For decades,it has been well accepted that every 1 wt.%Li addition to Al will reduce Al alloy’s density by 3%and increase its Young’s modulus by 6%.However,the fundamental mechanism of modulus improve-ments stays co...For decades,it has been well accepted that every 1 wt.%Li addition to Al will reduce Al alloy’s density by 3%and increase its Young’s modulus by 6%.However,the fundamental mechanism of modulus improve-ments stays controversial though all studies agreed that the contribution of such a substantial boosting comes from Li-rich clusters either in solid solution or precipitations.In this study,we experimentally produce nano-sized Li-rich clusters by non-equilibrium solidification using centrifugal casting and trace their evolutions as a function of subsequent heat treatments.High-resolution transmission electron mi-croscopy(HRTEM)reveals a further decrease in the lattice constants of Li-rich regions from the as-cast(0.406 nm),solid solution(0.405 nm)to the aged state(0.401 nm),while Young’s modulus of the Al-Li al-loy reaches 89.16 GPa.Small-angle neutron scattering(SANS)experiments and first-principle calculations based on density functional theory have shown both the bond strength around precipitates and the size of those Li-rich region dominate Young’s modulus.At the beginning,it is volumetric compression due to Li addition that increases modulus,tightening the Al-Al potential curves.In the end,it is the Al-Al and Al-Li valence bonds in Al 3 Li at large size and high-volume fraction which increase its second derivative of internal energy and thus Young’s modulus.展开更多
Through the solubility product theory of the ternary secondary phase,classical nucleation theory,and Ostwald ripening theory,a model was established to describe the thermodynamics and kinetics of(Ti,V)C precipitates i...Through the solubility product theory of the ternary secondary phase,classical nucleation theory,and Ostwald ripening theory,a model was established to describe the thermodynamics and kinetics of(Ti,V)C precipitates in austenite/ferrite(y/α)matrices.The model was used to calculate the volume fraction,precipitation-temperature-time(PTT)curve,and nucleation rate-temperature(NrT)curve of MC(M=Ti,V)precipitates in γ/α matrices in Ti-V microalloyed steels with various Ti/V ratios,which is verified by hardness tester,transmission electron microscopy and energy-dispersive X-ray spectroscopy.The calculations indicate that,by decreasing Ti/V ratio from Ti4V0 steel to Ti0V4 steel,the complete-dissolution temperature decreases monotonically from 1226 to 830℃,and the equilibrium volume fraction of MC pre-cipitated from austenite decreases from 0.333%to 0.091%at 900℃.Moreover,the maximum nucleation temperature of MC precipitated from α matrix decreases from 748 to 605℃and the fastest precipitation temperature decreases from 844 to 675℃as Ti/V ratio decreases.PTT and NrT diagrams of MC precipitated from α matrices in different Ti-V microalloyed steels all exhibit C-shaped and inverse C-shaped curves.In addition,both theoretical calculation and experimental results show that when tempered at 600℃for 100 h,Ti2V2 steel shows the largest hardness value of 312 HV among the three steels tested because it has a larger volume fraction(0.364%),a larger precipitate density(1689 μm-2),and the smallest average size(8.4 nm)of(Ti,V)C precipitates.The theoretical calculations are consistent with experimental results,which indicates that the thermodynamics and kinetics model for(Ti,V)C precipitates is reliable and accurate.展开更多
基金This work is financially supported by the National Natural Science Foundation of China(No.52073030).
文摘For decades,it has been well accepted that every 1 wt.%Li addition to Al will reduce Al alloy’s density by 3%and increase its Young’s modulus by 6%.However,the fundamental mechanism of modulus improve-ments stays controversial though all studies agreed that the contribution of such a substantial boosting comes from Li-rich clusters either in solid solution or precipitations.In this study,we experimentally produce nano-sized Li-rich clusters by non-equilibrium solidification using centrifugal casting and trace their evolutions as a function of subsequent heat treatments.High-resolution transmission electron mi-croscopy(HRTEM)reveals a further decrease in the lattice constants of Li-rich regions from the as-cast(0.406 nm),solid solution(0.405 nm)to the aged state(0.401 nm),while Young’s modulus of the Al-Li al-loy reaches 89.16 GPa.Small-angle neutron scattering(SANS)experiments and first-principle calculations based on density functional theory have shown both the bond strength around precipitates and the size of those Li-rich region dominate Young’s modulus.At the beginning,it is volumetric compression due to Li addition that increases modulus,tightening the Al-Al potential curves.In the end,it is the Al-Al and Al-Li valence bonds in Al 3 Li at large size and high-volume fraction which increase its second derivative of internal energy and thus Young’s modulus.
基金supported by the National Key Research and Development Program of China(Nos.2017YFB0305100 and 2017YFB0304700)the National Natural Science Foundation of China(Nos.51704008 and 51974003)+1 种基金the Open Research Fund of State Key Laboratory of Vanadium and Titanium Resources Comprehensive Utilization(No.18100009)the Open Research Fund from the State Key Laboratory of Rolling and Automation,Northeastern University(No.2018RALKFKT006).
文摘Through the solubility product theory of the ternary secondary phase,classical nucleation theory,and Ostwald ripening theory,a model was established to describe the thermodynamics and kinetics of(Ti,V)C precipitates in austenite/ferrite(y/α)matrices.The model was used to calculate the volume fraction,precipitation-temperature-time(PTT)curve,and nucleation rate-temperature(NrT)curve of MC(M=Ti,V)precipitates in γ/α matrices in Ti-V microalloyed steels with various Ti/V ratios,which is verified by hardness tester,transmission electron microscopy and energy-dispersive X-ray spectroscopy.The calculations indicate that,by decreasing Ti/V ratio from Ti4V0 steel to Ti0V4 steel,the complete-dissolution temperature decreases monotonically from 1226 to 830℃,and the equilibrium volume fraction of MC pre-cipitated from austenite decreases from 0.333%to 0.091%at 900℃.Moreover,the maximum nucleation temperature of MC precipitated from α matrix decreases from 748 to 605℃and the fastest precipitation temperature decreases from 844 to 675℃as Ti/V ratio decreases.PTT and NrT diagrams of MC precipitated from α matrices in different Ti-V microalloyed steels all exhibit C-shaped and inverse C-shaped curves.In addition,both theoretical calculation and experimental results show that when tempered at 600℃for 100 h,Ti2V2 steel shows the largest hardness value of 312 HV among the three steels tested because it has a larger volume fraction(0.364%),a larger precipitate density(1689 μm-2),and the smallest average size(8.4 nm)of(Ti,V)C precipitates.The theoretical calculations are consistent with experimental results,which indicates that the thermodynamics and kinetics model for(Ti,V)C precipitates is reliable and accurate.
