The precipitation and coarsening behavior of L12 nanophases in Al-0.15Zr-0.15Er-0.15Yb-xSc(wt.%)alloys during aging were characterized by Double-Cs-corrected STEM,TEM,microhardness testing,electrical conductivity meas...The precipitation and coarsening behavior of L12 nanophases in Al-0.15Zr-0.15Er-0.15Yb-xSc(wt.%)alloys during aging were characterized by Double-Cs-corrected STEM,TEM,microhardness testing,electrical conductivity measurement,and first-principles calculations.The results indicate that the Al_(3)(Yb,Er)initiates precipitation at approximately 175°C with substantial precipitation occurring at 250°C.The Al_(3)Sc and Al_(3)Zr precipitate at approximately 325 and 450°C,respectively.The core/shell precipitates initially form in Al-Zr-Er-Yb-Sc alloys consisting of an Al_(3)(Yb,Er)core,an Al_(3)Sc inner shell,and an Al_(3)Zr outer shell.Upon prolonged aging,the core and inner shell remain as the Al_(3)(Yb,Er)and Al_(3)Sc,respectively,with the outer shell transforming into Al_(3)(Yb,Er,Zr),and the interface withα-Al remaining as the Al_(3)Zr.The precipitation evolution of the core-shell phases aligns with thermodynamic predictions based on solute segregation energies and phase interface energies.The increase of Sc content can effectively improve the aging response rate and strength of alloys.A modest Sc addition notably improves the coarsening resistance of the precipitation,while an excessive amount does not further improve this resistance.展开更多
High-strength Al-Zn-Mg-Cu alloys are widely utilized,but their strength deteriorates as strengthening precipitates coarsen rapidly at elevated temperatures,limiting their applications above 150°C.This study syste...High-strength Al-Zn-Mg-Cu alloys are widely utilized,but their strength deteriorates as strengthening precipitates coarsen rapidly at elevated temperatures,limiting their applications above 150°C.This study systematically investigates the microstructure evolution and its impact on the properties of peak-aged Al-Zn-Mg-Cu alloys with varying Zn/Mg ratios during thermal exposure at a series of temperatures from 150 to 300°C for 500 h.The results reveal that alloys A1 and A2 with an optimal Zn/Mg ratio(1.50-2.14)and relatively lower(Zn+Mg)content(7.0-8.8 wt.%),exhibit superior heat resistance properties compared to the other three alloys.Despite having lower strength relative to alloys with higher solute content,peak-aged alloys A1 and A2 retain the highest strength after thermal exposure.This performance is attributed to the high proportion(over 80%)of T'/T phases in the precipitates for alloys A1 and A2,which demonstrate better thermal stability in comparison to η'/η phases.Additionally,the lower solute content reduces the driving force for diffusion of Zn and Mg atoms,thus inhibiting the coarsening of precipitates.Moreover,the study elucidates that the coarsening mechanism of precipitates transitions from interfacial diffusion control at 150°C to matrix diffusion control at 200-300°C.These insights into the composition-dependent coarsening behavior of precipitates in dual-phase strengthened Al-Zn-Mg-Cu alloys offer valuable guidance for designing heat-resistant aluminum alloys with enhanced performance at elevated temperatures.展开更多
The nano-scale L1_(2)-Ni_(3)Al precipitates significantly contribute to thermal stability of alumina-forming austenitic(AFA)steels.The coarsening behavior of L1_(2)-Ni_(3)Al precipitates in AFA steels during isotherma...The nano-scale L1_(2)-Ni_(3)Al precipitates significantly contribute to thermal stability of alumina-forming austenitic(AFA)steels.The coarsening behavior of L1_(2)-Ni_(3)Al precipitates in AFA steels during isothermal aging with considering the influence of alloying elements was investigated.The results show that the coarsening rate of L1_(2)-Ni_(3)Al precipitates increases with co-additions of Ni and Cu,and especially,the increase of Cu content promotes the nucleation of L1_(2)-Ni_(3)Al precipitates.A dynamic competition exists between Lifshitz-Slyozov-Wagner theory and transient interface diffusion-controlled theory for coarsening behavior of L1_(2)-Ni_(3)Al precipitates with duration of isothermal aging.Additionally,the transition from L1_(2)-Ni_(3)Al precipitates to B2-NiAl precipitates during isothermal aging results in the formation of a depleted zone of L1_(2)-Ni_(3)Al precipitates around B2-NiAl precipitates,which inhibits the growth of L1_(2)-Ni_(3)Al precipitates.The coarsening of L1_(2)-Ni_(3)Al precipitates significantly contributes to the yield strength of AFA steels.展开更多
A microscopic phase-field model was used to investigate a directional coarsening mechanism caused by the anisotropic growth of long period stacking and different effects of phases on precipitation in Ni-Al-V alloy.The...A microscopic phase-field model was used to investigate a directional coarsening mechanism caused by the anisotropic growth of long period stacking and different effects of phases on precipitation in Ni-Al-V alloy.The results show that DO22 mainly coarsens along its short axis,which may press the neighboring L12,leading to the interaction among atoms.Diffusion channels of Al are formed in the direction where the mismatch between γ＇ and γ reduces;the occupation probabilities are anisotropic in space;and direction coarsening of L12 occurs finally.With a rise of ageing temperature,phases appear later and DO22 is much later at a higher temperature,the average occupation probabilities of Al and V reduce,and Al changes more than V.展开更多
Phase-field modelling of microstructural evolution in polycrystalline systems with phase-associated grains has largely been confined to continuum-field models.In this study,a multiphase-field approach,with a provision...Phase-field modelling of microstructural evolution in polycrystalline systems with phase-associated grains has largely been confined to continuum-field models.In this study,a multiphase-field approach,with a provision for introducing grain boundary and interphase diffusion,is extended to analyse concurrent grain growth and coarsening in multicomponent polycrystalline microstructures with chemically-distinct grains.The effect of the number of phases and components on the kinetics of evolution is investigated by considering binary and ternary systems of duplex and triplex microstructures,along with a single phase system.It is realised that the mere increase in the number of phases minimises the rate of concurrent grain growth and coarsening.However,the effect of components is substantially dependent on the respective kinetic coefficients.This work unravels that the disparity in the influence of phases and components is primarily due to the corresponding change introduced in the transformation mechanism.While the raise in number of phases convolutes the diffusion paths,the increase in number of component effects the rate of evolution through the interdiffusion,which introduces interdependency in the diffusing chemical-species.Additionally,the role of phase-fractions on the transformation rate of triplex microstructure is studied,and correspondingly,the interplay of interface-and diffusion-governed evolution is elucidated.A representative evolution of three-dimensional triplex microstructure with equal phase-fraction is comparatively analysed with the evolution of corresponding two-dimensional setup.展开更多
Transformation and coarsening of carbides in 2.25Cr-lMo steel weld metal during tempering at 700 ℃ for different time intervals ranging from 1 to 150 h were examined by transmission electron microscopy and scanning e...Transformation and coarsening of carbides in 2.25Cr-lMo steel weld metal during tempering at 700 ℃ for different time intervals ranging from 1 to 150 h were examined by transmission electron microscopy and scanning electron microscopy. M3C carbides were observed in the as-welded specimens and when tempered, the precipitates were mainly composed of M3C, M7C3, and M23C6 carbides. A sequence for corresponding carbide transformation during tempering with initial precipitation of M3 C and the subsequent precipitation of M7 C3 and M23 C6 was proposed. The precipitation of M7C3 with higher chromium content was the main factor contributing to the decrease in coarsening rate of precipitates after prolonged tempering. The decrease in hardness of the tempered specimens agreed well with the prediction of the weakening of precipitation strengthening owing to the coarsening of carbides.展开更多
Recently,theγ’-strengthened superalloys are of great interests in high temperature applications due to their excellent high temperature strength which is derived from theγ’strengthening phase.For theseγ’-strengt...Recently,theγ’-strengthened superalloys are of great interests in high temperature applications due to their excellent high temperature strength which is derived from theγ’strengthening phase.For theseγ’-strengthened superalloys,the changes in morphology,size and distribution ofγ’precipitates due to coarsening during thermal exposure have a significant impact on the properties of alloys.This article briefly summarizes the recent advances on the coarsening behavior of gamma prime precipitates in the recently-developedγ’-strengthened superalloys and its effects on the hot deformation behavior of superalloys,drawing specific examples on Allvac^(■)718 Plus TM and Ni3 Al-based intermetallic superalloys.It is found that the particle size plays an important role in morphological evolution ofγ’precipitates.For instance,the morphology ofγ’precipitates evolves from cuboidal to strip-like or other complex structures in Ni3 Al-based intermetallic alloys,while theγ’precipitates in Allvac^(■)718 Plus alloy always present nearspherical morphology due to the relatively small initial particle size.The Lifshitz-Slyozof-Wagner(LSW)theory and its modifications,as well as Trans-Interface Diffusion Controlled(TIDC)theory have been applied to describing the coarsening kinetics ofγ’precipitates.Additionally,the hot deformation behavior ofγ’-strengthened superalloy is found to be greatly influenced by the coarsening ofγ’precipitates.展开更多
Tempering is an important process for T/P92 ferritic heat-resistant steel from the viewpoint of microstructure control, as it facili- tates the formation of final tempered martensite under serving conditions. In this ...Tempering is an important process for T/P92 ferritic heat-resistant steel from the viewpoint of microstructure control, as it facili- tates the formation of final tempered martensite under serving conditions. In this study, we have gained deeper insights on the mechanism underlying the microstructural evolution during tempering treatment, including the precipitation of carbides and the coarsening of martensite laths, as systematically analyzed by optical microscopy, transmission electron microscopy, and high-resolution transmission electron mi- croscopy. The chemical composition of the precipitates was analyzed using energy dispersive X-ray spectroscopy. Results indicate the for- mation of M3C （cementite） precipitates under normalized conditions. However, they tend to dissolve within a short time of tempering, owing to their low thermal stability. This phenomenon was substantiated by X-ray diffraction analysis. Besides, we could observe the precipitation of fine carbonitrides （MX） along the dislocations. The mechanism of carbon diffusion controlled growth of M23C6 can be expressed by the Zener＇s equation. The movement of Y-junctions was determined to be the fundamental mechanism underlying the martensite lath coarsening process. Vickers hardness was estimated to determine their mechanical properties. Based on the comprehensive analysis of both the micro- structural evolution and hardness variation, the process of tempering can be separated into three steps.展开更多
The solvus temperature,volume fraction,coarsening behavior of γ＇ precipitates and the partitioning behavior of alloying elements as well as lattice misfit of γ/γ＇ phases influence the creep behavior of Ni-and Co-...The solvus temperature,volume fraction,coarsening behavior of γ＇ precipitates and the partitioning behavior of alloying elements as well as lattice misfit of γ/γ＇ phases influence the creep behavior of Ni-and Co-base superalloys.However,few investigations about the microstructural characteristics and the coarsening behavior of γ＇ precipitates were reported in multicomponent novel Co-base superalloys during thermal exposure.Two alloys containing different contents of molybdenum and tungsten have been investigated to explore the effect of molybdenum on γ＇ solvus temperature,γ ＋γ＇ microstructure and γ＇ coarsening in Co–Al–W–Ta–Ti-base alloys.The results showed that the γ＇ solvus temperature decreases with the addition of Mo.Mo addition reduces the Y＇ volume fractions after aging above 1000℃,but results in negligible influence on the γ＇ volume fractions aging at 900℃.Meanwhile,γ＇ coarsening is controlled by diffusion in experimental alloys after aging at 900℃ and 1000℃,and the kinetics of Y＇growth in experimental alloys are consistent with the predictions of LSW theory.展开更多
It is generally considered that the Al3Sc nanoprecipitates are highly thermal stable,mainly due to quite slow Sc diffusion in theα-Al matrix.In this paper,we demonstrate in an Al-Cu-Sc alloy that the Cu atoms have du...It is generally considered that the Al3Sc nanoprecipitates are highly thermal stable,mainly due to quite slow Sc diffusion in theα-Al matrix.In this paper,we demonstrate in an Al-Cu-Sc alloy that the Cu atoms have dual effect on the coarsening of Al3Sc nanoprecipitates.On the one hand,the Cu atoms with high diffusivity tend to accelerate the Al3Sc coarsening,which results from the Cu-promoted Sc diffusion.On the other hand,some Cu atoms will segregate at the Al3Sc/matrix interface,which further stabilizes the Al3Sc nanoprecipitates by reducing the interfacial energy.Competition between these two effects is tailored by temperature,which rationalizes the experimental findings that the coarsening kinetics of Al3Sc nanoprecipitate is greatly boosted at 300℃-overaging while significantly suppressed at 400℃-overaging.展开更多
The coarsening behavior of γ particles in a nickel-base superalloy FGH95 was investigated by means of experimental observations and growth kinetics calculations. The results show that when aging at 1000,1080 and 1140...The coarsening behavior of γ particles in a nickel-base superalloy FGH95 was investigated by means of experimental observations and growth kinetics calculations. The results show that when aging at 1000,1080 and 1140°C for different times,the relation of average particle size to time obeys the cube law ( a /2)3= kt,where k is 15.49 × 103,77.5 × 103 and 230.04 × 103 nm3/min,respectively. The particle size distributions are better fit to the LSW theoretical distributions when aging at 1000°C within 1440 min....展开更多
The coarsening behavior of(Ni,Co)2Si particles in Cu-Ni-Co-Si alloy was investigated by experimental observations and coarsening kinetics calculations when aged at 450,500,550 and 600℃ for different durations.The res...The coarsening behavior of(Ni,Co)2Si particles in Cu-Ni-Co-Si alloy was investigated by experimental observations and coarsening kinetics calculations when aged at 450,500,550 and 600℃ for different durations.The results show that the critical particle radius for coherence mismatch is found to be 10.3 nm,and particles larger than 25 nm are generally semi-coherent.The relationship of(Ni,Co)2Si particles size and aging time follows Lifshitz,Slyosov and Wagner(LSW) theory.The particle size distributions fit well to the LSW theoretical distribution.The activation energy for(Ni,Co)2Si coarsening is accurately determined to be(216.21 ± 5.18)kJ mol-1 when considering the effect of temperature on the solution concentrations in matrix.The coarsening of(Ni,Co)2Si particles in Cu-Ni-Co-Si alloy is controlled by diffusion of Ni,Co and Si in Cu matrix.The growth of particles for long durations suggests that vacancies can be trapped within the structure for long time despite their mobility.展开更多
The coarsening and ageing hardening behaviors of γ′ particles in superalloy GH742 have been investigated by field emission scanning electron microscopy （FESEM）, coarsening kinetics calculation and microhardness te...The coarsening and ageing hardening behaviors of γ′ particles in superalloy GH742 have been investigated by field emission scanning electron microscopy （FESEM）, coarsening kinetics calculation and microhardness testing when ageing at 1 050, 950 and 900 ℃ for different time. The cube of γ′ particle size and ageing time follows a linear law as predicted by LSW theory. The particle size distributions give better fit to the LSW theoretical distribution. The activation energy for γ′ coarsening is accurately determined to have 245. 06±14.42 kJ/mol when considering the effect of temperature on the solution concentrations in matrix. Based on the activation energy, the coarsening kinetics of γ′ particle is predicted as r^-t^3=7.35×10^15 Ce/T exp -（245060±14420）/RT t.The microhardness studies indicate that microhardness decreases rapidly with increasing the ageing temperature and it has a maximum value corresponding to a critical particle size beyond which the microhardness increase stalls with increasing the ageing time.展开更多
The tailored nanoparticles with a complex core/shell structure can satisfy a variety of demands, such as lattice misfit, shearability and coarsening resistance. In this research, core-shell nanoscale Al3(Yb, Er, Sc,Zr...The tailored nanoparticles with a complex core/shell structure can satisfy a variety of demands, such as lattice misfit, shearability and coarsening resistance. In this research, core-shell nanoscale Al3(Yb, Er, Sc,Zr, Li) composite particles were precipitated in Al-2 Li-0.1 Yb-0.1 Er-0.1 Sc-0.1 Zr(wt%) alloy through the double-aging treatment, in which the core was(Yb, Er, Sc, Zr)-rich formed at 300°C and the shell was Li-rich formed at 150°C. The coarsening kinetics and precipitate size distributions(PSDs) of Al3(Yb, Er, Sc,Zr, Li) particles aged at 150°C previously aged at 300°C for 24 h showed a better fit to the relation of 2∝ kt and normal distribution, indicating that the coarsening of precipitates was controlled by interface reaction, not diffusion. The Orowan bypass strengthening was operative mechanism at 150°C.展开更多
In this work, the growth kinetics of MX （M - metal, X - C/N） nanoprecipitates in type 347H austenitic steel was systematically studied. To investigate the coarsening behavior and the growth mechanism of MX carbonitr...In this work, the growth kinetics of MX （M - metal, X - C/N） nanoprecipitates in type 347H austenitic steel was systematically studied. To investigate the coarsening behavior and the growth mechanism of MX carbonitrides during long-term aging, experiments were performed at 700, 800, 850, and 900℃ for different periods （1, 24, 70, and 100 h）. The precipitation behavior of carbonitrides in specimens subjected to various aging conditions was explored using carbon replicas and transmission electron microscopy （TEM） observations. The corresponding sizes ofMX carbonitrides were measured. The results demonstrates that MX carbonitrides precipitate in type 347H austenitic steel as Nb（C,N）. The coarsening rate constant is time-independent; however, an increase in aging temperature results in an increase in coarsening rate of Nb（C,N）. The coarsening process was analyzed according to the calculated diffusion activation energy of Nb（C,N）. When the aging temperature was 800-900℃, the mean activation energy was 294 kJ·mol -1, and the coarsening behavior was controlled primarily by the diffusion of Nb atoms.展开更多
Small addition of mischmetal （MM） into aluminum alloys can lead to grain refinement. However, it is still uncertain whether the same effect applies to Mg-Al alloys. This work indicated that small amount of mischmeta...Small addition of mischmetal （MM） into aluminum alloys can lead to grain refinement. However, it is still uncertain whether the same effect applies to Mg-Al alloys. This work indicated that small amount of mischmetal addition ranging from 0.1% to 1.2% （mass fraction） did not cause grain refinement in Mg-Al alloys. On the contrary, they tended to coarsen the grains. When added into Mg-Al alloys, MM reacted preferentially with Al to form Al11 MM3 phase. As Al11 MM3 phase mainly distributed within α-Mg grains than at grain boundaries, it had little effect in restricting grain growth. In addition, MM reacted with Al8（Mn, Fe）5 or ε-AlMn particles to form Al-MM-Mn compounds, thus it reduced the amount of heterogeneous nuclei in the melt and resulted in remarkable grain coarsening.展开更多
The morphological evolution and coarsening kinetics ofγ'precipitates in a Re-containing Ni-based single crystal superalloy were investigated during isothermal aging at 900,950 and 1000℃.After heat treatment,well...The morphological evolution and coarsening kinetics ofγ'precipitates in a Re-containing Ni-based single crystal superalloy were investigated during isothermal aging at 900,950 and 1000℃.After heat treatment,well-defined cuboidalγ'precipitates with low misfit was obtained within the experimental alloy.Then coarsening rate constants and particle size distribution(PSD)ofγ'phases were calculated and specified based on the measured precipitate sizes for va rying periods of aging times from 100 to 2000 h.After aging for 2000 h,γ'precipitates maintained cubical shape at 900℃,while exhibited sphere at 950 and 1000℃.Coarsening models based on diffusion-controlled process with a functional relationship of r^(3) vs.t(classic Lifshitz-Slyozov-Wagner coarsening model)and interface-controlled model with a function of r^(2) vs.t(trans-interface diffusion-controlled coarsening model)were investigated to fit between the experimental results and theoretical analysis.It was found that Re as the slowest diffusing solute in the alloy constituted the rate-limited step for coarsening based on LSW model,while the process limiting coarsening as governed by an interface diffusion process could possibly be related to the Al diffusion through theγ/γ'interface.The PSDs and coarsening exponent were discussed by comparing the experimental data with predictions of LSW and TIDC models.Finally,coarsening mechanism could be divided into four regimes:(i)coarsening by diffusion-controlled;(ii)coarsening by diffusion and interface cocontrolled;(iii)coarsening by interface-controlled;(iv)coarsening by interface-controlled accompanied withγ'coalescence.展开更多
By optical microscopy, transmission electron microscopy and energy dispersive spectroscopy, the changes with aging time in size, composition and distribution of nanometer-sized （Ti, Mo）C precipitated in Ti-Mo low-al...By optical microscopy, transmission electron microscopy and energy dispersive spectroscopy, the changes with aging time in size, composition and distribution of nanometer-sized （Ti, Mo）C precipitated in Ti-Mo low-alloy ferritc steel have been studied in comparison with that of nanometer-sized TiC precipitated in Ti low-alloy ferritc steel. It was found that the growth rate of （Ti,Mo）C in Ti-Mo steel was less than that of TiC in Ti steel. Nanometer- sized carbides formed at 650 ~C （or 550 ~C） for 55 h were at transitional stage from growth to coarsening. When aging time reaches 55 h, the coarsening rates of nanometer-sized carbides in Ti and Ti-Mo steel tend to be the same and in- variable. The influence of Mo on growth and coarsening of nanometer-sized carbides tends to decrease with increasing aging time, and Mo contents in nanometer-sized carbides with the same size at different aging time were different.展开更多
The rate of microstructural coarsening of recrystallisation and partial melting (RAP) and cooling slope (CS) processed materials in the semi-solid state is compared with rates found in the literature.The rate of coars...The rate of microstructural coarsening of recrystallisation and partial melting (RAP) and cooling slope (CS) processed materials in the semi-solid state is compared with rates found in the literature.The rate of coarsening depended on the liquid fraction but RAP route 2014 alloy with 37% liquid coarsened slightly more slowly than the CS route 2014 alloy with a lower liquid fraction of 17%, contrary to expectations.For the CS route, an increase in liquid fraction resulted in faster coarsening.A modified 2014 alloy with Fe, Mn and Zn stripped out of the composition gave a relatively high coarsening rate.The coarsening rate was also relatively high for CS 201 alloy in comparison with either RAP 2014 or CS 2014.Low coarsening rates are thought to be associated with the presence of particles which inhibit the migration of liquid film grain boundaries.This could be the result of pinning or of the liquid film impeding diffusion at the boundary.展开更多
With carbon extraction replica technique, electron diffraction and EDAX analysis, second particle size distribution in two Ti microalloyed steels and their heat-affected zones were investigated. The results show that ...With carbon extraction replica technique, electron diffraction and EDAX analysis, second particle size distribution in two Ti microalloyed steels and their heat-affected zones were investigated. The results show that the particles in the Ti microalloyed steels are TiN particles, and the TiN particles in the steel with lower Ti/N ratio exhibit smaller size and lower dissolution and coarsening rate and extent. Based on the investigation results, kinetic models for TiN particle dissolution and coarsening during welding thermal cycle were developed. The predicted values calculated by using the models are in good agreement with the experimental ones.展开更多
基金supported by the National Natural Science Foundation of China (No. 52274403).
文摘The precipitation and coarsening behavior of L12 nanophases in Al-0.15Zr-0.15Er-0.15Yb-xSc(wt.%)alloys during aging were characterized by Double-Cs-corrected STEM,TEM,microhardness testing,electrical conductivity measurement,and first-principles calculations.The results indicate that the Al_(3)(Yb,Er)initiates precipitation at approximately 175°C with substantial precipitation occurring at 250°C.The Al_(3)Sc and Al_(3)Zr precipitate at approximately 325 and 450°C,respectively.The core/shell precipitates initially form in Al-Zr-Er-Yb-Sc alloys consisting of an Al_(3)(Yb,Er)core,an Al_(3)Sc inner shell,and an Al_(3)Zr outer shell.Upon prolonged aging,the core and inner shell remain as the Al_(3)(Yb,Er)and Al_(3)Sc,respectively,with the outer shell transforming into Al_(3)(Yb,Er,Zr),and the interface withα-Al remaining as the Al_(3)Zr.The precipitation evolution of the core-shell phases aligns with thermodynamic predictions based on solute segregation energies and phase interface energies.The increase of Sc content can effectively improve the aging response rate and strength of alloys.A modest Sc addition notably improves the coarsening resistance of the precipitation,while an excessive amount does not further improve this resistance.
基金supported by the Natural Science Foundation of Chongqing(Nos.CSTB2022NSCQ-LZX0002 and CSTB2022NSCQ-BHX0624)the Chongqing Key Project for Technological Innovation and Application(No.CSTB2022TIAD-KPX0073)+1 种基金the Opening Project of State Key Laboratory for Advanced Metals and Materials(No.2022-Z03)the Science and Technology Research Program of Chongqing Municipal Education Commission(No.KJQN202200810).
文摘High-strength Al-Zn-Mg-Cu alloys are widely utilized,but their strength deteriorates as strengthening precipitates coarsen rapidly at elevated temperatures,limiting their applications above 150°C.This study systematically investigates the microstructure evolution and its impact on the properties of peak-aged Al-Zn-Mg-Cu alloys with varying Zn/Mg ratios during thermal exposure at a series of temperatures from 150 to 300°C for 500 h.The results reveal that alloys A1 and A2 with an optimal Zn/Mg ratio(1.50-2.14)and relatively lower(Zn+Mg)content(7.0-8.8 wt.%),exhibit superior heat resistance properties compared to the other three alloys.Despite having lower strength relative to alloys with higher solute content,peak-aged alloys A1 and A2 retain the highest strength after thermal exposure.This performance is attributed to the high proportion(over 80%)of T'/T phases in the precipitates for alloys A1 and A2,which demonstrate better thermal stability in comparison to η'/η phases.Additionally,the lower solute content reduces the driving force for diffusion of Zn and Mg atoms,thus inhibiting the coarsening of precipitates.Moreover,the study elucidates that the coarsening mechanism of precipitates transitions from interfacial diffusion control at 150°C to matrix diffusion control at 200-300°C.These insights into the composition-dependent coarsening behavior of precipitates in dual-phase strengthened Al-Zn-Mg-Cu alloys offer valuable guidance for designing heat-resistant aluminum alloys with enhanced performance at elevated temperatures.
基金financial supports from the National Natural Science Foundation of China(Nos.52471004,52171107,52201203)the Industry-University-Research Cooperation Project of Hebei Based Universities and Shijiazhuang City(No.241791237A)the Fundamental Research Funds for the Central Universities(No.N2423030)。
文摘The nano-scale L1_(2)-Ni_(3)Al precipitates significantly contribute to thermal stability of alumina-forming austenitic(AFA)steels.The coarsening behavior of L1_(2)-Ni_(3)Al precipitates in AFA steels during isothermal aging with considering the influence of alloying elements was investigated.The results show that the coarsening rate of L1_(2)-Ni_(3)Al precipitates increases with co-additions of Ni and Cu,and especially,the increase of Cu content promotes the nucleation of L1_(2)-Ni_(3)Al precipitates.A dynamic competition exists between Lifshitz-Slyozov-Wagner theory and transient interface diffusion-controlled theory for coarsening behavior of L1_(2)-Ni_(3)Al precipitates with duration of isothermal aging.Additionally,the transition from L1_(2)-Ni_(3)Al precipitates to B2-NiAl precipitates during isothermal aging results in the formation of a depleted zone of L1_(2)-Ni_(3)Al precipitates around B2-NiAl precipitates,which inhibits the growth of L1_(2)-Ni_(3)Al precipitates.The coarsening of L1_(2)-Ni_(3)Al precipitates significantly contributes to the yield strength of AFA steels.
基金Projects(51075335,10902086,50875217) supported by the National Natural Science Foundation of ChinaProject(JC201005) supported by the Northwestern Polytechnical University Foundation for Fundamental Research,ChinaProject(CX201007) supported by the Doctorate Foundation of Northwestern Polytechnical University,China
文摘A microscopic phase-field model was used to investigate a directional coarsening mechanism caused by the anisotropic growth of long period stacking and different effects of phases on precipitation in Ni-Al-V alloy.The results show that DO22 mainly coarsens along its short axis,which may press the neighboring L12,leading to the interaction among atoms.Diffusion channels of Al are formed in the direction where the mismatch between γ＇ and γ reduces;the occupation probabilities are anisotropic in space;and direction coarsening of L12 occurs finally.With a rise of ageing temperature,phases appear later and DO22 is much later at a higher temperature,the average occupation probabilities of Al and V reduce,and Al changes more than V.
基金financial support of the German Research Foundation(DFG)under the project AN 1245/1the support of the BMBF project‘Ker Solife100’the Helmholtz programme‘Renewable energies’(35.14.01)。
文摘Phase-field modelling of microstructural evolution in polycrystalline systems with phase-associated grains has largely been confined to continuum-field models.In this study,a multiphase-field approach,with a provision for introducing grain boundary and interphase diffusion,is extended to analyse concurrent grain growth and coarsening in multicomponent polycrystalline microstructures with chemically-distinct grains.The effect of the number of phases and components on the kinetics of evolution is investigated by considering binary and ternary systems of duplex and triplex microstructures,along with a single phase system.It is realised that the mere increase in the number of phases minimises the rate of concurrent grain growth and coarsening.However,the effect of components is substantially dependent on the respective kinetic coefficients.This work unravels that the disparity in the influence of phases and components is primarily due to the corresponding change introduced in the transformation mechanism.While the raise in number of phases convolutes the diffusion paths,the increase in number of component effects the rate of evolution through the interdiffusion,which introduces interdependency in the diffusing chemical-species.Additionally,the role of phase-fractions on the transformation rate of triplex microstructure is studied,and correspondingly,the interplay of interface-and diffusion-governed evolution is elucidated.A representative evolution of three-dimensional triplex microstructure with equal phase-fraction is comparatively analysed with the evolution of corresponding two-dimensional setup.
基金Financial support from Kobe Steel is gratefully acknowledged
文摘Transformation and coarsening of carbides in 2.25Cr-lMo steel weld metal during tempering at 700 ℃ for different time intervals ranging from 1 to 150 h were examined by transmission electron microscopy and scanning electron microscopy. M3C carbides were observed in the as-welded specimens and when tempered, the precipitates were mainly composed of M3C, M7C3, and M23C6 carbides. A sequence for corresponding carbide transformation during tempering with initial precipitation of M3 C and the subsequent precipitation of M7 C3 and M23 C6 was proposed. The precipitation of M7C3 with higher chromium content was the main factor contributing to the decrease in coarsening rate of precipitates after prolonged tempering. The decrease in hardness of the tempered specimens agreed well with the prediction of the weakening of precipitation strengthening owing to the coarsening of carbides.
基金supported financially by the National Natural Science Foundation of China(Nos.51474156,U1660201)the National High Technology Research and Development Program of China(No.2015AA042504)。
文摘Recently,theγ’-strengthened superalloys are of great interests in high temperature applications due to their excellent high temperature strength which is derived from theγ’strengthening phase.For theseγ’-strengthened superalloys,the changes in morphology,size and distribution ofγ’precipitates due to coarsening during thermal exposure have a significant impact on the properties of alloys.This article briefly summarizes the recent advances on the coarsening behavior of gamma prime precipitates in the recently-developedγ’-strengthened superalloys and its effects on the hot deformation behavior of superalloys,drawing specific examples on Allvac^(■)718 Plus TM and Ni3 Al-based intermetallic superalloys.It is found that the particle size plays an important role in morphological evolution ofγ’precipitates.For instance,the morphology ofγ’precipitates evolves from cuboidal to strip-like or other complex structures in Ni3 Al-based intermetallic alloys,while theγ’precipitates in Allvac^(■)718 Plus alloy always present nearspherical morphology due to the relatively small initial particle size.The Lifshitz-Slyozof-Wagner(LSW)theory and its modifications,as well as Trans-Interface Diffusion Controlled(TIDC)theory have been applied to describing the coarsening kinetics ofγ’precipitates.Additionally,the hot deformation behavior ofγ’-strengthened superalloy is found to be greatly influenced by the coarsening ofγ’precipitates.
基金financially supported by the China National Funds for Distinguished Young Scientists(No.51325401)the International Thermonuclear Experimental Reactor(ITER)Program Special Project(No.2014GB125006)+1 种基金the National Natural Science Foundation of China(No.51104107)the Major State Basic Research Development Program(No.2014CB046805)
文摘Tempering is an important process for T/P92 ferritic heat-resistant steel from the viewpoint of microstructure control, as it facili- tates the formation of final tempered martensite under serving conditions. In this study, we have gained deeper insights on the mechanism underlying the microstructural evolution during tempering treatment, including the precipitation of carbides and the coarsening of martensite laths, as systematically analyzed by optical microscopy, transmission electron microscopy, and high-resolution transmission electron mi- croscopy. The chemical composition of the precipitates was analyzed using energy dispersive X-ray spectroscopy. Results indicate the for- mation of M3C （cementite） precipitates under normalized conditions. However, they tend to dissolve within a short time of tempering, owing to their low thermal stability. This phenomenon was substantiated by X-ray diffraction analysis. Besides, we could observe the precipitation of fine carbonitrides （MX） along the dislocations. The mechanism of carbon diffusion controlled growth of M23C6 can be expressed by the Zener＇s equation. The movement of Y-junctions was determined to be the fundamental mechanism underlying the martensite lath coarsening process. Vickers hardness was estimated to determine their mechanical properties. Based on the comprehensive analysis of both the micro- structural evolution and hardness variation, the process of tempering can be separated into three steps.
基金financially supported by the National Natural Science Foundation of China (Grant Nos.50771012,51201006 and 51301014)the Aeronautical Science Foundation of China (Grant No.2009ZF74011)
文摘The solvus temperature,volume fraction,coarsening behavior of γ＇ precipitates and the partitioning behavior of alloying elements as well as lattice misfit of γ/γ＇ phases influence the creep behavior of Ni-and Co-base superalloys.However,few investigations about the microstructural characteristics and the coarsening behavior of γ＇ precipitates were reported in multicomponent novel Co-base superalloys during thermal exposure.Two alloys containing different contents of molybdenum and tungsten have been investigated to explore the effect of molybdenum on γ＇ solvus temperature,γ ＋γ＇ microstructure and γ＇ coarsening in Co–Al–W–Ta–Ti-base alloys.The results showed that the γ＇ solvus temperature decreases with the addition of Mo.Mo addition reduces the Y＇ volume fractions after aging above 1000℃,but results in negligible influence on the γ＇ volume fractions aging at 900℃.Meanwhile,γ＇ coarsening is controlled by diffusion in experimental alloys after aging at 900℃ and 1000℃,and the kinetics of Y＇growth in experimental alloys are consistent with the predictions of LSW theory.
基金financially supported by the National Natural Science Foundation of China(Nos.51621063,51625103,51722104,51790482,51761135031 and 51871033)the National Key Research and Development Program of China(No.2016YFB0700403)+1 种基金the 111 Project of China(No.BP2018008)supported by the International Joint Laboratory for Micro/Nano Manufacturing and Measurement Technologies.
文摘It is generally considered that the Al3Sc nanoprecipitates are highly thermal stable,mainly due to quite slow Sc diffusion in theα-Al matrix.In this paper,we demonstrate in an Al-Cu-Sc alloy that the Cu atoms have dual effect on the coarsening of Al3Sc nanoprecipitates.On the one hand,the Cu atoms with high diffusivity tend to accelerate the Al3Sc coarsening,which results from the Cu-promoted Sc diffusion.On the other hand,some Cu atoms will segregate at the Al3Sc/matrix interface,which further stabilizes the Al3Sc nanoprecipitates by reducing the interfacial energy.Competition between these two effects is tailored by temperature,which rationalizes the experimental findings that the coarsening kinetics of Al3Sc nanoprecipitate is greatly boosted at 300℃-overaging while significantly suppressed at 400℃-overaging.
基金the National Natural Science Foundation of China (No. 50471097)the Programme of Introducing Talents of Discipline to Chinese Universities (No. B07003).
文摘The coarsening behavior of γ particles in a nickel-base superalloy FGH95 was investigated by means of experimental observations and growth kinetics calculations. The results show that when aging at 1000,1080 and 1140°C for different times,the relation of average particle size to time obeys the cube law ( a /2)3= kt,where k is 15.49 × 103,77.5 × 103 and 230.04 × 103 nm3/min,respectively. The particle size distributions are better fit to the LSW theoretical distributions when aging at 1000°C within 1440 min....
基金financially supported by the National Natural Science Foundation of China(Nos.51561008 and 51461017)Jiangxi Yorth Major Natural Science Foundation(Nos.20171ACB21044 and 20161BBE50030)
文摘The coarsening behavior of(Ni,Co)2Si particles in Cu-Ni-Co-Si alloy was investigated by experimental observations and coarsening kinetics calculations when aged at 450,500,550 and 600℃ for different durations.The results show that the critical particle radius for coherence mismatch is found to be 10.3 nm,and particles larger than 25 nm are generally semi-coherent.The relationship of(Ni,Co)2Si particles size and aging time follows Lifshitz,Slyosov and Wagner(LSW) theory.The particle size distributions fit well to the LSW theoretical distribution.The activation energy for(Ni,Co)2Si coarsening is accurately determined to be(216.21 ± 5.18)kJ mol-1 when considering the effect of temperature on the solution concentrations in matrix.The coarsening of(Ni,Co)2Si particles in Cu-Ni-Co-Si alloy is controlled by diffusion of Ni,Co and Si in Cu matrix.The growth of particles for long durations suggests that vacancies can be trapped within the structure for long time despite their mobility.
基金Sponsored by National Natural Science Foundation of China(50471097)
文摘The coarsening and ageing hardening behaviors of γ′ particles in superalloy GH742 have been investigated by field emission scanning electron microscopy （FESEM）, coarsening kinetics calculation and microhardness testing when ageing at 1 050, 950 and 900 ℃ for different time. The cube of γ′ particle size and ageing time follows a linear law as predicted by LSW theory. The particle size distributions give better fit to the LSW theoretical distribution. The activation energy for γ′ coarsening is accurately determined to have 245. 06±14.42 kJ/mol when considering the effect of temperature on the solution concentrations in matrix. Based on the activation energy, the coarsening kinetics of γ′ particle is predicted as r^-t^3=7.35×10^15 Ce/T exp -（245060±14420）/RT t.The microhardness studies indicate that microhardness decreases rapidly with increasing the ageing temperature and it has a maximum value corresponding to a critical particle size beyond which the microhardness increase stalls with increasing the ageing time.
基金supported financially by the National Natural Science Foundation of China(Nos.51671063,51771060,51871068 and 51971071)the Domain Foundation of Equipment Advance Research of 13th Five-year Plan(No.61409220118)+2 种基金the Fundamental Research Funds for the Central Universities(No.HEUCFG201834)the Harbin City Application Technology Research and Development Project(Nos.2015RQXXJ001 and 2017RAQXJ032)the Ph.D.Student Research and Innovation Fund of the Fundamental Research Funds for the Central Universities(No.3072019GIP1013)。
文摘The tailored nanoparticles with a complex core/shell structure can satisfy a variety of demands, such as lattice misfit, shearability and coarsening resistance. In this research, core-shell nanoscale Al3(Yb, Er, Sc,Zr, Li) composite particles were precipitated in Al-2 Li-0.1 Yb-0.1 Er-0.1 Sc-0.1 Zr(wt%) alloy through the double-aging treatment, in which the core was(Yb, Er, Sc, Zr)-rich formed at 300°C and the shell was Li-rich formed at 150°C. The coarsening kinetics and precipitate size distributions(PSDs) of Al3(Yb, Er, Sc,Zr, Li) particles aged at 150°C previously aged at 300°C for 24 h showed a better fit to the relation of 2∝ kt and normal distribution, indicating that the coarsening of precipitates was controlled by interface reaction, not diffusion. The Orowan bypass strengthening was operative mechanism at 150°C.
基金the China National Funds for Distinguished Young Scientists (No.51325401)the National High Technology Research and Development Program of China (No.2015AA042504)the National Natural Science Foundation of China (No.51474156) for financial support
文摘In this work, the growth kinetics of MX （M - metal, X - C/N） nanoprecipitates in type 347H austenitic steel was systematically studied. To investigate the coarsening behavior and the growth mechanism of MX carbonitrides during long-term aging, experiments were performed at 700, 800, 850, and 900℃ for different periods （1, 24, 70, and 100 h）. The precipitation behavior of carbonitrides in specimens subjected to various aging conditions was explored using carbon replicas and transmission electron microscopy （TEM） observations. The corresponding sizes ofMX carbonitrides were measured. The results demonstrates that MX carbonitrides precipitate in type 347H austenitic steel as Nb（C,N）. The coarsening rate constant is time-independent; however, an increase in aging temperature results in an increase in coarsening rate of Nb（C,N）. The coarsening process was analyzed according to the calculated diffusion activation energy of Nb（C,N）. When the aging temperature was 800-900℃, the mean activation energy was 294 kJ·mol -1, and the coarsening behavior was controlled primarily by the diffusion of Nb atoms.
基金Project supported by Beijing Municipal Science & Technology Research Project (Y0405004040211)
文摘Small addition of mischmetal （MM） into aluminum alloys can lead to grain refinement. However, it is still uncertain whether the same effect applies to Mg-Al alloys. This work indicated that small amount of mischmetal addition ranging from 0.1% to 1.2% （mass fraction） did not cause grain refinement in Mg-Al alloys. On the contrary, they tended to coarsen the grains. When added into Mg-Al alloys, MM reacted preferentially with Al to form Al11 MM3 phase. As Al11 MM3 phase mainly distributed within α-Mg grains than at grain boundaries, it had little effect in restricting grain growth. In addition, MM reacted with Al8（Mn, Fe）5 or ε-AlMn particles to form Al-MM-Mn compounds, thus it reduced the amount of heterogeneous nuclei in the melt and resulted in remarkable grain coarsening.
基金the financial support by the National Natural Science Foundation of China(51631008)the National Key Research and Development Program of China(No.2016YFB0701400)+1 种基金Natural Science Foundation of Shaanxi Province 2020JM-122the National High Technology Research and Development Program of China(No.2012AA03A511)。
文摘The morphological evolution and coarsening kinetics ofγ'precipitates in a Re-containing Ni-based single crystal superalloy were investigated during isothermal aging at 900,950 and 1000℃.After heat treatment,well-defined cuboidalγ'precipitates with low misfit was obtained within the experimental alloy.Then coarsening rate constants and particle size distribution(PSD)ofγ'phases were calculated and specified based on the measured precipitate sizes for va rying periods of aging times from 100 to 2000 h.After aging for 2000 h,γ'precipitates maintained cubical shape at 900℃,while exhibited sphere at 950 and 1000℃.Coarsening models based on diffusion-controlled process with a functional relationship of r^(3) vs.t(classic Lifshitz-Slyozov-Wagner coarsening model)and interface-controlled model with a function of r^(2) vs.t(trans-interface diffusion-controlled coarsening model)were investigated to fit between the experimental results and theoretical analysis.It was found that Re as the slowest diffusing solute in the alloy constituted the rate-limited step for coarsening based on LSW model,while the process limiting coarsening as governed by an interface diffusion process could possibly be related to the Al diffusion through theγ/γ'interface.The PSDs and coarsening exponent were discussed by comparing the experimental data with predictions of LSW and TIDC models.Finally,coarsening mechanism could be divided into four regimes:(i)coarsening by diffusion-controlled;(ii)coarsening by diffusion and interface cocontrolled;(iii)coarsening by interface-controlled;(iv)coarsening by interface-controlled accompanied withγ'coalescence.
基金Item Sponsored by National Key Technology Research and Development Program in 11th Five-year Plan of China(2006BE03A0)
文摘By optical microscopy, transmission electron microscopy and energy dispersive spectroscopy, the changes with aging time in size, composition and distribution of nanometer-sized （Ti, Mo）C precipitated in Ti-Mo low-alloy ferritc steel have been studied in comparison with that of nanometer-sized TiC precipitated in Ti low-alloy ferritc steel. It was found that the growth rate of （Ti,Mo）C in Ti-Mo steel was less than that of TiC in Ti steel. Nanometer- sized carbides formed at 650 ~C （or 550 ~C） for 55 h were at transitional stage from growth to coarsening. When aging time reaches 55 h, the coarsening rates of nanometer-sized carbides in Ti and Ti-Mo steel tend to be the same and in- variable. The influence of Mo on growth and coarsening of nanometer-sized carbides tends to decrease with increasing aging time, and Mo contents in nanometer-sized carbides with the same size at different aging time were different.
基金the UK Engineering and Physical Sciences Research Council (EPSRC Grant GR/M89096) and the University of Leicester for financial support
文摘The rate of microstructural coarsening of recrystallisation and partial melting (RAP) and cooling slope (CS) processed materials in the semi-solid state is compared with rates found in the literature.The rate of coarsening depended on the liquid fraction but RAP route 2014 alloy with 37% liquid coarsened slightly more slowly than the CS route 2014 alloy with a lower liquid fraction of 17%, contrary to expectations.For the CS route, an increase in liquid fraction resulted in faster coarsening.A modified 2014 alloy with Fe, Mn and Zn stripped out of the composition gave a relatively high coarsening rate.The coarsening rate was also relatively high for CS 201 alloy in comparison with either RAP 2014 or CS 2014.Low coarsening rates are thought to be associated with the presence of particles which inhibit the migration of liquid film grain boundaries.This could be the result of pinning or of the liquid film impeding diffusion at the boundary.
基金The authors appreciate the financial support of the Fund for Doctor of Shandong University.
文摘With carbon extraction replica technique, electron diffraction and EDAX analysis, second particle size distribution in two Ti microalloyed steels and their heat-affected zones were investigated. The results show that the particles in the Ti microalloyed steels are TiN particles, and the TiN particles in the steel with lower Ti/N ratio exhibit smaller size and lower dissolution and coarsening rate and extent. Based on the investigation results, kinetic models for TiN particle dissolution and coarsening during welding thermal cycle were developed. The predicted values calculated by using the models are in good agreement with the experimental ones.
