The effect of the gradient content of Co element on the solidification process of Cu-based alloy under deep under cooling conditions was explored.The non-equilibrium solidification structure of the under cooled alloy ...The effect of the gradient content of Co element on the solidification process of Cu-based alloy under deep under cooling conditions was explored.The non-equilibrium solidification structure of the under cooled alloy samples were analyzed.It is found that the rapidly solidified alloy has undergone twice grain refinement during the undercooling process.Characterization and significance of the maximum undercooling refinement structure of Cu60Ni35Co5 at T=253 K were analyzed.High-density defects were observed,such as dislocations,stacking faults networks,and twinning structures.The standard FCC diffraction pattern represents that it is still a single-phase structure.Based on the metallographic diagram,EBSD and TEM data analysis,it is illustrated that the occurrence of grain refinement under high undercooling is due to stress induced recrystallization.In addition,the laser cladding technology is used to coat Co-based alloy(Stellite12) coating on 304 stainless steel substrate;the microstructure of the coating cross-section was analyzed.It was found that the microstructure of the cross-section is presented as columnar crystals,planar crystals,and disordered growth direction,so that the coating has better hardness and wear resistance.By electrochemical corrosion of the substrate and coating,it can be seen that the Co and Cr elements present in the coating are more likely to form a dense passivation film,which improved the corrosion resistance of the coating.展开更多
Undercoolings up to 397 K(0.283 T_E)have been obtained for Ni-32.5% Sn eutectic alloymelted by superheating-cooling cycles and denucleating with inorganic glasses.The predomi-nant dissipation of heat for highly underc...Undercoolings up to 397 K(0.283 T_E)have been obtained for Ni-32.5% Sn eutectic alloymelted by superheating-cooling cycles and denucleating with inorganic glasses.The predomi-nant dissipation of heat for highly undercooled alloy melt is through radiation.An approxi-mate method is consequently derived to calculate its mean specific heat from measured coolingcurves.With the aid of high speed cinematography,it is revealed that the surface or interfaceheterogeneous nucleation takes place in preference to homogeneous nucleation even though theundercooling has exceeded 0.2 T_E.展开更多
A new concept of undercooling heredity is developed to evaluate the undercooling ability in a non catalytic nucleation coated mould, where alloy melts were highly undercooled previously. Before the heredity experiment...A new concept of undercooling heredity is developed to evaluate the undercooling ability in a non catalytic nucleation coated mould, where alloy melts were highly undercooled previously. Before the heredity experiment a non catalytic nucleation composite glass lined coating (B F) was prepared on the inner surface of mould and the Cu 70 Ni 30 alloy was selected to perform undercooling experiment in the B F non catalytic coating mould. Its ratio of undercooling heredity was 0.76. The results prove that the B F coating is an ideal non catalytic media for purified Cu 70 Ni 30 alloy melts due to its small contact angle between the melt and coating layer. Considering that various microstructures form under different undercoolings, two critical undercoolings, Δ T 1 and Δ T 2, and their corresponding microstructures of Cu 70 Ni 30 alloy are well defined. Moreover, it is found that the manned trigging solidification in the non catalytic coating mould could be used to get directional undercooling dendrite structure while the melt undercooling is larger than the critical undercooling Δ T 2.展开更多
The rapid quenching and large undercooling phenomena and device working principle in the rapid solidification process are analyzed, and the working characteristics are presented in detail. The results show that these ...The rapid quenching and large undercooling phenomena and device working principle in the rapid solidification process are analyzed, and the working characteristics are presented in detail. The results show that these multistage device are ideal for making amorphous, quasicrystalline, microcrystalline and fine metallic powders.展开更多
The effects of a high magnetic field on the evolution of the single-phase interface and the liquid-solid interface energy in Al-Cu alloy were investigated experimentally.It is found that the application of the magneti...The effects of a high magnetic field on the evolution of the single-phase interface and the liquid-solid interface energy in Al-Cu alloy were investigated experimentally.It is found that the application of the magnetic field has a significant promotion effect on the migration of liquid droplets,accelerating the formation of the single-phase interface.This should be attributed to the thermoelectric(TE)magnetic convection in the droplets which has enhanced the diffusion and increased the migration speed of liquid droplets.Further,the effect of the high magnetic field on the solid-liquid interface energy is analyzed by an improved grain boundary groove(GBG)method.The average solid-liquid interface energy of theα-Al/Al-Cu and Al2Cu/Al-Cu systems increases and decreases with the increase of the magnetic field,respectively.The above experiment results are well explained based on the formation and interaction of the magnetic dipole at the solid-liquid interface.Moreover,experimental results reveal that the magnetic-field-induced interface energy increases and decreases the nucleation undercooling of the Al-30wt.%Cu alloy and Al-35wt.%Cu alloy,respectively.By studying the effect of the magnetic-field-induced interface energy on the nucleation undercooling,the understanding of the interface energy-induced nucleation undercooling deepens.展开更多
The solidification characteristics of highly undercooled Cu-7.77% Co peritectic alloy has been examined by glass fluxing technique. The obtained undercoolings vary from 93 to 203 K(0.14 T_L). It is found that the a(Co...The solidification characteristics of highly undercooled Cu-7.77% Co peritectic alloy has been examined by glass fluxing technique. The obtained undercoolings vary from 93 to 203 K(0.14 T_L). It is found that the a(Co) phase always nucleates and grows preferentially, which is followed by peritectic transformation. This means that the peritectic phase cannot form directly, even though the alloy melt is undercooled to a temperature far below its peritectic point. The maximum recalescence temperature measured experimentally decreases as undercooling increases, which is lower than the thermodynamic calculation result owing to the actual non-adiabatic nature of recalescence process. The dendritic fragmentation of primary α(Co) phase induced by high undercooling is found to enhance the completion of peritectic transformation. In addition, the LKT/BCT dendrite growth model is modified in order to make it applicable to those binary alloy systems with seriously curved liquidus and solidus lines. The dendrite展开更多
Rapid solidification of bulk Ag42.4Cu21.6Sb36 ternary eutectic alloy is accomplished by glass fluxing method,during which the maximum undercooling attains 114 K (0.16 TE). Under high undercooling conditions,the ternar...Rapid solidification of bulk Ag42.4Cu21.6Sb36 ternary eutectic alloy is accomplished by glass fluxing method,during which the maximum undercooling attains 114 K (0.16 TE). Under high undercooling conditions,the ternary eutectic consists ofε (Ag3Sb),(Sb)and θ(Cu2Sb)phases,instead of (Ag),(Sb)and θphases as predicted by the phase diagram.In the sample of small undercooling,the alloy microstructure is characterized by the mixture of primary θ(Cu2Sb),(ε+θ) and (ε+Sb) pseudobinary eutectics,and regular (ε+θ+Sb) ternary eutectic.With the increase of undercooling, θ (Cu2Sb) primary phase and pseudobinary eutectics disappear gradually,and ternary eutectic transfers from regular to anomalous structure.When undercooling exceeds 102 K,anomalous (ε+θ+Sb) ternary eutectic is the unique microstructure.Competitive nucleation and growth of these three eutectic phases is the main cause for the formation of complex growth morphologies.Based on the current experiments and theoretical calculations,it can be concluded that the intermetallic compound phaseθ(Cu2Sb) is the leading nucleating phase.展开更多
The effects of thermodynamic and dynamic factors on nucleation process have been integrated in a theoretical formula representing the dependence of undercooling on parameters concerned. Moreover, a method to determine...The effects of thermodynamic and dynamic factors on nucleation process have been integrated in a theoretical formula representing the dependence of undercooling on parameters concerned. Moreover, a method to determine the kind and amount of the most effective catalyst in an undercooled melt has been acquired. The results show that the undercooling increases with the decreasing surface area of the most effective catalyst and the increasing cooling rate as the kind of the most effective catalyst is constant. It increases to a maximum value when the ratio of the surface area of catalyst ( Sv V) to the cooling rate of melt ( Rc) decreases to a critical value. The maximum undecooling not only depends on the ratio of non-dimensional factor of activation energy for an atom to diffuse (φ) to non-dimensional factor of driving force for nucleus to form (ψ), but also depends on the contact angle of the most effective catalyst; the smaller the ratio of φ to ψ, the higher the maximum undercooling, but it does展开更多
Droplets of Cu-20%Sb hypoeutectic alloy has been rapidly solidified in drop tube within the containerless condition. With the decrease of droplet diameter, undercooling increases and the microstructures of primary cop...Droplets of Cu-20%Sb hypoeutectic alloy has been rapidly solidified in drop tube within the containerless condition. With the decrease of droplet diameter, undercooling increases and the microstructures of primary copper dendrite refines. Undercooling up to 207 K (0.17 TL) is obtained in experiment. Theoretic analysis indicated that, because of the broad temperature range of solidification, the rapid growth of primary copper dendrite is controlled by the solutal diffusion. Judging from the calculation of T0 curve in the phase diagram, it is shown that the critical undercooling of segregationless solidification is △T0=474 K. At the maximum undercooling of 207 K, the growth velocity of primary copper phase exceeds to 37 mm/s, and the distinct solute trapping occurs.展开更多
A mathematical model for the undercooling of the metal droplet during the rapid solidification is established, by which the factors that influence the undercooling of the metal droplet during the rapid solidification ...A mathematical model for the undercooling of the metal droplet during the rapid solidification is established, by which the factors that influence the undercooling of the metal droplet during the rapid solidification are analyzed, and the parameter ζ=σSL3/ (TL?H 2 ) is defined as the impact factor of the undercooling for the droplet solidification. Different undercoolings of droplets induced by various rapid solidification conditions are mainly ascribed to the change of the impact factor. Moreover, it is shown that the larger of ζ, the higher the relative undercooling can be gained. Meanwhile, the parameters such as solid-liquid interfacial energy σSL and latent heat of solidification ?H also vary with the rapid solidification conditions of droplets.展开更多
The experiments on undercooling of acoustically levitated water drops with the radius of 5-8 mm are carried out, and the maximum undercooling of 24 K is obtained in such a containerless state. Various factors influenc...The experiments on undercooling of acoustically levitated water drops with the radius of 5-8 mm are carried out, and the maximum undercooling of 24 K is obtained in such a containerless state. Various factors influencing the undercoolability of water under acoustic levitation are synthetically analyzed. The experimental results indicate that impurities tend to decrease the undercooling level, whereas the dominant factor is the effect of ultrasound. The stirring and cavitation effects of ultrasound tend to stimulate the nucleation of water and prevent further bulk undercooling in experiments. The stirring effect provides some extra energy fluctuation to overcome the thermodynamic barrier for nucleation. The local high pressure caused by cavitation effect increases the local undercooling in water and stimulates nucleation before the achievement of a large bulk undercooling. According to the cooling curves, the dendrite growth velocity of ice is estimated, which is in good agreement with the theoretical prediction at the lower undercooling. The theoretical calculation predicts a dendrite growth velocity of 0.23 m/s corresponding to the maximum undercooling of 24 K, at which the rapid solidification of ice occurs.展开更多
Differential scanning calorimeter combined with flux(dehydrated B2O3) processing was used to realize and precisely measure the undercooling of germanium melts.The highest undercooling obtained in this way was 190 K.Re...Differential scanning calorimeter combined with flux(dehydrated B2O3) processing was used to realize and precisely measure the undercooling of germanium melts.The highest undercooling obtained in this way was 190 K.Relations between the undercooling and cooling rate or overheating tem-perature are analyzed respectively.The undercooling obtained is found to be increased with increasing of the cooling rate in the range from 5 to 40 K/min.At a given cooling rate,the undercooling reached is increased with the increasing of the overheating temperature of the melt,but tends to be constant at last.Crystallization of the undercooled germanium melt is investigated at the same time.It is clearly shown that,the higher the cooling rate,the shorter time is needed for crystallization.展开更多
Both Cu60Ni38Co2 and Cu60Ni40 alloy were naturally cooled after rapid solidification from the liquid phase.The transformation law of the microstructure characteristics of the rapidly solidified alloy with the change o...Both Cu60Ni38Co2 and Cu60Ni40 alloy were naturally cooled after rapid solidification from the liquid phase.The transformation law of the microstructure characteristics of the rapidly solidified alloy with the change of undercooling(ΔT)was systematically studied.It is found that the two alloys experience the same transformation process.The refinement structures under different undercoolings were characterized by electron backscatter diffraction(EBSD).The results show that the characteristics of the refinement structure of the two alloys with low undercooling are the same,but the characteristics of the refinement structure with high undercooling are opposite.The transmission electron microscopy(TEM)results of Cu60Ni38Co2 alloy show that the dislocation network density of low undercooled microstructure is lower than that of high undercooled microstructure.By combining EBSD and TEM,it could be confirmed that the dendrite remelting fracture is the reason for the refinement of the low undercooled structure,while the high undercooled structure is refined due to recrystallization.On this basis,in the processing of copper base alloys,there will be serious work hardening phenomenon and machining hard problem of consciousness problems caused by excessive cutting force.A twodimensional orthogonal turning finite element model was established using ABAQUS software to analyze the changes in cutting speed and tool trajectory in copper based alloy ultrasonic elliptical vibration turning.The results show that in copper based alloy ultrasonic elliptical vibration turning,cutting process parameters have a significant impact on cutting force.Choosing reasonable process parameters can effectively reduce cutting force and improve machining quality.展开更多
Both Cu60Ni38Co2 and Cu60Ni40 alloy were naturally cooled after rapid solidification from the liquid phase.The transformation law of the microstructure characteristics of the rapidly solidified alloy with the change o...Both Cu60Ni38Co2 and Cu60Ni40 alloy were naturally cooled after rapid solidification from the liquid phase.The transformation law of the microstructure characteristics of the rapidly solidified alloy with the change of undercooling(DT)was systematically studied.It was found that the two alloys experienced the same transformation process.The refinement structures under different undercoolings were characterized by electron backscatter diffraction(EBSD).The experimental results show that the characteristics of the refinement structure of the two alloys with low undercooling are the same,whereas,the characteristics of the refinement structure with high undercooling are opposite.The transmission electron microscope(TEM)results of Cu60Ni38Co2 alloy show that the dislocation network density of low undercooled microstructure is lower than that of high undercooled microstructure.By combining EBSD and TEM,it can be confirmed that the dendrite remelting fracture is the reason for the refinement of the low undercooled structure,while the high undercooled structure is refined due to recrystallization.展开更多
In this work, the effects of Ti CN and γ-Al_(2)O_(3) nanoparticle(NP) addition on the microstructural evolution of cast AZ91 alloys at the cooling rate ranging from 15 to 120 K/s have been systematically investigated...In this work, the effects of Ti CN and γ-Al_(2)O_(3) nanoparticle(NP) addition on the microstructural evolution of cast AZ91 alloys at the cooling rate ranging from 15 to 120 K/s have been systematically investigated. Experimental results reveal that grain coarsening occurs in cast AZ91 alloys when the cooling rate exceeds 90 K/s, while it can be effectively inhibited upon addition of NPs. The marked inhibition effect may originate from the formation of Ti CN or γ-Al_(2)O_(3) NP-induced undercooling zone ahead of solid/liquid(S/L) front of α-Mg, which not only can restrict grain growth effectively, but also can reactivate the native nucleants that are inactive in AZ91 melts to participate in nucleation events. And if possessing high nucleation potency, NPs can also promote further nucleation events and lead to significant grain refinement. An analytical model has been established to quantitatively account for the restriction effect of NPs on grain growth. The present work may shed a new light on the grain coarsening of cast alloys during fast cooling and provide an effective approach to circumvent it.展开更多
To improve the competitive relationship between strength and toughness,the effect of low undercooling in austenite(γ)on the microstructure and mechanical properties of commercial vanadium-containing wheel steels was ...To improve the competitive relationship between strength and toughness,the effect of low undercooling in austenite(γ)on the microstructure and mechanical properties of commercial vanadium-containing wheel steels was studied using an optical microscope(OM),a scanning electron microscope(SEM),a transmission electron microscope(TEM),and mechanical property tests.The results show that when the wheel steel is slightly cooled to an appropriate temperature above A c3 point for a short time after it has been austenitized at an elevated temperature,the solid-solved vanadium is pre-precipitated in the form of V(C,N)second phase semicoherent with the matrix in the originalγgrain.This phase hardly participates in matrix strengthening.Due to the small mismatch between V(C,N)and ferrite(α),during the subsequent-cooling phase transformation stage,the pre-precipitated second phase becomes theαnucleation point,causing granular and ellipsoidal intragranular ferrite(IGF,with an average size of 4-6μm)to nucleate in the originalγ.The IGF production and strength loss increases with the increasing undercooling degree.Based on this,Masteel Co.,Ltd.has developed a new heat-treatment step-cooling process that can promote the formation of IGF,considerably improving the level and uniformity of fracture toughness on the premise that the strength and hardness of the wheel are almost unchanged.展开更多
Bulk Fe-30Ni alloy melt was nudercooled up to 337K by combining the glass fluxing technique with superheating-cooling cycle. Grain refinement at low undercoolings was observed in the experiment in addition to that at ...Bulk Fe-30Ni alloy melt was nudercooled up to 337K by combining the glass fluxing technique with superheating-cooling cycle. Grain refinement at low undercoolings was observed in the experiment in addition to that at high undercoolings. The current grain refinement mechanisms were examined, and it is concluded that the refined gains are all developed from dendrites, however the grain refinement at low undercoolings is due to chemical superheating, while that at high undercoolings due to rapid solidification contruction.展开更多
基金Funded by the Basic Research Projects in Shanxi Province(No.202103021224183)。
文摘The effect of the gradient content of Co element on the solidification process of Cu-based alloy under deep under cooling conditions was explored.The non-equilibrium solidification structure of the under cooled alloy samples were analyzed.It is found that the rapidly solidified alloy has undergone twice grain refinement during the undercooling process.Characterization and significance of the maximum undercooling refinement structure of Cu60Ni35Co5 at T=253 K were analyzed.High-density defects were observed,such as dislocations,stacking faults networks,and twinning structures.The standard FCC diffraction pattern represents that it is still a single-phase structure.Based on the metallographic diagram,EBSD and TEM data analysis,it is illustrated that the occurrence of grain refinement under high undercooling is due to stress induced recrystallization.In addition,the laser cladding technology is used to coat Co-based alloy(Stellite12) coating on 304 stainless steel substrate;the microstructure of the coating cross-section was analyzed.It was found that the microstructure of the cross-section is presented as columnar crystals,planar crystals,and disordered growth direction,so that the coating has better hardness and wear resistance.By electrochemical corrosion of the substrate and coating,it can be seen that the Co and Cr elements present in the coating are more likely to form a dense passivation film,which improved the corrosion resistance of the coating.
文摘Undercoolings up to 397 K(0.283 T_E)have been obtained for Ni-32.5% Sn eutectic alloymelted by superheating-cooling cycles and denucleating with inorganic glasses.The predomi-nant dissipation of heat for highly undercooled alloy melt is through radiation.An approxi-mate method is consequently derived to calculate its mean specific heat from measured coolingcurves.With the aid of high speed cinematography,it is revealed that the surface or interfaceheterogeneous nucleation takes place in preference to homogeneous nucleation even though theundercooling has exceeded 0.2 T_E.
文摘A new concept of undercooling heredity is developed to evaluate the undercooling ability in a non catalytic nucleation coated mould, where alloy melts were highly undercooled previously. Before the heredity experiment a non catalytic nucleation composite glass lined coating (B F) was prepared on the inner surface of mould and the Cu 70 Ni 30 alloy was selected to perform undercooling experiment in the B F non catalytic coating mould. Its ratio of undercooling heredity was 0.76. The results prove that the B F coating is an ideal non catalytic media for purified Cu 70 Ni 30 alloy melts due to its small contact angle between the melt and coating layer. Considering that various microstructures form under different undercoolings, two critical undercoolings, Δ T 1 and Δ T 2, and their corresponding microstructures of Cu 70 Ni 30 alloy are well defined. Moreover, it is found that the manned trigging solidification in the non catalytic coating mould could be used to get directional undercooling dendrite structure while the melt undercooling is larger than the critical undercooling Δ T 2.
文摘The rapid quenching and large undercooling phenomena and device working principle in the rapid solidification process are analyzed, and the working characteristics are presented in detail. The results show that these multistage device are ideal for making amorphous, quasicrystalline, microcrystalline and fine metallic powders.
基金financed by the National Natural Science Foun-dation of China(Nos.51904183 and 52130204)the Inde-pendent Research and Development Project of State Key Labora-tory of Advanced Special Steel,Shanghai Key Laboratory of Ad-vanced Ferrometallurgy,Shanghai University(SKLASS 2021-Z07)the Science and Technology Commission of Shanghai Munic-ipality(Nos.19DZ2270200,20511107700).
文摘The effects of a high magnetic field on the evolution of the single-phase interface and the liquid-solid interface energy in Al-Cu alloy were investigated experimentally.It is found that the application of the magnetic field has a significant promotion effect on the migration of liquid droplets,accelerating the formation of the single-phase interface.This should be attributed to the thermoelectric(TE)magnetic convection in the droplets which has enhanced the diffusion and increased the migration speed of liquid droplets.Further,the effect of the high magnetic field on the solid-liquid interface energy is analyzed by an improved grain boundary groove(GBG)method.The average solid-liquid interface energy of theα-Al/Al-Cu and Al2Cu/Al-Cu systems increases and decreases with the increase of the magnetic field,respectively.The above experiment results are well explained based on the formation and interaction of the magnetic dipole at the solid-liquid interface.Moreover,experimental results reveal that the magnetic-field-induced interface energy increases and decreases the nucleation undercooling of the Al-30wt.%Cu alloy and Al-35wt.%Cu alloy,respectively.By studying the effect of the magnetic-field-induced interface energy on the nucleation undercooling,the understanding of the interface energy-induced nucleation undercooling deepens.
文摘The solidification characteristics of highly undercooled Cu-7.77% Co peritectic alloy has been examined by glass fluxing technique. The obtained undercoolings vary from 93 to 203 K(0.14 T_L). It is found that the a(Co) phase always nucleates and grows preferentially, which is followed by peritectic transformation. This means that the peritectic phase cannot form directly, even though the alloy melt is undercooled to a temperature far below its peritectic point. The maximum recalescence temperature measured experimentally decreases as undercooling increases, which is lower than the thermodynamic calculation result owing to the actual non-adiabatic nature of recalescence process. The dendritic fragmentation of primary α(Co) phase induced by high undercooling is found to enhance the completion of peritectic transformation. In addition, the LKT/BCT dendrite growth model is modified in order to make it applicable to those binary alloy systems with seriously curved liquidus and solidus lines. The dendrite
基金This work was suported by the National Natural Science Foundation of China(Grant Nos.50121101,50395105 and 50201013)TCTPFT by SEC and NPU Youth Scientific and Technological Innovation Foundation.
文摘Rapid solidification of bulk Ag42.4Cu21.6Sb36 ternary eutectic alloy is accomplished by glass fluxing method,during which the maximum undercooling attains 114 K (0.16 TE). Under high undercooling conditions,the ternary eutectic consists ofε (Ag3Sb),(Sb)and θ(Cu2Sb)phases,instead of (Ag),(Sb)and θphases as predicted by the phase diagram.In the sample of small undercooling,the alloy microstructure is characterized by the mixture of primary θ(Cu2Sb),(ε+θ) and (ε+Sb) pseudobinary eutectics,and regular (ε+θ+Sb) ternary eutectic.With the increase of undercooling, θ (Cu2Sb) primary phase and pseudobinary eutectics disappear gradually,and ternary eutectic transfers from regular to anomalous structure.When undercooling exceeds 102 K,anomalous (ε+θ+Sb) ternary eutectic is the unique microstructure.Competitive nucleation and growth of these three eutectic phases is the main cause for the formation of complex growth morphologies.Based on the current experiments and theoretical calculations,it can be concluded that the intermetallic compound phaseθ(Cu2Sb) is the leading nucleating phase.
文摘The effects of thermodynamic and dynamic factors on nucleation process have been integrated in a theoretical formula representing the dependence of undercooling on parameters concerned. Moreover, a method to determine the kind and amount of the most effective catalyst in an undercooled melt has been acquired. The results show that the undercooling increases with the decreasing surface area of the most effective catalyst and the increasing cooling rate as the kind of the most effective catalyst is constant. It increases to a maximum value when the ratio of the surface area of catalyst ( Sv V) to the cooling rate of melt ( Rc) decreases to a critical value. The maximum undecooling not only depends on the ratio of non-dimensional factor of activation energy for an atom to diffuse (φ) to non-dimensional factor of driving force for nucleus to form (ψ), but also depends on the contact angle of the most effective catalyst; the smaller the ratio of φ to ψ, the higher the maximum undercooling, but it does
基金This work was supported by the National Natural Science Foundation of China (Grant Nos. 59901009 and 50101010) Huo Yingdong Education Foundation (Grant No. 71044).
文摘Droplets of Cu-20%Sb hypoeutectic alloy has been rapidly solidified in drop tube within the containerless condition. With the decrease of droplet diameter, undercooling increases and the microstructures of primary copper dendrite refines. Undercooling up to 207 K (0.17 TL) is obtained in experiment. Theoretic analysis indicated that, because of the broad temperature range of solidification, the rapid growth of primary copper dendrite is controlled by the solutal diffusion. Judging from the calculation of T0 curve in the phase diagram, it is shown that the critical undercooling of segregationless solidification is △T0=474 K. At the maximum undercooling of 207 K, the growth velocity of primary copper phase exceeds to 37 mm/s, and the distinct solute trapping occurs.
基金This work was supported by the National Natural Science Foundation of China(Grant No.50401023)the Science and Technology Committee of Shanghai Municipality(Grant No.0452nm062).
文摘A mathematical model for the undercooling of the metal droplet during the rapid solidification is established, by which the factors that influence the undercooling of the metal droplet during the rapid solidification are analyzed, and the parameter ζ=σSL3/ (TL?H 2 ) is defined as the impact factor of the undercooling for the droplet solidification. Different undercoolings of droplets induced by various rapid solidification conditions are mainly ascribed to the change of the impact factor. Moreover, it is shown that the larger of ζ, the higher the relative undercooling can be gained. Meanwhile, the parameters such as solid-liquid interfacial energy σSL and latent heat of solidification ?H also vary with the rapid solidification conditions of droplets.
基金the National Natural Science Foundation of China(Grant Nos.50101010&502211011)Fok Ying Tung Education Foundation(Grant No.71044)
文摘The experiments on undercooling of acoustically levitated water drops with the radius of 5-8 mm are carried out, and the maximum undercooling of 24 K is obtained in such a containerless state. Various factors influencing the undercoolability of water under acoustic levitation are synthetically analyzed. The experimental results indicate that impurities tend to decrease the undercooling level, whereas the dominant factor is the effect of ultrasound. The stirring and cavitation effects of ultrasound tend to stimulate the nucleation of water and prevent further bulk undercooling in experiments. The stirring effect provides some extra energy fluctuation to overcome the thermodynamic barrier for nucleation. The local high pressure caused by cavitation effect increases the local undercooling in water and stimulates nucleation before the achievement of a large bulk undercooling. According to the cooling curves, the dendrite growth velocity of ice is estimated, which is in good agreement with the theoretical prediction at the lower undercooling. The theoretical calculation predicts a dendrite growth velocity of 0.23 m/s corresponding to the maximum undercooling of 24 K, at which the rapid solidification of ice occurs.
基金This work was supported by the National Natural Science Foundation of China(Grant No.50171059)by the Nature Science Foundation of Hebei Province,China(Grant No.503278).
文摘Differential scanning calorimeter combined with flux(dehydrated B2O3) processing was used to realize and precisely measure the undercooling of germanium melts.The highest undercooling obtained in this way was 190 K.Relations between the undercooling and cooling rate or overheating tem-perature are analyzed respectively.The undercooling obtained is found to be increased with increasing of the cooling rate in the range from 5 to 40 K/min.At a given cooling rate,the undercooling reached is increased with the increasing of the overheating temperature of the melt,but tends to be constant at last.Crystallization of the undercooled germanium melt is investigated at the same time.It is clearly shown that,the higher the cooling rate,the shorter time is needed for crystallization.
基金Funded by the Basic Research Projects in Shanxi Province(202103021224183)。
文摘Both Cu60Ni38Co2 and Cu60Ni40 alloy were naturally cooled after rapid solidification from the liquid phase.The transformation law of the microstructure characteristics of the rapidly solidified alloy with the change of undercooling(ΔT)was systematically studied.It is found that the two alloys experience the same transformation process.The refinement structures under different undercoolings were characterized by electron backscatter diffraction(EBSD).The results show that the characteristics of the refinement structure of the two alloys with low undercooling are the same,but the characteristics of the refinement structure with high undercooling are opposite.The transmission electron microscopy(TEM)results of Cu60Ni38Co2 alloy show that the dislocation network density of low undercooled microstructure is lower than that of high undercooled microstructure.By combining EBSD and TEM,it could be confirmed that the dendrite remelting fracture is the reason for the refinement of the low undercooled structure,while the high undercooled structure is refined due to recrystallization.On this basis,in the processing of copper base alloys,there will be serious work hardening phenomenon and machining hard problem of consciousness problems caused by excessive cutting force.A twodimensional orthogonal turning finite element model was established using ABAQUS software to analyze the changes in cutting speed and tool trajectory in copper based alloy ultrasonic elliptical vibration turning.The results show that in copper based alloy ultrasonic elliptical vibration turning,cutting process parameters have a significant impact on cutting force.Choosing reasonable process parameters can effectively reduce cutting force and improve machining quality.
基金Funded by the Basic Research Projects in Shanxi Province(No.202103021224183)。
文摘Both Cu60Ni38Co2 and Cu60Ni40 alloy were naturally cooled after rapid solidification from the liquid phase.The transformation law of the microstructure characteristics of the rapidly solidified alloy with the change of undercooling(DT)was systematically studied.It was found that the two alloys experienced the same transformation process.The refinement structures under different undercoolings were characterized by electron backscatter diffraction(EBSD).The experimental results show that the characteristics of the refinement structure of the two alloys with low undercooling are the same,whereas,the characteristics of the refinement structure with high undercooling are opposite.The transmission electron microscope(TEM)results of Cu60Ni38Co2 alloy show that the dislocation network density of low undercooled microstructure is lower than that of high undercooled microstructure.By combining EBSD and TEM,it can be confirmed that the dendrite remelting fracture is the reason for the refinement of the low undercooled structure,while the high undercooled structure is refined due to recrystallization.
基金was sponsored by the National Natural Science Foundation of China,People’s Republic of China (NSFC) under Grant no.51804197,Grant no.51674166 and U1902220Startup Fund for Youngman Research at SJTU (SFYR at SJTU)。
文摘In this work, the effects of Ti CN and γ-Al_(2)O_(3) nanoparticle(NP) addition on the microstructural evolution of cast AZ91 alloys at the cooling rate ranging from 15 to 120 K/s have been systematically investigated. Experimental results reveal that grain coarsening occurs in cast AZ91 alloys when the cooling rate exceeds 90 K/s, while it can be effectively inhibited upon addition of NPs. The marked inhibition effect may originate from the formation of Ti CN or γ-Al_(2)O_(3) NP-induced undercooling zone ahead of solid/liquid(S/L) front of α-Mg, which not only can restrict grain growth effectively, but also can reactivate the native nucleants that are inactive in AZ91 melts to participate in nucleation events. And if possessing high nucleation potency, NPs can also promote further nucleation events and lead to significant grain refinement. An analytical model has been established to quantitatively account for the restriction effect of NPs on grain growth. The present work may shed a new light on the grain coarsening of cast alloys during fast cooling and provide an effective approach to circumvent it.
文摘To improve the competitive relationship between strength and toughness,the effect of low undercooling in austenite(γ)on the microstructure and mechanical properties of commercial vanadium-containing wheel steels was studied using an optical microscope(OM),a scanning electron microscope(SEM),a transmission electron microscope(TEM),and mechanical property tests.The results show that when the wheel steel is slightly cooled to an appropriate temperature above A c3 point for a short time after it has been austenitized at an elevated temperature,the solid-solved vanadium is pre-precipitated in the form of V(C,N)second phase semicoherent with the matrix in the originalγgrain.This phase hardly participates in matrix strengthening.Due to the small mismatch between V(C,N)and ferrite(α),during the subsequent-cooling phase transformation stage,the pre-precipitated second phase becomes theαnucleation point,causing granular and ellipsoidal intragranular ferrite(IGF,with an average size of 4-6μm)to nucleate in the originalγ.The IGF production and strength loss increases with the increasing undercooling degree.Based on this,Masteel Co.,Ltd.has developed a new heat-treatment step-cooling process that can promote the formation of IGF,considerably improving the level and uniformity of fracture toughness on the premise that the strength and hardness of the wheel are almost unchanged.
文摘Bulk Fe-30Ni alloy melt was nudercooled up to 337K by combining the glass fluxing technique with superheating-cooling cycle. Grain refinement at low undercoolings was observed in the experiment in addition to that at high undercoolings. The current grain refinement mechanisms were examined, and it is concluded that the refined gains are all developed from dendrites, however the grain refinement at low undercoolings is due to chemical superheating, while that at high undercoolings due to rapid solidification contruction.
