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 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
The effects of cooling rates corresponding to different diameters of the steel mould and laser surface melting(LSM)on the as-cast microstructures of Mg-9Al-xSi(x=1,3)(mass fraction,%)alloys were investigated by XRD an...The effects of cooling rates corresponding to different diameters of the steel mould and laser surface melting(LSM)on the as-cast microstructures of Mg-9Al-xSi(x=1,3)(mass fraction,%)alloys were investigated by XRD and OM.The results show that obvious refinement of the alloy microstructure is obtained with increasing cooling rate by conventional ingot metallurgy.However, no evident modified morphologies of both dendritic primary Mg2Si and Chinese script eutectic Mg2Si in the Mg-Al-Si alloy occurs. Surprisingly,the morphologies of Mg2Si phases within the laser-melted Mg-Al-Si alloy transform drastically from both coarse Chinese script shape for the eutectic Mg2Si and dendrite for the primary Mg2Si to fine spherical particles with an average size of about 3μm due to the rapid cooling of the melted layer,and the Mg2Si particulates distribute more uniformly in theα-Mg matrix.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
During the direct chill(DC)casting process,primary cooling from the mold and bottom block,and secondary cooling from the waterjets produce a concave solid shell.The depth of this liquid pocket and mushy zone not only ...During the direct chill(DC)casting process,primary cooling from the mold and bottom block,and secondary cooling from the waterjets produce a concave solid shell.The depth of this liquid pocket and mushy zone not only depends on the solidification range of the alloy but also the boundary conditions such as cooling rates.Al-Li alloys solidify in a long solidification range increasing the susceptibility of porosity nucleation in the semi-solid region.In this study,the effects of cooling rate on the porosity formation were quantified for the large ingot casting using X-ray computed tomography(XCT).By characterizing pore size distributions at four different cooling conditions,the correlation between the mechanical properties at both room and high temperatures and the microstructure features was identified.The constitutive equations were constructed.It is found that increasing the cooling rate reduces the grain size,increases the number density of micropores,and minimizes the number of large pores,thereby improving the mechanical performance.Therefore,long mushy zones and deep liquid pockets in Al-Li alloys can be effectively controlled by controlling the boundary conditions of the DC casting solidification process,thereby obtaining castings with excellent mechanical properties.展开更多
Solidification process of 231 t 30Cr2Ni4MoV ingot manufactured by slow cooling process was studied using experimental and numerical simulations, which tackled the problems of high cost and long period in large ingot s...Solidification process of 231 t 30Cr2Ni4MoV ingot manufactured by slow cooling process was studied using experimental and numerical simulations, which tackled the problems of high cost and long period in large ingot studying. Based on the numerical results of large ingot, five characteristic locations under different temperature gradients and cooling rates chosen from the large ingot were simulated. The experiments were performed under the same temperature conditions as in numerical simulations with specialized instrument. The influences of temperature gradient in the solid-liquid interface and solidification rate on the size and morphology of solidification structure were analyzed at cooling rate ranging from 10-3 to 10 2℃ s-1. Solidification macrostructure and microstructure showed that no columnar dendrite was found in any specimen. The grain size and secondary dendrite arm spacing decreased at larger cooling rate, and the relationship between secondary dendrite arm spacing and local solidification time or cooling rate was determined.展开更多
The effect of different cooling rates(2.7,5.5,17.1,and 57.5℃/s)on the solidification parameters,microstructure,and mechanical properties of Al-15Mg_(2)Si composites was studied.The results showed that a high cooling ...The effect of different cooling rates(2.7,5.5,17.1,and 57.5℃/s)on the solidification parameters,microstructure,and mechanical properties of Al-15Mg_(2)Si composites was studied.The results showed that a high cooling rate refined the Mg_(2)Si particles and changed their morphology to more compacted forms with less microcracking tendency.The average radius and fraction of primary Mg_(2)Si particles decreased from 20μm and 13.5%to about 10μm and 7.3%,respectively,as the cooling rate increased from 2.7 to 57.5℃/s.Increasing the cooling rate also improved the distribution of microconstituents and decreased the grain size and volume fraction of micropores.The mechanical properties results revealed that augmenting the cooling rate from 2.7 to about 57.5℃/s increased the hardness and quality index by 25%and245%,respectively.The high cooling rate also changed the fracture mechanism from a brittle-dominated mode to a high-energy ductile mode comprising extensive dimpled zones.展开更多
The corrosion and passive behaviour of duplex stainless steel 2205 at six cooling rates （1, 5, 10, 15, 20 ℃ s^-1 and water quenched） in a simulated marine-environment solution was investigated using electrochemical...The corrosion and passive behaviour of duplex stainless steel 2205 at six cooling rates （1, 5, 10, 15, 20 ℃ s^-1 and water quenched） in a simulated marine-environment solution was investigated using electrochemical measurements of potentiostatic critical pitting temperature, potentiodynamic polarisation curves, electrochemical impedance spectroscopy and Mott-Schottky curves. The microstructural evolution and pitting morphologies of the specimens were visualised using an optical microscope and scanning electron microscope. The electrochemical responses of the passive film show that passivity of the steel was enhanced as the cooling rate increased; however, the threshold cooling rate was 20 ℃ s^-1, beyond which pitting corrosion resistance remained stable. Based on the analyses of microstructural evolution and pit morphologies, the proportion of the ferrite phase increased with the cooling rate and the ratio of austenite and ferrite was close to 1：1. The pitting size decreased as the cooling rate increased, and most metastable pits on specimens were located in the ferrite phase and on the ferrite-austenite interface. Thus, pitting resistance of steel is governed by the phase that provides the lowest pitting resistance equivalent number. The optimised pitting corrosion resistance for steel 2205 was achieved when it was greater than or equal to 20℃ s^-1.展开更多
The mechanical peratures and cooling rates properties and microstructure of as-cast Nb-Ti microalloyed steel at different tern- are investigated in this paper. The III brittle zone （700--900~C） is revealed. The redu...The mechanical peratures and cooling rates properties and microstructure of as-cast Nb-Ti microalloyed steel at different tern- are investigated in this paper. The III brittle zone （700--900~C） is revealed. The reduction of the hot ductility is due to the film-like proeutectoid ferrite or the Nb（C, N） precipitates along the austenitic grain boundaries. In the temperature range of 850 1000~C, with the increase of the cooling rate, the hot ductility decreases. However, in the range of 650--850 ~C, the appearance of large volume fractions of ferrite on austenite grain boundaries minimizes the effect of cooling rate on hot ductility. When the cooling rate is 10 ℃/s, austenite transforms more quickly to ferrite and at a lower temperature a larger amount of ferrite nucleates and precipitates in the grain, which leads to a sharper improvement in the hot ductility at 650 ℃.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
The recrystallization behaviors of a nickel-based single crystal superalloy during heat treatment at 1,200℃ for 4 h with various cooling rates were studied.Results show that the thickness of recrystallization layer d...The recrystallization behaviors of a nickel-based single crystal superalloy during heat treatment at 1,200℃ for 4 h with various cooling rates were studied.Results show that the thickness of recrystallization layer decreases with the increase of cooling rate.In addition,the microstructures ofγ′phase in the recrystallization region are different in various cooling rates.In the high cooling rates(70,100℃·min^(-1)),small size and high volume fraction ofγ′phases are formed in the recrystallization region.It is also found that irregular fine secondaryγ′phases are precipitated between matrix channels with an average size of 150 nm in the original matric(100℃·min^(-1)).The sizes of the secondaryγ′phase decrease with the increase of cooling rate.In contrast,large size and small volume fraction ofγ′phases are formed in the recrystallization region,and a grain boundary layer is formed under a low cooling rate(10℃·min^(-1)).The evolution mechanism of recrystallization at various cooling rates during heat treatment is analyzed.展开更多
The 3D morphologies and growth mechanisms of proeutectic FeAl_(3) at the Al/Fe interface under different cooling rates were studied by synchrotron X-ray tomography.With increasing cooling rate,FeAl_(3) crystals develo...The 3D morphologies and growth mechanisms of proeutectic FeAl_(3) at the Al/Fe interface under different cooling rates were studied by synchrotron X-ray tomography.With increasing cooling rate,FeAl_(3) crystals developed from faceted polygonal prism,plates with flat surface,thin ribbon-like with periodic undulating surface to non-faceted rods with radial dendrites in cross section,indicating a gradual interface growth mode transition from two-dimensional layer growth to continuous growth.At a higher cooling rate,twinning mechanism plays a leading role in the formation and growth of FeAl_(3).A link between the morphologies,twinning and crystallographic structure was established based on quantitative analyses on the 3D structures.展开更多
Ti-48Al-xNb alloys were solidified by containerless electromagnetic levitation with quenching system of the conical copper mold.The influence of cooling rates on phase selection of T1-48Al-xNb alloys was investigated....Ti-48Al-xNb alloys were solidified by containerless electromagnetic levitation with quenching system of the conical copper mold.The influence of cooling rates on phase selection of T1-48Al-xNb alloys was investigated.In near-equilibrium solidification condition,the dendrite βphase is observed as the leading phase.No other metastable phase(e.g.,α phase) is observed.In contrast,in rapid solidification condition,the metastable α phase is observed in as-quenched Ti-48Al-2Nb alloy.Furthermore,the metastable α phase is replaced by the primary β phase with Nb addition increasing.For Ti-48Al-(x=4,6,8)Nb alloys,increasing cooling rate results in a solidification path transition.The peritectic reaction(L+β→α) is therefore significantly suppressed.The relationships between primary dendrite arm spacing(λ_(1)) and cooling rate(τ) can be described.展开更多
In this work, AI-7.5Si-4Cu alloy melt modified by AI-10Sr, RE and AI-5-13-B master alloys was poured into multi-step moulds made from three moulding sands, including quartz, alumina and chromite, to investigate compar...In this work, AI-7.5Si-4Cu alloy melt modified by AI-10Sr, RE and AI-5-13-B master alloys was poured into multi-step moulds made from three moulding sands, including quartz, alumina and chromite, to investigate comparatively the effects of different cooling rates of the casting mould on the alloy＇s microstructures and mechanical properties. The results show that with an increase in wall thickness, the cooling rate decreases, the dendrite arm spacing （DAS） increases significantly and the mechanical properties decrease steadily. The elongation is more sensitive to the cooling rate than the tensile strength. No obvious trend of the effect of wall thickness on hardness of the alloy was found. When the cooling rate is at its greatest, the microstructures and mechanical properties are the best when using chromite sand. The improvement of the properties is mainly attributed to the decrease of the DAS, the grain refinement and the metamorphic effect. Each of the three has a strong impact on the microstructures. Furthermore, a series of fitting models was established based on the data of the DAS to predict the mechanical properties of the multivariate sand-cast AI-7.5Si-4Cu alloy,展开更多
The effects of cooling rates on solidification behaviors,segregation characteristics and tensile property of GH4151 alloy were investigated using microstructure characterization and tensile test.Firstly,a relationship...The effects of cooling rates on solidification behaviors,segregation characteristics and tensile property of GH4151 alloy were investigated using microstructure characterization and tensile test.Firstly,a relationship between the secondary dendrite arm spacing and cooling rate was determined and it was confirmed to be valid.Secondly,it can be found from microstructure observations that the morphology of(Nb,Ti)C carbides transits from blocky and script type to fine script type and spotty type,and the refinedγ'phase was observed due to decrease of segregation with increasing cooling rates.Thirdly,the solidification microstructures of the industrial-scale samples were analyzed.The morphology ofηphase changes from indistinguishable shape,fine needle-like shape to large block-like shape with increasing ingot diameter.As a result,the mechanical properties of alloy decrease due to increase of brittle precipitations.The experimental results show that the precipitation behavior of GH4151 is affected by segregation degree of elements,and the segregation degree is determined by solute distribution process and solid back-diffusion process.展开更多
The extruded AZ80+0.4%Ce magnesium alloy was twisted in the temperature range of 300-380℃by using a Gleeble 3500 thermal simulation test machine with a torsion unit.The deformed cylindrical specimens were cooled at a...The extruded AZ80+0.4%Ce magnesium alloy was twisted in the temperature range of 300-380℃by using a Gleeble 3500 thermal simulation test machine with a torsion unit.The deformed cylindrical specimens were cooled at a cooling rate of 10℃/s or 0.1℃/s,respectively,and aged at 170℃.The microstructure analysis results showed that the grain size decreased with increasing specimen radial position from center(SRPC),and that the strong initial basal texture of the extruded magnesium alloy was weakened.Both continuous and discontinuous dynamic recrystallization mechanisms were involved in contributing to the grain refinement for all specimens investigated.And a novel extension twinning induced dynamic recrystallization mechanism was proposed for specimen deformed at 300℃.For the specimens deformed at 300℃and 340℃followed by a slow cooling rate(0.1℃/s),precipitates of various shapes(β-Mg_(17)Al_(12)),with the dominant precipitates being on the grains boundaries,appeared on the surface section.For specimen deformed at 380℃,lamellar precipitates(LPS)in the interiors of the grains were predominant.After aging,the LPS still dominated for specimens twisted at 380℃;however,the LPS gradually decreased with decreasing deformation temperatures from 380℃to 300℃.Dynamically precipitatedβ,especially those decorating the grain boundaries,changed the competition pictures for the LPS and precipitates of other shapes after aging.Interestingly,LPS dominated the areas for the center section of the specimens after aging regardless of deformation temperatures.Low temperature deformation with high SRPC followed by rapid cooling rate increased the micro hardness of the alloy after aging due to refined grain,reduced precipitates size,decreased lamellar spacing as well as strain hardening.展开更多
Influence of different cooling rates on the microstructure and the precipitation behavior of Nb-Ti microalloyed steel was investigated by CSLM, OM, SEM and EDS. The results show that the precipitation process of carbo...Influence of different cooling rates on the microstructure and the precipitation behavior of Nb-Ti microalloyed steel was investigated by CSLM, OM, SEM and EDS. The results show that the precipitation process of carbonitrides can be in-situ observed by CSLM, and with the increase of the cooling rate, the distribution of precipitates changes from along the austenitic grain boundaries to within the grains. With the increase of the cooling rate, the proeutectoid ferritic film becomes smaller and smaller and then disappears, and the original austenitic grains become finer and finer. In order to obtain non-film like proeutectoid ferrites or non-chain like precipitates along the austenitic grain boundaries and finer austenitic grains,the cooling rate should be at least 5℃/s.展开更多
Quenching experiments were performed at different cooling rates under non-directional solidification by differential thermal analysis, and the morphologic variation of primary phase, phase transition temperature and h...Quenching experiments were performed at different cooling rates under non-directional solidification by differential thermal analysis, and the morphologic variation of primary phase, phase transition temperature and hardness change at the same quenching temperature were investigated. The experimental results show that, with the gradual decrease of the cooling rate from 25 K/min, the morphology of ferrite starts to transform experiencing the dendrite, radial pattern, Widmanstatten-like and wire-net. Sample starts to present the Widmanstatten-like microstructure at 10 K/min which does not exist at higher or lower cooling rates, and this microstructure is detrimental to the mechanical property. Except 10 K/min, the hardness decreases with decreasing cooling rate.展开更多
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Project(LRB05-311)supported by the Postdoctoral Foundation of Heilongjiang Province,ChinaProject(HEUFT05038)supported by the Basic Research Foundation of Harbin Engineering University,ChinaProject(2009AA03Z423)supported by the National High-TechResearch and Development Program of China
文摘The effects of cooling rates corresponding to different diameters of the steel mould and laser surface melting(LSM)on the as-cast microstructures of Mg-9Al-xSi(x=1,3)(mass fraction,%)alloys were investigated by XRD and OM.The results show that obvious refinement of the alloy microstructure is obtained with increasing cooling rate by conventional ingot metallurgy.However, no evident modified morphologies of both dendritic primary Mg2Si and Chinese script eutectic Mg2Si in the Mg-Al-Si alloy occurs. Surprisingly,the morphologies of Mg2Si phases within the laser-melted Mg-Al-Si alloy transform drastically from both coarse Chinese script shape for the eutectic Mg2Si and dendrite for the primary Mg2Si to fine spherical particles with an average size of about 3μm due to the rapid cooling of the melted layer,and the Mg2Si particulates distribute more uniformly in theα-Mg matrix.
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.
基金supported by the National Natural Science Foundation of China(Project number:52073030).
文摘During the direct chill(DC)casting process,primary cooling from the mold and bottom block,and secondary cooling from the waterjets produce a concave solid shell.The depth of this liquid pocket and mushy zone not only depends on the solidification range of the alloy but also the boundary conditions such as cooling rates.Al-Li alloys solidify in a long solidification range increasing the susceptibility of porosity nucleation in the semi-solid region.In this study,the effects of cooling rate on the porosity formation were quantified for the large ingot casting using X-ray computed tomography(XCT).By characterizing pore size distributions at four different cooling conditions,the correlation between the mechanical properties at both room and high temperatures and the microstructure features was identified.The constitutive equations were constructed.It is found that increasing the cooling rate reduces the grain size,increases the number density of micropores,and minimizes the number of large pores,thereby improving the mechanical performance.Therefore,long mushy zones and deep liquid pockets in Al-Li alloys can be effectively controlled by controlling the boundary conditions of the DC casting solidification process,thereby obtaining castings with excellent mechanical properties.
基金Acknowledgements This work was financially supported by the National Natural Science Foundation of China （51704210）, the National Key Research and Development Program of China （No. 2017YFB0701802） and Natural Science Foundation of Hebei Province （E2017105016）.
文摘Solidification process of 231 t 30Cr2Ni4MoV ingot manufactured by slow cooling process was studied using experimental and numerical simulations, which tackled the problems of high cost and long period in large ingot studying. Based on the numerical results of large ingot, five characteristic locations under different temperature gradients and cooling rates chosen from the large ingot were simulated. The experiments were performed under the same temperature conditions as in numerical simulations with specialized instrument. The influences of temperature gradient in the solid-liquid interface and solidification rate on the size and morphology of solidification structure were analyzed at cooling rate ranging from 10-3 to 10 2℃ s-1. Solidification macrostructure and microstructure showed that no columnar dendrite was found in any specimen. The grain size and secondary dendrite arm spacing decreased at larger cooling rate, and the relationship between secondary dendrite arm spacing and local solidification time or cooling rate was determined.
文摘The effect of different cooling rates(2.7,5.5,17.1,and 57.5℃/s)on the solidification parameters,microstructure,and mechanical properties of Al-15Mg_(2)Si composites was studied.The results showed that a high cooling rate refined the Mg_(2)Si particles and changed their morphology to more compacted forms with less microcracking tendency.The average radius and fraction of primary Mg_(2)Si particles decreased from 20μm and 13.5%to about 10μm and 7.3%,respectively,as the cooling rate increased from 2.7 to 57.5℃/s.Increasing the cooling rate also improved the distribution of microconstituents and decreased the grain size and volume fraction of micropores.The mechanical properties results revealed that augmenting the cooling rate from 2.7 to about 57.5℃/s increased the hardness and quality index by 25%and245%,respectively.The high cooling rate also changed the fracture mechanism from a brittle-dominated mode to a high-energy ductile mode comprising extensive dimpled zones.
文摘The corrosion and passive behaviour of duplex stainless steel 2205 at six cooling rates （1, 5, 10, 15, 20 ℃ s^-1 and water quenched） in a simulated marine-environment solution was investigated using electrochemical measurements of potentiostatic critical pitting temperature, potentiodynamic polarisation curves, electrochemical impedance spectroscopy and Mott-Schottky curves. The microstructural evolution and pitting morphologies of the specimens were visualised using an optical microscope and scanning electron microscope. The electrochemical responses of the passive film show that passivity of the steel was enhanced as the cooling rate increased; however, the threshold cooling rate was 20 ℃ s^-1, beyond which pitting corrosion resistance remained stable. Based on the analyses of microstructural evolution and pit morphologies, the proportion of the ferrite phase increased with the cooling rate and the ratio of austenite and ferrite was close to 1：1. The pitting size decreased as the cooling rate increased, and most metastable pits on specimens were located in the ferrite phase and on the ferrite-austenite interface. Thus, pitting resistance of steel is governed by the phase that provides the lowest pitting resistance equivalent number. The optimised pitting corrosion resistance for steel 2205 was achieved when it was greater than or equal to 20℃ s^-1.
文摘The mechanical peratures and cooling rates properties and microstructure of as-cast Nb-Ti microalloyed steel at different tern- are investigated in this paper. The III brittle zone （700--900~C） is revealed. The reduction of the hot ductility is due to the film-like proeutectoid ferrite or the Nb（C, N） precipitates along the austenitic grain boundaries. In the temperature range of 850 1000~C, with the increase of the cooling rate, the hot ductility decreases. However, in the range of 650--850 ~C, the appearance of large volume fractions of ferrite on austenite grain boundaries minimizes the effect of cooling rate on hot ductility. When the cooling rate is 10 ℃/s, austenite transforms more quickly to ferrite and at a lower temperature a larger amount of ferrite nucleates and precipitates in the grain, which leads to a sharper improvement in the hot ductility at 650 ℃.
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financially supported by the National Natural Science Foundation of China(No.92060104)the National Science and Technology Major Project(No.2017-VII-00080102)the Shanghai Municipal Science and Technology Committee Grant(No.20511107700)。
文摘The recrystallization behaviors of a nickel-based single crystal superalloy during heat treatment at 1,200℃ for 4 h with various cooling rates were studied.Results show that the thickness of recrystallization layer decreases with the increase of cooling rate.In addition,the microstructures ofγ′phase in the recrystallization region are different in various cooling rates.In the high cooling rates(70,100℃·min^(-1)),small size and high volume fraction ofγ′phases are formed in the recrystallization region.It is also found that irregular fine secondaryγ′phases are precipitated between matrix channels with an average size of 150 nm in the original matric(100℃·min^(-1)).The sizes of the secondaryγ′phase decrease with the increase of cooling rate.In contrast,large size and small volume fraction ofγ′phases are formed in the recrystallization region,and a grain boundary layer is formed under a low cooling rate(10℃·min^(-1)).The evolution mechanism of recrystallization at various cooling rates during heat treatment is analyzed.
基金supported by the National Natural Science Foundation of China-Excellent Young Scholars(No.51922068)National Natural Science Foundation of China(Nos.51821001,51904186,and 51904187)。
文摘The 3D morphologies and growth mechanisms of proeutectic FeAl_(3) at the Al/Fe interface under different cooling rates were studied by synchrotron X-ray tomography.With increasing cooling rate,FeAl_(3) crystals developed from faceted polygonal prism,plates with flat surface,thin ribbon-like with periodic undulating surface to non-faceted rods with radial dendrites in cross section,indicating a gradual interface growth mode transition from two-dimensional layer growth to continuous growth.At a higher cooling rate,twinning mechanism plays a leading role in the formation and growth of FeAl_(3).A link between the morphologies,twinning and crystallographic structure was established based on quantitative analyses on the 3D structures.
基金financially supported by the National Natural Science Foundation of China (No.51401168)the 111 Project (the Discipline Innovative Engineering Plan) of Northwestern Polytechnical University (No.B08040)。
文摘Ti-48Al-xNb alloys were solidified by containerless electromagnetic levitation with quenching system of the conical copper mold.The influence of cooling rates on phase selection of T1-48Al-xNb alloys was investigated.In near-equilibrium solidification condition,the dendrite βphase is observed as the leading phase.No other metastable phase(e.g.,α phase) is observed.In contrast,in rapid solidification condition,the metastable α phase is observed in as-quenched Ti-48Al-2Nb alloy.Furthermore,the metastable α phase is replaced by the primary β phase with Nb addition increasing.For Ti-48Al-(x=4,6,8)Nb alloys,increasing cooling rate results in a solidification path transition.The peritectic reaction(L+β→α) is therefore significantly suppressed.The relationships between primary dendrite arm spacing(λ_(1)) and cooling rate(τ) can be described.
基金financially supported by the Technology Innovation Fund Project of High-tech Small and Medium Enterprises,Ministry of Science and Technology of China(No:09C26279200863)the Special Fund Project on Science and Technology Achievement Transformation of Jiangsu Province(No:BA201184)
文摘In this work, AI-7.5Si-4Cu alloy melt modified by AI-10Sr, RE and AI-5-13-B master alloys was poured into multi-step moulds made from three moulding sands, including quartz, alumina and chromite, to investigate comparatively the effects of different cooling rates of the casting mould on the alloy＇s microstructures and mechanical properties. The results show that with an increase in wall thickness, the cooling rate decreases, the dendrite arm spacing （DAS） increases significantly and the mechanical properties decrease steadily. The elongation is more sensitive to the cooling rate than the tensile strength. No obvious trend of the effect of wall thickness on hardness of the alloy was found. When the cooling rate is at its greatest, the microstructures and mechanical properties are the best when using chromite sand. The improvement of the properties is mainly attributed to the decrease of the DAS, the grain refinement and the metamorphic effect. Each of the three has a strong impact on the microstructures. Furthermore, a series of fitting models was established based on the data of the DAS to predict the mechanical properties of the multivariate sand-cast AI-7.5Si-4Cu alloy,
文摘The effects of cooling rates on solidification behaviors,segregation characteristics and tensile property of GH4151 alloy were investigated using microstructure characterization and tensile test.Firstly,a relationship between the secondary dendrite arm spacing and cooling rate was determined and it was confirmed to be valid.Secondly,it can be found from microstructure observations that the morphology of(Nb,Ti)C carbides transits from blocky and script type to fine script type and spotty type,and the refinedγ'phase was observed due to decrease of segregation with increasing cooling rates.Thirdly,the solidification microstructures of the industrial-scale samples were analyzed.The morphology ofηphase changes from indistinguishable shape,fine needle-like shape to large block-like shape with increasing ingot diameter.As a result,the mechanical properties of alloy decrease due to increase of brittle precipitations.The experimental results show that the precipitation behavior of GH4151 is affected by segregation degree of elements,and the segregation degree is determined by solute distribution process and solid back-diffusion process.
基金supported by key technology research and development project of Shan Xi province(20201102019)Natural science foundation of Shanxi Province(201901D111167)+1 种基金Shanxi Scholarship Council of China(2020–117)JCKY2018408B003Magnesium alloy high-performance XXX multi-directional extrusion technology and XX supporting scientific research project(xxxx-2019-021)。
文摘The extruded AZ80+0.4%Ce magnesium alloy was twisted in the temperature range of 300-380℃by using a Gleeble 3500 thermal simulation test machine with a torsion unit.The deformed cylindrical specimens were cooled at a cooling rate of 10℃/s or 0.1℃/s,respectively,and aged at 170℃.The microstructure analysis results showed that the grain size decreased with increasing specimen radial position from center(SRPC),and that the strong initial basal texture of the extruded magnesium alloy was weakened.Both continuous and discontinuous dynamic recrystallization mechanisms were involved in contributing to the grain refinement for all specimens investigated.And a novel extension twinning induced dynamic recrystallization mechanism was proposed for specimen deformed at 300℃.For the specimens deformed at 300℃and 340℃followed by a slow cooling rate(0.1℃/s),precipitates of various shapes(β-Mg_(17)Al_(12)),with the dominant precipitates being on the grains boundaries,appeared on the surface section.For specimen deformed at 380℃,lamellar precipitates(LPS)in the interiors of the grains were predominant.After aging,the LPS still dominated for specimens twisted at 380℃;however,the LPS gradually decreased with decreasing deformation temperatures from 380℃to 300℃.Dynamically precipitatedβ,especially those decorating the grain boundaries,changed the competition pictures for the LPS and precipitates of other shapes after aging.Interestingly,LPS dominated the areas for the center section of the specimens after aging regardless of deformation temperatures.Low temperature deformation with high SRPC followed by rapid cooling rate increased the micro hardness of the alloy after aging due to refined grain,reduced precipitates size,decreased lamellar spacing as well as strain hardening.
文摘Influence of different cooling rates on the microstructure and the precipitation behavior of Nb-Ti microalloyed steel was investigated by CSLM, OM, SEM and EDS. The results show that the precipitation process of carbonitrides can be in-situ observed by CSLM, and with the increase of the cooling rate, the distribution of precipitates changes from along the austenitic grain boundaries to within the grains. With the increase of the cooling rate, the proeutectoid ferritic film becomes smaller and smaller and then disappears, and the original austenitic grains become finer and finer. In order to obtain non-film like proeutectoid ferrites or non-chain like precipitates along the austenitic grain boundaries and finer austenitic grains,the cooling rate should be at least 5℃/s.
基金Funded by the National Natural Science Foundation of China(No.2010DFB70630)
文摘Quenching experiments were performed at different cooling rates under non-directional solidification by differential thermal analysis, and the morphologic variation of primary phase, phase transition temperature and hardness change at the same quenching temperature were investigated. The experimental results show that, with the gradual decrease of the cooling rate from 25 K/min, the morphology of ferrite starts to transform experiencing the dendrite, radial pattern, Widmanstatten-like and wire-net. Sample starts to present the Widmanstatten-like microstructure at 10 K/min which does not exist at higher or lower cooling rates, and this microstructure is detrimental to the mechanical property. Except 10 K/min, the hardness decreases with decreasing cooling rate.
