta potential of the main galvanic source also contributed to improved corrosion resistance.The HPDC AZXW9100 alloy produced in this study exhibited the lowest corrosion rate compared to other alloys.These findings suggest that controlling the cooling rate is a promising strategy for enhancing the corrosion resistance of AZXW9100 alloys.展开更多
The influence of cooling rate on the intergranular corrosion(IGC)susceptibility of Nb-Ti dual-stabilized super ferritic stainless steel S44660 following cold rolling and annealing was investigated using an optimized d...The influence of cooling rate on the intergranular corrosion(IGC)susceptibility of Nb-Ti dual-stabilized super ferritic stainless steel S44660 following cold rolling and annealing was investigated using an optimized double-loop electrochemical potentiokinetic reactivation(DL-EPR)test,complemented by microstructural characterization.The results revealed that the optimal DL-EPR test conditions consisted of a 2 mol/L H_(2)SO_(4)and 3 mol/L HCl solution,with a scanning rate of 0.1 V min^(−1)at 30℃.Notably,the resistance to IGC in S44660 steel increased progressively with higher cooling rates after annealing.Transmission electron microscopy and electron probe microanalysis showed that the sensitization of S44660 steel was attributed to the formation of Cr-depleted zones along grain boundaries due to M_(23)C_(6)precipitates.IGC was no longer observed in S44660 steel when the cooling rate after annealing reached or exceeded 90℃s^(-1).It was confirmed that the nucleation and growth of carbides require a certain amount of time.Increasing the cooling rate after annealing effectively inhibits carbide precipitation and growth,thereby reducing the degree of intergranular Cr depletion and enhancing IGC resistance of S44660 steel.展开更多
Annealing treatment is an effective strategy to enhance the comprehensive properties of Mg-8Li-3Al-2Zn(LAZ832)alloy,where the cooling rate plays a decisive role in tailoring microstructure and performance.This study s...Annealing treatment is an effective strategy to enhance the comprehensive properties of Mg-8Li-3Al-2Zn(LAZ832)alloy,where the cooling rate plays a decisive role in tailoring microstructure and performance.This study systematically investigates the effects of cooling rates,controlled via water quenching(WC),air cooling(AC),and furnace cooling(FC),on the phase evolution,mechanical properties,and corrosion resistance of LAZ832.The annealed microstructure consists ofα-Mg,β-Li,AlLi,and MgLi_(2)Al phases,and the volume fraction of Al-Li phases(AlLi and MgLi_(2)Al)increases as the cooling rate decreases.Strengthening mechanisms are dominated by solid solution strengthening,driven by the dissolution of Al and Zn atoms into the matrix,which significantly enhances tensile strength.However,excessive solute content leads to a marked decline in ductility.Scanning probe microscope(SPM)reveals an elevated work function due to the dissolution of Al and Zn atoms into the matrix phase,correlating with improved corrosion resistance.Comprehensive analysis demonstrates that air cooling achieves an optimal balance between tensile strength,ductility,and corrosion resistance,outperforming furnace-cooled samples and offering a pragmatic compromise compared to water-quenched specimens with higher strength but brittle failure.These findings establish a robust framework for designing LAZ832 alloys with tailored microstructures and multi-property optimization,advancing their application in lightweight engineering fields.展开更多
The crystallization behavior of polymers is significantly influenced by molecular chain length and the dispersion of varying chain lengths.The complexity of studying crystallization arises from the dispersity of polym...The crystallization behavior of polymers is significantly influenced by molecular chain length and the dispersion of varying chain lengths.The complexity of studying crystallization arises from the dispersity of polymer materials and the typically slow cooling rates.Recent advancements in fast cooling techniques have rendered the investigation of polymer crystallization at varying cooling rates an attractive area of research;however,a systematic quantitative framework for this process is still lacking.We employ a coarse-grained model for polyvinyl alcohol(CGPVA)in molecular dynamics simulations to study the crystallization of linear polymers with varying chain lengths under variable cooling rates.Monodisperse,bidisperse and polydisperse samples are simulated.We propose two formulae based on a two-phase assumption to fit the exothermal curves obtained during cooling.Based on these formulae,better estimations of crystallization temperatures are obtained and the effects of chain lengths and cooling rates are studied.It is found that the crystallization temperature increases with chain length,similar to the Gibbs-Thomson relation formelting temperature,indicating a strong relation between fast crystallization and glass formation in linear polymers.Extrapolation to the infinitely slow cooling rate provides an easy way in simulations to estimate the equilibrium crystallization temperature.The effective chain lengths of polydisperse and bidisperse samples are found to be the number-averaged chain lengths compared to the weight-averaged ones.The chain length-dependent crystallization exhibits crossover behavior near the entanglement length,indicating the effects of entanglements under fast cooling conditions.The effect of chain length dispersity on crystallization becomes more obvious under fast cooling conditions.展开更多
The cooling rate of the center and edge of vacuum induction melting(VIM)or vacuum arc remelting(VAR)ingots exhibit substantial difference,leading to markedly distinct dendritic structures and precipitates.The current ...The cooling rate of the center and edge of vacuum induction melting(VIM)or vacuum arc remelting(VAR)ingots exhibit substantial difference,leading to markedly distinct dendritic structures and precipitates.The current lack of precise predictions for dendritic segregation and the distribution of precipitates in ingot makes it difficult to determine the annealing and homogenization heat treatment process.Thus,clarifying the impact of cooling rate on the solidification behavior of alloy is significantly important.The dendritic structure and precipitation characteristics of as-cast C-HRA-3 Ni–Cr–Co–Mo-based heat-resistant alloy were investigated using Thermo-Calc thermodynamic calculations,scanning electron microscopy observations,and electron probe microanalyzer.Based on high temperature observation system,the effects of cooling rate on the dendritic structure,dendritic segregation,and precipitation in this alloy were explored.The results showed that the precipitates in the as-cast C-HRA-3 alloy primarily consist of blocky Ti(C,N)phases,large-sized Ti(C,N)–M_(6)C–M_(23)C_(6) symbiotic phases and M_(6)C–M_(23)C_(6) carbides,and small-sized dispersed M_(6)C and M_(23)C_(6) carbides surronding these symbiotic phases.The primary constituent elements of these precipitates are Mo,Cr,C,and Ti,which predominantly concentrate in the interdendritic regions of the as-cast alloy.There is a clear power-law relationship between the secondary dendrite arm spacing and the cooling rate.The dendritic segregation ratio of Mo,Cr,and Ti exhibits a piecewise functional relationship with the cooling rate,under equiaxed dendritic solidification condition.These predictive models and theoretical analyses were validated using numerical simulations and experimental results from the 200 kg grade VIM electrode.展开更多
Microstructural evolution features have been systematically investigated for the weld metal of EH36 shipbuilding steel under an in situ confocal scanning laser microscope.The influence of cooling rate on microstructur...Microstructural evolution features have been systematically investigated for the weld metal of EH36 shipbuilding steel under an in situ confocal scanning laser microscope.The influence of cooling rate on microstructural changes during the transformation from austenite to ferrite has been clarified.It is found that ferrite side plates form preceding to acicular ferrites,although the starting temperature of respective component decreases as the cooling rate is raised.In particular,the growth rate of acicular ferrite is measured to increase significantly,rising from 30.4μm/s at a cooling rate of 3 K/s to 109.0μm/s at 15 K/s,driven primarily by an ever-increasing degree of undercooling.These findings highlight the critical role of cooling rate in dictating the sequence and growth rate of microstructural transformations,which is crucial for optimizing welding processes to obtain desired microstructures while avoiding the formation of deleterious components.展开更多
The effects of cooling rate on the solidification parameters and microstructure of Al-7Si-0.3Mg-0.15 Fe alloy during solidification process were studied.To obtain different cooling rates,the step casting with five dif...The effects of cooling rate on the solidification parameters and microstructure of Al-7Si-0.3Mg-0.15 Fe alloy during solidification process were studied.To obtain different cooling rates,the step casting with five different thicknesses was used and the cooling rates and solidification parameters were determined by computer-aided thermal analysis method.The results show that at higher cooling rates,the primary α（Al） dendrite nucleation temperature,eutectic reaction temperature and solidus temperature shift to lower temperatures.Besides,with increasing cooling rate from 0.19 ℃/s up to 6.25 ℃/s,the secondary dendritic arm spacing decreases from 68 μm to 20 μm,and the primary dendritic volume fraction declines by approximately 5%.In addition,it reduces the length of Fe-bearing phase from 28 μm to 18 μm with a better uniform distribution.It is also found that high cooling rates make for modifying eutectic silicon into fibrous branched morphology,and decreasing block or lamella shape eutectic silicon.展开更多
Mg69Zn27Ca4 alloys with diameters of 1.5, 2 and 3 mm were fabricated using copper mold injection casting method. Microstructural analysis reveals that the alloy with a diameter of 1.5 mm is almost completely composed ...Mg69Zn27Ca4 alloys with diameters of 1.5, 2 and 3 mm were fabricated using copper mold injection casting method. Microstructural analysis reveals that the alloy with a diameter of 1.5 mm is almost completely composed of amorphous phase. However, with the cooling rate decline, a little α-Mg and MgZn dendrites can be found in the amorphous matrix. Based on the microstructural and tensile results, the ductile dendrites are conceived to be highly responsible for the enhanced compressive strain from 1.3% to 3.1% by increasing the sample diameter from 1.5 mm to 3 mm. In addition, the Mg69Zn27Ca4 alloy with 1.5 mm diameter has the best corrosion properties. The current Mg-based alloys show much better corrosion resistance than the traditionally commercial wrought magnesium alloy ZK60 in simulated sea-water.展开更多
The effect of cooling rate of the solidification process on the following solution heat treatment of A356 alloy was investigated,where the cooling rates of 96 K/s and 3 K/s were obtained by the step-like metal mold.Th...The effect of cooling rate of the solidification process on the following solution heat treatment of A356 alloy was investigated,where the cooling rates of 96 K/s and 3 K/s were obtained by the step-like metal mold.Then the eutectic silicon morphology evolution and tensile properties of the alloy samples were observed and analyzed after solution heat treatment at 540 °C for different time.The results show that the high cooling rate of the solidification process can not only reduce the solid solution heat treatment time to rapidly modify the eutectic silicon morphology,but also improve the alloy tensile properties.Specially,it is found that the disintegration,the spheroidization and coarsening of eutectic silicon of A356 alloy are completed during solution heat treatment through two stages,i.e.,at first,the disintegration and spheroidization of the eutectic silicon mainly takes place,then the eutectic silicon will coarsen.展开更多
The effect of the cooling rate ranging from 1.4 °C/s to 3.5 °C/s on the solidification behavior of the sand-cast Mg?10Gd?3Y?0.4Zr alloy was studied by computer aided cooling curve analysis (CA-CCA). With the...The effect of the cooling rate ranging from 1.4 °C/s to 3.5 °C/s on the solidification behavior of the sand-cast Mg?10Gd?3Y?0.4Zr alloy was studied by computer aided cooling curve analysis (CA-CCA). With the increase in cooling rate, the nucleation temperature (Tα,N) increases from 634.8 °C to 636.3 °C, the minimum temperature (Tα,Min) decreases from 631.9 °C to 630.7 °C, the nucleation undercooling (ΔTN) increases from 2.9 °C to 5.6 °C, the beginning temperature of the eutectic reaction (Teut,N) increases, the time of the eutectic reaction shortens, solidus temperature decreases from 546.0 °C to 541.4 °C, and solidification temperature range (ΔTS) increases by 6.1 °C. The increased nucleation rate (N&) is supposed to be the main reason for the increased?TN. Increased value (Teut,N?Teut,G) and shortened time of the eutectic reaction cause the change in the volume fraction and morphology of the second phase.展开更多
Heat treatment of Ti-50.9%Ni （mole fraction） alloy was studied by differential scanning calorimetry, X-ray diffraction, scanning electron microscopey and energy dispersive X-ray analysis to investigate the influence...Heat treatment of Ti-50.9%Ni （mole fraction） alloy was studied by differential scanning calorimetry, X-ray diffraction, scanning electron microscopey and energy dispersive X-ray analysis to investigate the influence of cooling rate on transformation behavior and microstructures of NiTi shape memory alloy. The experimental results show that three-stage phase transformation can be induced at a very low cooling rate such as cooling in furnace. The cooling rate also has a great influence on the phase transformation temperatures. Both martensitic start transformation temperature （Ms） and martensitic finish transformation temperature （Mf） decrease with the decrease of the cooling rate, and decreasing the cooling rate contributes to enhancing the M→A austenite transformation temperature. The phase transformation hysteresis （Af-Mf） increases with the decrease of the cooling rate. Heat treatment is unable to eliminate the textures formed in hot working of NiTi sample, but can weaken the intensity of them. The cooling rate has little influence on the grain size.展开更多
Al-1.0%Sc-1.0%Zr （mass fraction） master alloy was prepared at different cooling rates. The morphology and thermodynamics data of the primary particles of the master alloy were investigated by X-ray diffraction （XRD...Al-1.0%Sc-1.0%Zr （mass fraction） master alloy was prepared at different cooling rates. The morphology and thermodynamics data of the primary particles of the master alloy were investigated by X-ray diffraction （XRD）, scanning electron microscopy （SEM） and differential scanning calorimetry （DSC）. It shows that the primary particles are dendrite-shaped particles comprised of several attached small cubic, cusped-cubic or crucifer shape particles at slow cooling rate. However, the primary particles are separated with crucifer shape at intermediate cooling rate, and they are cubic with cusped-cubic shape at high cooling rate. Meanwhile, the separated and attached particles present AlaSc/AlaZr1-xScx core-shell structure. The formation mechanism of the structure was systematically investigated by a mathematical model.展开更多
The rapid solidification processes of liquid Cu56Zr44 alloys at different cooling rates （γ） were simulated by a molecular dynamics （MD） method. In order to assess the influence of cooling rate on the clustering t...The rapid solidification processes of liquid Cu56Zr44 alloys at different cooling rates （γ） were simulated by a molecular dynamics （MD） method. In order to assess the influence of cooling rate on the clustering tendency and degree towards icosahedrons, a ten-indices＇ cluster-type index method was suggested to characterize the local atomic structures in the super-cooled liquid and the rapidly solidified solid. And their clustering and ordering degrees as well as the packing density of ieosahedral clusters were also evaluated by an icosahedral clustering degree （fI）, the chemical order parameter （ηαβ） and densification coefficients （D0, DI and DIS）, respectively. Results show that the main local atomic configurations in Cu56Zr44 alloy system are Z12 clusters centered by Cu, and most of which are （12 0 12 0 0 0 0 0 0 0） standard icosahedra and （12 0 8 0 0 0 2 2 0 0） as well as （12 2 8 2 0 0 0 0 0 0） defective icosahedra. Below glass transition temperature （Tg）, these icosahedral clusters will be coalesced to various icosahedral medium-range orders （IMROs） by IS linkages, namely, icosahedral bond, and their number N, size n, order parameter ηαβ as well as spatial distributions vary with y. As the cooling rate exceeds the critical value （γc） at which a glassy transition can take place, a lower cooling rate, e.g., γ1=10^1K/ns, is demonstrated to be favorable to uplift the number of icosahedra and enlarge the size of IMROs compared with the higher cooling rates, e.g., γ5=10^5 K/ns, and their packing density and clustering degree towards icosahedra in the rapidly solidified solid can also benefit from the slow cooling process.展开更多
Effects of cooling rate on thermal expansion of Cu49Hf42Al9 metallic glass were studied. Five types of amorphous samples with different sizes were prepared in order to get a broad range of cooling rates （from 102 to ...Effects of cooling rate on thermal expansion of Cu49Hf42Al9 metallic glass were studied. Five types of amorphous samples with different sizes were prepared in order to get a broad range of cooling rates （from 102 to 107 K/s）. The average thermal expansion coefficients （αaver） of as-quenched samples range from 6.14×10-6 to 9.20×10-6 K-1. When the temperature is below the glass transformation temperature （Tg）, αaver of as-quenched samples has a negative correlation with cooling rate; the values of αaver of annealed and crystallized samples are closed to each other. The results indicate that the amount and motion of free volume play important roles in thermal expansion of metallic glasses.展开更多
The effects of temperature and pressure on density, microstructure and mechanical properties of powder compacts during hot isostatic pressing（HIPping） were investigated. Optimized HIPping parameters of temperature r...The effects of temperature and pressure on density, microstructure and mechanical properties of powder compacts during hot isostatic pressing（HIPping） were investigated. Optimized HIPping parameters of temperature range from 900 to 940℃, pressure over 100 MPa and holding time of 3 h, were obtained. Tensile properties after different heat treatments show that both the geometry of samples and cooling rate have a significant influence on mechanical properties. Finite element method was used to predict the temperature field distribution during HIPped sample cooling, and the experimental results are in agreement with simulation prediction. The interaction of HIPping parameters was analyzed based on the response surface methodology（RSM） in this study.展开更多
Mg?1Zn?0.5Ca alloys were prepared by traditional steel mould casting and water-cooled copper mould injection casting at higher cooling rate. Microstructure, mechanical properties and bio-corrosion resistance of two al...Mg?1Zn?0.5Ca alloys were prepared by traditional steel mould casting and water-cooled copper mould injection casting at higher cooling rate. Microstructure, mechanical properties and bio-corrosion resistance of two alloys were contrastively investigated. Grain size reduces remarkably and microstructure becomes homogenous when raising cooling rate. The bio-corrosion behaviour in 3.5% sodium chloride solution (3.5% NaCl) and Hank’s solution at 37°C was investigated using electrochemical polarization measurement and the results indicate that the alloy prepared at higher cooling rates has better corrosion resistance in both types of solution. Further mass loss immersion test in Hank’s solution reveals the same result. The reason of corrosion resistance improvement is that raising cooling rate brings about homogeneous microstructure, which leads to micro-galvanic corrosion alleviation. The tensile test results show that yield strength, ultimate tensile strength and elongation are improved by raising cooling rate and the improvement is mainly due to grain refinement.展开更多
The effects of different solidification rates after pouring on the microstructures,microsegregation and mechanical properties of cast superalloy K417 G were investigated.Scheil-model was applied to calculate the tempe...The effects of different solidification rates after pouring on the microstructures,microsegregation and mechanical properties of cast superalloy K417 G were investigated.Scheil-model was applied to calculate the temperature range of solidification.The casting mould with different casting runners was designed to obtain three different cooling rates.The microstructures were observed and the microsegregation was investigated.Also,high temperature tensile test was performed at 900℃ and stress rupture test was performed at 950℃ with the stress of 235 MPa.The results showed that the secondary dendrite arm spacing,microsegregation,the size and volume fraction of γ＇phase and the size of γ/γ＇eutectic increased with decreasing cooling rate,but the volume fraction of γ/γ＇ eutectic decreased.In the cooling rate range of 1.42℃s^-1–0.849 s^-1,the cast micro-porosities and carbides varied little,while the volume fraction and size of phase and γ/γ＇ eutectic played a decisive role on mechanical properties.The specimen with the slowest cooling rate of 0.84℃ s^-1 showed the best comprehensive mechanical properties.展开更多
The dendrite morphologies and spacings of directionally solidified DZ125 superalloy were investigated under high thermal gradient about 500 K/cm. The results reveal that, with increasing cooling rate, both the spacing...The dendrite morphologies and spacings of directionally solidified DZ125 superalloy were investigated under high thermal gradient about 500 K/cm. The results reveal that, with increasing cooling rate, both the spacings of primary and secondary dendrite arms decrease, and the dendrite morphologies transit from coarse to superfme dendrite. The secondary dendrite arms trend to be refined and be well developed, and the tertiary dendrite will occur. The predictions of the Kurz/Fisher model and the Hunt/Lu model accord basically with the experimental data for primary dendrite arm spacing. The regression equation of the primary dendrite arm spacings 21 and the cooling rate Vc is λ1 = 0.013 Vc^-0.32. The regression equation of the secondary dendrite arm spacing λ2 and the cooling rate Vc is λ2 = 0.00258 Vc^-0.31, which gives good agreement with the Feurer/Wunderlin model.展开更多
The effect of cooling rate on the transition of dendrite morphology of a Mg-6Gd （wt%） alloy was semiquantitatively analyzed under a constant temperature gradient by using synchrotron X-ray radiographic technique. Re...The effect of cooling rate on the transition of dendrite morphology of a Mg-6Gd （wt%） alloy was semiquantitatively analyzed under a constant temperature gradient by using synchrotron X-ray radiographic technique. Results show that equiaxed dendrites, including exotic ＇butterfly-shaped＇ dendrite morphology, dominate at high cooling rate （〉1 K/s）. When the cooling rate decreases in the range of 0.5-1 K/s, the equiaxed-to-columnar transition takes place, and solute segregates at the center of two long dendrite arms （LDA） of the ＇butterfly-shaped＇ dendrite. When the cooling rate is lower than 0.3 K/s, directional solidification occurs and the columnar dendritic growth direction gradually rotates from the crystalline axis to the thermal gradient direction with an increase in cooling rate. Meanwhile, interface moves faster but the dendrite arm spacing decreases. Floating, collision and rotation of dendrites under convection were also studied in this work.2018 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science ＆ Technology.展开更多
基金financially supported by the National Natural Science Foundation of China(Grant Nos.52474362,52174317 and 51904146)the General Project Funded by Liaoning Province Education Department(Grant No.JYTMS20230943)。
文摘The high-alloyed wrought superalloy GH4975 tends to form coarse MC carbides and eutectic(γ+γ′)phases,which adversely affect the cogging and homogenization process.To provide theoretical guidance for control of MC carbides and eutectic(γ+γ′)formation,differential thermal analysis(DTA)was utilized to investigate the effect of cooing rate(10-90℃·min^(-1))on solidification behavior and micro-segregation of GH4975 alloy.According to the thermodynamic calculation and distribution characteristics of precipitates,the MC carbides can act as nucleation sites forγdendrites,but the nucleation ofγdendrites becomes less dependent on the MC carbide primers at higher cooling rates.As theγdendrites grow,the elements including Ti and Nb gradually accumulate in the residual liquid and leads to the formation of more MC carbides near the interdendritic region.Finally,the solidification is terminated with the formation of eutectic(γ+γ′).With an increase in cooling rate,the liquidus temperature rises,but the solidus temperature decreases,and thus the solidification range is obviously enlarged.The dendritic structure is significantly refined by the increase of cooling rate.The secondary dendrite arm spacing,λ_(2),as a function of cooling rate,T,can be expressed asλ_(2)=216.78T^(-0.42).Moreover,the increasing cooling rate weakens the back diffusion of Al,Ti,and Nb,increases the undercooling,and limits the growth of precipitates.Consequently,the sizes of MC carbides,eutectic(γ+γ′),and primaryγ′significantly decrease,but the area fraction of eutectic(γ+γ′)linerly increases as the cooling rate rises.Thus moderate cooling rate(such as 30℃·min^(-1))should be selected during the solidification process of GH4975 alloy.
基金supported by the Materials and Components Technology Development Program of the Ministry of Trade,Industry,and Energy(MOTIE,South Korea)(No.20024843)。
文摘Micro-alloying is an effective approach for improving the corrosion resistance of cast AZ91.However,the effect of micro-alloyed elements on corrosion resistance can be varied depending on the solidification rate influencing the diffusion and precipitation behavior of micro-alloying elements.This study investigated the effects of the cooling rate on the microstructure and corrosion behavior of micro-Ca and-Y alloyed cast AZ91 alloy(i.e.,AZXW9100).To achieve various cooling rates,the alloys were prepared using three methods:steel mold casting(SMC),copper step mold casting(CSMC),and high-pressure die casting(HPDC).The corrosion behavior was analyzed through weight loss measurements,electrochemical impedance spectroscopy,and corrosion morphology observations.The results showed that the key microstructural factors influencing corrosion resistance differed between short-and long-term corrosion.As the cooling rate increased,the short-term corrosion rate was lowered from 0.91 mm/y(SMC)to 0.38 mm/y(HPDC),which was attributed to the decrease in the total area fractions of the eutecticαandβphases acting as galvanic corrosion sources.The long-term corrosion rate was reduced from 17.20 mm/y(SMC)to 0.71 mm/y(HPDC),which was revealed to be due to the enhanced connectivity of theβphase acting as corrosion barriers.Meanwhile,the increase in the cooling rate led to a modification of the Zn molar ratio in theβphase,reducing the Volta potential of theβphase from 101.8 m V to 66.9 m V.This reduction in the Volta potential of the main galvanic source also contributed to improved corrosion resistance.The HPDC AZXW9100 alloy produced in this study exhibited the lowest corrosion rate compared to other alloys.These findings suggest that controlling the cooling rate is a promising strategy for enhancing the corrosion resistance of AZXW9100 alloys.
基金supported by the National Natural Science Foundation of China(Grant No.52375364)Central Guiding Local Science and Technology Development Fund Project(Grant No.YDZJSX2021A036)+1 种基金Basic Research Program of Shanxi Province(Grant No.TZLH20230818001)Shanxi Province key research and development project(Grant No.202302050201011).
文摘The influence of cooling rate on the intergranular corrosion(IGC)susceptibility of Nb-Ti dual-stabilized super ferritic stainless steel S44660 following cold rolling and annealing was investigated using an optimized double-loop electrochemical potentiokinetic reactivation(DL-EPR)test,complemented by microstructural characterization.The results revealed that the optimal DL-EPR test conditions consisted of a 2 mol/L H_(2)SO_(4)and 3 mol/L HCl solution,with a scanning rate of 0.1 V min^(−1)at 30℃.Notably,the resistance to IGC in S44660 steel increased progressively with higher cooling rates after annealing.Transmission electron microscopy and electron probe microanalysis showed that the sensitization of S44660 steel was attributed to the formation of Cr-depleted zones along grain boundaries due to M_(23)C_(6)precipitates.IGC was no longer observed in S44660 steel when the cooling rate after annealing reached or exceeded 90℃s^(-1).It was confirmed that the nucleation and growth of carbides require a certain amount of time.Increasing the cooling rate after annealing effectively inhibits carbide precipitation and growth,thereby reducing the degree of intergranular Cr depletion and enhancing IGC resistance of S44660 steel.
基金the financial support of the Scientific Research Starting Foundation of Anhui Polytechnic University of China(Grant No.2200YQQ049)the Excellent Scientific Research and Innovation Teams of Anhui Province,China(Grant No.2022AH010059).
文摘Annealing treatment is an effective strategy to enhance the comprehensive properties of Mg-8Li-3Al-2Zn(LAZ832)alloy,where the cooling rate plays a decisive role in tailoring microstructure and performance.This study systematically investigates the effects of cooling rates,controlled via water quenching(WC),air cooling(AC),and furnace cooling(FC),on the phase evolution,mechanical properties,and corrosion resistance of LAZ832.The annealed microstructure consists ofα-Mg,β-Li,AlLi,and MgLi_(2)Al phases,and the volume fraction of Al-Li phases(AlLi and MgLi_(2)Al)increases as the cooling rate decreases.Strengthening mechanisms are dominated by solid solution strengthening,driven by the dissolution of Al and Zn atoms into the matrix,which significantly enhances tensile strength.However,excessive solute content leads to a marked decline in ductility.Scanning probe microscope(SPM)reveals an elevated work function due to the dissolution of Al and Zn atoms into the matrix phase,correlating with improved corrosion resistance.Comprehensive analysis demonstrates that air cooling achieves an optimal balance between tensile strength,ductility,and corrosion resistance,outperforming furnace-cooled samples and offering a pragmatic compromise compared to water-quenched specimens with higher strength but brittle failure.These findings establish a robust framework for designing LAZ832 alloys with tailored microstructures and multi-property optimization,advancing their application in lightweight engineering fields.
基金National Natural Science Foundation of China No.22341302.
文摘The crystallization behavior of polymers is significantly influenced by molecular chain length and the dispersion of varying chain lengths.The complexity of studying crystallization arises from the dispersity of polymer materials and the typically slow cooling rates.Recent advancements in fast cooling techniques have rendered the investigation of polymer crystallization at varying cooling rates an attractive area of research;however,a systematic quantitative framework for this process is still lacking.We employ a coarse-grained model for polyvinyl alcohol(CGPVA)in molecular dynamics simulations to study the crystallization of linear polymers with varying chain lengths under variable cooling rates.Monodisperse,bidisperse and polydisperse samples are simulated.We propose two formulae based on a two-phase assumption to fit the exothermal curves obtained during cooling.Based on these formulae,better estimations of crystallization temperatures are obtained and the effects of chain lengths and cooling rates are studied.It is found that the crystallization temperature increases with chain length,similar to the Gibbs-Thomson relation formelting temperature,indicating a strong relation between fast crystallization and glass formation in linear polymers.Extrapolation to the infinitely slow cooling rate provides an easy way in simulations to estimate the equilibrium crystallization temperature.The effective chain lengths of polydisperse and bidisperse samples are found to be the number-averaged chain lengths compared to the weight-averaged ones.The chain length-dependent crystallization exhibits crossover behavior near the entanglement length,indicating the effects of entanglements under fast cooling conditions.The effect of chain length dispersity on crystallization becomes more obvious under fast cooling conditions.
基金funded by the National Key R&D Program Funded Projects(No.2021YFB3704102).
文摘The cooling rate of the center and edge of vacuum induction melting(VIM)or vacuum arc remelting(VAR)ingots exhibit substantial difference,leading to markedly distinct dendritic structures and precipitates.The current lack of precise predictions for dendritic segregation and the distribution of precipitates in ingot makes it difficult to determine the annealing and homogenization heat treatment process.Thus,clarifying the impact of cooling rate on the solidification behavior of alloy is significantly important.The dendritic structure and precipitation characteristics of as-cast C-HRA-3 Ni–Cr–Co–Mo-based heat-resistant alloy were investigated using Thermo-Calc thermodynamic calculations,scanning electron microscopy observations,and electron probe microanalyzer.Based on high temperature observation system,the effects of cooling rate on the dendritic structure,dendritic segregation,and precipitation in this alloy were explored.The results showed that the precipitates in the as-cast C-HRA-3 alloy primarily consist of blocky Ti(C,N)phases,large-sized Ti(C,N)–M_(6)C–M_(23)C_(6) symbiotic phases and M_(6)C–M_(23)C_(6) carbides,and small-sized dispersed M_(6)C and M_(23)C_(6) carbides surronding these symbiotic phases.The primary constituent elements of these precipitates are Mo,Cr,C,and Ti,which predominantly concentrate in the interdendritic regions of the as-cast alloy.There is a clear power-law relationship between the secondary dendrite arm spacing and the cooling rate.The dendritic segregation ratio of Mo,Cr,and Ti exhibits a piecewise functional relationship with the cooling rate,under equiaxed dendritic solidification condition.These predictive models and theoretical analyses were validated using numerical simulations and experimental results from the 200 kg grade VIM electrode.
基金support from the National Natural Science Foundation of China(Grant Nos.U20A20277 and 52350610226)National Key Research and Development Plan of China(Grant No.2022YFE0123300).
文摘Microstructural evolution features have been systematically investigated for the weld metal of EH36 shipbuilding steel under an in situ confocal scanning laser microscope.The influence of cooling rate on microstructural changes during the transformation from austenite to ferrite has been clarified.It is found that ferrite side plates form preceding to acicular ferrites,although the starting temperature of respective component decreases as the cooling rate is raised.In particular,the growth rate of acicular ferrite is measured to increase significantly,rising from 30.4μm/s at a cooling rate of 3 K/s to 109.0μm/s at 15 K/s,driven primarily by an ever-increasing degree of undercooling.These findings highlight the critical role of cooling rate in dictating the sequence and growth rate of microstructural transformations,which is crucial for optimizing welding processes to obtain desired microstructures while avoiding the formation of deleterious components.
基金Projects (2005CB724105,2011CB706801) supported by the National Basic Research Program of ChinaProjects (10477010,51171089) supported by the National Natural Science Foundation of ChinaProjects (2009ZX04006-041-04,2011ZX04014-052) supported by the Important National Science&Technology Specific,China
文摘The effects of cooling rate on the solidification parameters and microstructure of Al-7Si-0.3Mg-0.15 Fe alloy during solidification process were studied.To obtain different cooling rates,the step casting with five different thicknesses was used and the cooling rates and solidification parameters were determined by computer-aided thermal analysis method.The results show that at higher cooling rates,the primary α（Al） dendrite nucleation temperature,eutectic reaction temperature and solidus temperature shift to lower temperatures.Besides,with increasing cooling rate from 0.19 ℃/s up to 6.25 ℃/s,the secondary dendritic arm spacing decreases from 68 μm to 20 μm,and the primary dendritic volume fraction declines by approximately 5%.In addition,it reduces the length of Fe-bearing phase from 28 μm to 18 μm with a better uniform distribution.It is also found that high cooling rates make for modifying eutectic silicon into fibrous branched morphology,and decreasing block or lamella shape eutectic silicon.
基金Project(NCET-11-0554)supported by the Program for New Century Excellent Talents in UniversityProject(2011BAE22B04)supportedby the National Key Technology R&D Program of ChinaProject(51271206)supported by the National Natural Science Foundation of China
文摘Mg69Zn27Ca4 alloys with diameters of 1.5, 2 and 3 mm were fabricated using copper mold injection casting method. Microstructural analysis reveals that the alloy with a diameter of 1.5 mm is almost completely composed of amorphous phase. However, with the cooling rate decline, a little α-Mg and MgZn dendrites can be found in the amorphous matrix. Based on the microstructural and tensile results, the ductile dendrites are conceived to be highly responsible for the enhanced compressive strain from 1.3% to 3.1% by increasing the sample diameter from 1.5 mm to 3 mm. In addition, the Mg69Zn27Ca4 alloy with 1.5 mm diameter has the best corrosion properties. The current Mg-based alloys show much better corrosion resistance than the traditionally commercial wrought magnesium alloy ZK60 in simulated sea-water.
基金Project(3102014KYJD002)supported by the Fundamental Research Funds for the Central Universities of ChinaProjects(50901059,51431008,51134011)supported by the National Natural Science Foundation of China+2 种基金Project(2011CB610403)supported by the National Basic Research Program of ChinaProject(51125002)supported by the China National Funds for Distinguished Young ScientistsProject(JC20120223)supported by the Fundamental Research Fund of Northwestern Polytechnical University,China
文摘The effect of cooling rate of the solidification process on the following solution heat treatment of A356 alloy was investigated,where the cooling rates of 96 K/s and 3 K/s were obtained by the step-like metal mold.Then the eutectic silicon morphology evolution and tensile properties of the alloy samples were observed and analyzed after solution heat treatment at 540 °C for different time.The results show that the high cooling rate of the solidification process can not only reduce the solid solution heat treatment time to rapidly modify the eutectic silicon morphology,but also improve the alloy tensile properties.Specially,it is found that the disintegration,the spheroidization and coarsening of eutectic silicon of A356 alloy are completed during solution heat treatment through two stages,i.e.,at first,the disintegration and spheroidization of the eutectic silicon mainly takes place,then the eutectic silicon will coarsen.
基金Project(51275295)supported by the National Natural Science Foundation of ChinaProject(USCAST2012-15)supported by the SAST-SJTU Joint Research Centre of Advanced Aerospace Technology,ChinaProjects(20120073120011,20130073110052)supported by the Research Fund for the Doctoral Program of Higher Education of China
文摘The effect of the cooling rate ranging from 1.4 °C/s to 3.5 °C/s on the solidification behavior of the sand-cast Mg?10Gd?3Y?0.4Zr alloy was studied by computer aided cooling curve analysis (CA-CCA). With the increase in cooling rate, the nucleation temperature (Tα,N) increases from 634.8 °C to 636.3 °C, the minimum temperature (Tα,Min) decreases from 631.9 °C to 630.7 °C, the nucleation undercooling (ΔTN) increases from 2.9 °C to 5.6 °C, the beginning temperature of the eutectic reaction (Teut,N) increases, the time of the eutectic reaction shortens, solidus temperature decreases from 546.0 °C to 541.4 °C, and solidification temperature range (ΔTS) increases by 6.1 °C. The increased nucleation rate (N&) is supposed to be the main reason for the increased?TN. Increased value (Teut,N?Teut,G) and shortened time of the eutectic reaction cause the change in the volume fraction and morphology of the second phase.
基金Project (51071056) supported by the National Natural Science Foundation of ChinaProjects (HEUCFR1132, HEUCF121712) supported by the Fundamental Research Funds for the Central Universities of China
文摘Heat treatment of Ti-50.9%Ni （mole fraction） alloy was studied by differential scanning calorimetry, X-ray diffraction, scanning electron microscopey and energy dispersive X-ray analysis to investigate the influence of cooling rate on transformation behavior and microstructures of NiTi shape memory alloy. The experimental results show that three-stage phase transformation can be induced at a very low cooling rate such as cooling in furnace. The cooling rate also has a great influence on the phase transformation temperatures. Both martensitic start transformation temperature （Ms） and martensitic finish transformation temperature （Mf） decrease with the decrease of the cooling rate, and decreasing the cooling rate contributes to enhancing the M→A austenite transformation temperature. The phase transformation hysteresis （Af-Mf） increases with the decrease of the cooling rate. Heat treatment is unable to eliminate the textures formed in hot working of NiTi sample, but can weaken the intensity of them. The cooling rate has little influence on the grain size.
基金Project(2012CB619503)supported by the National Basic Research Program of ChinaProject(2013AA031001)supported by the National High-tech Research and Development Program of ChinaProject(2012DFA50630)supported by the International Science and Technology Cooperation Program of China
文摘Al-1.0%Sc-1.0%Zr （mass fraction） master alloy was prepared at different cooling rates. The morphology and thermodynamics data of the primary particles of the master alloy were investigated by X-ray diffraction （XRD）, scanning electron microscopy （SEM） and differential scanning calorimetry （DSC）. It shows that the primary particles are dendrite-shaped particles comprised of several attached small cubic, cusped-cubic or crucifer shape particles at slow cooling rate. However, the primary particles are separated with crucifer shape at intermediate cooling rate, and they are cubic with cusped-cubic shape at high cooling rate. Meanwhile, the separated and attached particles present AlaSc/AlaZr1-xScx core-shell structure. The formation mechanism of the structure was systematically investigated by a mathematical model.
基金Project(51071065)supported by the National Natural Science Foundation of ChinaProject(20100161110001)supported by the Specialized Research Fund for the Doctoral Program of Higher Education of China
文摘The rapid solidification processes of liquid Cu56Zr44 alloys at different cooling rates （γ） were simulated by a molecular dynamics （MD） method. In order to assess the influence of cooling rate on the clustering tendency and degree towards icosahedrons, a ten-indices＇ cluster-type index method was suggested to characterize the local atomic structures in the super-cooled liquid and the rapidly solidified solid. And their clustering and ordering degrees as well as the packing density of ieosahedral clusters were also evaluated by an icosahedral clustering degree （fI）, the chemical order parameter （ηαβ） and densification coefficients （D0, DI and DIS）, respectively. Results show that the main local atomic configurations in Cu56Zr44 alloy system are Z12 clusters centered by Cu, and most of which are （12 0 12 0 0 0 0 0 0 0） standard icosahedra and （12 0 8 0 0 0 2 2 0 0） as well as （12 2 8 2 0 0 0 0 0 0） defective icosahedra. Below glass transition temperature （Tg）, these icosahedral clusters will be coalesced to various icosahedral medium-range orders （IMROs） by IS linkages, namely, icosahedral bond, and their number N, size n, order parameter ηαβ as well as spatial distributions vary with y. As the cooling rate exceeds the critical value （γc） at which a glassy transition can take place, a lower cooling rate, e.g., γ1=10^1K/ns, is demonstrated to be favorable to uplift the number of icosahedra and enlarge the size of IMROs compared with the higher cooling rates, e.g., γ5=10^5 K/ns, and their packing density and clustering degree towards icosahedra in the rapidly solidified solid can also benefit from the slow cooling process.
基金Projects(50831003,50871062)supported by the National Natural Science Foundation of ChinaProject(Z2008F08)supported by the Natural Science Foundation of Shandong Province,China
文摘Effects of cooling rate on thermal expansion of Cu49Hf42Al9 metallic glass were studied. Five types of amorphous samples with different sizes were prepared in order to get a broad range of cooling rates （from 102 to 107 K/s）. The average thermal expansion coefficients （αaver） of as-quenched samples range from 6.14×10-6 to 9.20×10-6 K-1. When the temperature is below the glass transformation temperature （Tg）, αaver of as-quenched samples has a negative correlation with cooling rate; the values of αaver of annealed and crystallized samples are closed to each other. The results indicate that the amount and motion of free volume play important roles in thermal expansion of metallic glasses.
文摘The effects of temperature and pressure on density, microstructure and mechanical properties of powder compacts during hot isostatic pressing（HIPping） were investigated. Optimized HIPping parameters of temperature range from 900 to 940℃, pressure over 100 MPa and holding time of 3 h, were obtained. Tensile properties after different heat treatments show that both the geometry of samples and cooling rate have a significant influence on mechanical properties. Finite element method was used to predict the temperature field distribution during HIPped sample cooling, and the experimental results are in agreement with simulation prediction. The interaction of HIPping parameters was analyzed based on the response surface methodology（RSM） in this study.
基金Project(20921002)supported by the Innovative Research Groups of the National Natural Science Foundation of ChinaProject(21221061)supported by the National Natural Science Foundation of China+1 种基金Project(201105007)supported by the Science and Technology Program of Jilin Province,ChinaProject(20140325003GX)supported by the Science and Technology Support Project of Jilin Province,China
文摘Mg?1Zn?0.5Ca alloys were prepared by traditional steel mould casting and water-cooled copper mould injection casting at higher cooling rate. Microstructure, mechanical properties and bio-corrosion resistance of two alloys were contrastively investigated. Grain size reduces remarkably and microstructure becomes homogenous when raising cooling rate. The bio-corrosion behaviour in 3.5% sodium chloride solution (3.5% NaCl) and Hank’s solution at 37°C was investigated using electrochemical polarization measurement and the results indicate that the alloy prepared at higher cooling rates has better corrosion resistance in both types of solution. Further mass loss immersion test in Hank’s solution reveals the same result. The reason of corrosion resistance improvement is that raising cooling rate brings about homogeneous microstructure, which leads to micro-galvanic corrosion alleviation. The tensile test results show that yield strength, ultimate tensile strength and elongation are improved by raising cooling rate and the improvement is mainly due to grain refinement.
文摘The effects of different solidification rates after pouring on the microstructures,microsegregation and mechanical properties of cast superalloy K417 G were investigated.Scheil-model was applied to calculate the temperature range of solidification.The casting mould with different casting runners was designed to obtain three different cooling rates.The microstructures were observed and the microsegregation was investigated.Also,high temperature tensile test was performed at 900℃ and stress rupture test was performed at 950℃ with the stress of 235 MPa.The results showed that the secondary dendrite arm spacing,microsegregation,the size and volume fraction of γ＇phase and the size of γ/γ＇eutectic increased with decreasing cooling rate,but the volume fraction of γ/γ＇ eutectic decreased.In the cooling rate range of 1.42℃s^-1–0.849 s^-1,the cast micro-porosities and carbides varied little,while the volume fraction and size of phase and γ/γ＇ eutectic played a decisive role on mechanical properties.The specimen with the slowest cooling rate of 0.84℃ s^-1 showed the best comprehensive mechanical properties.
基金supported by the National Natural Science Foundation of China (Nos.50771081,50827102)the National Basic Research Program of China (No.2006CB605202)
文摘The dendrite morphologies and spacings of directionally solidified DZ125 superalloy were investigated under high thermal gradient about 500 K/cm. The results reveal that, with increasing cooling rate, both the spacings of primary and secondary dendrite arms decrease, and the dendrite morphologies transit from coarse to superfme dendrite. The secondary dendrite arms trend to be refined and be well developed, and the tertiary dendrite will occur. The predictions of the Kurz/Fisher model and the Hunt/Lu model accord basically with the experimental data for primary dendrite arm spacing. The regression equation of the primary dendrite arm spacings 21 and the cooling rate Vc is λ1 = 0.013 Vc^-0.32. The regression equation of the secondary dendrite arm spacing λ2 and the cooling rate Vc is λ2 = 0.00258 Vc^-0.31, which gives good agreement with the Feurer/Wunderlin model.
基金financially supported by the Program of Shanghai Subject Chief Engineering (No. 14XD1425000)the National Natural Science Foundation of China (No. 51304135)the Chinese Scholarship Council and DOE (No. DE-FG02-07ER46417)
文摘The effect of cooling rate on the transition of dendrite morphology of a Mg-6Gd （wt%） alloy was semiquantitatively analyzed under a constant temperature gradient by using synchrotron X-ray radiographic technique. Results show that equiaxed dendrites, including exotic ＇butterfly-shaped＇ dendrite morphology, dominate at high cooling rate （〉1 K/s）. When the cooling rate decreases in the range of 0.5-1 K/s, the equiaxed-to-columnar transition takes place, and solute segregates at the center of two long dendrite arms （LDA） of the ＇butterfly-shaped＇ dendrite. When the cooling rate is lower than 0.3 K/s, directional solidification occurs and the columnar dendritic growth direction gradually rotates from the crystalline axis to the thermal gradient direction with an increase in cooling rate. Meanwhile, interface moves faster but the dendrite arm spacing decreases. Floating, collision and rotation of dendrites under convection were also studied in this work.2018 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science ＆ Technology.
