The creep behaviour of β-Si3N4 whisker reinforced Al-8.5Fe-1.3V-1.7Si composite has been investigated at the temperature 773 and 823 K. The results are characterized by high stress exponent and high apparent creep ac...The creep behaviour of β-Si3N4 whisker reinforced Al-8.5Fe-1.3V-1.7Si composite has been investigated at the temperature 773 and 823 K. The results are characterized by high stress exponent and high apparent creep activation energy The creep data can be interpreted based on the incorporation of a threshold Stress and a load transfer coefficient into the power-law creep equation. A good correlation between the normalized creep rate and normalized effective stress is available which demonstrates that the creep behaviour of both the alloy and the composite is controlled by the matrix lattice self-diffusion in AI. EXamination on microstructure shows that edge dislocations exist at the interfaces between two adjacent whiskers and the intedeces emit edge dislocations in parallel paired-columns.展开更多
Carbon is an important alloying element in improving high temperature mechanical properties of various metallic materials.The effects of carbon on high temperature mechanical properties of aβ-stabilized Ti?45Al?3Fe?2...Carbon is an important alloying element in improving high temperature mechanical properties of various metallic materials.The effects of carbon on high temperature mechanical properties of aβ-stabilized Ti?45Al?3Fe?2Mo(molar fraction,%)alloy were studied through compressive and creep tests.The results show that the carbon addition(0.5%,molar fraction)obviously enhances the high temperature compressive strength and creep resistance of theβ-stabilized Ti?45Al?3Fe?2Mo alloy.A lot of nano-scaled Ti3AlC carbides precipitate in theβ-stabilized alloy and these carbides pin the dislocations,and greatly increase the high temperature properties.At the same time,the carbon addition decreases the amount of?phase,refines the lamellar spacing,and causes solution strengthening,which also contribute to the improvement of the high temperature properties.展开更多
Dislocation creep at elevated temperatures plays an important role for plastic deformation in crystalline metals.When using traditional discrete dislocation dynamics(DDD)to capture this process,we often need to update...Dislocation creep at elevated temperatures plays an important role for plastic deformation in crystalline metals.When using traditional discrete dislocation dynamics(DDD)to capture this process,we often need to update the forces on N dislocations involving~N 2 interactions.In this letter,we introduce a multi-scale algorithm to speed up the calculations by dividing a sample of interest into sub-domain grids:dislocations within a characteristic area interact following the conventional way,but their interaction with dislocations in other grids are simplified by lumping all dislocations in another grid as a super one.Such a multi-scale algorithm lowers the computational load to~N 1.5.We employed this algorithm to model dislocation creep in Al-Mg alloy.The simulation leads to a power-law creep rate in consistent with experimental observations.The stress exponent of the power-law creep is a resultant of dislocations climb for~5 and viscous dislocations glide for~3.展开更多
Precipitates in an 11% Cr ferritic/martensitic steel containing Nd with tempering and creep conditions were investigated using transmission electron microscope with energy-dispersive X-ray spectroscopy. The precipitat...Precipitates in an 11% Cr ferritic/martensitic steel containing Nd with tempering and creep conditions were investigated using transmission electron microscope with energy-dispersive X-ray spectroscopy. The precipitates in the steel with a tempering condition were identified to be Cr-rich M23C6 carbide, Nb-rich/V-rich/Ta–Nb-rich MX carbides, Nbrich MX carbonitride, and Fe-rich M5C2 carbide. Nd-rich carbonitride, which is not known to have been reported previously in steels, was also detected in the steel after tempering. Most of the Nb-rich MX precipitates were dissolved, whereas the amount of Ta-rich MX precipitates was increased significantly in the steel after a creep test at 600 °C at an applied stress of180 MPa for 1,100 h. No Fe2 W Laves phase has been detected in the steel after tempering.（Fe, Cr）2W Laves phase with a relatively large size was observed in the steel after the creep test.展开更多
Two alloys with different Ta and Al contents were applied to study the influence of Ta/Al ratio on the microstructural evolution and creep deformation under high temperature.The increase of Ta/Al ratio made theγ/γ&#...Two alloys with different Ta and Al contents were applied to study the influence of Ta/Al ratio on the microstructural evolution and creep deformation under high temperature.The increase of Ta/Al ratio made theγ/γ'lattice misfit more negative and enhanced the volume fraction ofγ'phase,which produced cubic and smallγ'phase in the initial microstructures.These initial tinyγ'phases impeded the dislocations movement and delayed the course of complete raftedγ'phase during the origination of creep deformation,which prolonged the time of the primary creep stage.Moreover,the increase of Ta/Al ratio and addition of Ru produced the denser and stable dislocation networks,the high APB energy and better solution strengthening,which hindered the climbing and sliding of dislocations,and restrained the formation of superdislocations in theγ'precipitate.The second creep stage was extended,and the minimum creep rate was reduced.Hence,the increase of whole creep life of the alloy containing high Ta/Al ratio was attributed to the prolongation of the primary and second creep stages,and the low minimum creep rate.The appearance of the topological inversion phenomenon during the tertiary creep stage was the primary cause for the sudden increase of the creep strain rate of the alloy containing low Ta/Al ratio.However,the high creep strain rate of the alloy containing high Ta/Al ratio during the tertiary creep stage was related to the occurrence and extending of the cracks near the voids.Both alloys would lose efficacy within 20 h-30 h.展开更多
A new ferritic creep resistant steel has been developed by eliminating Nb and adding 1.5 mass % Re to a ferritic steel grade T/P23 with the aim of enhancing its mechanical properties at high temperature.Cast ingots of...A new ferritic creep resistant steel has been developed by eliminating Nb and adding 1.5 mass % Re to a ferritic steel grade T/P23 with the aim of enhancing its mechanical properties at high temperature.Cast ingots of both steels, new grade and ASTM T/P 23, were hot rolled at 900℃ and then submitted to a thermal treatment consisting of solubilization at 1050℃ and tempering at 700℃. Tempered bainitic microstructures obtained contain second phases reinforcing carbide particles, mainly M_6C and M_（23）C_6 at the boundaries of both, prior austenite grains and bainitic ferrite laths, as well as MC within the grains. Mechanical properties at temperatures ranging from 540 to 600℃ were studied by strain-ratechange tests in compression at strain rates between 10^（-7） and 10^（-4）s^（-1）. These tests showed high stress exponents（n ≥ 20） and activation energies（Q ≈ 400 k J/mol） for both alloys, which were associated with a dislocation movement mechanism with a strong interaction between dislocations and precipitates. On the other hand, a creep exponent of 5 was derived for the stress dependence of minimum creep rate from conventional-type creep tests at 600℃. Although this stress exponent is usually related to a dislocation climb controlled creep mechanism, remarkable microstructural degradation observed with increasing creep time makes difficult to elucidate the true deformation mechanism controlling creep.展开更多
文摘The creep behaviour of β-Si3N4 whisker reinforced Al-8.5Fe-1.3V-1.7Si composite has been investigated at the temperature 773 and 823 K. The results are characterized by high stress exponent and high apparent creep activation energy The creep data can be interpreted based on the incorporation of a threshold Stress and a load transfer coefficient into the power-law creep equation. A good correlation between the normalized creep rate and normalized effective stress is available which demonstrates that the creep behaviour of both the alloy and the composite is controlled by the matrix lattice self-diffusion in AI. EXamination on microstructure shows that edge dislocations exist at the interfaces between two adjacent whiskers and the intedeces emit edge dislocations in parallel paired-columns.
基金Project(2014CB6644002)supported by the National Basic Research Program of ChinaProject(2016YFB0700302)supported by the National Major Scientific Research Development Program,ChinaProject(2017JJ2311)supported by the Hunan Natural Science Foundation of China
文摘Carbon is an important alloying element in improving high temperature mechanical properties of various metallic materials.The effects of carbon on high temperature mechanical properties of aβ-stabilized Ti?45Al?3Fe?2Mo(molar fraction,%)alloy were studied through compressive and creep tests.The results show that the carbon addition(0.5%,molar fraction)obviously enhances the high temperature compressive strength and creep resistance of theβ-stabilized Ti?45Al?3Fe?2Mo alloy.A lot of nano-scaled Ti3AlC carbides precipitate in theβ-stabilized alloy and these carbides pin the dislocations,and greatly increase the high temperature properties.At the same time,the carbon addition decreases the amount of?phase,refines the lamellar spacing,and causes solution strengthening,which also contribute to the improvement of the high temperature properties.
基金support from the National Key Research and Development Program of China (Grant 2017YFB0202800)the National Natural Science Foundation of China, Basic Science Center for “Multiscale Problems in Nonlinear Mechanics” (Grant 11988102)+1 种基金the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDB22020200)the Chinese Academy of Sciences Center for Excellence in Complex System Mechanics
文摘Dislocation creep at elevated temperatures plays an important role for plastic deformation in crystalline metals.When using traditional discrete dislocation dynamics(DDD)to capture this process,we often need to update the forces on N dislocations involving~N 2 interactions.In this letter,we introduce a multi-scale algorithm to speed up the calculations by dividing a sample of interest into sub-domain grids:dislocations within a characteristic area interact following the conventional way,but their interaction with dislocations in other grids are simplified by lumping all dislocations in another grid as a super one.Such a multi-scale algorithm lowers the computational load to~N 1.5.We employed this algorithm to model dislocation creep in Al-Mg alloy.The simulation leads to a power-law creep rate in consistent with experimental observations.The stress exponent of the power-law creep is a resultant of dislocations climb for~5 and viscous dislocations glide for~3.
基金financially supported by Key Program of National Natural Science Foundation of China (No. 51034011)ITER-National Magnetic Confinement Fusion Program (No.2011GB113001)National Science and Technology Major Project (No.2011ZX06004-009)
文摘Precipitates in an 11% Cr ferritic/martensitic steel containing Nd with tempering and creep conditions were investigated using transmission electron microscope with energy-dispersive X-ray spectroscopy. The precipitates in the steel with a tempering condition were identified to be Cr-rich M23C6 carbide, Nb-rich/V-rich/Ta–Nb-rich MX carbides, Nbrich MX carbonitride, and Fe-rich M5C2 carbide. Nd-rich carbonitride, which is not known to have been reported previously in steels, was also detected in the steel after tempering. Most of the Nb-rich MX precipitates were dissolved, whereas the amount of Ta-rich MX precipitates was increased significantly in the steel after a creep test at 600 °C at an applied stress of180 MPa for 1,100 h. No Fe2 W Laves phase has been detected in the steel after tempering.（Fe, Cr）2W Laves phase with a relatively large size was observed in the steel after the creep test.
基金financially supported by the National Science and Technology Major Project under(No.2017-Ⅵ-0002-0072)the National Key R&D Program of China(No.2017YFA0700704)+1 种基金the National Natural Science Foundation of China(Nos.51671188,51601192 and 51701210)the Youth Innovation Promotion Association,Chinese Academy of Sciences and State Key Lab of Advanced Metals and Materials Open Fund(No.2018-Z07)。
文摘Two alloys with different Ta and Al contents were applied to study the influence of Ta/Al ratio on the microstructural evolution and creep deformation under high temperature.The increase of Ta/Al ratio made theγ/γ'lattice misfit more negative and enhanced the volume fraction ofγ'phase,which produced cubic and smallγ'phase in the initial microstructures.These initial tinyγ'phases impeded the dislocations movement and delayed the course of complete raftedγ'phase during the origination of creep deformation,which prolonged the time of the primary creep stage.Moreover,the increase of Ta/Al ratio and addition of Ru produced the denser and stable dislocation networks,the high APB energy and better solution strengthening,which hindered the climbing and sliding of dislocations,and restrained the formation of superdislocations in theγ'precipitate.The second creep stage was extended,and the minimum creep rate was reduced.Hence,the increase of whole creep life of the alloy containing high Ta/Al ratio was attributed to the prolongation of the primary and second creep stages,and the low minimum creep rate.The appearance of the topological inversion phenomenon during the tertiary creep stage was the primary cause for the sudden increase of the creep strain rate of the alloy containing low Ta/Al ratio.However,the high creep strain rate of the alloy containing high Ta/Al ratio during the tertiary creep stage was related to the occurrence and extending of the cracks near the voids.Both alloys would lose efficacy within 20 h-30 h.
基金supported by the Spanish Ministry of Economy and Competitiveness(MINECO)under Grant MAT2012-39124,MAT2015-68919,and MAT2016-80875
文摘A new ferritic creep resistant steel has been developed by eliminating Nb and adding 1.5 mass % Re to a ferritic steel grade T/P23 with the aim of enhancing its mechanical properties at high temperature.Cast ingots of both steels, new grade and ASTM T/P 23, were hot rolled at 900℃ and then submitted to a thermal treatment consisting of solubilization at 1050℃ and tempering at 700℃. Tempered bainitic microstructures obtained contain second phases reinforcing carbide particles, mainly M_6C and M_（23）C_6 at the boundaries of both, prior austenite grains and bainitic ferrite laths, as well as MC within the grains. Mechanical properties at temperatures ranging from 540 to 600℃ were studied by strain-ratechange tests in compression at strain rates between 10^（-7） and 10^（-4）s^（-1）. These tests showed high stress exponents（n ≥ 20） and activation energies（Q ≈ 400 k J/mol） for both alloys, which were associated with a dislocation movement mechanism with a strong interaction between dislocations and precipitates. On the other hand, a creep exponent of 5 was derived for the stress dependence of minimum creep rate from conventional-type creep tests at 600℃. Although this stress exponent is usually related to a dislocation climb controlled creep mechanism, remarkable microstructural degradation observed with increasing creep time makes difficult to elucidate the true deformation mechanism controlling creep.
