The long-period-stacking-ordered(LPSO)structure affects the mechanical,corrosion and hydrolysis properties of Mg alloys.The current work employs high angle annular dark field-scanning transmission electron microscopy(...The long-period-stacking-ordered(LPSO)structure affects the mechanical,corrosion and hydrolysis properties of Mg alloys.The current work employs high angle annular dark field-scanning transmission electron microscopy(HAADF-STEM)and atom probe tomography(APT)to investigate the structural and local chemical information of LPSO phases formed in Mg-Ni-Y/Sm ternary alloys after extended isothermal annealing.Depending on the alloying elements and their concentrations,Mg-Ni-Y/Sm develops a two-phase LPSO+α-Mg structure in which the LPSO phase contains defects,hybrid LPSO structure,and Mg insertions.HAADF-STEM and APT indicate non-stoichiometric LPSO with incomplete Ni_(6)(Y/Sm)_(8) clusters.In addition,the APT quantitatively determines the local composition of LPSO and confirms the presence of Ni within the Mg bonding layers.These results provide insight into a better understanding of the structure and hydrolysis properties of LPSO-Mg alloys.展开更多
Aluminum-based alloys play a key role in modern engineering and are widely used in construction components in aircraft, automobiles and other means of transportation due to their light weight and superior mechanical p...Aluminum-based alloys play a key role in modern engineering and are widely used in construction components in aircraft, automobiles and other means of transportation due to their light weight and superior mechanical properties. Introduction of different nano-structure features can improve the service and the physical properties of such alloys. An improvement of an Al-based alloy has been performed based on the understanding of the relationships among compositions, processing, microstructural characteristics and properties. Knowledge of the decomposition process of the microstructure during the precipitation reaction is particularly important for future technical developments. The objective of this study is to investigate the nano-scale chemical composition in the Al-Cu, Al-Li and Al-Li-Cu alloys during the early stage of the precipitation sequence and to describe whether this compositional difference correlates with variations in the observed precipitation kinetics. Investigation of the fine scale segregation effects of dilute solutes in aluminum alloys which were experienced different heat treatments by using atom probe tomography has been achieved. The results show that an Al-1.7 at.% Cu alloy requires a long ageing time of approximately 8 h at 160°C to allow the diffusion of Cu atoms into Al matrix. For the Al-8.2 at.% Li alloy, a combination of both the natural ageing condition (48 h at room temperature) and a short artificial ageing condition (5 min at 160°C) induces increasing on the number density of the Li clusters and hence increase number of precipitated particles. Applying this combination of natural ageing and short artificial ageing conditions onto the ternary Al-4 at.% Li-1.7 at.% Cu alloy induces the formation of a Cu-rich phase. Increasing the Li content in the ternary alloy up to 8 at.% and increasing the ageing time to 30 min resulted in the precipitation processes ending with δ' particles. Thus the results contribute to the understanding of Al-alloy design.展开更多
The redistribution of C and N atoms during cryogenic treatment is crucial for the microstructure evolution and properties of high nitrogen martensitic steel.Here,the distinct redistribution behavior of C and N atoms i...The redistribution of C and N atoms during cryogenic treatment is crucial for the microstructure evolution and properties of high nitrogen martensitic steel.Here,the distinct redistribution behavior of C and N atoms in a martensitic stainless steel with 0.3 wt%C and 0.5 wt%N after cryogenic treatment were investigated by the atom probe tomography.Carbon clusters begin to form after cryogenic treatment at-60℃and gradually increase with the decrease of cryogenic treatment temperature.While Mo–N and Cr–N pairs are homogeneously distributed in the matrix even after cryogenic treatment at-120℃,and then form enrichment phenomenon when the cryogenic temperature is deeply lowered to-190℃.It is found that the distinct redistributions of C and N atoms are associated with the different interaction energy between substitutional atoms and them.The stronger interaction between Cr,Mo atoms and N delays the segregation of N during the cryogenic treatment.Finally,the mechanical properties results confirmed that the deep lower cryogenic treatment is a promising method to improve the hardness and strength in the high nitrogen martensitic stainless steel.展开更多
We investigate the effect of aging temperature on precipitation behavior and mechanical properties of an Al-7.6Zn-2.7Mg-2.0Cu-0.1Zr-0.07Ti(wt.%)alloy by evaluating the matrix’s microhardness,electrical resistivity,an...We investigate the effect of aging temperature on precipitation behavior and mechanical properties of an Al-7.6Zn-2.7Mg-2.0Cu-0.1Zr-0.07Ti(wt.%)alloy by evaluating the matrix’s microhardness,electrical resistivity,and tensile properties:additionally,employing X-ray diffraction(XRD),differential scanning calorimetry(DSC),transmission electron microscopy(TEM),and atom-probe tomography(APT)to charac-terize this alloy.The nanoprecipitates forming under peak-aging conditions vary with aging temperature,forming coherent GPI zones at 80℃,GPII zones with minorη’at 120-150℃,andη’/ηwith minor GP zones at 180-220℃.GPI and GPII zones forming at 80-150℃ contain similar concentrations of solute atoms(11Zn-9Mg-(<1.0)Cu(at.%)),whereas theη’/ηnanoprecipitates forming at 180℃ contain larger concentrations of solute atoms(28Zn-24Mg-3.4Cu(at.%)).The strength of the peak-aged alloy decreases with increasing aging temperature owing to the increasing size and decreasing number density of the nanoprecipitates.Under peak-aging conditions,precipitation strengthening originates mainly from dislo-cation shearing at 80-150℃ and from Orowan bypassing at temperatures above 180℃.The shearable to non-shearable transition of the nanoprecipitates at 180℃ reduces the strain hardening rate,thereby decreasing the alloy’s ductility.展开更多
Correlation between precipitates evolution and mechanical properties of Al-Sc-Zr alloy with Er additions during isothermal ageing were investigated by microhardness measurements,transmission electron microscopy,atom p...Correlation between precipitates evolution and mechanical properties of Al-Sc-Zr alloy with Er additions during isothermal ageing were investigated by microhardness measurements,transmission electron microscopy,atom probe tomography and density functional theory-based simulations.The results demonstrate that the Er additions significantly improve the hardness during elevated temperature ageing,especially at 400℃.This is mainly because Er additions increase the nucleation rate of the Al_(3)(Er,Sc,Zr)precipitates,resulting in a higher density of fine and uniform dispersion of L1_(2)structured nanoparticles.First-principles calculations demonstrate that the second nearest neighboring solute-solute interactions for the species Sc,Zr and Er are energetically favored–a key feature to rationalize the observed precipitate structure and the underlying formation mechanism.The sequential formation of the core/shell precipitates in the Er-free alloy and core/double-shell precipitates in the Er-containing alloy arises due to the different solute-solute and solute-vacancy interaction energies,and the relative diffusivities of the Er,Sc and Zr species in Al.These results shed light on the beneficial effects of Er additions on the agehardening behavior of Al-Sc-Zr alloy and provide guidance for designing the ageing treatments for the Al-Sc-Zr(-Er)alloys.展开更多
The microstructural evolution in Re-containing Ni-based single crystal superalloys with different Tantalum(Ta)content(2 Ta,5 Ta and 8 Ta in wt%)was investigated.Ta addition significantly affected theγ’precipitate mo...The microstructural evolution in Re-containing Ni-based single crystal superalloys with different Tantalum(Ta)content(2 Ta,5 Ta and 8 Ta in wt%)was investigated.Ta addition significantly affected theγ’precipitate morphology,γ/γ’lattice misfit and microstructural stability during long-term aging.Results showed that the partitioning behaviors of solutes were enhanced by Ta addition,meanwhile,the reversal partitioning behavior of W was triggered which partitioned fromγ’precipitate to matrix.The elemental concentration redistribution caused variations in lattice misfit from positive to negative,the values of lattice misfit were measured to be 0.16%for 2 Ta alloy,then decreased to-0.07%for 5 Ta alloy and negatively increased to-0.23%for 8 Ta alloy.These variations in the lattice misfit were reflected on the transition ofγ’morphology from round-cornered cuboidal shape to cuboidal with sharp corners,accomplished with increasing shape parameter ratioη.Consequently,the optimalγ’shape could be obtained at lattice misfit of approximately 0.3%.Theγ’coarsening investigation at 900℃(up to 2000 h)indicated that Ta addition was beneficial for improving the microstructural stability by reducing the coarsening rate and interfacial energy,accompanied by the enhanced capability of resistingγ’coalescence.By incorporating the calculated interfacial energy,computational modeling,Thermo-Calc and PrecipiCalc,were employed to elucidate theγ’kinetic pathways,the simulation results agreed with experiments,indicating that the model and parameters were reasonable.Additionally,it was found that there was no overlap betweenγ’nucleation and coarsening when theγ/γ’interfacial energy increased to a critical value.展开更多
Influence mechanism of B segregation on the microstructure evolution and hot ductility of S32654 at850-1250℃was systematically investigated through experimental research and theoretical calculation.The results demons...Influence mechanism of B segregation on the microstructure evolution and hot ductility of S32654 at850-1250℃was systematically investigated through experimental research and theoretical calculation.The results demonstrated that the segregation of B at grain boundary(GB)played different roles in the microstructure evolution and hot ductility at various temperatures.At 850℃,B segregation inhibited Mo segregation at the GB and enhanced the GB cohesion.At 900-950℃,B segregation restricted the diffusion and segregation of Mo to the GB,inhibiting the precipitation ofσphase.At 1000-1050℃,B segregation accelerated the dislocation accumulation and limited the GB migration,promoting the nucleation and inhibiting the growth of DRX grains.At 1100-1150℃,B has little effect on the DRX due to sufficient energy supply by higher temperature.Under the above beneficial effects of B,the hot ductility of S32654 was improved to varying degrees at 850-1150℃.However,as the temperature increased to1200-1250℃,B segregation decreased the solidus temperature and enhanced the liquefaction cracking tendency,resulting in a deterioration of the hot ductility.展开更多
Atom probe tomography was utilized to investigate Cu precipitation in a high-strength low-alloy steel isothermally aged at 500℃ for 1, 4, 16, and 64 h after water-quenching from 900℃. With prolonged aging time, the ...Atom probe tomography was utilized to investigate Cu precipitation in a high-strength low-alloy steel isothermally aged at 500℃ for 1, 4, 16, and 64 h after water-quenching from 900℃. With prolonged aging time, the Curich precipitates（CRPs） increased in size and decreased in number density, and gradually evolved from spheroidal to elliptical in morphology. The small CRPs were rich in a high amount of Fe and a certain amount of Ni and Mn at their early nucleation stage. The large CRPs with increased size due to extensive aging contained less Fe and more Cu at their later growth stage. Additionally, Ni and Mn were both readily to segregate at the CRP/matrix heterophase interfaces, and Mn was higher in content than Ni in the precipitate interior especially when the CRPs were large in size.展开更多
In this paper,we were devoted to quantitatively analyzing the dimensional change of 2024 Al alloy during the aging process.Types of precipitates have been defined and the corresponding volume fraction has been measure...In this paper,we were devoted to quantitatively analyzing the dimensional change of 2024 Al alloy during the aging process.Types of precipitates have been defined and the corresponding volume fraction has been measured with Transmission Electron Microscopy(TEM)and Three-Dimensional Atom Probe Tomography(3 D-APT).Guinier-Preston-Bagaryatsky(GPB)zone(0.86 vol.%)was found after aging for 8 h.GPB zone(0.17 vol.%)and S'phase(1.02 vol.%)was found after aging for 24 h.The reduced dimensional change was attributed to the reduction of matrix lattice parameter.Relative to the initial supersaturated solid solution state,the lattice strain of-2.79×10^(-5)and-5.16×10^(-5)was produced after 8 h and 24 h aging respectively.The difference of the lattice parameters between the precipitates and matrix was also considered.A model was established to quantitatively describe the relationship between the precipitation process and dimensional change of 2024 Al alloy.展开更多
Radiation-induced segregation(RIS) is a typical non-equilibrium process that can dramatically alter the behavior of defect sinks and material properties under irradiation. However, RIS mechanisms have been rarely stud...Radiation-induced segregation(RIS) is a typical non-equilibrium process that can dramatically alter the behavior of defect sinks and material properties under irradiation. However, RIS mechanisms have been rarely studied around small He bubbles owing to the technical challenges involved in direct measurements of local chemistry. Here, using state-of-the-art atom probe tomography, we report the RIS behavior near He bubbles in the Fe Ni Co Cr high-entropy alloy that indicates Co segregates most strongly, followed by weaker Ni segregation, whereas Fe and Cr are depleted almost to the same degree. Exceptionally, the magnitude of Co segregation around He bubbles is higher than previously measured values at voids and dislocation loops. Electron energy-loss spectroscopy was used to measure the He density and pressure inside individual bubbles. We demonstrate that He bubbles are over-pressurized at the irradiation temperature that could result in the vacancy bias and the subsequent vacancy-dominated RIS mechanism.First-principles calculations further reveal that there are repulsive interactions between He and Co atoms that may reduce the frequency of Co-vacancy exchange. As a result, He atoms likely retard Co diffusion via the vacancy mechanism and enhance the heterogeneity of RIS in Co-containing multicomponent alloys. These insights could provide the basis for understanding He effects in nuclear materials and open an avenue for tailoring the local chemical order of medium-and high-entropy alloys.展开更多
Corrosion resistance behavior of Co-containing maraging stainless steels was investigated. Neutral salt spray and polarization test showed that maraging stainless steel with high Co content showed poor corrosion behav...Corrosion resistance behavior of Co-containing maraging stainless steels was investigated. Neutral salt spray and polarization test showed that maraging stainless steel with high Co content showed poor corrosion behavior. Microstructure observation proved that segregation of Cr in the matrix deteriorated its corrosion resistance. The surface morphology of the aged maraging stainless steel with high Co content indicated that during passivation process, the newly formed passive film with sinusoidal distribution readily destroyed by the corrosive medium, hence, causing poor corrosion resistance. Moreover, through first-principles calculation it was proved that Co increased Fe–Fe ferromagnetic interaction which facilitated the formation Cr-rich clusters.展开更多
Precipitation-hardened high entropy alloys(HEAs)with carefully tuned compositions have shown excellent mechanical properties,demonstrating great potential for engineering applications.However,due to the lack of precis...Precipitation-hardened high entropy alloys(HEAs)with carefully tuned compositions have shown excellent mechanical properties,demonstrating great potential for engineering applications.However,due to the lack of precise multiple phase diagrams,the composition design of multi-principal-component HEAs still inevitably relies on the extremely time-consuming trial-and-error approach.The present study,on the basis of powerful composition quantification ability of atom probe tomography(APT)technology,proposed a framework to guide the quantitative design of precipitation-hardened HEAs.In this framework,the elemental partitioning was used as a crucial route to avoid the thermodynamic challenge of designing precipitation-hardened HEAs.As a case study,the role of Ti/Al ratio in the design ofγ-γ’HEAs was predicted through the proposed framework and then validated by experimental studies.The framework predicted that when the total content of Ti and Al is fixed,a higher Ti/Al ratio makesγ-γ’HEA stronger.APT and mechanical results agreed well with these predictions and validated the feasibility of the framework.These findings provided a new route to design the precipitation-hardened alloys and a deeper insight into the design ofγ-γ’HEA.展开更多
The segregation behavior of trace amount of phosphorus(P)and the mechanical properties of dual-phase(DP)steels have been systematically studied.The microstructure of DP steels is mainly composed of martensite,ferrite ...The segregation behavior of trace amount of phosphorus(P)and the mechanical properties of dual-phase(DP)steels have been systematically studied.The microstructure of DP steels is mainly composed of martensite,ferrite and nanoscale carbides.For the DP steels with diff erent trace amounts of P(≤0.015 wt%),P has almost no effect on the mechanical properties.Atom probe technology(APT)analyses confirm that P segregation was only found at the precipitate/matrix interface.Moreover,the precipitates of(Ti,Mo)C are widely distributed in the ferrite,martensite and ferrite/martensite interface regions.The special segregation feature of P would not concentrate at specific regions such as ferrite/martensite interface and/or martensite lath interface,which reveals that trace amounts of P(≤0.015 wt%)have almost no effect on the mechanical properties of DP steels.It is proved for the first time that the MC-type carbides of(Ti,Mo)C can reduce or eliminate the damage effect of P on the mechanical properties of steels,which provides a new way for the design of alloys to reduce P damage.This work will promote to increase the P content control standard in DP steels from 0.01 to 0.015 wt%,which will not change the mechanical properties,but greatly reduce the scrap rate and increase the energy e fficiency of the manufacturing process.展开更多
Al 5083 alloys(5.25 at.% Mg) of different tempers(H131 and H116) were aged at low temperatures(50and 70?C) for 41 months. Scanning transmission electron microscopy(STEM), energy-dispersive X-ray spectroscopy(EDS), and...Al 5083 alloys(5.25 at.% Mg) of different tempers(H131 and H116) were aged at low temperatures(50and 70?C) for 41 months. Scanning transmission electron microscopy(STEM), energy-dispersive X-ray spectroscopy(EDS), and atom probe tomography(APT) were applied to characterize precipitates formed in the sensitized samples. Experimental results revealed that the size of Mg-rich precipitates increased with aging time at 70?C for both alloys. APT results showed that Mg-rich precipitates of different Mg concentrations and morphologies formed in Al matrix and on the interface of Al matrix/pre-existing particles. In addition, a model based on local equilibrium of chemical potential and multi-class precipitates number evolution was adopted to predict the multiphase precipitation process in the Al-Mg binary system. The overall trend of precipitate radius and number density predicted by the model matched well with the experimental results. Moreover, modeling results revealed that nucleation and coarsening occurred faster in Al 5083 H131 than in Al 5083 H116 when aged at same temperature. The high density of dislocations and the pipe diffusion mechanism of dislocations can be used to explain such behavior.展开更多
Transmission electron microscopy(TEM)and atom probe tomography(APT)techniques were used to investigate the nanoscale orderedα_(2)(Ti_(3)Al)precipitates in Ti–Al binary alloys.Ti–6Al and Ti–8Al binary alloys were s...Transmission electron microscopy(TEM)and atom probe tomography(APT)techniques were used to investigate the nanoscale orderedα_(2)(Ti_(3)Al)precipitates in Ti–Al binary alloys.Ti–6Al and Ti–8Al binary alloys were solution treated and aged to obtain Widmanstatten microstructure and promoteα_(2)precipitates.The TEM results displayed strong short-range ordering ofα_(2)precipitates in Ti–8Al alloy,while no evidence of the superlattice reflections ofα_(2)in Ti–6Al alloy.The results acquired from APT showed theα_(2)clusters and atoms distribution at the interface between the matrix andα_(2)precipitates.The size and morphology ofα_(2)particles in Ti–8Al alloy,respectively,obtained by TEM and APT are closely consistent.Meanwhile,the APT results displayed tiny size clusters in Ti–6Al alloy,which supposed to give evidence of the initial ordering process ofα_(2)precipitates in the absence of correlative results from TEM.展开更多
The influence of minor Ag on the precipitation evolution of the Al-4.2Zn-2.8Mg-1.0Cu(wt.%)alloy from early stages to over-aged stages at 150°C was investigated.Surprisingly,co-precipitation of strengthening phase...The influence of minor Ag on the precipitation evolution of the Al-4.2Zn-2.8Mg-1.0Cu(wt.%)alloy from early stages to over-aged stages at 150°C was investigated.Surprisingly,co-precipitation of strengthening phases T′andη′are found in both Ag-free and Ag-added alloys.With Ag addition,precipitation of both T′andη′is refined and increased,such that the age-hardening capabilities and peak-aged tensile strength are improved.In addition,the quantitative proportion ofη′precipitates increases with the increase of Ag content due to the increase in the(Zn+Cu)/Mg ratio of nucleating particles.The narrowed precipitate-free zones(PFZs)are considered responsible for the undiminished fracture elongation in Ag-added alloys.Essentially,these effects of Ag are closely related to the strong Ag-vacancy and Ag-solute interactions.In over-aged stages,the Ag-added alloys still possess higher hardness values compared to the Ag-free alloy,which is related to precipitate coarsening mechanisms.The Ag-free alloy follows classical coarsening behavior by solid solution mediated diffusion,while the Ag-added alloy follows two possible coarsening mechanisms,coalescence of aggregates and diffusion of atoms.The smaller average size and higher residual number density of precipitates benefited from the slow diffusion-controlled coarsening behavior depending on the precipitate composition characteristics of the two-stage differentiation and the precipitate distribution characteristics of high-density dispersion in early-aged stages could explain why the hardness of Ag-added alloy keeps at a higher level than that of Ag-free alloy even after 1000 h ageing.Meanwhile,the transformation of metastable phases to stable phases is inhibited due to the addition of Ag,such that GP zones,T′,η′,ηand T phases coexist even after 14 d of ageing.In terms of phase composition,the addition of Ag decreases the ratio of Mg/(Al+Zn)in T-type phase.For the Ag-added alloy,the sum concentration of Zn+Mg inη′phase is about 10 at.%higher compared to T′phase,andηphase continues to have a high sum concentration of Zn+Mg,besides,the Zn/Mg ratio and Cu concentration exhibit obvious differences from T phase.展开更多
Tailoring nanostructures is a general approach used to obtain enhanced thermoelectric properties for halfHeusler compounds because the wide areas of grain and phase boundaries could be scattering centers that lower la...Tailoring nanostructures is a general approach used to obtain enhanced thermoelectric properties for halfHeusler compounds because the wide areas of grain and phase boundaries could be scattering centers that lower lattice thermal conductivity.However,a common fabrication method based on the sintering of crystalline precursors crushed from as-cast alloy ingots has limitations in obtaining a homogeneous microstructure without microsized impurity phases,owing to residual elemental segregation from casting.In this study,we used amorphous NbCoSn alloys as a precursor for the sintered specimen to obtain a homogeneous NbCoSn bulk specimen without microsized impurity phases and segregation,which led to the enhanced Seebeck coefficient due to the high purity of the half-Heusler phase after crystallization.Moreover,superplasticity originating from amorphous features enabled the powders to be largely deformed during the sintering process,even at a low sintering temperature(953 K).This resulted in less oxidation at both,the grain boundary and the interior,as the O diffusion pathway was blocked during the sintering process.As a result,the NbCoSn0.95Sb0.05 specimen using an amorphous precursor exhibited an enhanced zT of 0.7,due to the increase in the power factor and a decrease in lattice thermal conductivity compared to the specimen using a crystalline precursor.展开更多
The nano-bainitic microstructures were compared in a 0.79C-1.5Si-1.98Mn-0.24Mo-1.06Al (wt%) steel after isothermal heat-treatment and a Fe-0.2C-1.5Mn-1.2Si-0.3Mo-0.6Al-0.02Nb (wt%) steel after controlled thermomechani...The nano-bainitic microstructures were compared in a 0.79C-1.5Si-1.98Mn-0.24Mo-1.06Al (wt%) steel after isothermal heat-treatment and a Fe-0.2C-1.5Mn-1.2Si-0.3Mo-0.6Al-0.02Nb (wt%) steel after controlled thermomechanical processing.The microstructure for both steels consisted of bainite.The microstructural characteristics of bainite,such as the morphology of the nano-bainite and thicknesses of bainitic ferrite and retained austenite layers,as a function of steel composition and processing was studied using transmission electron microscopy (TEM).It was found that the nano-bainitic structure can be formed in the low alloy steel through thermomechanical processing.Atom probe tomography (APT) was employed as a powerful technique to determine local composition distributions in three dimensions with atomic resolution.The important conclusions from the APT research were that the carbon content of bainitic ferrite is higher than expected from paraequilibrium level of carbon in ferrite for both steels and that Fe-C clusters and fine particles are formed in the bainitic ferrite in both steels despite the high level of Si.展开更多
The effect of pre-deformation on nanoscale precipitates and hardness of a maraging stainless steel strengthened by the coprecipitation of Ni_(3)Ti,Mo-enriched and Cr-enriched precipitates was systematically studied us...The effect of pre-deformation on nanoscale precipitates and hardness of a maraging stainless steel strengthened by the coprecipitation of Ni_(3)Ti,Mo-enriched and Cr-enriched precipitates was systematically studied using electron back scattered diffraction,transmission electron microscopy and atom probe tomography(APT).Hardness measurements showed that the hardness of specimen with a deformation ratio of 90%peaked at HV 718 aged for 24 h,which is higher than that in the undeformed specimen(HV 603)aged for 72 h at 480℃.APT characterization revealed that pre-deformation could shorten the incubation time of the Mo-enriched and Cr-enriched precipitates.At the early-aged stage,pre-deformation increased the stain energy that inhibited the nucleation of Ni_(3)Ti precipitates,but accelerated the rejection of Mo from Ni-Ti clusters.Besides,the strengthening model indicated that strain hardening(43%)makes a larger contribution to the hardness at the early-aged condition,while precipitation hardening(58%)has most contribution to the hardness at the peak-aged conditition.展开更多
Herein,the evolution of long-period stacking ordered(LPSO)phases in the as-cast Mg-6Gd-1Zn-0.6Zr(wt.%)alloy are investigated via transmission electron microscopy(TEM)and atom probe tomography(APT).The TEM results reve...Herein,the evolution of long-period stacking ordered(LPSO)phases in the as-cast Mg-6Gd-1Zn-0.6Zr(wt.%)alloy are investigated via transmission electron microscopy(TEM)and atom probe tomography(APT).The TEM results reveal that two types of LPSO phase(a bulky interdendritic phase and a plate-like matrix LPSO phase)are formed in the as-cast sample.Most of the LPSO phases are confirmed to be of the 14H type,with a smaller proportion being of the 18R LPSO.Further,the APT results reveal that the composition of the interdendritic LPSO phase is closer to that of the ideal 14H phase compared to the matrix LPSO phase,and both the interdendritic and matrix LPSO phases exhibit a Gd/Zn ratio of 2.5,thereby indicating a deficient Zn content compared to the ideal 14H phase(i.e.,1.3).In addition,the influence of the LPSO phases on the deformation behavior is investigated at different compressive plastic strains using electron backscatter diffraction(EBSD)analysis to reveal twinning and slip behavior during deformation.The results indicate that the LPSO phase induces additional work hardening in the late stage of deformation via the suppression of{1011}compressive twinning and the activation of non-basal slip systems.展开更多
基金the financial support provided by ANR(Project ANR-22-PEHY-0007)DGA(French Direction Générale des Armées,2018600045)Région Nouvelle Aquitaine(agreement 2018–1R10126).
文摘The long-period-stacking-ordered(LPSO)structure affects the mechanical,corrosion and hydrolysis properties of Mg alloys.The current work employs high angle annular dark field-scanning transmission electron microscopy(HAADF-STEM)and atom probe tomography(APT)to investigate the structural and local chemical information of LPSO phases formed in Mg-Ni-Y/Sm ternary alloys after extended isothermal annealing.Depending on the alloying elements and their concentrations,Mg-Ni-Y/Sm develops a two-phase LPSO+α-Mg structure in which the LPSO phase contains defects,hybrid LPSO structure,and Mg insertions.HAADF-STEM and APT indicate non-stoichiometric LPSO with incomplete Ni_(6)(Y/Sm)_(8) clusters.In addition,the APT quantitatively determines the local composition of LPSO and confirms the presence of Ni within the Mg bonding layers.These results provide insight into a better understanding of the structure and hydrolysis properties of LPSO-Mg alloys.
文摘Aluminum-based alloys play a key role in modern engineering and are widely used in construction components in aircraft, automobiles and other means of transportation due to their light weight and superior mechanical properties. Introduction of different nano-structure features can improve the service and the physical properties of such alloys. An improvement of an Al-based alloy has been performed based on the understanding of the relationships among compositions, processing, microstructural characteristics and properties. Knowledge of the decomposition process of the microstructure during the precipitation reaction is particularly important for future technical developments. The objective of this study is to investigate the nano-scale chemical composition in the Al-Cu, Al-Li and Al-Li-Cu alloys during the early stage of the precipitation sequence and to describe whether this compositional difference correlates with variations in the observed precipitation kinetics. Investigation of the fine scale segregation effects of dilute solutes in aluminum alloys which were experienced different heat treatments by using atom probe tomography has been achieved. The results show that an Al-1.7 at.% Cu alloy requires a long ageing time of approximately 8 h at 160°C to allow the diffusion of Cu atoms into Al matrix. For the Al-8.2 at.% Li alloy, a combination of both the natural ageing condition (48 h at room temperature) and a short artificial ageing condition (5 min at 160°C) induces increasing on the number density of the Li clusters and hence increase number of precipitated particles. Applying this combination of natural ageing and short artificial ageing conditions onto the ternary Al-4 at.% Li-1.7 at.% Cu alloy induces the formation of a Cu-rich phase. Increasing the Li content in the ternary alloy up to 8 at.% and increasing the ageing time to 30 min resulted in the precipitation processes ending with δ' particles. Thus the results contribute to the understanding of Al-alloy design.
基金supported by the National Natural Science Foundation of China(No.51871212)the Strategic Priority Research Program of the Chinese Academy of Sciences(No.XDC04000000)+1 种基金the Major Scientific and Technological Projects of Jiangxi Province(No.20194ABC28011)the Project to Strengthen Industrial Development at the Grass-roots Level(TC190A4DA/35)。
文摘The redistribution of C and N atoms during cryogenic treatment is crucial for the microstructure evolution and properties of high nitrogen martensitic steel.Here,the distinct redistribution behavior of C and N atoms in a martensitic stainless steel with 0.3 wt%C and 0.5 wt%N after cryogenic treatment were investigated by the atom probe tomography.Carbon clusters begin to form after cryogenic treatment at-60℃and gradually increase with the decrease of cryogenic treatment temperature.While Mo–N and Cr–N pairs are homogeneously distributed in the matrix even after cryogenic treatment at-120℃,and then form enrichment phenomenon when the cryogenic temperature is deeply lowered to-190℃.It is found that the distinct redistributions of C and N atoms are associated with the different interaction energy between substitutional atoms and them.The stronger interaction between Cr,Mo atoms and N delays the segregation of N during the cryogenic treatment.Finally,the mechanical properties results confirmed that the deep lower cryogenic treatment is a promising method to improve the hardness and strength in the high nitrogen martensitic stainless steel.
基金financially supported by the Industrial Strategic Technology Development Program(No.10062304)funded by the Ministry of Trade,Industry,and Energy(MOTIE,Republic of Korea)the Main Research Program(Nos.PNK8370&PNK8650)funded by the Korea Institute of Materials Science(KIMS,Republic of Korea)+6 种基金a National Research Foundation of Korea(NRF)grant funded by the Korean government(MSIT)(No.RS-2023-00217415)performed at the Northwestern University Center for Atom-Probe Tomography(NUCAPT)The LEAP tomo-graph at NUCAPT was purchased and upgraded with grants from the NSF-MRI(No.DMR-0420532)ONRDURIP(Nos.N00014-0400798,N00014-0610539,N00014-0910781,N00014-1712870)programssupport from the MRSEC program(No.NSF DMR-1720139)the Materials Research Center,SHyNE Re-source(No.NSF ECCS-1542205)the Initiative for Sustainability and Energy(ISEN)at Northwestern University.We thank the Cen-ter for University-wide Research Facilities(CURF)at Jeonbuk Na-tional University and Institute of Next-generation Semiconductor convergence Technology(INST)at DGIST for their assistance with the experiments.We also thank Prof.Dieter Isheim and Dr.Amir Farkoosh for their helpful discussions concerning APT analyses.
文摘We investigate the effect of aging temperature on precipitation behavior and mechanical properties of an Al-7.6Zn-2.7Mg-2.0Cu-0.1Zr-0.07Ti(wt.%)alloy by evaluating the matrix’s microhardness,electrical resistivity,and tensile properties:additionally,employing X-ray diffraction(XRD),differential scanning calorimetry(DSC),transmission electron microscopy(TEM),and atom-probe tomography(APT)to charac-terize this alloy.The nanoprecipitates forming under peak-aging conditions vary with aging temperature,forming coherent GPI zones at 80℃,GPII zones with minorη’at 120-150℃,andη’/ηwith minor GP zones at 180-220℃.GPI and GPII zones forming at 80-150℃ contain similar concentrations of solute atoms(11Zn-9Mg-(<1.0)Cu(at.%)),whereas theη’/ηnanoprecipitates forming at 180℃ contain larger concentrations of solute atoms(28Zn-24Mg-3.4Cu(at.%)).The strength of the peak-aged alloy decreases with increasing aging temperature owing to the increasing size and decreasing number density of the nanoprecipitates.Under peak-aging conditions,precipitation strengthening originates mainly from dislo-cation shearing at 80-150℃ and from Orowan bypassing at temperatures above 180℃.The shearable to non-shearable transition of the nanoprecipitates at 180℃ reduces the strain hardening rate,thereby decreasing the alloy’s ductility.
基金supported by the National Natural Science Foundation of China(Grant Nos.U1737206 and 51971077)the AUSMURI program(Grant No.AUSMURI000005)funded under the auspices of the Commonwealth Department of Industry,Innovation and Sciencesupported by the Science and Technology Plan Project of Guangzhou City(Grant No.201807010063)
文摘Correlation between precipitates evolution and mechanical properties of Al-Sc-Zr alloy with Er additions during isothermal ageing were investigated by microhardness measurements,transmission electron microscopy,atom probe tomography and density functional theory-based simulations.The results demonstrate that the Er additions significantly improve the hardness during elevated temperature ageing,especially at 400℃.This is mainly because Er additions increase the nucleation rate of the Al_(3)(Er,Sc,Zr)precipitates,resulting in a higher density of fine and uniform dispersion of L1_(2)structured nanoparticles.First-principles calculations demonstrate that the second nearest neighboring solute-solute interactions for the species Sc,Zr and Er are energetically favored–a key feature to rationalize the observed precipitate structure and the underlying formation mechanism.The sequential formation of the core/shell precipitates in the Er-free alloy and core/double-shell precipitates in the Er-containing alloy arises due to the different solute-solute and solute-vacancy interaction energies,and the relative diffusivities of the Er,Sc and Zr species in Al.These results shed light on the beneficial effects of Er additions on the agehardening behavior of Al-Sc-Zr alloy and provide guidance for designing the ageing treatments for the Al-Sc-Zr(-Er)alloys.
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)Natural Science Foundation of Shaanxi Province2020JM-122。
文摘The microstructural evolution in Re-containing Ni-based single crystal superalloys with different Tantalum(Ta)content(2 Ta,5 Ta and 8 Ta in wt%)was investigated.Ta addition significantly affected theγ’precipitate morphology,γ/γ’lattice misfit and microstructural stability during long-term aging.Results showed that the partitioning behaviors of solutes were enhanced by Ta addition,meanwhile,the reversal partitioning behavior of W was triggered which partitioned fromγ’precipitate to matrix.The elemental concentration redistribution caused variations in lattice misfit from positive to negative,the values of lattice misfit were measured to be 0.16%for 2 Ta alloy,then decreased to-0.07%for 5 Ta alloy and negatively increased to-0.23%for 8 Ta alloy.These variations in the lattice misfit were reflected on the transition ofγ’morphology from round-cornered cuboidal shape to cuboidal with sharp corners,accomplished with increasing shape parameter ratioη.Consequently,the optimalγ’shape could be obtained at lattice misfit of approximately 0.3%.Theγ’coarsening investigation at 900℃(up to 2000 h)indicated that Ta addition was beneficial for improving the microstructural stability by reducing the coarsening rate and interfacial energy,accompanied by the enhanced capability of resistingγ’coalescence.By incorporating the calculated interfacial energy,computational modeling,Thermo-Calc and PrecipiCalc,were employed to elucidate theγ’kinetic pathways,the simulation results agreed with experiments,indicating that the model and parameters were reasonable.Additionally,it was found that there was no overlap betweenγ’nucleation and coarsening when theγ/γ’interfacial energy increased to a critical value.
基金financially supported by the National Natural Science Foundation of China(Grant Nos.U1860204,52004061)the Talent Project of Revitalizing Liaoning(Grant No.XLYC1802101)+4 种基金by China Postdoctoral Science Foundation(Grant Nos.2020M670777,2021T140100)the Northeastern University Postdoctoral Funds(Grant No.20200321)the Fundamental Research Funds for the Central Universities(Grant No.N2125017)the Elite Program of Southern Taihu Lakethe instrumental and data analysis from Analytical and Testing Center,Northeastern University。
文摘Influence mechanism of B segregation on the microstructure evolution and hot ductility of S32654 at850-1250℃was systematically investigated through experimental research and theoretical calculation.The results demonstrated that the segregation of B at grain boundary(GB)played different roles in the microstructure evolution and hot ductility at various temperatures.At 850℃,B segregation inhibited Mo segregation at the GB and enhanced the GB cohesion.At 900-950℃,B segregation restricted the diffusion and segregation of Mo to the GB,inhibiting the precipitation ofσphase.At 1000-1050℃,B segregation accelerated the dislocation accumulation and limited the GB migration,promoting the nucleation and inhibiting the growth of DRX grains.At 1100-1150℃,B has little effect on the DRX due to sufficient energy supply by higher temperature.Under the above beneficial effects of B,the hot ductility of S32654 was improved to varying degrees at 850-1150℃.However,as the temperature increased to1200-1250℃,B segregation decreased the solidus temperature and enhanced the liquefaction cracking tendency,resulting in a deterioration of the hot ductility.
文摘Atom probe tomography was utilized to investigate Cu precipitation in a high-strength low-alloy steel isothermally aged at 500℃ for 1, 4, 16, and 64 h after water-quenching from 900℃. With prolonged aging time, the Curich precipitates（CRPs） increased in size and decreased in number density, and gradually evolved from spheroidal to elliptical in morphology. The small CRPs were rich in a high amount of Fe and a certain amount of Ni and Mn at their early nucleation stage. The large CRPs with increased size due to extensive aging contained less Fe and more Cu at their later growth stage. Additionally, Ni and Mn were both readily to segregate at the CRP/matrix heterophase interfaces, and Mn was higher in content than Ni in the precipitate interior especially when the CRPs were large in size.
基金sponsored by the National Natural Science Foundation of China with grant No.U1637201。
文摘In this paper,we were devoted to quantitatively analyzing the dimensional change of 2024 Al alloy during the aging process.Types of precipitates have been defined and the corresponding volume fraction has been measured with Transmission Electron Microscopy(TEM)and Three-Dimensional Atom Probe Tomography(3 D-APT).Guinier-Preston-Bagaryatsky(GPB)zone(0.86 vol.%)was found after aging for 8 h.GPB zone(0.17 vol.%)and S'phase(1.02 vol.%)was found after aging for 24 h.The reduced dimensional change was attributed to the reduction of matrix lattice parameter.Relative to the initial supersaturated solid solution state,the lattice strain of-2.79×10^(-5)and-5.16×10^(-5)was produced after 8 h and 24 h aging respectively.The difference of the lattice parameters between the precipitates and matrix was also considered.A model was established to quantitatively describe the relationship between the precipitation process and dimensional change of 2024 Al alloy.
基金the funding support from the Hong Kong Research Grant Council (Grant Nos. City U11214820 and City U11205018)the funding support from the National Natural Science Foundation of China (NSFC) under project 11922215+2 种基金the funding support from the Natural Science Foundation of Guangdong Province in China (Grant No. 2019A1515011836)the support from National Natural Science Foundation of China (Grant No. 11974156)Guangdong International Science Collaboration Project (Grant No. 2019A050510001)。
文摘Radiation-induced segregation(RIS) is a typical non-equilibrium process that can dramatically alter the behavior of defect sinks and material properties under irradiation. However, RIS mechanisms have been rarely studied around small He bubbles owing to the technical challenges involved in direct measurements of local chemistry. Here, using state-of-the-art atom probe tomography, we report the RIS behavior near He bubbles in the Fe Ni Co Cr high-entropy alloy that indicates Co segregates most strongly, followed by weaker Ni segregation, whereas Fe and Cr are depleted almost to the same degree. Exceptionally, the magnitude of Co segregation around He bubbles is higher than previously measured values at voids and dislocation loops. Electron energy-loss spectroscopy was used to measure the He density and pressure inside individual bubbles. We demonstrate that He bubbles are over-pressurized at the irradiation temperature that could result in the vacancy bias and the subsequent vacancy-dominated RIS mechanism.First-principles calculations further reveal that there are repulsive interactions between He and Co atoms that may reduce the frequency of Co-vacancy exchange. As a result, He atoms likely retard Co diffusion via the vacancy mechanism and enhance the heterogeneity of RIS in Co-containing multicomponent alloys. These insights could provide the basis for understanding He effects in nuclear materials and open an avenue for tailoring the local chemical order of medium-and high-entropy alloys.
基金sponsored by the Youth Innovation Promotion Association of Chinese Academy of Sciences (No. 2017233)the Innovation Project of Institute of Metal Research (No. 2015-ZD04)
文摘Corrosion resistance behavior of Co-containing maraging stainless steels was investigated. Neutral salt spray and polarization test showed that maraging stainless steel with high Co content showed poor corrosion behavior. Microstructure observation proved that segregation of Cr in the matrix deteriorated its corrosion resistance. The surface morphology of the aged maraging stainless steel with high Co content indicated that during passivation process, the newly formed passive film with sinusoidal distribution readily destroyed by the corrosive medium, hence, causing poor corrosion resistance. Moreover, through first-principles calculation it was proved that Co increased Fe–Fe ferromagnetic interaction which facilitated the formation Cr-rich clusters.
基金financially supported by the Hong Kong Research Grant Council(Nos.CityU 11212915 and CityU 11205018)the National Natural Science foundation of China(Nos.51771149,52001266+1 种基金51901119)Natural Science Foundation of ShaanXi Province in China(No.2020JQ-720)。
文摘Precipitation-hardened high entropy alloys(HEAs)with carefully tuned compositions have shown excellent mechanical properties,demonstrating great potential for engineering applications.However,due to the lack of precise multiple phase diagrams,the composition design of multi-principal-component HEAs still inevitably relies on the extremely time-consuming trial-and-error approach.The present study,on the basis of powerful composition quantification ability of atom probe tomography(APT)technology,proposed a framework to guide the quantitative design of precipitation-hardened HEAs.In this framework,the elemental partitioning was used as a crucial route to avoid the thermodynamic challenge of designing precipitation-hardened HEAs.As a case study,the role of Ti/Al ratio in the design ofγ-γ’HEAs was predicted through the proposed framework and then validated by experimental studies.The framework predicted that when the total content of Ti and Al is fixed,a higher Ti/Al ratio makesγ-γ’HEA stronger.APT and mechanical results agreed well with these predictions and validated the feasibility of the framework.These findings provided a new route to design the precipitation-hardened alloys and a deeper insight into the design ofγ-γ’HEA.
基金financially supported by the National Natural Science Foundation of China(Nos.51871012,52071021)the Fundamental Research Funds for the Central Universities(No.FRF-GF-20-20B)。
文摘The segregation behavior of trace amount of phosphorus(P)and the mechanical properties of dual-phase(DP)steels have been systematically studied.The microstructure of DP steels is mainly composed of martensite,ferrite and nanoscale carbides.For the DP steels with diff erent trace amounts of P(≤0.015 wt%),P has almost no effect on the mechanical properties.Atom probe technology(APT)analyses confirm that P segregation was only found at the precipitate/matrix interface.Moreover,the precipitates of(Ti,Mo)C are widely distributed in the ferrite,martensite and ferrite/martensite interface regions.The special segregation feature of P would not concentrate at specific regions such as ferrite/martensite interface and/or martensite lath interface,which reveals that trace amounts of P(≤0.015 wt%)have almost no effect on the mechanical properties of DP steels.It is proved for the first time that the MC-type carbides of(Ti,Mo)C can reduce or eliminate the damage effect of P on the mechanical properties of steels,which provides a new way for the design of alloys to reduce P damage.This work will promote to increase the P content control standard in DP steels from 0.01 to 0.015 wt%,which will not change the mechanical properties,but greatly reduce the scrap rate and increase the energy e fficiency of the manufacturing process.
基金supported by Office of Naval Research, Award Number: N000141210507the Oak Ridge National Laboratory’s High Flux Isotope Reactor, which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of EnergyUniversity of Utah USTAR shared facilities support, in part, by the MRSEC Program of NSF under Award No. DMR-1121252
文摘Al 5083 alloys(5.25 at.% Mg) of different tempers(H131 and H116) were aged at low temperatures(50and 70?C) for 41 months. Scanning transmission electron microscopy(STEM), energy-dispersive X-ray spectroscopy(EDS), and atom probe tomography(APT) were applied to characterize precipitates formed in the sensitized samples. Experimental results revealed that the size of Mg-rich precipitates increased with aging time at 70?C for both alloys. APT results showed that Mg-rich precipitates of different Mg concentrations and morphologies formed in Al matrix and on the interface of Al matrix/pre-existing particles. In addition, a model based on local equilibrium of chemical potential and multi-class precipitates number evolution was adopted to predict the multiphase precipitation process in the Al-Mg binary system. The overall trend of precipitate radius and number density predicted by the model matched well with the experimental results. Moreover, modeling results revealed that nucleation and coarsening occurred faster in Al 5083 H131 than in Al 5083 H116 when aged at same temperature. The high density of dislocations and the pipe diffusion mechanism of dislocations can be used to explain such behavior.
基金financially supported by the Strategic Priority Research Program of the Chinese Academy of Sciences(No.XDA22010101)the Natural Key Research and Development Program of China(Nos.2016YFC0304201 and 2016YFC0304206)+1 种基金the Natural Science Foundation of China(No.51871225)the Youth Innovation Promotion Association CAS and Liaoning Revitalization Talents Program(No.XLYC1907005)。
文摘Transmission electron microscopy(TEM)and atom probe tomography(APT)techniques were used to investigate the nanoscale orderedα_(2)(Ti_(3)Al)precipitates in Ti–Al binary alloys.Ti–6Al and Ti–8Al binary alloys were solution treated and aged to obtain Widmanstatten microstructure and promoteα_(2)precipitates.The TEM results displayed strong short-range ordering ofα_(2)precipitates in Ti–8Al alloy,while no evidence of the superlattice reflections ofα_(2)in Ti–6Al alloy.The results acquired from APT showed theα_(2)clusters and atoms distribution at the interface between the matrix andα_(2)precipitates.The size and morphology ofα_(2)particles in Ti–8Al alloy,respectively,obtained by TEM and APT are closely consistent.Meanwhile,the APT results displayed tiny size clusters in Ti–6Al alloy,which supposed to give evidence of the initial ordering process ofα_(2)precipitates in the absence of correlative results from TEM.
基金supported by the Chongqing Key Project for Technological Innovation and Application(No.CSTB2022TIAD-KPX0073)the Natural Science Foundation of Chongqing(No.CSTB2022NSCQ-LZX0002)+1 种基金the National Natural Science Foundation of China(No.51871033)the Opening Project of State Key Laboratory for Advanced Metals and Materials(Nos.2022-Z03 and 2020-ZD02).
文摘The influence of minor Ag on the precipitation evolution of the Al-4.2Zn-2.8Mg-1.0Cu(wt.%)alloy from early stages to over-aged stages at 150°C was investigated.Surprisingly,co-precipitation of strengthening phases T′andη′are found in both Ag-free and Ag-added alloys.With Ag addition,precipitation of both T′andη′is refined and increased,such that the age-hardening capabilities and peak-aged tensile strength are improved.In addition,the quantitative proportion ofη′precipitates increases with the increase of Ag content due to the increase in the(Zn+Cu)/Mg ratio of nucleating particles.The narrowed precipitate-free zones(PFZs)are considered responsible for the undiminished fracture elongation in Ag-added alloys.Essentially,these effects of Ag are closely related to the strong Ag-vacancy and Ag-solute interactions.In over-aged stages,the Ag-added alloys still possess higher hardness values compared to the Ag-free alloy,which is related to precipitate coarsening mechanisms.The Ag-free alloy follows classical coarsening behavior by solid solution mediated diffusion,while the Ag-added alloy follows two possible coarsening mechanisms,coalescence of aggregates and diffusion of atoms.The smaller average size and higher residual number density of precipitates benefited from the slow diffusion-controlled coarsening behavior depending on the precipitate composition characteristics of the two-stage differentiation and the precipitate distribution characteristics of high-density dispersion in early-aged stages could explain why the hardness of Ag-added alloy keeps at a higher level than that of Ag-free alloy even after 1000 h ageing.Meanwhile,the transformation of metastable phases to stable phases is inhibited due to the addition of Ag,such that GP zones,T′,η′,ηand T phases coexist even after 14 d of ageing.In terms of phase composition,the addition of Ag decreases the ratio of Mg/(Al+Zn)in T-type phase.For the Ag-added alloy,the sum concentration of Zn+Mg inη′phase is about 10 at.%higher compared to T′phase,andηphase continues to have a high sum concentration of Zn+Mg,besides,the Zn/Mg ratio and Cu concentration exhibit obvious differences from T phase.
基金the Basic Science Research Program of the National Research Foundation of Korea(NRF)(Nos.2021R1A4A2001658 and 2021R1A6A3A03045488).
文摘Tailoring nanostructures is a general approach used to obtain enhanced thermoelectric properties for halfHeusler compounds because the wide areas of grain and phase boundaries could be scattering centers that lower lattice thermal conductivity.However,a common fabrication method based on the sintering of crystalline precursors crushed from as-cast alloy ingots has limitations in obtaining a homogeneous microstructure without microsized impurity phases,owing to residual elemental segregation from casting.In this study,we used amorphous NbCoSn alloys as a precursor for the sintered specimen to obtain a homogeneous NbCoSn bulk specimen without microsized impurity phases and segregation,which led to the enhanced Seebeck coefficient due to the high purity of the half-Heusler phase after crystallization.Moreover,superplasticity originating from amorphous features enabled the powders to be largely deformed during the sintering process,even at a low sintering temperature(953 K).This resulted in less oxidation at both,the grain boundary and the interior,as the O diffusion pathway was blocked during the sintering process.As a result,the NbCoSn0.95Sb0.05 specimen using an amorphous precursor exhibited an enhanced zT of 0.7,due to the increase in the power factor and a decrease in lattice thermal conductivity compared to the specimen using a crystalline precursor.
文摘The nano-bainitic microstructures were compared in a 0.79C-1.5Si-1.98Mn-0.24Mo-1.06Al (wt%) steel after isothermal heat-treatment and a Fe-0.2C-1.5Mn-1.2Si-0.3Mo-0.6Al-0.02Nb (wt%) steel after controlled thermomechanical processing.The microstructure for both steels consisted of bainite.The microstructural characteristics of bainite,such as the morphology of the nano-bainite and thicknesses of bainitic ferrite and retained austenite layers,as a function of steel composition and processing was studied using transmission electron microscopy (TEM).It was found that the nano-bainitic structure can be formed in the low alloy steel through thermomechanical processing.Atom probe tomography (APT) was employed as a powerful technique to determine local composition distributions in three dimensions with atomic resolution.The important conclusions from the APT research were that the carbon content of bainitic ferrite is higher than expected from paraequilibrium level of carbon in ferrite for both steels and that Fe-C clusters and fine particles are formed in the bainitic ferrite in both steels despite the high level of Si.
基金sponsored by the Youth Innovation Promotion Association of Chinese Academy of Sciences(No.2017233)。
文摘The effect of pre-deformation on nanoscale precipitates and hardness of a maraging stainless steel strengthened by the coprecipitation of Ni_(3)Ti,Mo-enriched and Cr-enriched precipitates was systematically studied using electron back scattered diffraction,transmission electron microscopy and atom probe tomography(APT).Hardness measurements showed that the hardness of specimen with a deformation ratio of 90%peaked at HV 718 aged for 24 h,which is higher than that in the undeformed specimen(HV 603)aged for 72 h at 480℃.APT characterization revealed that pre-deformation could shorten the incubation time of the Mo-enriched and Cr-enriched precipitates.At the early-aged stage,pre-deformation increased the stain energy that inhibited the nucleation of Ni_(3)Ti precipitates,but accelerated the rejection of Mo from Ni-Ti clusters.Besides,the strengthening model indicated that strain hardening(43%)makes a larger contribution to the hardness at the early-aged condition,while precipitation hardening(58%)has most contribution to the hardness at the peak-aged conditition.
基金This work was supported by the National Research Foundation of Korea(Grant number:NRF-2019K1A3A1A18116059 and NRF-2023R1A2C200529811)Austrian Science Fund(FWF)(P 32378-N37)Federal Ministry of Austria Education,Science and Research(BMBWF)(KR 06/2020).
文摘Herein,the evolution of long-period stacking ordered(LPSO)phases in the as-cast Mg-6Gd-1Zn-0.6Zr(wt.%)alloy are investigated via transmission electron microscopy(TEM)and atom probe tomography(APT).The TEM results reveal that two types of LPSO phase(a bulky interdendritic phase and a plate-like matrix LPSO phase)are formed in the as-cast sample.Most of the LPSO phases are confirmed to be of the 14H type,with a smaller proportion being of the 18R LPSO.Further,the APT results reveal that the composition of the interdendritic LPSO phase is closer to that of the ideal 14H phase compared to the matrix LPSO phase,and both the interdendritic and matrix LPSO phases exhibit a Gd/Zn ratio of 2.5,thereby indicating a deficient Zn content compared to the ideal 14H phase(i.e.,1.3).In addition,the influence of the LPSO phases on the deformation behavior is investigated at different compressive plastic strains using electron backscatter diffraction(EBSD)analysis to reveal twinning and slip behavior during deformation.The results indicate that the LPSO phase induces additional work hardening in the late stage of deformation via the suppression of{1011}compressive twinning and the activation of non-basal slip systems.
