The modified precipitation theory was employed to directly predict the multi-variantε-carbide precipitation from thermodynamics and growing and ripening kinetics.Three distinct variants were identified:Variants 1 and...The modified precipitation theory was employed to directly predict the multi-variantε-carbide precipitation from thermodynamics and growing and ripening kinetics.Three distinct variants were identified:Variants 1 and 2 were the perpendicular plate-likeε-carbides,while the granularε-carbides were Variant 3.The particle sizes of Variants 1 and 2 were usually larger than those of Variant 3.The mean aspect ratios of Variants 1 and 2 were 4.96,4.62 and 4.35 larger than those(1.72,1.63 and 1.56)for the granularε-carbides when coiled at 140,200 and 250℃,respectively.Thermodynamic analysis indicated that Variants 1 and 2 are easier to nucleate than Variant 3.The growing kinetic analysis implied that the relative nucleation time and precipitation time for Variants 1 and 2 were about 8 and 5 orders of magnitude less than those for Variant 3,respectively.The ripening kinetics further displayed that the ripening rate was similar for Variants 1,2 and 3.In addition,the dislocation density has weak influence onε-carbide nucleation.These findings suggest that the precipitation thermodynamic and kinetic models could be extended to second phase precipitation in other materials systems.Besides,nano-scaleε-carbides,fine block size and nano-twins,as well as medium-density dislocations,jointly caused the optimal match between strength and total elongation when coiled at 140℃.展开更多
304H austenitic stainless steel wire was investigated,emphasizing microstructural deformation,martensite phase transformation,and residual magnetic properties during drawing.Utilizing several microstructural observati...304H austenitic stainless steel wire was investigated,emphasizing microstructural deformation,martensite phase transformation,and residual magnetic properties during drawing.Utilizing several microstructural observation techniques,the volume fraction of martensite,modes of grain deformation in distinct regions,and the phase relationship between austenite and martensite were comprehensively characterized.In addition,a finite element simulation with representative volume elements specific to different zones also offers insights into strain responses during the drawing process.Results from the first-pass drawing reveal that there exists a higher volume fraction of martensite in the central region of 304H austenitic stainless steel wire compared to edge areas.This discrepancy is attributed to a concentrated presence of shear slip system{111}<110>γcrystallographic orientation,primarily accumulating in the central region obeying the Kurdjumov-Sachs path.Subsequent to the second drawing pass,the cumulative shear deformation within distinct regions of the steel wire became more pronounced.This resulted in a progressive augmentation of the volume fraction of martensite in both the central and peripheral regions of the steel wire.Concurrently,this led to a discernible elevation in the overall residual magnetism of the steel wire.展开更多
Nano-scale chemical inhomogeneity in surface oxide films formed on a V-and N-containing martensite stainless steel and tempering heating induced changes are investigated by a combination of synchrotron-based hard X-ra...Nano-scale chemical inhomogeneity in surface oxide films formed on a V-and N-containing martensite stainless steel and tempering heating induced changes are investigated by a combination of synchrotron-based hard X-ray Photoelectron emission spectroscopy(HAXPES)and microscopy(HAXPEEM)as well as microscopic X-ray absorption spectroscopy(μ-XAS)techniques.The results reveal the inhomogeneity in the oxide films on the micron-sized Cr_(2)N-and VN-type particles,while the inhomogeneity on the martensite matrix phase exists due to localised formation of nano-sized tempering nitride particles at 600℃.The oxide film formed on Cr_(2)N-type particles is rich in Cr_(2)O_(3) compared with that on the martensite matrix and VN-type particles.With the increase of tempering temperature,Cr_(2)O_(3) formation is faster for the oxidation of Cr in the martensite matrix than the oxidation of Cr nitride-rich particles.展开更多
Oxygen is crucial in influencing the microstructure evolution of selective laser melted(SLMed)Ti–6Al–4V,significantly impacting its applicability in various sectors.Therefore,this study investigates the influnce of ...Oxygen is crucial in influencing the microstructure evolution of selective laser melted(SLMed)Ti–6Al–4V,significantly impacting its applicability in various sectors.Therefore,this study investigates the influnce of oxygen on microstructure evolution,particularlyα′martensite transformation and refinement mechanisms.Four alloys,Ti–6Al–4V–xO(x=0.11,0.16,0.21,and 0.25 wt%),were fabricated by the SLM process.The martensite start temperature(M_(s))of Ti–6Al–4V,as evaluated by computation,is 656.8°C,and oxygen was found to increase the M_(s) by about 10°C per 0.1 wt%.The SLMed alloy samples exhibit[001]_(β)growth texture along the build direction.Crystallographic analysis of martensite morphology suggests internal twinning on{1011}planes as the lattice invariant strain,which becomes more predominant with increasing oxygen content.Refinement of α′martensite plates by oxygen is due to increased lattice distortion,reduced prior β grain size,and oxygen segregation toβgrain boundaries.Our findings contribute to improving the understanding of the effect of oxygen on the transformation mechanism ofα′martensite during SLM of Ti–6Al–4V.展开更多
1.Introduction Quenched Fe-C martensite is very strong but brittle due to su-persaturated carbon atoms located at octahedron sites[1].In order to improve the ductility,tempering is necessary.With increasing tempering ...1.Introduction Quenched Fe-C martensite is very strong but brittle due to su-persaturated carbon atoms located at octahedron sites[1].In order to improve the ductility,tempering is necessary.With increasing tempering temperature,the saturated carbon can be sequentially arranged as clusters,segregation into defects like dislocations and interfaces,transition carbides,and cementite[2].As a result,the strength is reduced and the ductility is improved predominantly due to the decreased amount of solid solution carbon[3].展开更多
The effect of the amount of isothermal martensite and bainite on the microstructure and properties in a medium-carbon quenching and partitioning(Q&P)steel was investigated by designing the different Q&P treatm...The effect of the amount of isothermal martensite and bainite on the microstructure and properties in a medium-carbon quenching and partitioning(Q&P)steel was investigated by designing the different Q&P treatment parameters.The results show that the amount of isothermal martensite increased gradually with the increase in quenching time.The increase in isothermal martensite amount improved the product of strength and elongation(PSE)of Q&P steels.In addition,the increase in carbides amount and the recovery in prior martensite with longer partitioning time led to an increase in PSE first and then,a decrease.It implies that a higher PSE could be obtained by the selection of a suitable partitioning time.Furthermore,the effect of bainite transformation during partitioning on PSE was investigated by designing the different partitioning temperatures,including 300,400(below bainite starting temperature,B_(s))and 480℃(above B_(s)).The results show that compared with the samples partitioned at temperature above B_(s),the bainite transformation was only detected when the samples were partitioned at temperature below B_(s).The bainite transformation amount increased with the decreasing partitioning temperature,leading to the inhibition of carbides precipitation and more stable RA and thus,resulting in the highest PSE.展开更多
Martensites distributed along the austenite grain boundaries and twin boundaries have been examined in Fe-C,Fe-Ni-C and Fe-Cr-Mn-Mo-C alloys.The martensites may nucleate preferentially and grow easily along these boun...Martensites distributed along the austenite grain boundaries and twin boundaries have been examined in Fe-C,Fe-Ni-C and Fe-Cr-Mn-Mo-C alloys.The martensites may nucleate preferentially and grow easily along these boundaries.In the mixed martensites,the preferentially formed one is plate or butterfly martensite展开更多
The apparent morphologies of packet martensite in eight high carbon steels were researched by using optical microscope, scanning electron microscope, and transmission electron microscope. It was found that the apparen...The apparent morphologies of packet martensite in eight high carbon steels were researched by using optical microscope, scanning electron microscope, and transmission electron microscope. It was found that the apparent morphologies, substructures, and habit plane of packet martensite in high carbon steels are entirely different from that in low carbon steels; the substructures of packet martensite in high carbon steels possess fully twinned structure, while the substructures of individual coarse martensite plates in these steels bear both fully and partially twinned structures. The formation reason for apparent morphologies, substructures and two habit planes （i. e, { 111 }, and { 225}r） of high carbon martensite were discussed in detail.展开更多
The as-quenched microstructures of low carbon steels were observed by scanning electron microscope, and the thin foil specimen was examined by transmission electron microscopy. It is found that the space morphology of...The as-quenched microstructures of low carbon steels were observed by scanning electron microscope, and the thin foil specimen was examined by transmission electron microscopy. It is found that the space morphology of low carbon martensite is not lath-like but thin sheet-like, which is designated as sheet-like martensite or packet thin sheet martensite. A three-dimensional model was presented. The reason for exhibiting two apparent morphologies, i.e. double contrast packet and simple contrast packet, in as-quenched low carbon steels was analyzed in detail. It is suggested that the data of martensitic habit plane measured by other procedures should be further inspected using optical metallographic method. The apparent morphologies of low carbon martensite confirm that its habit plane should be {557}_r, rather than {111}_r, {345}_r, nor {213}_r.展开更多
Designing of alloy concept and process for DP,TRIP and TWIP steels stressing at martensite transformation are analyzed.For DP steel,austenite volume percent and its carbon content at different intercritical temperatur...Designing of alloy concept and process for DP,TRIP and TWIP steels stressing at martensite transformation are analyzed.For DP steel,austenite volume percent and its carbon content at different intercritical temperatures are calculated as well as the tensile strength of the steel,which meet well with the experimental result.The condition for dissolution of carbide is discussed by experiments and predicted by kinetic estimation.Several sample TRIP steels are prepared and their concentration profiles are calculated showing different diffusion characteristics of elements.Calculation also shows carbon enrichment is successful in this stage through the quick diffusion of carbon from ferrite to austenie.In order to maintain the austenite stability or to prevent precipitation of cementite,minimum cooling rate from the intercritical zone to over aging stage is obtained through kinetic simulation.Bainite transformation is estimated,which indicates the carbon rerichment from ferrite of bainite structure to austenite in this stage is also successful.Thermal HCP martensite transformation and the strain induced martensite transformation in TWIP steel is introduced.Relationship between transformation and mechanical properties in the steel is also mentioned.展开更多
It is not clear whether a shape memory effect （SME） can be realized by stress-induced α＇ martensite in metastable austenitic stainless steels although the stress-induced ε martensite in these materials can result...It is not clear whether a shape memory effect （SME） can be realized by stress-induced α＇ martensite in metastable austenitic stainless steels although the stress-induced ε martensite in these materials can result in the SME. To clarify this problem, the relationship between the shape recovery and the reverse transformation of the stress-induced ε and α＇ martensite in a 304 stainless steel was investigated. The results show that the stress-induced α＇ martensite can result in the SME when heating above 773 K. After deformation at 77 K and step heating or directly holding at 1073 K, two-stage shape recoveries below 440 K and above 773 K can be obtained due to the reverse transformation of the stress-induced ε martensite and α＇ martensite, respectively. After deformation at room temperature, the α＇ martensite produced can result in the SME only when directly holding at 1073 K. The intrusion of more dislocations before the formation of the α＇ martensite at room temperature than at 77 K is the reason that the α＇ martensite induced at room temperature cannot result in the SME in the case of slow heating. The recovered strains resulting from the stress-induced ε and α＇ martensite are proportional to the amounts of their reverse transformation, respectively.展开更多
1 Introduction The method of acoustic emission (AE)as a new technique to study the processesin metals and alloys in the solid phase hasbeen widely used and developed since thelast 20 years. Martensite transformationis...1 Introduction The method of acoustic emission (AE)as a new technique to study the processesin metals and alloys in the solid phase hasbeen widely used and developed since thelast 20 years. Martensite transformationis diffusionless. Because of changing fromone crystal structure to another by coopera-tive shear process the interfacial coherency展开更多
The morphology and formation mechanism of the substructure of martensite in TC21 alloy was investigated by XRD and TEM. The results showed that the martensitic transformation from β to α＂ occurs upon quenching afte...The morphology and formation mechanism of the substructure of martensite in TC21 alloy was investigated by XRD and TEM. The results showed that the martensitic transformation from β to α＂ occurs upon quenching after solution treatment between 960-1000 ℃. The antiphase boundary （APB）-like structure was observed clearly in the α＂ martensite plates. The APB-like contrasts exist along the （001） and （020） planes of α＂ martensite. This APB-like structure of α＂ martensite was identified as a kind of stacking fault with an APB-like morphology induced by martensitic transformation and not by order/disorder transition. During martensitic transformation, martensitic domains nucleate and grow, eventually encounter each other, resulting in the formation of the APBdike contrast.展开更多
The quantitative analysis of substructure in the martensite/bainite mixed structure, which is obtained from low-carbon NiCrMoV steels under different cooling conditions, was made by means of optical microscope （OM）,...The quantitative analysis of substructure in the martensite/bainite mixed structure, which is obtained from low-carbon NiCrMoV steels under different cooling conditions, was made by means of optical microscope （OM）, scanning electron microscope （SEM）, electron backscatter diffraction （EBSD）, and transmission electron microscope （TEM）, in order to research the effect on toughness. The test results indicate that the toughness of the steel is en- hanced with the decrease in the packet and block size under the condition of the same prior austenite grain size mixed with different ratios of martensite and bainite while the lath width is about 0.38μm. The calculation shows that both the packet and block boundaries have the same hindering effect on crack extension. Furthermore, the effect of the block width on impact energy is much larger than that of the packet. Therefore, the block can be used as microstruc- tural substructure to affect the toughness in low-carbon martensite steels, suggesting that the block size is ＂the effective grain size＂ for controlling toughness.展开更多
An Fe-0.2C-1.5Si-1.67Mn steel was subjected to quenching and partitioning （Q＆P） process, and the interface migration between martensite and austenite at an elevated partitioning temperature was observed. The interf...An Fe-0.2C-1.5Si-1.67Mn steel was subjected to quenching and partitioning （Q＆P） process, and the interface migration between martensite and austenite at an elevated partitioning temperature was observed. The interface migration is excluded in constrained paraequilibrium （CPE） model. Based on ＂endpoint＂ predicted by CPE model the thermodynamic condition of interface migration is analyzed, that is, the difference in the chemical potential of iron in both ferrite （martenisite） and austenite produces the driving force of the iron atoms to migrate from one phase to the other phase. In addition, the interface migration can change the austenite fraction; as a result, the austenite fraction at partitioning temperature may be higher than that at quenching temperature through the interface migration, but this phenomenon cannot be explained by CPE model.展开更多
The high strength martensite steels are widely used in aerospace,ocean engineering,etc.,due to their high strength,good ductility and acceptable corrosion resistance.This paper provides a review for the influence of m...The high strength martensite steels are widely used in aerospace,ocean engineering,etc.,due to their high strength,good ductility and acceptable corrosion resistance.This paper provides a review for the influence of microstructure on corrosion behavior of high strength martensite steels.Pitting is the most common corrosion type of high strength stainless steels,which always occurs at weak area of passive film such as inclusions,carbide/intermetallic interfaces.Meanwhile,the chromium carbide precipitations in the martensitic lath/prior austenite boundaries always result in intergranular corrosion.The precipitation,dislocation and grain/lath boundary are also used as crack nucleation and hydrogen traps,leading to hydrogen embrittlement and stress corrosion cracking for high strength martensite steels.Yet,the retained/reversed austenite has beneficial effects on the corrosion resistance and could reduce the sensitivity of stress corrosion cracking for high strength martensite steels.Finally,the corrosion mechanisms of additive manufacturing high strength steels and the ideas for designing new high strength martensite steel are explored.展开更多
Two near single-phase NiTiNb alloys--NisoTi4sNb2 and Ni49.5Ti46.5Nb4-are prepared and studied by means of scanning electron microscopy （SEM）, energy dispersive spectroscopy （EDS）, differential scanning calorimetry...Two near single-phase NiTiNb alloys--NisoTi4sNb2 and Ni49.5Ti46.5Nb4-are prepared and studied by means of scanning electron microscopy （SEM）, energy dispersive spectroscopy （EDS）, differential scanning calorimetry （DSC） and tensile tests in order to unearth the effects of Nb-atom solid solution in NiTi phase on the yield strength induced by self-accommodation of martensite variants. The results show that the yield strength of near single-phase NiTiNb alloys varies inversely with the amount of Nb-atoms solid-dissolved in NiTi phase. From the results out of the prior and current studies, it can be surmised that the effects of Nb content on the yield strength of NiTiNb alloys in martensite state depend on the coaction. Nb solid solution weakening mechanism and β-Nb phase composite strengthening mechanism. This inference might be a satisfactory explanation to the fact that the yield strength of （NiTi）50-0.5xNbx alloys in martensite state begins with decline and then rises when the Nb content increases.展开更多
The crystallography of martensite formed in 0.2C-2.0Mn-1,5Si-0.6Cr steel was studied using the electron backscattered diffraction （EBSD） technique. The results showed that the observed orientation relationship （OR...The crystallography of martensite formed in 0.2C-2.0Mn-1,5Si-0.6Cr steel was studied using the electron backscattered diffraction （EBSD） technique. The results showed that the observed orientation relationship （OR） was closer to that of Nishiyama-Wassermann （N-W） than Kurdjumov-Sachs. The martensite consisted of parallel laths forming morphological packets. Typically, there were three different lath orientations in a morphological packet consisting of three specific N-W OR variants sharing the same {111} austenite plane. A packet of martensite laths with a common {111} austenite plane was termed a crystallographic packet. Generally, the crystallographic packet size corresponded to the morphological packet size, but occasionally the morphological packet was found to consist of two or more crystallographic packets. Therefore, the crystallographic packet size appeared to be finer than the morphological packet size. The relative orientation between the variants in crystallographic packets was found to be near 60°〈110〉, which explains the strong peak observed near 60° in the grain boundary misorientation distribution. Martensite also contained a high fraction of boundaries with a misorientation in the range 2.5-8°. Typically these boundaries were found to be located inside the martensite laths forming sub-laths.展开更多
Tempering is an important process for T/P92 ferritic heat-resistant steel from the viewpoint of microstructure control, as it facili- tates the formation of final tempered martensite under serving conditions. In this ...Tempering is an important process for T/P92 ferritic heat-resistant steel from the viewpoint of microstructure control, as it facili- tates the formation of final tempered martensite under serving conditions. In this study, we have gained deeper insights on the mechanism underlying the microstructural evolution during tempering treatment, including the precipitation of carbides and the coarsening of martensite laths, as systematically analyzed by optical microscopy, transmission electron microscopy, and high-resolution transmission electron mi- croscopy. The chemical composition of the precipitates was analyzed using energy dispersive X-ray spectroscopy. Results indicate the for- mation of M3C （cementite） precipitates under normalized conditions. However, they tend to dissolve within a short time of tempering, owing to their low thermal stability. This phenomenon was substantiated by X-ray diffraction analysis. Besides, we could observe the precipitation of fine carbonitrides （MX） along the dislocations. The mechanism of carbon diffusion controlled growth of M23C6 can be expressed by the Zener＇s equation. The movement of Y-junctions was determined to be the fundamental mechanism underlying the martensite lath coarsening process. Vickers hardness was estimated to determine their mechanical properties. Based on the comprehensive analysis of both the micro- structural evolution and hardness variation, the process of tempering can be separated into three steps.展开更多
基金supported by the National Natural Science Foundation of China(No.52293395)National Key R&D Program of China(No.2021YFB3702403).
文摘The modified precipitation theory was employed to directly predict the multi-variantε-carbide precipitation from thermodynamics and growing and ripening kinetics.Three distinct variants were identified:Variants 1 and 2 were the perpendicular plate-likeε-carbides,while the granularε-carbides were Variant 3.The particle sizes of Variants 1 and 2 were usually larger than those of Variant 3.The mean aspect ratios of Variants 1 and 2 were 4.96,4.62 and 4.35 larger than those(1.72,1.63 and 1.56)for the granularε-carbides when coiled at 140,200 and 250℃,respectively.Thermodynamic analysis indicated that Variants 1 and 2 are easier to nucleate than Variant 3.The growing kinetic analysis implied that the relative nucleation time and precipitation time for Variants 1 and 2 were about 8 and 5 orders of magnitude less than those for Variant 3,respectively.The ripening kinetics further displayed that the ripening rate was similar for Variants 1,2 and 3.In addition,the dislocation density has weak influence onε-carbide nucleation.These findings suggest that the precipitation thermodynamic and kinetic models could be extended to second phase precipitation in other materials systems.Besides,nano-scaleε-carbides,fine block size and nano-twins,as well as medium-density dislocations,jointly caused the optimal match between strength and total elongation when coiled at 140℃.
基金funded by National Natural Science Foundation of China(52201084 and 52231003)Major Program(JD)of Hubei Province(2023BAA019)+2 种基金China Scholarship Council(CSC)Postdoctoral Station of metallurgical Engineering of Wuhan University of Science and Technology(WUST)Postdoctoral workstation of Zhejiang Jincheng New Material Co.,Ltd.
文摘304H austenitic stainless steel wire was investigated,emphasizing microstructural deformation,martensite phase transformation,and residual magnetic properties during drawing.Utilizing several microstructural observation techniques,the volume fraction of martensite,modes of grain deformation in distinct regions,and the phase relationship between austenite and martensite were comprehensively characterized.In addition,a finite element simulation with representative volume elements specific to different zones also offers insights into strain responses during the drawing process.Results from the first-pass drawing reveal that there exists a higher volume fraction of martensite in the central region of 304H austenitic stainless steel wire compared to edge areas.This discrepancy is attributed to a concentrated presence of shear slip system{111}<110>γcrystallographic orientation,primarily accumulating in the central region obeying the Kurdjumov-Sachs path.Subsequent to the second drawing pass,the cumulative shear deformation within distinct regions of the steel wire became more pronounced.This resulted in a progressive augmentation of the volume fraction of martensite in both the central and peripheral regions of the steel wire.Concurrently,this led to a discernible elevation in the overall residual magnetism of the steel wire.
基金supported by the Vinnova(project number 2020-03778)supported by the Swedish Research Council(Vetenskapsradet,project number 2021-04157).
文摘Nano-scale chemical inhomogeneity in surface oxide films formed on a V-and N-containing martensite stainless steel and tempering heating induced changes are investigated by a combination of synchrotron-based hard X-ray Photoelectron emission spectroscopy(HAXPES)and microscopy(HAXPEEM)as well as microscopic X-ray absorption spectroscopy(μ-XAS)techniques.The results reveal the inhomogeneity in the oxide films on the micron-sized Cr_(2)N-and VN-type particles,while the inhomogeneity on the martensite matrix phase exists due to localised formation of nano-sized tempering nitride particles at 600℃.The oxide film formed on Cr_(2)N-type particles is rich in Cr_(2)O_(3) compared with that on the martensite matrix and VN-type particles.With the increase of tempering temperature,Cr_(2)O_(3) formation is faster for the oxidation of Cr in the martensite matrix than the oxidation of Cr nitride-rich particles.
基金financially supported by the National Natural Science Foundation of China(No.52205431)the CAS Interdisciplinary Innovation Team Project(No.JCTD-2020-10)。
文摘Oxygen is crucial in influencing the microstructure evolution of selective laser melted(SLMed)Ti–6Al–4V,significantly impacting its applicability in various sectors.Therefore,this study investigates the influnce of oxygen on microstructure evolution,particularlyα′martensite transformation and refinement mechanisms.Four alloys,Ti–6Al–4V–xO(x=0.11,0.16,0.21,and 0.25 wt%),were fabricated by the SLM process.The martensite start temperature(M_(s))of Ti–6Al–4V,as evaluated by computation,is 656.8°C,and oxygen was found to increase the M_(s) by about 10°C per 0.1 wt%.The SLMed alloy samples exhibit[001]_(β)growth texture along the build direction.Crystallographic analysis of martensite morphology suggests internal twinning on{1011}planes as the lattice invariant strain,which becomes more predominant with increasing oxygen content.Refinement of α′martensite plates by oxygen is due to increased lattice distortion,reduced prior β grain size,and oxygen segregation toβgrain boundaries.Our findings contribute to improving the understanding of the effect of oxygen on the transformation mechanism ofα′martensite during SLM of Ti–6Al–4V.
基金National Natural Science Foundation of China(Nos.52271004 and 51901021).
文摘1.Introduction Quenched Fe-C martensite is very strong but brittle due to su-persaturated carbon atoms located at octahedron sites[1].In order to improve the ductility,tempering is necessary.With increasing tempering temperature,the saturated carbon can be sequentially arranged as clusters,segregation into defects like dislocations and interfaces,transition carbides,and cementite[2].As a result,the strength is reduced and the ductility is improved predominantly due to the decreased amount of solid solution carbon[3].
基金The authors gratefully acknowledge the financial supports from the National Natural Science Foundation of China(No.52104381)the China Postdoctoral Science Foundation(No.2023M732721)also the help on microstructure analysis from Dr.Zhen Wang at the Analytical and Testing Center of Wuhan University of Science and Technology.
文摘The effect of the amount of isothermal martensite and bainite on the microstructure and properties in a medium-carbon quenching and partitioning(Q&P)steel was investigated by designing the different Q&P treatment parameters.The results show that the amount of isothermal martensite increased gradually with the increase in quenching time.The increase in isothermal martensite amount improved the product of strength and elongation(PSE)of Q&P steels.In addition,the increase in carbides amount and the recovery in prior martensite with longer partitioning time led to an increase in PSE first and then,a decrease.It implies that a higher PSE could be obtained by the selection of a suitable partitioning time.Furthermore,the effect of bainite transformation during partitioning on PSE was investigated by designing the different partitioning temperatures,including 300,400(below bainite starting temperature,B_(s))and 480℃(above B_(s)).The results show that compared with the samples partitioned at temperature above B_(s),the bainite transformation was only detected when the samples were partitioned at temperature below B_(s).The bainite transformation amount increased with the decreasing partitioning temperature,leading to the inhibition of carbides precipitation and more stable RA and thus,resulting in the highest PSE.
文摘Martensites distributed along the austenite grain boundaries and twin boundaries have been examined in Fe-C,Fe-Ni-C and Fe-Cr-Mn-Mo-C alloys.The martensites may nucleate preferentially and grow easily along these boundaries.In the mixed martensites,the preferentially formed one is plate or butterfly martensite
文摘The apparent morphologies of packet martensite in eight high carbon steels were researched by using optical microscope, scanning electron microscope, and transmission electron microscope. It was found that the apparent morphologies, substructures, and habit plane of packet martensite in high carbon steels are entirely different from that in low carbon steels; the substructures of packet martensite in high carbon steels possess fully twinned structure, while the substructures of individual coarse martensite plates in these steels bear both fully and partially twinned structures. The formation reason for apparent morphologies, substructures and two habit planes （i. e, { 111 }, and { 225}r） of high carbon martensite were discussed in detail.
文摘The as-quenched microstructures of low carbon steels were observed by scanning electron microscope, and the thin foil specimen was examined by transmission electron microscopy. It is found that the space morphology of low carbon martensite is not lath-like but thin sheet-like, which is designated as sheet-like martensite or packet thin sheet martensite. A three-dimensional model was presented. The reason for exhibiting two apparent morphologies, i.e. double contrast packet and simple contrast packet, in as-quenched low carbon steels was analyzed in detail. It is suggested that the data of martensitic habit plane measured by other procedures should be further inspected using optical metallographic method. The apparent morphologies of low carbon martensite confirm that its habit plane should be {557}_r, rather than {111}_r, {345}_r, nor {213}_r.
基金financially supported by VANITECNational Key Technology R&D Program of the Eleventh Five-year Plan+1 种基金National Key Basic research development project of China (973 Programme, No.2010CB630802)NSFC (No.50934011 and No.50971137)
文摘Designing of alloy concept and process for DP,TRIP and TWIP steels stressing at martensite transformation are analyzed.For DP steel,austenite volume percent and its carbon content at different intercritical temperatures are calculated as well as the tensile strength of the steel,which meet well with the experimental result.The condition for dissolution of carbide is discussed by experiments and predicted by kinetic estimation.Several sample TRIP steels are prepared and their concentration profiles are calculated showing different diffusion characteristics of elements.Calculation also shows carbon enrichment is successful in this stage through the quick diffusion of carbon from ferrite to austenie.In order to maintain the austenite stability or to prevent precipitation of cementite,minimum cooling rate from the intercritical zone to over aging stage is obtained through kinetic simulation.Bainite transformation is estimated,which indicates the carbon rerichment from ferrite of bainite structure to austenite in this stage is also successful.Thermal HCP martensite transformation and the strain induced martensite transformation in TWIP steel is introduced.Relationship between transformation and mechanical properties in the steel is also mentioned.
基金supported by the National Natural Science Foundation of China(Nos.51271128 and 51671138)
文摘It is not clear whether a shape memory effect （SME） can be realized by stress-induced α＇ martensite in metastable austenitic stainless steels although the stress-induced ε martensite in these materials can result in the SME. To clarify this problem, the relationship between the shape recovery and the reverse transformation of the stress-induced ε and α＇ martensite in a 304 stainless steel was investigated. The results show that the stress-induced α＇ martensite can result in the SME when heating above 773 K. After deformation at 77 K and step heating or directly holding at 1073 K, two-stage shape recoveries below 440 K and above 773 K can be obtained due to the reverse transformation of the stress-induced ε martensite and α＇ martensite, respectively. After deformation at room temperature, the α＇ martensite produced can result in the SME only when directly holding at 1073 K. The intrusion of more dislocations before the formation of the α＇ martensite at room temperature than at 77 K is the reason that the α＇ martensite induced at room temperature cannot result in the SME in the case of slow heating. The recovered strains resulting from the stress-induced ε and α＇ martensite are proportional to the amounts of their reverse transformation, respectively.
文摘1 Introduction The method of acoustic emission (AE)as a new technique to study the processesin metals and alloys in the solid phase hasbeen widely used and developed since thelast 20 years. Martensite transformationis diffusionless. Because of changing fromone crystal structure to another by coopera-tive shear process the interfacial coherency
基金Project (2011AA030101) supported by the High-tech Research and Development Program of China
文摘The morphology and formation mechanism of the substructure of martensite in TC21 alloy was investigated by XRD and TEM. The results showed that the martensitic transformation from β to α＂ occurs upon quenching after solution treatment between 960-1000 ℃. The antiphase boundary （APB）-like structure was observed clearly in the α＂ martensite plates. The APB-like contrasts exist along the （001） and （020） planes of α＂ martensite. This APB-like structure of α＂ martensite was identified as a kind of stacking fault with an APB-like morphology induced by martensitic transformation and not by order/disorder transition. During martensitic transformation, martensitic domains nucleate and grow, eventually encounter each other, resulting in the formation of the APBdike contrast.
文摘The quantitative analysis of substructure in the martensite/bainite mixed structure, which is obtained from low-carbon NiCrMoV steels under different cooling conditions, was made by means of optical microscope （OM）, scanning electron microscope （SEM）, electron backscatter diffraction （EBSD）, and transmission electron microscope （TEM）, in order to research the effect on toughness. The test results indicate that the toughness of the steel is en- hanced with the decrease in the packet and block size under the condition of the same prior austenite grain size mixed with different ratios of martensite and bainite while the lath width is about 0.38μm. The calculation shows that both the packet and block boundaries have the same hindering effect on crack extension. Furthermore, the effect of the block width on impact energy is much larger than that of the packet. Therefore, the block can be used as microstruc- tural substructure to affect the toughness in low-carbon martensite steels, suggesting that the block size is ＂the effective grain size＂ for controlling toughness.
文摘An Fe-0.2C-1.5Si-1.67Mn steel was subjected to quenching and partitioning （Q＆P） process, and the interface migration between martensite and austenite at an elevated partitioning temperature was observed. The interface migration is excluded in constrained paraequilibrium （CPE） model. Based on ＂endpoint＂ predicted by CPE model the thermodynamic condition of interface migration is analyzed, that is, the difference in the chemical potential of iron in both ferrite （martenisite） and austenite produces the driving force of the iron atoms to migrate from one phase to the other phase. In addition, the interface migration can change the austenite fraction; as a result, the austenite fraction at partitioning temperature may be higher than that at quenching temperature through the interface migration, but this phenomenon cannot be explained by CPE model.
文摘The high strength martensite steels are widely used in aerospace,ocean engineering,etc.,due to their high strength,good ductility and acceptable corrosion resistance.This paper provides a review for the influence of microstructure on corrosion behavior of high strength martensite steels.Pitting is the most common corrosion type of high strength stainless steels,which always occurs at weak area of passive film such as inclusions,carbide/intermetallic interfaces.Meanwhile,the chromium carbide precipitations in the martensitic lath/prior austenite boundaries always result in intergranular corrosion.The precipitation,dislocation and grain/lath boundary are also used as crack nucleation and hydrogen traps,leading to hydrogen embrittlement and stress corrosion cracking for high strength martensite steels.Yet,the retained/reversed austenite has beneficial effects on the corrosion resistance and could reduce the sensitivity of stress corrosion cracking for high strength martensite steels.Finally,the corrosion mechanisms of additive manufacturing high strength steels and the ideas for designing new high strength martensite steel are explored.
基金National Natural Science Foundation of China （50971009） Science Fund for Creative Research Groups （50921003） Aviation Science Foundation of China （2009ZF51059）
文摘Two near single-phase NiTiNb alloys--NisoTi4sNb2 and Ni49.5Ti46.5Nb4-are prepared and studied by means of scanning electron microscopy （SEM）, energy dispersive spectroscopy （EDS）, differential scanning calorimetry （DSC） and tensile tests in order to unearth the effects of Nb-atom solid solution in NiTi phase on the yield strength induced by self-accommodation of martensite variants. The results show that the yield strength of near single-phase NiTiNb alloys varies inversely with the amount of Nb-atoms solid-dissolved in NiTi phase. From the results out of the prior and current studies, it can be surmised that the effects of Nb content on the yield strength of NiTiNb alloys in martensite state depend on the coaction. Nb solid solution weakening mechanism and β-Nb phase composite strengthening mechanism. This inference might be a satisfactory explanation to the fact that the yield strength of （NiTi）50-0.5xNbx alloys in martensite state begins with decline and then rises when the Nb content increases.
基金Funding from The Finnish Funding Agency for Technology and Innovation (Tekes) is gratefully acknowledged.(QaMiS project,No. 1691/31/07No. 40197/07)
文摘The crystallography of martensite formed in 0.2C-2.0Mn-1,5Si-0.6Cr steel was studied using the electron backscattered diffraction （EBSD） technique. The results showed that the observed orientation relationship （OR） was closer to that of Nishiyama-Wassermann （N-W） than Kurdjumov-Sachs. The martensite consisted of parallel laths forming morphological packets. Typically, there were three different lath orientations in a morphological packet consisting of three specific N-W OR variants sharing the same {111} austenite plane. A packet of martensite laths with a common {111} austenite plane was termed a crystallographic packet. Generally, the crystallographic packet size corresponded to the morphological packet size, but occasionally the morphological packet was found to consist of two or more crystallographic packets. Therefore, the crystallographic packet size appeared to be finer than the morphological packet size. The relative orientation between the variants in crystallographic packets was found to be near 60°〈110〉, which explains the strong peak observed near 60° in the grain boundary misorientation distribution. Martensite also contained a high fraction of boundaries with a misorientation in the range 2.5-8°. Typically these boundaries were found to be located inside the martensite laths forming sub-laths.
基金financially supported by the China National Funds for Distinguished Young Scientists(No.51325401)the International Thermonuclear Experimental Reactor(ITER)Program Special Project(No.2014GB125006)+1 种基金the National Natural Science Foundation of China(No.51104107)the Major State Basic Research Development Program(No.2014CB046805)
文摘Tempering is an important process for T/P92 ferritic heat-resistant steel from the viewpoint of microstructure control, as it facili- tates the formation of final tempered martensite under serving conditions. In this study, we have gained deeper insights on the mechanism underlying the microstructural evolution during tempering treatment, including the precipitation of carbides and the coarsening of martensite laths, as systematically analyzed by optical microscopy, transmission electron microscopy, and high-resolution transmission electron mi- croscopy. The chemical composition of the precipitates was analyzed using energy dispersive X-ray spectroscopy. Results indicate the for- mation of M3C （cementite） precipitates under normalized conditions. However, they tend to dissolve within a short time of tempering, owing to their low thermal stability. This phenomenon was substantiated by X-ray diffraction analysis. Besides, we could observe the precipitation of fine carbonitrides （MX） along the dislocations. The mechanism of carbon diffusion controlled growth of M23C6 can be expressed by the Zener＇s equation. The movement of Y-junctions was determined to be the fundamental mechanism underlying the martensite lath coarsening process. Vickers hardness was estimated to determine their mechanical properties. Based on the comprehensive analysis of both the micro- structural evolution and hardness variation, the process of tempering can be separated into three steps.
