Recent studies have indicated that precipitation-strengthened high-entropy alloys(HEAs)show superior mechanical performance and have been successfully fabricated by additive manufacturing.However,the lack of fatigue a...Recent studies have indicated that precipitation-strengthened high-entropy alloys(HEAs)show superior mechanical performance and have been successfully fabricated by additive manufacturing.However,the lack of fatigue and fracture research has limited the engineering applications of additive manufacturing HEAs.This work explored a dual precipitation-strengthened(FeCoNi)_(86)Al_(7)Ti_(7) HEA with excellent tensile and fatigue strength,prepared through selective laser melting and heat treatment.Compared with the as-built samples,the tensile properties and fatigue endurance limit improved through aging by 48.7%and 30%,respectively.The strengthening mechanism and enhanced fatigue performance were clarified in detail.The improvement in fatigue strength was attributed to the improved resistance of the L1_(2) and L2_(1) precipitates.During deformation,the dislocation shear coherent L1_(2) precipitates reduced slip band energy and inhibited slip band expansion,while the L2_(1) particles acted as obstructions for further slip band propagation,severely limiting the rapid formation and propagation of crack growth.In-situ TEM cyclic tensile-tensile testing also clarified the fatigue crack growth behavior,demonstrating that crack deflection due to L2_(1) precipitate obstruction slowed down the crack growth rate and efficiently promoted the closure of the microcrack tips.This work offers im plications for a new strategy to develop additive manufacturing HEAs.展开更多
The crystal structure evolution of the Cu-rich nano precipitates from bcc to 9R during thermal aging was studied in nuclear reactor pressure vessel(RPV)model steels.The specimens,contained higher copper and nickel con...The crystal structure evolution of the Cu-rich nano precipitates from bcc to 9R during thermal aging was studied in nuclear reactor pressure vessel(RPV)model steels.The specimens,contained higher copper and nickel contents than commercially available one,were heated at 890 C for 0.5 h and then water quenched followed by tempering at 660 C for 10 h and aging at 400 C for 1000 h.It was observed that bcc and 9R orthogonal structure,as well as 9R orthogonal and 9R monoclinic structure,coexist in a single Cu-rich nano precipitate.Further analyses pointed out that Cu-rich nano precipitates of bcc structure were not stable,it may preferentially transform to 9R orthogonal structure and then to 9R monoclinic structure.This results showed that the crystal structure evolution of the Cu-rich nano precipitates was complex.展开更多
The mechanical-property improvement of directionally-solidified Nickel-based single crystal(SC)superalloy with the single-direction magnetic fields is limited by their destructiveness on the dendritic microstructure.H...The mechanical-property improvement of directionally-solidified Nickel-based single crystal(SC)superalloy with the single-direction magnetic fields is limited by their destructiveness on the dendritic microstructure.Here,the work present breaks through the bottleneck.It shows that the application of the cusp magnetic field(CMF)ensures that the dendrites are not destroyed.This feature embodies that the primary dendrite trunks arrange regularly and orderly,as well the secondary dendrite arms grow symmetrically.By contrast,both the unidirectional transverse and longitudinal magnetic field destroy the dendrite morphology,and there are a number of stray grains near the totally-re melted interface.The nondestructive effect is achieved mainly by the combined action of the thermoelectromagnetic force on the dendrites and thermoelectromagnetic convection in the melt during directional solidification.The investigation should contribute a new route for dramatically and effectively improving the crystal quality and mechanical properties of the directionally-solidified alloys.展开更多
Over recent years,eutectic high-entropy alloys(EHEAs)have intrigued substantial research enthusiasms due to their good castability as well as balanced strength-ductility synergy.In this study,a bulk cast Al_(19.25)Co_...Over recent years,eutectic high-entropy alloys(EHEAs)have intrigued substantial research enthusiasms due to their good castability as well as balanced strength-ductility synergy.In this study,a bulk cast Al_(19.25)Co_(18.86)Fe_(18.36)Ni_(43.53)EHEA is developed with fine in-situ lamellar eutectics.The eutectics comprise alternating ordered face-centered-cubic(L1_(2))and ordered body-centered-cubic(B2)phases with semicoherent interfaces.The resulting microstructure resembles that of most reported as-cast EHEAs,but the B2 lamellae are devoid of nano-precipitates because of the Cr-element removal in current tailored eutectic composition.Surprisingly,the B2 lamellae still feature much higher deformation resistance than the L1_(2) lamellae,so that less lattice defects are detected in the B2 lamellae until the fracture.More interestingly,in the L1_(2) lamellae we identify a dynamic microstructure refinement that correlates to extraordinary strain hardening in tension.The precipitate-free EHEA consequently shows excellent tensile ductility of~10%and high ultimate strength up to~956 MPa.展开更多
The stray crystal in the platform region is one of the common main defects in single-crystal superalloy blades. The simple and effective method to eliminate this defect is urgent to be explored. This work found that t...The stray crystal in the platform region is one of the common main defects in single-crystal superalloy blades. The simple and effective method to eliminate this defect is urgent to be explored. This work found that the Cusp magnetic field can effectively inhibit the stray-crystal formation in the platform. The tendency of stray-crystal formation decreases as the magnetic-field strength increases at a certain withdrawal rate and temperature-gradient. The suppressing effect decreases as the withdrawal rate or the temperature-gradient increases. Finally, the inhibiting mechanism on the stray-crystal formation from the Cusp magnetic field is proposed based on the experiments and the numerical simulation. The magneticfield application strengthens the flow velocity and changes the flow structure near the liquid-solid interface, and further reduces the radial temperature difference. Accordingly, the secondary dendrites in the heat-conduction undercooled zone expands towards the corner in a faster speed, which reduces the stray-crystal formation in the platform corner. This study provides an effective and simple method for decreasing the stray-crystal formation during the preparation of single-crystal with platform region.展开更多
基金financially supported by the National Natural Sci-ence Foundation of China(Nos.51971123 and 51925103)the In-novation Program of Shanghai Municipal Education Commission(No.2021-01-07-00-09-E00114)+3 种基金the Program 173(No.2020-JCIQ-ZD-186-01)the Key Program of Science and Technology of Yun nan Province(No.202002AB080001-2)the 111 Project(No.D16002)the Space Utilization System of China Manned Space Engineer-ing(No.KJZ-YY-NCL08)。
文摘Recent studies have indicated that precipitation-strengthened high-entropy alloys(HEAs)show superior mechanical performance and have been successfully fabricated by additive manufacturing.However,the lack of fatigue and fracture research has limited the engineering applications of additive manufacturing HEAs.This work explored a dual precipitation-strengthened(FeCoNi)_(86)Al_(7)Ti_(7) HEA with excellent tensile and fatigue strength,prepared through selective laser melting and heat treatment.Compared with the as-built samples,the tensile properties and fatigue endurance limit improved through aging by 48.7%and 30%,respectively.The strengthening mechanism and enhanced fatigue performance were clarified in detail.The improvement in fatigue strength was attributed to the improved resistance of the L1_(2) and L2_(1) precipitates.During deformation,the dislocation shear coherent L1_(2) precipitates reduced slip band energy and inhibited slip band expansion,while the L2_(1) particles acted as obstructions for further slip band propagation,severely limiting the rapid formation and propagation of crack growth.In-situ TEM cyclic tensile-tensile testing also clarified the fatigue crack growth behavior,demonstrating that crack deflection due to L2_(1) precipitate obstruction slowed down the crack growth rate and efficiently promoted the closure of the microcrack tips.This work offers im plications for a new strategy to develop additive manufacturing HEAs.
基金financially supported by the National Basic Research Program of China(No.2011CB610503)National Natural Science Foundation of China(No.50931003)Ministry of Major Subject of Shanghai(No.S30107)
文摘The crystal structure evolution of the Cu-rich nano precipitates from bcc to 9R during thermal aging was studied in nuclear reactor pressure vessel(RPV)model steels.The specimens,contained higher copper and nickel contents than commercially available one,were heated at 890 C for 0.5 h and then water quenched followed by tempering at 660 C for 10 h and aging at 400 C for 1000 h.It was observed that bcc and 9R orthogonal structure,as well as 9R orthogonal and 9R monoclinic structure,coexist in a single Cu-rich nano precipitate.Further analyses pointed out that Cu-rich nano precipitates of bcc structure were not stable,it may preferentially transform to 9R orthogonal structure and then to 9R monoclinic structure.This results showed that the crystal structure evolution of the Cu-rich nano precipitates was complex.
基金financially supported by the National Natural Science Foundation of China(No.51871142)。
文摘The mechanical-property improvement of directionally-solidified Nickel-based single crystal(SC)superalloy with the single-direction magnetic fields is limited by their destructiveness on the dendritic microstructure.Here,the work present breaks through the bottleneck.It shows that the application of the cusp magnetic field(CMF)ensures that the dendrites are not destroyed.This feature embodies that the primary dendrite trunks arrange regularly and orderly,as well the secondary dendrite arms grow symmetrically.By contrast,both the unidirectional transverse and longitudinal magnetic field destroy the dendrite morphology,and there are a number of stray grains near the totally-re melted interface.The nondestructive effect is achieved mainly by the combined action of the thermoelectromagnetic force on the dendrites and thermoelectromagnetic convection in the melt during directional solidification.The investigation should contribute a new route for dramatically and effectively improving the crystal quality and mechanical properties of the directionally-solidified alloys.
基金financial support from the National Key Research and Development Program of China(Nos.2018YFF0109404,2016YFB0300401 and 2016YFB0301401)the National Natural Science Foundation of China(Nos.U1732276 and U1860202)+4 种基金financial support from the National Natural Science Foundation of China(No.51704193)financial support from the National Natural Science Foundation of China(No.52004156)financial support from the National Natural Science Foundation of China(No.51904184)the supports from the National Science Foundation(DMR-1611180)the US Army Research Office(W911NF-19-2-0049)。
文摘Over recent years,eutectic high-entropy alloys(EHEAs)have intrigued substantial research enthusiasms due to their good castability as well as balanced strength-ductility synergy.In this study,a bulk cast Al_(19.25)Co_(18.86)Fe_(18.36)Ni_(43.53)EHEA is developed with fine in-situ lamellar eutectics.The eutectics comprise alternating ordered face-centered-cubic(L1_(2))and ordered body-centered-cubic(B2)phases with semicoherent interfaces.The resulting microstructure resembles that of most reported as-cast EHEAs,but the B2 lamellae are devoid of nano-precipitates because of the Cr-element removal in current tailored eutectic composition.Surprisingly,the B2 lamellae still feature much higher deformation resistance than the L1_(2) lamellae,so that less lattice defects are detected in the B2 lamellae until the fracture.More interestingly,in the L1_(2) lamellae we identify a dynamic microstructure refinement that correlates to extraordinary strain hardening in tension.The precipitate-free EHEA consequently shows excellent tensile ductility of~10%and high ultimate strength up to~956 MPa.
基金financially supported by the National Natural Science Foundation of China (No.51871142)the Independent Research and Development Project of State Key Laboratory of Advanced Special Steel,Shanghai Key Laboratory of Advanced Ferrometallurgy,Shanghai University (No.SKLASS 2020-Z04)the Science and Technology Commission of Shanghai Municipality (No.19DZ2270200)。
文摘The stray crystal in the platform region is one of the common main defects in single-crystal superalloy blades. The simple and effective method to eliminate this defect is urgent to be explored. This work found that the Cusp magnetic field can effectively inhibit the stray-crystal formation in the platform. The tendency of stray-crystal formation decreases as the magnetic-field strength increases at a certain withdrawal rate and temperature-gradient. The suppressing effect decreases as the withdrawal rate or the temperature-gradient increases. Finally, the inhibiting mechanism on the stray-crystal formation from the Cusp magnetic field is proposed based on the experiments and the numerical simulation. The magneticfield application strengthens the flow velocity and changes the flow structure near the liquid-solid interface, and further reduces the radial temperature difference. Accordingly, the secondary dendrites in the heat-conduction undercooled zone expands towards the corner in a faster speed, which reduces the stray-crystal formation in the platform corner. This study provides an effective and simple method for decreasing the stray-crystal formation during the preparation of single-crystal with platform region.