Integrating a heterogeneous structure can significantly enhance the strength-ductility synergy of composites.However,the relationship between hetero-deformation induced(HDI)strain hardening and dislocation activity ca...Integrating a heterogeneous structure can significantly enhance the strength-ductility synergy of composites.However,the relationship between hetero-deformation induced(HDI)strain hardening and dislocation activity caused by heterogeneous structures in the magnesium matrix composite remains unclear.In this study,a dual-heterogeneous TiC/AZ61 composite exhibits significantly improved plastic elongation(PEL)by nearly one time compared to uniform FG composite,meanwhile maintaining a high strength(UTS:417 MPa).This is because more severe deformation inhomogeneity in heterogeneous structure leads to more geometrically necessary dislocations(GNDs)accumulation and stronger HDI stress,resulting in higher HDI hardening compared to FG and CG composites.During the early stage of plastic deformation,the pile-up types of GND in the FG zone and CG zone are significantly different.GNDs tend to form substructures in the FG zone instead of the CG zone.They only accumulate at grain boundaries of the CG region,thereby leading to obviously increased back stress in the CG region.In the late deformation stage,the elevated HDI stress activates the new〈c+a〉dislocations in the CG region,resulting in dislocation entanglements and even the formation of substructures,further driving the high hardening in the heterogeneous composite.However,For CG composite,〈c+a〉dislocations are not activated even under large plastic strains,and only〈a〉dislocations pile up at grain boundaries and twin boundaries.Our work provides an in-depth understanding of dislocation variation and HDI hardening in heterogeneous magnesium-based composites.展开更多
In this paper,the work hardening and softening behavior of AZ31 magnesium alloy sheets by hard plate accumulative roll bonding(HP-ARB)process in a specific temperature range was studied for the first time,and the cycl...In this paper,the work hardening and softening behavior of AZ31 magnesium alloy sheets by hard plate accumulative roll bonding(HP-ARB)process in a specific temperature range was studied for the first time,and the cyclic stress relaxation test,EBSD,TEM and other characterization methods were used.When the rolling temperature is 350℃,the grain size of magnesium sheets is refined to 4.32(±0.36)μm on average,and it shows an excellent combination of strength and plasticity.The tensile strength reaches 307(±8.52)MPa and the elongation is 12.73(±0.84)%.At this time,the curve of work hardening rate decreases smoothly and the degree of hardening is the lowest,and the amplitude of stress drop △σ_(p) in work softening test is the smallest with the increase of cycle times,which shows that the well coordination between work hardening and softening behavior has been achieved.Research has found that the combined effect of grain boundary strengthening and fine grain strengthening enhances the yield and tensile strength of magnesium sheets after three passes HP-ARB process at 350℃.This is attributed to the high degree of dislocation slip opening in the pyramidal surfaceand<c+a>,which not only coordinates the c-axis strain of the entire grain,but also promotes the slip transfer of dislocations in the fine-grained region,significantly improving the elongation of the sheets.This study provides a new idea for the forming and manufacturing of high performance magnesium alloy sheets.展开更多
To obtain the Ti_(p)with different aspect ratios,the Ti_(p)/Mg-5Zn-0.3Ca composite prepared by semi-solid stir casting was subjected to extrusion at 220℃,180℃,and 140℃,respectively.Then,the effect of the Ti_(p)’s ...To obtain the Ti_(p)with different aspect ratios,the Ti_(p)/Mg-5Zn-0.3Ca composite prepared by semi-solid stir casting was subjected to extrusion at 220℃,180℃,and 140℃,respectively.Then,the effect of the Ti_(p)’s aspect ratio on the microstructure,mechanical properties,work hardening and softening behaviors of Ti_(p)/Mg-5Zn-0.3Ca composites was investigated.The results indicated that the Ti_(p)could be elongated obviously after low-temperature extrusion,and the aspect ratio of which would reach to 13.7:1 as the extrusion temperature deceased to 140℃.Then the“Ti/Mg”layer-like structure was formed in the Ti_(p)/Mg-5Zn-0.3Ca composite.Accompanied with the elongation of Ti_(p),the dynamic recrystallized grains and dynamic precipitates were both refined significantly,however,the dynamic recrystallization rate changed a little.The elongated Ti_(p)endowed the Ti_(p)/Mg-5Zn-0.3Ca composites with better matching of strength and toughness without the sacrifice of elongation and bending strain.Both the work hardening rate and softening rate of Ti_(p)/Mg-5Zn-0.3Ca composites increased with the increasing aspect ratio of Ti_(p).The formation of“Ti/Mg”layer-like structure contributed to the redistribution of strain from large aggregations to a network-like distribution,which effectively suppresses the initiation and propagation of micro-cracks,thus enhancing the plasticity of the Ti_(p)/Mg-5Zn-0.3Ca composites.展开更多
High-entropy alloys(HEAs)exhibit the excellent elevated-temperature performance and irradiation resistance due to the important core effect of serious lattice distortion for impeding dislocation motion,as candidate ma...High-entropy alloys(HEAs)exhibit the excellent elevated-temperature performance and irradiation resistance due to the important core effect of serious lattice distortion for impeding dislocation motion,as candidate materials for nuclear applications.Despite the growth of the nuclear power sector,the effects of high-temperature and high-dose irradiation-induced voids on the mechanical properties of HEA in higher power nuclear reactors remain insufficiently researched,hindering its industrial application.In this study,we establish a consistent parameterization crystal plastic constitutive model for the hardening and creep behaviors of HEA,incorporating the spatial distribution of void size and shape effects,in contrast to traditional creep models that rely on temperature-related fitting parameters of the phenomenological power law equation.The model matches well with experimental data at different temperatures and irradiation doses,demonstrating its robustness.The effects of irradiation dose,temperature,and degree of lattice distortion on irradiation hardening and creep behavior of void-containing HEA are investigated.The results indicate that HEA with high lattice distortion exhibits better creep resistance under higher stress loads.The yield stress of irradiated HEA increases with increasing irradiation dose and temperature.The creep resistance increases with increasing irradiation dose and decreases with increasing irradiation temperature.The increase in irradiation dose causes a specific morphological transformation from spherical to cubic voids.The modeling and results could provide an effective theoretical way for tuning the yield strength and alloy design in advanced HEAs to meet irradiation properties.展开更多
Mg-3Gd(wt.%)samples with different initial grain sizes were prepared to evaluate the grain size effect on microstructural evolution during cold rolling and subsequent annealing hardening response.The deformation behav...Mg-3Gd(wt.%)samples with different initial grain sizes were prepared to evaluate the grain size effect on microstructural evolution during cold rolling and subsequent annealing hardening response.The deformation behavior and mechanical response of the as-rolled and annealed samples were systematically investigated by a combination of electron microscopy and microhardness characterization.The results show that the twinning activities were highly suppressed in the fine-grained samples during rolling.Upon increasing the rolling reduction to 40%,ultra-fine grain structures with a volume fraction of∼28%were formed due to the activation of multiple slip systems.Conversely,twinning dominated the early stages of deformation in the coarse-grained samples.After a 10%rolling reduction,numerous twins with a volume fraction of∼23%were formed.Further increasing the rolling reduction to 40%,high-density dislocations were activated and twin structures with a volume fraction of∼36%were formed.The annealing hardening response of deformed samples was effectively enhanced compared to that of the non-deformed samples,which was attributed to the enhanced Gd segregation along grain boundaries,twin boundaries and dislocation cores.Moreover,the grain size and rolling reduction were found to affect the microstructure evolution during annealing,resulting in a notable difference in the annealing hardening response of Mg-3Gd alloy between samples of different grain sizes deformed to different strains.These findings highlight the crucial importance of microstructural and processing parameters in the design of high-strength,cost-effective Mg alloys.展开更多
This paper explored how the work hardening process enhances the yield strength(YS)of magnesium alloy and established a novel quantitative relationship between microstructure and YS.The YS of an extruded Mg–Nd–Zn–Zr...This paper explored how the work hardening process enhances the yield strength(YS)of magnesium alloy and established a novel quantitative relationship between microstructure and YS.The YS of an extruded Mg–Nd–Zn–Zr alloy was nearly tripled by rotary swaging at 200℃,attributing to dislocation accumulation,grain fragmentation,and basal texture formation.A modified Hall–Petch relationship(GM–HP)was established by introducing the equivalent orientation factor and dislocation density to accurately quantify the YS and the contribution of each strengthening mechanism.The contributions of dislocation hardening and grain boundary strengthening were multiplied by the strong basal texture.This paper offered valuable insights and guidance for designing and optimizing high-strength wrought magnesium alloys and their plastic processing methods.展开更多
Nacre exhibits exceptional mechanical properties,which are attributed to its brick-mortar microstructure with an integration of stiff mineral platelets and soft organic interfaces.The rapidly developing 3D printing te...Nacre exhibits exceptional mechanical properties,which are attributed to its brick-mortar microstructure with an integration of stiff mineral platelets and soft organic interfaces.The rapidly developing 3D printing technique has been used to make nacreinspired composites with similar brick-mortar structure.It is known that the strain hardening phenomenon plays an important role in the high strength and toughness of natural nacre.However,the role of strain hardening on the mechanical properties of biomimetic nacreous composites still lacks theoretical evaluation and experimental confirmation.Based on a mesomechanical theoretical model,we derive the stress-strain response and macroscopic strength of the brick-mortar structure under uniaxial tension.The brick-mortar structure shows three typical failure modes,according to the occurrence of strain hardening and platelet fracture.Furthermore,we investigate how the occurrence of strain hardening depends on its geometry and constituent properties.It is found that increasing the aspect ratio of the platelets promotes strain hardening,while increasing the stiffness of the soft phase leads to the disappearance of strain hardening.Furthermore,we utilize bi-material 3D printing technology to prepare biomimetic nacre samples and conduct uniaxial tensile mechanical tests.We observe the occurrence of strain hardening with the increase in the length of the platelets,resulting in a significant increase in the strength and fracture strain of artificial nacre.Our result highlights the significant role of strain hardening in regulating the mechanical properties of nacre-like composite materials.展开更多
Existing creep constitutive models rarely incorporate studies on the coupling mechanism between creep damage and rock strain softening/hardening.This study analyzed the strain softening and hardening behaviors of argi...Existing creep constitutive models rarely incorporate studies on the coupling mechanism between creep damage and rock strain softening/hardening.This study analyzed the strain softening and hardening behaviors of argillaceous sandstone and sandy mudstone during load-induced failure based on the plastic increment theory.These behaviors were then coupled with an improved Burgers creep model to establish a coupled creep-damage and plastic softening/hardening model.Finally,the validity and engineering applicability of the proposed model were verified through FLAC~(3D)numerical simulations.The numerical simulation results of standard cylindrical specimens show that the established coupling model can effectively reflect the unloading creep deformation law and failure characteristics of argillaceous sandstone and sandy mudstone.Taking the diversion tunnel of a hydropower station in Northwest China as an example for engineering application,the coupled creep-damage and plastic softening/hardening model is introduced into FLAC~(3D)to carry out numerical simulation calculation of the tunnel under excavation and unsupported creep conditions.The results show that the uncoordinated deformation of the upper and lower walls of the surrounding rock of the tunnel is more prominent.When the buried depth of the tunnel increases to 80 m,the monitoring point C in the sandy mudstone area of the upper wall shows nonlinear accelerated deformation under unsupported creep conditions,and the maximum displacement in the horizontal direction reaches 44.5 mm,and the maximum displacement in the vertical direction reaches 53.5 mm.The coupled creep-damage and plastic softening/hardening model established in the research results can well describe the whole process of uncoordinated deformation and failure in the unloading creep process of soft-hard interbedded rock mass.展开更多
Multi-pass hot processing methods are commonly used in magnesium(Mg)alloys to overcome the poor workability due to limited slip systems,which generally involve complicated post-deformation softening and hardening beha...Multi-pass hot processing methods are commonly used in magnesium(Mg)alloys to overcome the poor workability due to limited slip systems,which generally involve complicated post-deformation softening and hardening behaviors.In this work,to reveal post-deformation softening and hardening mechanisms of a Mg-2Y-1Zn alloy,double-stage hot compression tests and microstructural observations were conducted.The results showed that the softening fraction of Mg-2Y-1Zn alloy showed a non-linear dependence on deformation conditions and could be general coupled by Z parameter.Due to the formation and cross-overlapping of twins and kinks,only static recovery(SRV)occurred during holding process at 300℃/0.001 s^(-1) which led to the least static softening:5.52% after 10 s of holding.For samples at 400℃/0.001s^(-1),the enhanced post-deformation softening,which is 11.93% after 10 s of holding,was attributed to static recrystallization(SRX)followed continuous dynamic recrystallization(CDRX)happened during first deformation stage as well as SRV influenced by the LPSO phases.Under deformation condition of 400℃/0.1 s^(-1),the coupled meta-dynamic recrystallization(MDRX)and SRX resulted in serious stress relaxation,which is 42.83% after 10 s of holding,and caused hardening phenomenon at reloading stage.The 18R-LPSO and 14H-LPSO phases synchronously worked on deformation behaviors and limited the growth of recrystallized grains.Further,a simplified static softening kinetics model was established based on Johnson-Mehl-Avrami-Kolmogorov equation and employed to rationalize experimental data.展开更多
By developing high comprehensive performance((BH)_(max)+H_(cj)),Nd-Fe-B magnets can operate stably in high-temperature applications,greatly expanding the application scenarios of them.Unfortunately,there is a constrai...By developing high comprehensive performance((BH)_(max)+H_(cj)),Nd-Fe-B magnets can operate stably in high-temperature applications,greatly expanding the application scenarios of them.Unfortunately,there is a constraint relationship between coercivity(H_(cj))and maximum magnetic energy product((BH)_(max)),and an increase in H_(cj) always accompanies a decrease in(BH)_(max).Here,the excellent comprehensive magnetic performance of up to 86.54,namely(BH)_(max) of 42.33 MGOe and H_(cj) of 44.21 kOe,is unprecedented in the sintered Nd-Fe-B magnets.This magnet is obtained by designing a unique grain structure through micrometallurgical reactions to prepare a matrix with excellent comprehensive performance,and then by stepwise diffusion,the(BH)_(max) and H_(cj) of the magnet are simultaneously enhanced.The magnet prepared in this way has a“double-shell core”structure and Tb segregation distribution inside the core.The working temperature of the magnet in this work reached 280℃,providing a new approach for the development of high-performance Nd-Fe-B magnets.展开更多
In this work,Ti_(p)/Mg-7Gd-2Y-3Zn(Ti_(p)/GWZ723)composites with various Ti_(p) sizes(~10μm,~20μm and~35μm)were fabricated using semi-solid stirring casting method,the composites were subjected to hot extrusion,and ...In this work,Ti_(p)/Mg-7Gd-2Y-3Zn(Ti_(p)/GWZ723)composites with various Ti_(p) sizes(~10μm,~20μm and~35μm)were fabricated using semi-solid stirring casting method,the composites were subjected to hot extrusion,and the infuence of Ti_(p) size on long-period stacking ordered(LPSO)phase,dynamic recrystallization(DRX),mechanical properties,and work hardening behavior of the Ti_(p)/GWZ723 composites was investigated.The results indicate that with the increase in Ti_(p) size,the grain size of the as-cast Ti_(p)/GWZ723 composites increases,and the lamellar 14H LPSO phase precipitates within the matrix after homogenization treatment.With the increase in Ti_(p) size,the reduction in the Ti_(p) surface area leads to a decrease in surface energy.Consequently,the enrichment of RE element is reduced,which facilitates the formation of the 14H LPSO phase.Moreover,the layer spacing of the 14H LPSO phase decreases.Particle deformation zone(PDZ)is formed around the Ti_(p) after extrusion,promoting the nucleation of DRX.The PDZ size increases with the increase in the Ti_(p) size.Nevertheless,the elongation of the Ti_(p) releases stress and reduces the PDZ size.Simultaneously,the 14H LPSO phase with a small interlayer spacing inhibits the non-basal slip,and the volume fraction of DRX(VDRX)decreases with the increase in the Ti_(p) size.With the increase in Ti_(p) size,the refned grain size and the 14H LPSO phase with smaller interlayer spacing contribute to enhancing the work hardening rate and dynamic recovery rate of the Ti_(p)/GWZ723 composites.The Ti_(p)/Mg laminar-like interface formed in the Ti_(p)/GWZ723 composites can alleviate local stress concentration and inhibit the initiation and propagation of cracks.展开更多
The flow stress behavior of ZK60 alloy at elevated temperature was investigated. The strain hardening and dynamic recrystallization of the alloy were modeled by Kocks-Meching model and Avrami equation, respectively. A...The flow stress behavior of ZK60 alloy at elevated temperature was investigated. The strain hardening and dynamic recrystallization of the alloy were modeled by Kocks-Meching model and Avrami equation, respectively. A new constitutive equation during hot deformation was constructed to predict the flow stress considering the dynamic recrystallization. The results show that the flow stress curves predicted by the proposed equation have high correlation coefficients with the experimental data, which confirms that the developed model is accurate and effective to establish the flow stress equation of ZK60 magnesium alloy during hot deformation. Microstructure observation shows that dynamic recovery occurs in the initial stage of hot deformation. However, the microstructure turns to recrvstallization structure as the strain increases.展开更多
The effects of the deformation temperature and the strain rate on the hot deformation behavior of pure copper were investigated based on compression tests. The expressions of strain hardening rate, dynamic recrystalli...The effects of the deformation temperature and the strain rate on the hot deformation behavior of pure copper were investigated based on compression tests. The expressions of strain hardening rate, dynamic recrystallization critical stress, saturated stress, dynamic recovery volume fraction and dynamic recrystallization volume fraction were determined. According to the processing map, the instability regions occur in regions of 400?450 °C, 0.001?0.05 s?1 and 450?750 °C, 0.05?1 s?1. The deformation mechanism in the stability region is dynamic recrystallization. The flow stress was predicted. The results also show that the true stress–true strain curves predicted by the extracted model are in good agreement with the experimental results.展开更多
The aim of the present work is to develop a model for simulating double-peak precipitation hardening kinetics in Al-Zn-Mg alloy with the simultaneous formation of different types of precipitates at elevated temperatur...The aim of the present work is to develop a model for simulating double-peak precipitation hardening kinetics in Al-Zn-Mg alloy with the simultaneous formation of different types of precipitates at elevated temperatures based on the modified Langer-Schwartz approach. The double aging peaks are present in the long time age-hardening curves of Al-Zn-Mg alloys. The physically-based model, while taking explicitly into account nucleation, growth, coarsening of the new phase precipitations and two strengthening mechanisms associated with particle-dislocation interaction （shearing and bypassing）, was used for the analysis of precipitates evolution and precipitation hardening during aging of Al-Zn-Mg alloy. Model predictions were compared with the measurements of Al-Zn-Mg alloy. The systematic and quantitative results show that the predicted hardness profiles of double peaks via adding a shape dependent parameter in the growth equation for growth and coarsening generally agree well with the measured ones. Two strengthening mechanisms associated with particle-dislocation interaction （shearing and bypassing） were considered operating simultaneously in view of the particle size-distribution. The transition from shearing to bypassing strengthening mechanism was found to occur at rather early stage of the particle growth. The bypassing was found to be the prevailing strengthening mechanism in the investigated alloys.展开更多
The influences of high temperature pre-straining (HT-PS) on the natural aging and bake hardening of Al?Mg?Si alloys were investigated by Vickers microhardness measurements, differential scanning calorimetry (DSC) anal...The influences of high temperature pre-straining (HT-PS) on the natural aging and bake hardening of Al?Mg?Si alloys were investigated by Vickers microhardness measurements, differential scanning calorimetry (DSC) analysis and transmission electron microscopy (TEM) characterization. The results show that pre-straining at 170 °C immediately after quenching can effectively resolve the rather high T4 temper hardness caused by the conventional room temperature (RT) pre-straining treatment, and give a better bake hardening response (BHR) after paint-bake cycle. HT-PS 7% at 170 °C for 10 min is chosen as the optimum process as it provides lower T4 temper hardness and better BHR. The simultaneous introduction of dislocations and Cluster (2) can significantly suppress the natural aging and promote the precipitation of β″ phase, and reduce the effects of deformation hardening by dynamic recovery.展开更多
The mathematical model of the grinding temperature is established. The grinding temperature and the cooling rate are measured in the grind-hardening process of 40Cr steel under different conditions. Moreover, the grin...The mathematical model of the grinding temperature is established. The grinding temperature and the cooling rate are measured in the grind-hardening process of 40Cr steel under different conditions. Moreover, the grind-hardening effects are investigated. Experimental results show that the calculated temperatures are comparatively close to the measured ones, and the required temperature and cooling rate can be achieved. Furthermore, the microstructure of the hardened zone is similar to that obtained through the high-frequency induction technique. The average hardness of the entirely hardened zone is HV670 and the thickness of the hardened layer is adjacent to 1.3 mm. It indicates that the hardening mechanism induced by the grinding heat and high-frequency heating is identical. Finally, the fine needlelike martensite is obtained.展开更多
The analysis of plane strain elastic-plastic bending of a linear strain hardening curved beam with a narrow rectangular cross section subjected to couples at its end is conducted based on a unified yield criterion. Th...The analysis of plane strain elastic-plastic bending of a linear strain hardening curved beam with a narrow rectangular cross section subjected to couples at its end is conducted based on a unified yield criterion. The solutions for the mechanical properties of plane strain bending are derived, which are adapted for various kinds of non-strength differential materials and can be degenerated to those based on the Tresca, von Mises, and twin-shear yield criteria. The dependences of the two critical bending moments, the radii of the interfaces between the elastic and plastic regions and the radial displacements of the points at the symmetrical plane on different yield criteria and Poisson’s ratios are discussed. The results show that the influences of different yield criteria and Poisson’s ratio on the two critical bending moments, the radii of the interfaces between the elastic and plastic regions and the radial displacements of the points at the symmetrical plane of the curved beam are significant. Once the value of bis obtained by experiments, the yield criterion and the corresponding solution for the materials of interest are then determined.展开更多
The precipitation hardening behavior in dilute Al-Yb alloys upon annealing at different temperatures was investigated to shed light on the mechanism of micro-alloying element in aluminum alloys. When aging at differen...The precipitation hardening behavior in dilute Al-Yb alloys upon annealing at different temperatures was investigated to shed light on the mechanism of micro-alloying element in aluminum alloys. When aging at different temperatures, the samples showed their corresponding peak hardness in the range of 400-416 MPa due to the precipitation of Al3Yb with L 12 crystal structure. The coarsening kinetics of the Al3Yb precipitates obeyed the LSW theory, which indicated that the coarsening process was controlled by the diffusion of Yb. The coherence between Al3Yb particles and matrix was maintained until the particle size reached 11 nm. When the particle size increased to about 2 nm, the shearing mechanism started to change to Orowan mechanism.展开更多
A novel Al-alloyed press-hardening steel(PHS)was developed,which exhibits excellent tensile,bending and antioxidation properties.Al is a ferrite-forming element that can hinder the formation of cementite and enhance t...A novel Al-alloyed press-hardening steel(PHS)was developed,which exhibits excellent tensile,bending and antioxidation properties.Al is a ferrite-forming element that can hinder the formation of cementite and enhance the stability of austenite.The incorporation of Al not only induces the formation of ferrite within martensitic matrix but also enhances the stability of retained austenite(RA).The microstructure of novel steel consists of martensite,ferrite,and RA after press hardening.Investigations into the role of Al in RA development were supported by thermo-kinetic calculations.The simultaneous introduction of ferrite and RA into the martensitic matrix via tailored chemical compositions significantly enhances the elongation and bending toughness of the novel PHS.Additionally,Al can form a dense Al oxide at the bottom of oxide layer,resulting in the improved antioxidant properties.Compared to 22MnB5 steel,it is an exciting discovery as there is a significant improvement in total elongation and bending toughness of novel PHS without compromising strength.The novel PHS,with its exceptional balance of strength and ductility,will play a crucial role in reducing weight when it replaces the existing class 22MnB5 PHS in different structural components of vehicle bodies.展开更多
Specimens of two different kinds of bake hardening steels （BH-Mn and BH-P） were prepared and treated with different annealing processes （water quenching and overaging）. A novel technique of three dimensional atom ...Specimens of two different kinds of bake hardening steels （BH-Mn and BH-P） were prepared and treated with different annealing processes （water quenching and overaging）. A novel technique of three dimensional atom probe was used to investigate solute distributions in these steels. The results indicate that C concentration decreases, whereas V increases during overaging in both bake hardening steels. The conclusion that no vanadium carbides pre- cipitate during the overaging is therefore originally obtained by microanalysis in bake hardening steels. Moreover, bake hardening values of all the specimens were tested by tensile experiments with 2 0/~ pre-deformation. However, those of overaged specimens were further measured with higher levels of pre-deformation because no bake hardening phenomenon was present at 2% pre-deformation. As the pre-deformation increases from 2% to 6% and 8%, both overaged steels show bake hardening values, and the value data are almost the same.展开更多
基金support from the National Natural Science Foundation of China(No:52061040)China Postdoctoral Science Foundation(No:2021M692512)+1 种基金Opening Project of Material Corrosion and Protection Key Laboratory of Sichuan Province(No:2023CL01)Open Projects of Key Laboratory of Advanced Technologies of Materials,Ministry of Education China,Southwest Jiaotong University(No:KLATM202003).
文摘Integrating a heterogeneous structure can significantly enhance the strength-ductility synergy of composites.However,the relationship between hetero-deformation induced(HDI)strain hardening and dislocation activity caused by heterogeneous structures in the magnesium matrix composite remains unclear.In this study,a dual-heterogeneous TiC/AZ61 composite exhibits significantly improved plastic elongation(PEL)by nearly one time compared to uniform FG composite,meanwhile maintaining a high strength(UTS:417 MPa).This is because more severe deformation inhomogeneity in heterogeneous structure leads to more geometrically necessary dislocations(GNDs)accumulation and stronger HDI stress,resulting in higher HDI hardening compared to FG and CG composites.During the early stage of plastic deformation,the pile-up types of GND in the FG zone and CG zone are significantly different.GNDs tend to form substructures in the FG zone instead of the CG zone.They only accumulate at grain boundaries of the CG region,thereby leading to obviously increased back stress in the CG region.In the late deformation stage,the elevated HDI stress activates the new〈c+a〉dislocations in the CG region,resulting in dislocation entanglements and even the formation of substructures,further driving the high hardening in the heterogeneous composite.However,For CG composite,〈c+a〉dislocations are not activated even under large plastic strains,and only〈a〉dislocations pile up at grain boundaries and twin boundaries.Our work provides an in-depth understanding of dislocation variation and HDI hardening in heterogeneous magnesium-based composites.
基金supported by the Natural Science Foundation of Heilongjiang Province(No.JQ2022E004).
文摘In this paper,the work hardening and softening behavior of AZ31 magnesium alloy sheets by hard plate accumulative roll bonding(HP-ARB)process in a specific temperature range was studied for the first time,and the cyclic stress relaxation test,EBSD,TEM and other characterization methods were used.When the rolling temperature is 350℃,the grain size of magnesium sheets is refined to 4.32(±0.36)μm on average,and it shows an excellent combination of strength and plasticity.The tensile strength reaches 307(±8.52)MPa and the elongation is 12.73(±0.84)%.At this time,the curve of work hardening rate decreases smoothly and the degree of hardening is the lowest,and the amplitude of stress drop △σ_(p) in work softening test is the smallest with the increase of cycle times,which shows that the well coordination between work hardening and softening behavior has been achieved.Research has found that the combined effect of grain boundary strengthening and fine grain strengthening enhances the yield and tensile strength of magnesium sheets after three passes HP-ARB process at 350℃.This is attributed to the high degree of dislocation slip opening in the pyramidal surfaceand<c+a>,which not only coordinates the c-axis strain of the entire grain,but also promotes the slip transfer of dislocations in the fine-grained region,significantly improving the elongation of the sheets.This study provides a new idea for the forming and manufacturing of high performance magnesium alloy sheets.
基金supported by the“National Natural Science Foundation of China”(Grants.52271109 and 52001223)Support from the“National Key Research and Development Program for Young Scientists”(Grant.2021YFB3703300)+1 种基金the Major Special Plan for Science and Technology in Shanxi Province(202201050201012)the Special Fund Project for Guiding Local Science and Technology Development by the Central Government(Grant.YDZJSX2021B019)。
文摘To obtain the Ti_(p)with different aspect ratios,the Ti_(p)/Mg-5Zn-0.3Ca composite prepared by semi-solid stir casting was subjected to extrusion at 220℃,180℃,and 140℃,respectively.Then,the effect of the Ti_(p)’s aspect ratio on the microstructure,mechanical properties,work hardening and softening behaviors of Ti_(p)/Mg-5Zn-0.3Ca composites was investigated.The results indicated that the Ti_(p)could be elongated obviously after low-temperature extrusion,and the aspect ratio of which would reach to 13.7:1 as the extrusion temperature deceased to 140℃.Then the“Ti/Mg”layer-like structure was formed in the Ti_(p)/Mg-5Zn-0.3Ca composite.Accompanied with the elongation of Ti_(p),the dynamic recrystallized grains and dynamic precipitates were both refined significantly,however,the dynamic recrystallization rate changed a little.The elongated Ti_(p)endowed the Ti_(p)/Mg-5Zn-0.3Ca composites with better matching of strength and toughness without the sacrifice of elongation and bending strain.Both the work hardening rate and softening rate of Ti_(p)/Mg-5Zn-0.3Ca composites increased with the increasing aspect ratio of Ti_(p).The formation of“Ti/Mg”layer-like structure contributed to the redistribution of strain from large aggregations to a network-like distribution,which effectively suppresses the initiation and propagation of micro-cracks,thus enhancing the plasticity of the Ti_(p)/Mg-5Zn-0.3Ca composites.
基金support from the National Natural Science Foundation of China(Nos.12302083,U2267252,12372069,and 12172123)the China Postdoctoral Science Foundation(Nos.2023M731061 and BX20230109)+1 种基金the Natural Science Foundation of Hunan Province(No.2022JJ20001)the Hunan Provincial Innovation Foundation for Postgraduate(No.CX20230420).
文摘High-entropy alloys(HEAs)exhibit the excellent elevated-temperature performance and irradiation resistance due to the important core effect of serious lattice distortion for impeding dislocation motion,as candidate materials for nuclear applications.Despite the growth of the nuclear power sector,the effects of high-temperature and high-dose irradiation-induced voids on the mechanical properties of HEA in higher power nuclear reactors remain insufficiently researched,hindering its industrial application.In this study,we establish a consistent parameterization crystal plastic constitutive model for the hardening and creep behaviors of HEA,incorporating the spatial distribution of void size and shape effects,in contrast to traditional creep models that rely on temperature-related fitting parameters of the phenomenological power law equation.The model matches well with experimental data at different temperatures and irradiation doses,demonstrating its robustness.The effects of irradiation dose,temperature,and degree of lattice distortion on irradiation hardening and creep behavior of void-containing HEA are investigated.The results indicate that HEA with high lattice distortion exhibits better creep resistance under higher stress loads.The yield stress of irradiated HEA increases with increasing irradiation dose and temperature.The creep resistance increases with increasing irradiation dose and decreases with increasing irradiation temperature.The increase in irradiation dose causes a specific morphological transformation from spherical to cubic voids.The modeling and results could provide an effective theoretical way for tuning the yield strength and alloy design in advanced HEAs to meet irradiation properties.
基金financial support from the National Key Research and Development Program of China(No.2021YFB3702101)National Natural Science Foundation of China(No.52130107,52071038)+5 种基金Fundamental Research Funds for the Central Universities(No.2023CDJXY-018)the“111”Project(No.B16007)by the Ministry of Education and the State Administration of Foreign Experts Affairs of Chinasupport to the Norwegian Micro-and Nano-Fabrication Facility,NorFab(No.295864)the Norwegian Laboratory for Mineral and Materials Characterization,MiMaC(No.269842/F50)the RCN INRPART project IntMat(No.309724)the Center for Research based Innovation SFI PhysMet(No.309584).
文摘Mg-3Gd(wt.%)samples with different initial grain sizes were prepared to evaluate the grain size effect on microstructural evolution during cold rolling and subsequent annealing hardening response.The deformation behavior and mechanical response of the as-rolled and annealed samples were systematically investigated by a combination of electron microscopy and microhardness characterization.The results show that the twinning activities were highly suppressed in the fine-grained samples during rolling.Upon increasing the rolling reduction to 40%,ultra-fine grain structures with a volume fraction of∼28%were formed due to the activation of multiple slip systems.Conversely,twinning dominated the early stages of deformation in the coarse-grained samples.After a 10%rolling reduction,numerous twins with a volume fraction of∼23%were formed.Further increasing the rolling reduction to 40%,high-density dislocations were activated and twin structures with a volume fraction of∼36%were formed.The annealing hardening response of deformed samples was effectively enhanced compared to that of the non-deformed samples,which was attributed to the enhanced Gd segregation along grain boundaries,twin boundaries and dislocation cores.Moreover,the grain size and rolling reduction were found to affect the microstructure evolution during annealing,resulting in a notable difference in the annealing hardening response of Mg-3Gd alloy between samples of different grain sizes deformed to different strains.These findings highlight the crucial importance of microstructural and processing parameters in the design of high-strength,cost-effective Mg alloys.
基金supported by National Natural Science Foundation of China(Grant No.51975146)Natural Science Foundation of Shandong Province(Grant No.ZR2020QE171)。
文摘This paper explored how the work hardening process enhances the yield strength(YS)of magnesium alloy and established a novel quantitative relationship between microstructure and YS.The YS of an extruded Mg–Nd–Zn–Zr alloy was nearly tripled by rotary swaging at 200℃,attributing to dislocation accumulation,grain fragmentation,and basal texture formation.A modified Hall–Petch relationship(GM–HP)was established by introducing the equivalent orientation factor and dislocation density to accurately quantify the YS and the contribution of each strengthening mechanism.The contributions of dislocation hardening and grain boundary strengthening were multiplied by the strong basal texture.This paper offered valuable insights and guidance for designing and optimizing high-strength wrought magnesium alloys and their plastic processing methods.
基金supported by the National Natural Science Foundation of China(Grant No.12372325).
文摘Nacre exhibits exceptional mechanical properties,which are attributed to its brick-mortar microstructure with an integration of stiff mineral platelets and soft organic interfaces.The rapidly developing 3D printing technique has been used to make nacreinspired composites with similar brick-mortar structure.It is known that the strain hardening phenomenon plays an important role in the high strength and toughness of natural nacre.However,the role of strain hardening on the mechanical properties of biomimetic nacreous composites still lacks theoretical evaluation and experimental confirmation.Based on a mesomechanical theoretical model,we derive the stress-strain response and macroscopic strength of the brick-mortar structure under uniaxial tension.The brick-mortar structure shows three typical failure modes,according to the occurrence of strain hardening and platelet fracture.Furthermore,we investigate how the occurrence of strain hardening depends on its geometry and constituent properties.It is found that increasing the aspect ratio of the platelets promotes strain hardening,while increasing the stiffness of the soft phase leads to the disappearance of strain hardening.Furthermore,we utilize bi-material 3D printing technology to prepare biomimetic nacre samples and conduct uniaxial tensile mechanical tests.We observe the occurrence of strain hardening with the increase in the length of the platelets,resulting in a significant increase in the strength and fracture strain of artificial nacre.Our result highlights the significant role of strain hardening in regulating the mechanical properties of nacre-like composite materials.
基金funded by the Natural Science Foundation of China(Grant No.U22A20600)。
文摘Existing creep constitutive models rarely incorporate studies on the coupling mechanism between creep damage and rock strain softening/hardening.This study analyzed the strain softening and hardening behaviors of argillaceous sandstone and sandy mudstone during load-induced failure based on the plastic increment theory.These behaviors were then coupled with an improved Burgers creep model to establish a coupled creep-damage and plastic softening/hardening model.Finally,the validity and engineering applicability of the proposed model were verified through FLAC~(3D)numerical simulations.The numerical simulation results of standard cylindrical specimens show that the established coupling model can effectively reflect the unloading creep deformation law and failure characteristics of argillaceous sandstone and sandy mudstone.Taking the diversion tunnel of a hydropower station in Northwest China as an example for engineering application,the coupled creep-damage and plastic softening/hardening model is introduced into FLAC~(3D)to carry out numerical simulation calculation of the tunnel under excavation and unsupported creep conditions.The results show that the uncoordinated deformation of the upper and lower walls of the surrounding rock of the tunnel is more prominent.When the buried depth of the tunnel increases to 80 m,the monitoring point C in the sandy mudstone area of the upper wall shows nonlinear accelerated deformation under unsupported creep conditions,and the maximum displacement in the horizontal direction reaches 44.5 mm,and the maximum displacement in the vertical direction reaches 53.5 mm.The coupled creep-damage and plastic softening/hardening model established in the research results can well describe the whole process of uncoordinated deformation and failure in the unloading creep process of soft-hard interbedded rock mass.
基金supported by the National Natural Science Foundation of China(52174361,52074114)Science and Technology Innovation Program of Hunan Province(2023RC3106)+2 种基金Open Fund of the China Spallation Neutron Source Songshan Lake Science City(KFKT2023B13)Graduate Training and Innovation Practice Base of Hunan Province,China Scholarship Council(202106130051)Postgraduate Scientific Research Innovation Project of Hunan Province(QL20220100,QL20230094).
文摘Multi-pass hot processing methods are commonly used in magnesium(Mg)alloys to overcome the poor workability due to limited slip systems,which generally involve complicated post-deformation softening and hardening behaviors.In this work,to reveal post-deformation softening and hardening mechanisms of a Mg-2Y-1Zn alloy,double-stage hot compression tests and microstructural observations were conducted.The results showed that the softening fraction of Mg-2Y-1Zn alloy showed a non-linear dependence on deformation conditions and could be general coupled by Z parameter.Due to the formation and cross-overlapping of twins and kinks,only static recovery(SRV)occurred during holding process at 300℃/0.001 s^(-1) which led to the least static softening:5.52% after 10 s of holding.For samples at 400℃/0.001s^(-1),the enhanced post-deformation softening,which is 11.93% after 10 s of holding,was attributed to static recrystallization(SRX)followed continuous dynamic recrystallization(CDRX)happened during first deformation stage as well as SRV influenced by the LPSO phases.Under deformation condition of 400℃/0.1 s^(-1),the coupled meta-dynamic recrystallization(MDRX)and SRX resulted in serious stress relaxation,which is 42.83% after 10 s of holding,and caused hardening phenomenon at reloading stage.The 18R-LPSO and 14H-LPSO phases synchronously worked on deformation behaviors and limited the growth of recrystallized grains.Further,a simplified static softening kinetics model was established based on Johnson-Mehl-Avrami-Kolmogorov equation and employed to rationalize experimental data.
基金supported by the National Key Research and Development Program of China(No.2021YFB3502802)Zhejiang Provincial Department of Science and Technology of China(No.2022C01020)+3 种基金the Key Research and Development Program of Ningbo City(No.2023Z093)Ningbo Natural Science Foundation(No.2023J344)the Natural Science Foundation of Zhejiang Province(No.LQ23E010001)Ningbo Young Science and Technology Innovation Leading Talents(No.2023QL040).
文摘By developing high comprehensive performance((BH)_(max)+H_(cj)),Nd-Fe-B magnets can operate stably in high-temperature applications,greatly expanding the application scenarios of them.Unfortunately,there is a constraint relationship between coercivity(H_(cj))and maximum magnetic energy product((BH)_(max)),and an increase in H_(cj) always accompanies a decrease in(BH)_(max).Here,the excellent comprehensive magnetic performance of up to 86.54,namely(BH)_(max) of 42.33 MGOe and H_(cj) of 44.21 kOe,is unprecedented in the sintered Nd-Fe-B magnets.This magnet is obtained by designing a unique grain structure through micrometallurgical reactions to prepare a matrix with excellent comprehensive performance,and then by stepwise diffusion,the(BH)_(max) and H_(cj) of the magnet are simultaneously enhanced.The magnet prepared in this way has a“double-shell core”structure and Tb segregation distribution inside the core.The working temperature of the magnet in this work reached 280℃,providing a new approach for the development of high-performance Nd-Fe-B magnets.
基金supported by the National Natural Science Foundation of China(Grant Nos.52271109 and 52401162)support from the Natural Science Foundation of Shanxi(Grant Nos.202403021211064 and 202403011212003)the Major Special Plan for Science and Technology in Shanxi Province(202201050201012).
文摘In this work,Ti_(p)/Mg-7Gd-2Y-3Zn(Ti_(p)/GWZ723)composites with various Ti_(p) sizes(~10μm,~20μm and~35μm)were fabricated using semi-solid stirring casting method,the composites were subjected to hot extrusion,and the infuence of Ti_(p) size on long-period stacking ordered(LPSO)phase,dynamic recrystallization(DRX),mechanical properties,and work hardening behavior of the Ti_(p)/GWZ723 composites was investigated.The results indicate that with the increase in Ti_(p) size,the grain size of the as-cast Ti_(p)/GWZ723 composites increases,and the lamellar 14H LPSO phase precipitates within the matrix after homogenization treatment.With the increase in Ti_(p) size,the reduction in the Ti_(p) surface area leads to a decrease in surface energy.Consequently,the enrichment of RE element is reduced,which facilitates the formation of the 14H LPSO phase.Moreover,the layer spacing of the 14H LPSO phase decreases.Particle deformation zone(PDZ)is formed around the Ti_(p) after extrusion,promoting the nucleation of DRX.The PDZ size increases with the increase in the Ti_(p) size.Nevertheless,the elongation of the Ti_(p) releases stress and reduces the PDZ size.Simultaneously,the 14H LPSO phase with a small interlayer spacing inhibits the non-basal slip,and the volume fraction of DRX(VDRX)decreases with the increase in the Ti_(p) size.With the increase in Ti_(p) size,the refned grain size and the 14H LPSO phase with smaller interlayer spacing contribute to enhancing the work hardening rate and dynamic recovery rate of the Ti_(p)/GWZ723 composites.The Ti_(p)/Mg laminar-like interface formed in the Ti_(p)/GWZ723 composites can alleviate local stress concentration and inhibit the initiation and propagation of cracks.
文摘The flow stress behavior of ZK60 alloy at elevated temperature was investigated. The strain hardening and dynamic recrystallization of the alloy were modeled by Kocks-Meching model and Avrami equation, respectively. A new constitutive equation during hot deformation was constructed to predict the flow stress considering the dynamic recrystallization. The results show that the flow stress curves predicted by the proposed equation have high correlation coefficients with the experimental data, which confirms that the developed model is accurate and effective to establish the flow stress equation of ZK60 magnesium alloy during hot deformation. Microstructure observation shows that dynamic recovery occurs in the initial stage of hot deformation. However, the microstructure turns to recrvstallization structure as the strain increases.
基金Project(cstc2015jcyj BX0115)supported by the Chongqing Research Program of Basic Research and Frontier Technology,China
文摘The effects of the deformation temperature and the strain rate on the hot deformation behavior of pure copper were investigated based on compression tests. The expressions of strain hardening rate, dynamic recrystallization critical stress, saturated stress, dynamic recovery volume fraction and dynamic recrystallization volume fraction were determined. According to the processing map, the instability regions occur in regions of 400?450 °C, 0.001?0.05 s?1 and 450?750 °C, 0.05?1 s?1. The deformation mechanism in the stability region is dynamic recrystallization. The flow stress was predicted. The results also show that the true stress–true strain curves predicted by the extracted model are in good agreement with the experimental results.
基金Project(51021063)supported by the Creative Research Group of the National Natural Science Foundation of ChinaProject(50831007)supported by the National Natural Science Foundation of China+1 种基金Project(2011CB610401)supported by the National Basic Research Program of ChinaProject(12C1142)supported by the Education Department of Hunan Province,China
文摘The aim of the present work is to develop a model for simulating double-peak precipitation hardening kinetics in Al-Zn-Mg alloy with the simultaneous formation of different types of precipitates at elevated temperatures based on the modified Langer-Schwartz approach. The double aging peaks are present in the long time age-hardening curves of Al-Zn-Mg alloys. The physically-based model, while taking explicitly into account nucleation, growth, coarsening of the new phase precipitations and two strengthening mechanisms associated with particle-dislocation interaction （shearing and bypassing）, was used for the analysis of precipitates evolution and precipitation hardening during aging of Al-Zn-Mg alloy. Model predictions were compared with the measurements of Al-Zn-Mg alloy. The systematic and quantitative results show that the predicted hardness profiles of double peaks via adding a shape dependent parameter in the growth equation for growth and coarsening generally agree well with the measured ones. Two strengthening mechanisms associated with particle-dislocation interaction （shearing and bypassing） were considered operating simultaneously in view of the particle size-distribution. The transition from shearing to bypassing strengthening mechanism was found to occur at rather early stage of the particle growth. The bypassing was found to be the prevailing strengthening mechanism in the investigated alloys.
基金Project(2014DFA51270)supported by the International Science and Technology Cooperation Program of ChinaProject(51421001)supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China
文摘The influences of high temperature pre-straining (HT-PS) on the natural aging and bake hardening of Al?Mg?Si alloys were investigated by Vickers microhardness measurements, differential scanning calorimetry (DSC) analysis and transmission electron microscopy (TEM) characterization. The results show that pre-straining at 170 °C immediately after quenching can effectively resolve the rather high T4 temper hardness caused by the conventional room temperature (RT) pre-straining treatment, and give a better bake hardening response (BHR) after paint-bake cycle. HT-PS 7% at 170 °C for 10 min is chosen as the optimum process as it provides lower T4 temper hardness and better BHR. The simultaneous introduction of dislocations and Cluster (2) can significantly suppress the natural aging and promote the precipitation of β″ phase, and reduce the effects of deformation hardening by dynamic recovery.
文摘The mathematical model of the grinding temperature is established. The grinding temperature and the cooling rate are measured in the grind-hardening process of 40Cr steel under different conditions. Moreover, the grind-hardening effects are investigated. Experimental results show that the calculated temperatures are comparatively close to the measured ones, and the required temperature and cooling rate can be achieved. Furthermore, the microstructure of the hardened zone is similar to that obtained through the high-frequency induction technique. The average hardness of the entirely hardened zone is HV670 and the thickness of the hardened layer is adjacent to 1.3 mm. It indicates that the hardening mechanism induced by the grinding heat and high-frequency heating is identical. Finally, the fine needlelike martensite is obtained.
基金The Project of the Ministry of Housing and Urban-Rural Development(No.2014-K4-010)
文摘The analysis of plane strain elastic-plastic bending of a linear strain hardening curved beam with a narrow rectangular cross section subjected to couples at its end is conducted based on a unified yield criterion. The solutions for the mechanical properties of plane strain bending are derived, which are adapted for various kinds of non-strength differential materials and can be degenerated to those based on the Tresca, von Mises, and twin-shear yield criteria. The dependences of the two critical bending moments, the radii of the interfaces between the elastic and plastic regions and the radial displacements of the points at the symmetrical plane on different yield criteria and Poisson’s ratios are discussed. The results show that the influences of different yield criteria and Poisson’s ratio on the two critical bending moments, the radii of the interfaces between the elastic and plastic regions and the radial displacements of the points at the symmetrical plane of the curved beam are significant. Once the value of bis obtained by experiments, the yield criterion and the corresponding solution for the materials of interest are then determined.
基金Project(2013AA031301)supported by Hi-tech Research and Development Program of ChinaProject(S2013ZR0611)supported by the National International Scientific and Technological Cooperation Program of China
文摘The precipitation hardening behavior in dilute Al-Yb alloys upon annealing at different temperatures was investigated to shed light on the mechanism of micro-alloying element in aluminum alloys. When aging at different temperatures, the samples showed their corresponding peak hardness in the range of 400-416 MPa due to the precipitation of Al3Yb with L 12 crystal structure. The coarsening kinetics of the Al3Yb precipitates obeyed the LSW theory, which indicated that the coarsening process was controlled by the diffusion of Yb. The coherence between Al3Yb particles and matrix was maintained until the particle size reached 11 nm. When the particle size increased to about 2 nm, the shearing mechanism started to change to Orowan mechanism.
基金supported by the Fundamental Research Funds for the Central Universities(N2107001)the National Natural Science Foundation of China(52001060).
文摘A novel Al-alloyed press-hardening steel(PHS)was developed,which exhibits excellent tensile,bending and antioxidation properties.Al is a ferrite-forming element that can hinder the formation of cementite and enhance the stability of austenite.The incorporation of Al not only induces the formation of ferrite within martensitic matrix but also enhances the stability of retained austenite(RA).The microstructure of novel steel consists of martensite,ferrite,and RA after press hardening.Investigations into the role of Al in RA development were supported by thermo-kinetic calculations.The simultaneous introduction of ferrite and RA into the martensitic matrix via tailored chemical compositions significantly enhances the elongation and bending toughness of the novel PHS.Additionally,Al can form a dense Al oxide at the bottom of oxide layer,resulting in the improved antioxidant properties.Compared to 22MnB5 steel,it is an exciting discovery as there is a significant improvement in total elongation and bending toughness of novel PHS without compromising strength.The novel PHS,with its exceptional balance of strength and ductility,will play a crucial role in reducing weight when it replaces the existing class 22MnB5 PHS in different structural components of vehicle bodies.
基金sponsored by National Natural Science Foundation of China(50934011,50971137)National Basic Research Program of China(2010CB630802)
文摘Specimens of two different kinds of bake hardening steels （BH-Mn and BH-P） were prepared and treated with different annealing processes （water quenching and overaging）. A novel technique of three dimensional atom probe was used to investigate solute distributions in these steels. The results indicate that C concentration decreases, whereas V increases during overaging in both bake hardening steels. The conclusion that no vanadium carbides pre- cipitate during the overaging is therefore originally obtained by microanalysis in bake hardening steels. Moreover, bake hardening values of all the specimens were tested by tensile experiments with 2 0/~ pre-deformation. However, those of overaged specimens were further measured with higher levels of pre-deformation because no bake hardening phenomenon was present at 2% pre-deformation. As the pre-deformation increases from 2% to 6% and 8%, both overaged steels show bake hardening values, and the value data are almost the same.
