The evolution of precipitates of S31042 steel during 700 ℃ aging was investigated by using a scanning elec- tron microscope, a transmission electron microscope, and electron energy spectrum technology. The various co...The evolution of precipitates of S31042 steel during 700 ℃ aging was investigated by using a scanning elec- tron microscope, a transmission electron microscope, and electron energy spectrum technology. The various combi nations of M23C6, MX, NbCrN, and σ and G phases in the steel were found at different aging states. In the begin ning of aging, M23C6 precipitates swiftly along the grain boundaries. When the aging time exceeds 6 000 h, precipita- ted M23C6 carbides along the grain boundaries turn to be granular. It was found that Si element segregates to grain boundaries during above process, which may enhance the granular shape of M23C6 carbides and its transformation to and G phases. When the aging time exceeds 10 000 h, various shaped a phase and granular G phase appear along the grain boundaries and there are no continuous M23C6 carbides along the grain boundaries. Meanwhile, a large quantity of granular M23C6 carbides and a minor amount of G phase precipitate near the grain boundaries. Based on the segre- gation of silicon to the grain boundaries, a precipitation evolution model during aging was concluded.展开更多
We discussed the decrease in residual stress,precipitation evolution,and mechanical properties of GH4151 alloy in different annealing temperatures,which were studied by the scanning electron microscope(SEM),high-resol...We discussed the decrease in residual stress,precipitation evolution,and mechanical properties of GH4151 alloy in different annealing temperatures,which were studied by the scanning electron microscope(SEM),high-resolution transmission electron microscopy(HRTEM),and electron backscatter diffraction(EBSD).The findings reveal that annealing processing has a significant impact on diminishing residual stresses.As the annealing temperature rose from 950 to 1150℃,the majority of the residual stresses were relieved from 60.1 MPa down to 10.9 MPa.Moreover,the stress relaxation mechanism transitioned from being mainly controlled by dislocation slip to a combination of dislocation slip and grain boundary migration.Meanwhile,the annealing treatment promotes the decomposition of the Laves,accompanied by the precipitation ofμ-(Mo_(6)Co_(7))starting at 950℃ and reaching a maximum value at 1050℃.The tensile strength and plasticity of the annealing alloy at 1150℃ reached the maximum(1394 MPa,56.1%)which was 131%,200%fold than those of the as-cast alloy(1060 MPa,26.6%),but the oxidation process in the alloy was accelerated at 1150℃.The enhancement in durability and flexibility is primarily due to the dissolution of the brittle phase,along with the shape and dispersal of theγ′phase.展开更多
The precipitation evolution and mechanical properties of a commercial Al-Cu-Li-Mg alloy in natural aging(NA)tempers(T3 and T4)were investigated in detail.It was demonstrated that GPI zones were the pre-dominant precip...The precipitation evolution and mechanical properties of a commercial Al-Cu-Li-Mg alloy in natural aging(NA)tempers(T3 and T4)were investigated in detail.It was demonstrated that GPI zones were the pre-dominant precipitate throughout the NA process.In the T3-aged sample,the precipitation sequence was determined:supersaturated solid solution(SSSS)→atom clusters→GPI zones,whereas in the T4-aged sample,it was SSSS→atom clusters→GPI zones→GPI zones+δ/GPI zones/δ.For samples without NA,the strength of the T3 sample was 76.0 MPa higher than that in the T4 sample,which was attributed to the pre-deformation-introduced dislocations.During the early stage of NA,GPI zone nucleation was substantially restricted in the T3 sample,resulting in a much lower number density of GPI zones.It re-tarded the aging response and strength improvement,consequently narrowing the strength difference between T3 and T4 samples.As the NA process progressed,the average diameter of GPI zones in the T4 sample increased to approximately 4.5 nm,accompanied by the formation ofδ/GPI zones/δcomposite precipitate.However,GPI zone growth andδ/GPI zones/δcomposite precipitate formation were signifi-cantly inhibited in the T3 sample,as evidenced by its much smaller average diameter of approximately 2.1 nm and the absence of composite precipitates.It inhibited the strength improvement of the T3 sam-ple.Therefore,after a long period(5 months)of NA,the strength of the T4 sample was about 13.0 MPa higher than that of the T3 samples,instead.展开更多
The influence of minor Ag on the precipitation evolution of the Al-4.2Zn-2.8Mg-1.0Cu(wt.%)alloy from early stages to over-aged stages at 150°C was investigated.Surprisingly,co-precipitation of strengthening phase...The influence of minor Ag on the precipitation evolution of the Al-4.2Zn-2.8Mg-1.0Cu(wt.%)alloy from early stages to over-aged stages at 150°C was investigated.Surprisingly,co-precipitation of strengthening phases T′andη′are found in both Ag-free and Ag-added alloys.With Ag addition,precipitation of both T′andη′is refined and increased,such that the age-hardening capabilities and peak-aged tensile strength are improved.In addition,the quantitative proportion ofη′precipitates increases with the increase of Ag content due to the increase in the(Zn+Cu)/Mg ratio of nucleating particles.The narrowed precipitate-free zones(PFZs)are considered responsible for the undiminished fracture elongation in Ag-added alloys.Essentially,these effects of Ag are closely related to the strong Ag-vacancy and Ag-solute interactions.In over-aged stages,the Ag-added alloys still possess higher hardness values compared to the Ag-free alloy,which is related to precipitate coarsening mechanisms.The Ag-free alloy follows classical coarsening behavior by solid solution mediated diffusion,while the Ag-added alloy follows two possible coarsening mechanisms,coalescence of aggregates and diffusion of atoms.The smaller average size and higher residual number density of precipitates benefited from the slow diffusion-controlled coarsening behavior depending on the precipitate composition characteristics of the two-stage differentiation and the precipitate distribution characteristics of high-density dispersion in early-aged stages could explain why the hardness of Ag-added alloy keeps at a higher level than that of Ag-free alloy even after 1000 h ageing.Meanwhile,the transformation of metastable phases to stable phases is inhibited due to the addition of Ag,such that GP zones,T′,η′,ηand T phases coexist even after 14 d of ageing.In terms of phase composition,the addition of Ag decreases the ratio of Mg/(Al+Zn)in T-type phase.For the Ag-added alloy,the sum concentration of Zn+Mg inη′phase is about 10 at.%higher compared to T′phase,andηphase continues to have a high sum concentration of Zn+Mg,besides,the Zn/Mg ratio and Cu concentration exhibit obvious differences from T phase.展开更多
In the precipitation-hardened Ni-based superalloy,typified by ATI 718 Plus,the nano-scaleγandγphase in duplet or triple coprecipitate morphology can provide superior high-temperature strength.Thus,it is of great sen...In the precipitation-hardened Ni-based superalloy,typified by ATI 718 Plus,the nano-scaleγandγphase in duplet or triple coprecipitate morphology can provide superior high-temperature strength.Thus,it is of great sense to study the evolution ofγ’/γ’’coprecipitate during long term service at elevated temperature.In this study,the new-typeγ’/γ’’coprecipitates with a sandwich or compact configuration were found firstly in wrought ATI 718 Plus superalloy during long term thermal exposure at 705℃.These co-structure of theγ’/γ’’precipitates evidently inhibit the coarsening ofγ’phase.The increase of thermal exposure time evidently leads to the increase of the volume fraction ofγ’/γ’’coprecipitate and transformation of sandwich-typeγ’/γ’’coprecipitate to compact-typeγ’/γ’’coprecipitate,which is characterized asγphase precipitate at several faces of theγphase.The main evolution mechanism ofγ’/γ’’coprecipitates is element segregation,especially the composition variations of Al+Ti and Nb and their ratio of Al+Ti/Nb.In addition,the interfacial energy betweenγ’’phase andγmatrix also plays a key role on theγ’/γ’’coprecipitates evolution.The calculated results show that the longer thermal exposure time leads to the higher interfacial energy,which is beneficial for nucleation and precipitation ofγ’’phase on the faces ofγ’phase.展开更多
In this study, we used the stop-action technique to experimentally investigate the material flow and microstructural evolution of alclad 2A12-T4 aluminum alloy during refill friction stir spot welding.There are two ma...In this study, we used the stop-action technique to experimentally investigate the material flow and microstructural evolution of alclad 2A12-T4 aluminum alloy during refill friction stir spot welding.There are two material flow components, i.e., the inward-or outward-directed spiral flow on the horizontal plane and the upward-or downward-directed flow on the vertical plane.In the plunge stage, the flow of plasticized metal into the cavity is similar to that of a stack, whereby the upper layer is pushed upward by the lower layer.In the refill stage, this is process reversed.As such, there is no obvious vertical plasticized metal flow between adjacent layers.Welding leads to the coarsening of S(Al2CuMg) in the thermo-mechanically affected zone and the diminishing of S in the stir zone.Continuous dynamic recrystallization results in the formation of fine equiaxed grains in the stir zone, but this process becomes difficult in the thermo-mechanically affected zone due to the lower deformation rate and the pinning action of S precipitates on the dislocations and sub-grain boundaries, which leads to a high fraction of low-angle grain boundaries in this zone.展开更多
Gadolinium(Gd)is one of the most effective strengthening elements for magnesium alloys.The development of commercially available Mg-Gd alloys with high Gd content and the optimization of their preparation processes ha...Gadolinium(Gd)is one of the most effective strengthening elements for magnesium alloys.The development of commercially available Mg-Gd alloys with high Gd content and the optimization of their preparation processes have been a major focus in magnesium alloy research.In this study,a Mg-23Gd-2Zn-0.4Zr alloy with ultra-high Gd content is designed,and high-quality fabrication is achieved using laser-directed energy deposition(LDED)technology.Through heat treatment and microstructure control,a balance between tensile strength(425 MPa)and elongation(3.4%)is achieved.The ultra-high strength of the LDED-T6 VZ232K alloy is primarily attributed to precipitation strengthening caused by the ultra-high density(2.4×10^(4)μm^(-2))ofβphase.The high ductility is mainly due to the modification of the fracture mode,facilitated by the introduction of a substantial number of stacking fault structures during solution heat treatment.The extended hardness plateau(exceeding 138 Hv)and high yield strength(exceeding 300 MPa)are associated with the three-directional cross-interlocked structure of theβphase in the over-aged state at 220℃ and 250℃.The analysis of the LDED-VZ232K alloy indicates that reduced heat input during the additive manufacturing(AM)process is critical for the defect-free fabrication of alloys with ultra-high Gd content.展开更多
The formation and evolution of Gd-rich precipitates companying with the matrix structure ordering in a Mg_(97)Gd_(2)Cd_(1)(at.%)solid-solution alloy aged at 200℃have been systemically investigated using high-angle an...The formation and evolution of Gd-rich precipitates companying with the matrix structure ordering in a Mg_(97)Gd_(2)Cd_(1)(at.%)solid-solution alloy aged at 200℃have been systemically investigated using high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM).The results show that Gd-rich precipitation dynamics during the aging treatment are noticeably affected by a continuous ordering transformation in the matrix.The ordering transformation process involving mainly re-distribution of Cd atoms was revealed to occur in the following way:random super-saturated solid solution(S.S.S.S.)→B19-type ordered domains→D019-type ordered domains.Four Gd(Cd)-rich precipitates,G.P.Ⅰ zone,G.P.Ⅱ zone,β′andβ_(1)phases,have been observed to be formed in sequence to coexist with the various ordered domains.Based on the HAADF-STEM characterization on the aging microstructures at different aging stages,it can be concluded that the Cd-addition and related matrix structure ordering can play significant roles in modifying the early-stage G.P.zone structure,altering the morphology ofβ′precipitates and promoting the forming ability of theβ_(1)precipitate.展开更多
The high temperature(HT)thermal stability and mechanical properties of Al-5%Cu(AC)and Al-5%Cu-0.2%Mn-0.2 Zr%(ACMZ)alloys from 573 to 673 K were systematically studied.The results displayed that micro-alloying addition...The high temperature(HT)thermal stability and mechanical properties of Al-5%Cu(AC)and Al-5%Cu-0.2%Mn-0.2 Zr%(ACMZ)alloys from 573 to 673 K were systematically studied.The results displayed that micro-alloying additions of Zr and Mn elements have presented a significant role in stabilizing the main strengthening metastableθ′precipitates at a temperature as high as 573 K.Simultaneously,the HT tensile test demonstrated that ACMZ alloy retained their strength of(88.6±8.8)MPa,which was much higher than that of AC alloy((32.5±0.8)MPa)after the thermal exposure at 573 K for 200 h.Finally,the underlying mechanisms of strength and ductility enhancement mechanism of the ACMZ alloy at HT were discussed in detail.展开更多
A 16Cr-25Ni superaustenitic stainless steel weld metal for austenitic stainless steel/ferrite heat-resistance steel dissimilar metal weld was designed and prepared through tungsten inert-gas welding.The precipitate ev...A 16Cr-25Ni superaustenitic stainless steel weld metal for austenitic stainless steel/ferrite heat-resistance steel dissimilar metal weld was designed and prepared through tungsten inert-gas welding.The precipitate evolution and its correlation with mechanical properties were investigated during post-weld heat treatment(PWHT)at 690℃ for up to 12 h.The primary precipitates in the as-welded weld metal were identified as Mo-rich M6C carbides in the interdendritic region and semicontinuous fine-sized M23C6 carbides along grain boundary.After PWHT,three types of precipitates coexisted in the interdendritic region:primary M6C carbides,newly precipitated Mo-rich M2X carbonitrides and some of the secondary M23C6 carbides.Additionally,mass secondary M23C6 carbides formed and coarsened along grain boundary.No undesirable intermetallic phases formed during the whole period.The M2X and interdendritic M23C6 improved the strength of the weld metal after PWHT,but the elongation and impact toughness degraded,which were mainly owing to the intergranular M23C6 carbides that changed the fracture mode from ductile transgranular mode to mixed mode of transgranular and intergranular fracture.Meanwhile,the coarsening of M2X carbonitrides may lead to the elongation loss during 8 h to 12 h.Evolution of impact toughness was also related to the M2X carbonitrides,which made the crack easier to propagate compared with austenitic matrix and contributed to the decline of impact toughness.However,due to the sluggish precipitation of M2X carbonitrides with longer holding time,the decreasing trend became slow from 4 to 12 h.The results showed that PWHT should be controlled within 8 h to obtain better combination of strength and ductility.展开更多
Creep ageing forming(CAF)has been widely used in the aerospace engineering,but how to optimize the processing conditions,especially under complex stress state of the CAF process for large-size components produced by f...Creep ageing forming(CAF)has been widely used in the aerospace engineering,but how to optimize the processing conditions,especially under complex stress state of the CAF process for large-size components produced by friction-stir welding is still a great challenge to now.In this work,the creep ageing behaviors and underlying microstructure evolution of a thick friction-stir welded Al-Cu alloy plate after CAF process under different stress levels are systematically investigated.The creep strain and the strength of the joint are both significantly increased when the stress is close to the average yield strength of the initial weld joint.The grain size reduces while the local strain and dislocation density increase from top to bottom of the NZ;hence,the bottom layer of the weld joint exhibits higher creep strain and steady-stage creep strain rate during the CAF process.The results reveal that the gradient microstructures sensitive to the stress level effectively govern the creep-ageing performance from the upper to the bottom layer in a thick friction stir welded Al-Cu alloy plate.Rationally increasing the initial dislocation density of the weld joint can both enhance the tensile properties and promote the creep deformation of the weld joint for CAF process.展开更多
文摘The evolution of precipitates of S31042 steel during 700 ℃ aging was investigated by using a scanning elec- tron microscope, a transmission electron microscope, and electron energy spectrum technology. The various combi nations of M23C6, MX, NbCrN, and σ and G phases in the steel were found at different aging states. In the begin ning of aging, M23C6 precipitates swiftly along the grain boundaries. When the aging time exceeds 6 000 h, precipita- ted M23C6 carbides along the grain boundaries turn to be granular. It was found that Si element segregates to grain boundaries during above process, which may enhance the granular shape of M23C6 carbides and its transformation to and G phases. When the aging time exceeds 10 000 h, various shaped a phase and granular G phase appear along the grain boundaries and there are no continuous M23C6 carbides along the grain boundaries. Meanwhile, a large quantity of granular M23C6 carbides and a minor amount of G phase precipitate near the grain boundaries. Based on the segre- gation of silicon to the grain boundaries, a precipitation evolution model during aging was concluded.
基金This work was financially supported by the National Science and Technology Major Project of China(No.J2019-VI-0006-0120)the National Key R&D Program of China(No.2021YFB3700402)the National Natural Science Foundation of China(Nos.52074092 and 52274330).
文摘We discussed the decrease in residual stress,precipitation evolution,and mechanical properties of GH4151 alloy in different annealing temperatures,which were studied by the scanning electron microscope(SEM),high-resolution transmission electron microscopy(HRTEM),and electron backscatter diffraction(EBSD).The findings reveal that annealing processing has a significant impact on diminishing residual stresses.As the annealing temperature rose from 950 to 1150℃,the majority of the residual stresses were relieved from 60.1 MPa down to 10.9 MPa.Moreover,the stress relaxation mechanism transitioned from being mainly controlled by dislocation slip to a combination of dislocation slip and grain boundary migration.Meanwhile,the annealing treatment promotes the decomposition of the Laves,accompanied by the precipitation ofμ-(Mo_(6)Co_(7))starting at 950℃ and reaching a maximum value at 1050℃.The tensile strength and plasticity of the annealing alloy at 1150℃ reached the maximum(1394 MPa,56.1%)which was 131%,200%fold than those of the as-cast alloy(1060 MPa,26.6%),but the oxidation process in the alloy was accelerated at 1150℃.The enhancement in durability and flexibility is primarily due to the dissolution of the brittle phase,along with the shape and dispersal of theγ′phase.
文摘The precipitation evolution and mechanical properties of a commercial Al-Cu-Li-Mg alloy in natural aging(NA)tempers(T3 and T4)were investigated in detail.It was demonstrated that GPI zones were the pre-dominant precipitate throughout the NA process.In the T3-aged sample,the precipitation sequence was determined:supersaturated solid solution(SSSS)→atom clusters→GPI zones,whereas in the T4-aged sample,it was SSSS→atom clusters→GPI zones→GPI zones+δ/GPI zones/δ.For samples without NA,the strength of the T3 sample was 76.0 MPa higher than that in the T4 sample,which was attributed to the pre-deformation-introduced dislocations.During the early stage of NA,GPI zone nucleation was substantially restricted in the T3 sample,resulting in a much lower number density of GPI zones.It re-tarded the aging response and strength improvement,consequently narrowing the strength difference between T3 and T4 samples.As the NA process progressed,the average diameter of GPI zones in the T4 sample increased to approximately 4.5 nm,accompanied by the formation ofδ/GPI zones/δcomposite precipitate.However,GPI zone growth andδ/GPI zones/δcomposite precipitate formation were signifi-cantly inhibited in the T3 sample,as evidenced by its much smaller average diameter of approximately 2.1 nm and the absence of composite precipitates.It inhibited the strength improvement of the T3 sam-ple.Therefore,after a long period(5 months)of NA,the strength of the T4 sample was about 13.0 MPa higher than that of the T3 samples,instead.
基金supported by the Chongqing Key Project for Technological Innovation and Application(No.CSTB2022TIAD-KPX0073)the Natural Science Foundation of Chongqing(No.CSTB2022NSCQ-LZX0002)+1 种基金the National Natural Science Foundation of China(No.51871033)the Opening Project of State Key Laboratory for Advanced Metals and Materials(Nos.2022-Z03 and 2020-ZD02).
文摘The influence of minor Ag on the precipitation evolution of the Al-4.2Zn-2.8Mg-1.0Cu(wt.%)alloy from early stages to over-aged stages at 150°C was investigated.Surprisingly,co-precipitation of strengthening phases T′andη′are found in both Ag-free and Ag-added alloys.With Ag addition,precipitation of both T′andη′is refined and increased,such that the age-hardening capabilities and peak-aged tensile strength are improved.In addition,the quantitative proportion ofη′precipitates increases with the increase of Ag content due to the increase in the(Zn+Cu)/Mg ratio of nucleating particles.The narrowed precipitate-free zones(PFZs)are considered responsible for the undiminished fracture elongation in Ag-added alloys.Essentially,these effects of Ag are closely related to the strong Ag-vacancy and Ag-solute interactions.In over-aged stages,the Ag-added alloys still possess higher hardness values compared to the Ag-free alloy,which is related to precipitate coarsening mechanisms.The Ag-free alloy follows classical coarsening behavior by solid solution mediated diffusion,while the Ag-added alloy follows two possible coarsening mechanisms,coalescence of aggregates and diffusion of atoms.The smaller average size and higher residual number density of precipitates benefited from the slow diffusion-controlled coarsening behavior depending on the precipitate composition characteristics of the two-stage differentiation and the precipitate distribution characteristics of high-density dispersion in early-aged stages could explain why the hardness of Ag-added alloy keeps at a higher level than that of Ag-free alloy even after 1000 h ageing.Meanwhile,the transformation of metastable phases to stable phases is inhibited due to the addition of Ag,such that GP zones,T′,η′,ηand T phases coexist even after 14 d of ageing.In terms of phase composition,the addition of Ag decreases the ratio of Mg/(Al+Zn)in T-type phase.For the Ag-added alloy,the sum concentration of Zn+Mg inη′phase is about 10 at.%higher compared to T′phase,andηphase continues to have a high sum concentration of Zn+Mg,besides,the Zn/Mg ratio and Cu concentration exhibit obvious differences from T phase.
基金the National Natural Science Foundation of China(Nos.52034004,51974201 and 52122409)for grant and financial support。
文摘In the precipitation-hardened Ni-based superalloy,typified by ATI 718 Plus,the nano-scaleγandγphase in duplet or triple coprecipitate morphology can provide superior high-temperature strength.Thus,it is of great sense to study the evolution ofγ’/γ’’coprecipitate during long term service at elevated temperature.In this study,the new-typeγ’/γ’’coprecipitates with a sandwich or compact configuration were found firstly in wrought ATI 718 Plus superalloy during long term thermal exposure at 705℃.These co-structure of theγ’/γ’’precipitates evidently inhibit the coarsening ofγ’phase.The increase of thermal exposure time evidently leads to the increase of the volume fraction ofγ’/γ’’coprecipitate and transformation of sandwich-typeγ’/γ’’coprecipitate to compact-typeγ’/γ’’coprecipitate,which is characterized asγphase precipitate at several faces of theγphase.The main evolution mechanism ofγ’/γ’’coprecipitates is element segregation,especially the composition variations of Al+Ti and Nb and their ratio of Al+Ti/Nb.In addition,the interfacial energy betweenγ’’phase andγmatrix also plays a key role on theγ’/γ’’coprecipitates evolution.The calculated results show that the longer thermal exposure time leads to the higher interfacial energy,which is beneficial for nucleation and precipitation ofγ’’phase on the faces ofγ’phase.
基金financially supported by the National Science and Technology Major Project of China (No.2017ZX04005001)the Key Research & Development program of Shandong Province (2018GGX103053)。
文摘In this study, we used the stop-action technique to experimentally investigate the material flow and microstructural evolution of alclad 2A12-T4 aluminum alloy during refill friction stir spot welding.There are two material flow components, i.e., the inward-or outward-directed spiral flow on the horizontal plane and the upward-or downward-directed flow on the vertical plane.In the plunge stage, the flow of plasticized metal into the cavity is similar to that of a stack, whereby the upper layer is pushed upward by the lower layer.In the refill stage, this is process reversed.As such, there is no obvious vertical plasticized metal flow between adjacent layers.Welding leads to the coarsening of S(Al2CuMg) in the thermo-mechanically affected zone and the diminishing of S in the stir zone.Continuous dynamic recrystallization results in the formation of fine equiaxed grains in the stir zone, but this process becomes difficult in the thermo-mechanically affected zone due to the lower deformation rate and the pinning action of S precipitates on the dislocations and sub-grain boundaries, which leads to a high fraction of low-angle grain boundaries in this zone.
基金financially supported by the National Key Research and Development Pragram of China(Grant No.2023YFB4603300)。
文摘Gadolinium(Gd)is one of the most effective strengthening elements for magnesium alloys.The development of commercially available Mg-Gd alloys with high Gd content and the optimization of their preparation processes have been a major focus in magnesium alloy research.In this study,a Mg-23Gd-2Zn-0.4Zr alloy with ultra-high Gd content is designed,and high-quality fabrication is achieved using laser-directed energy deposition(LDED)technology.Through heat treatment and microstructure control,a balance between tensile strength(425 MPa)and elongation(3.4%)is achieved.The ultra-high strength of the LDED-T6 VZ232K alloy is primarily attributed to precipitation strengthening caused by the ultra-high density(2.4×10^(4)μm^(-2))ofβphase.The high ductility is mainly due to the modification of the fracture mode,facilitated by the introduction of a substantial number of stacking fault structures during solution heat treatment.The extended hardness plateau(exceeding 138 Hv)and high yield strength(exceeding 300 MPa)are associated with the three-directional cross-interlocked structure of theβphase in the over-aged state at 220℃ and 250℃.The analysis of the LDED-VZ232K alloy indicates that reduced heat input during the additive manufacturing(AM)process is critical for the defect-free fabrication of alloys with ultra-high Gd content.
基金The authors would like to acknowledge the financial supports from the National Natural Science Foundation of China[grant number 11274027]Beijing Municipal Natural Science Foundation[grant number 2092005].
文摘The formation and evolution of Gd-rich precipitates companying with the matrix structure ordering in a Mg_(97)Gd_(2)Cd_(1)(at.%)solid-solution alloy aged at 200℃have been systemically investigated using high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM).The results show that Gd-rich precipitation dynamics during the aging treatment are noticeably affected by a continuous ordering transformation in the matrix.The ordering transformation process involving mainly re-distribution of Cd atoms was revealed to occur in the following way:random super-saturated solid solution(S.S.S.S.)→B19-type ordered domains→D019-type ordered domains.Four Gd(Cd)-rich precipitates,G.P.Ⅰ zone,G.P.Ⅱ zone,β′andβ_(1)phases,have been observed to be formed in sequence to coexist with the various ordered domains.Based on the HAADF-STEM characterization on the aging microstructures at different aging stages,it can be concluded that the Cd-addition and related matrix structure ordering can play significant roles in modifying the early-stage G.P.zone structure,altering the morphology ofβ′precipitates and promoting the forming ability of theβ_(1)precipitate.
基金financial supports from the National Natural Science Foundation of China(No.52071207)the China Postdoctoral Science Foundation(Nos.2019TQ0193,2019M661497)+1 种基金the National Key Research and Development Program of China(No.2018YFB1106302)Anhui Provincial Engineering Research Center of Aluminum Matrix Composites,China(No.2017WAMC002)。
文摘The high temperature(HT)thermal stability and mechanical properties of Al-5%Cu(AC)and Al-5%Cu-0.2%Mn-0.2 Zr%(ACMZ)alloys from 573 to 673 K were systematically studied.The results displayed that micro-alloying additions of Zr and Mn elements have presented a significant role in stabilizing the main strengthening metastableθ′precipitates at a temperature as high as 573 K.Simultaneously,the HT tensile test demonstrated that ACMZ alloy retained their strength of(88.6±8.8)MPa,which was much higher than that of AC alloy((32.5±0.8)MPa)after the thermal exposure at 573 K for 200 h.Finally,the underlying mechanisms of strength and ductility enhancement mechanism of the ACMZ alloy at HT were discussed in detail.
基金supported by the innovation project of Shenyang National Laboratory for Materials Science(No.SYNL-2019)。
文摘A 16Cr-25Ni superaustenitic stainless steel weld metal for austenitic stainless steel/ferrite heat-resistance steel dissimilar metal weld was designed and prepared through tungsten inert-gas welding.The precipitate evolution and its correlation with mechanical properties were investigated during post-weld heat treatment(PWHT)at 690℃ for up to 12 h.The primary precipitates in the as-welded weld metal were identified as Mo-rich M6C carbides in the interdendritic region and semicontinuous fine-sized M23C6 carbides along grain boundary.After PWHT,three types of precipitates coexisted in the interdendritic region:primary M6C carbides,newly precipitated Mo-rich M2X carbonitrides and some of the secondary M23C6 carbides.Additionally,mass secondary M23C6 carbides formed and coarsened along grain boundary.No undesirable intermetallic phases formed during the whole period.The M2X and interdendritic M23C6 improved the strength of the weld metal after PWHT,but the elongation and impact toughness degraded,which were mainly owing to the intergranular M23C6 carbides that changed the fracture mode from ductile transgranular mode to mixed mode of transgranular and intergranular fracture.Meanwhile,the coarsening of M2X carbonitrides may lead to the elongation loss during 8 h to 12 h.Evolution of impact toughness was also related to the M2X carbonitrides,which made the crack easier to propagate compared with austenitic matrix and contributed to the decline of impact toughness.However,due to the sluggish precipitation of M2X carbonitrides with longer holding time,the decreasing trend became slow from 4 to 12 h.The results showed that PWHT should be controlled within 8 h to obtain better combination of strength and ductility.
基金Project(2021YFB3400903) supported by the National Key R&D Program of ChinaProject(1053320211480) supported by the Science and Technology Innovation Project of Graduate Students of Central South University,China。
文摘Creep ageing forming(CAF)has been widely used in the aerospace engineering,but how to optimize the processing conditions,especially under complex stress state of the CAF process for large-size components produced by friction-stir welding is still a great challenge to now.In this work,the creep ageing behaviors and underlying microstructure evolution of a thick friction-stir welded Al-Cu alloy plate after CAF process under different stress levels are systematically investigated.The creep strain and the strength of the joint are both significantly increased when the stress is close to the average yield strength of the initial weld joint.The grain size reduces while the local strain and dislocation density increase from top to bottom of the NZ;hence,the bottom layer of the weld joint exhibits higher creep strain and steady-stage creep strain rate during the CAF process.The results reveal that the gradient microstructures sensitive to the stress level effectively govern the creep-ageing performance from the upper to the bottom layer in a thick friction stir welded Al-Cu alloy plate.Rationally increasing the initial dislocation density of the weld joint can both enhance the tensile properties and promote the creep deformation of the weld joint for CAF process.
