A method is proposed to enhance the thermal stability of laser-powder bed fusion fabricated(L-PBFed)FeCoCrNi alloy by introducing Al element segregation through in-situ alloying.The introduced Al segregation exists in...A method is proposed to enhance the thermal stability of laser-powder bed fusion fabricated(L-PBFed)FeCoCrNi alloy by introducing Al element segregation through in-situ alloying.The introduced Al segregation exists in two forms of B2/BCC phases,one in banded shape within the FCC matrix and the other as particles at grain boundaries(GBs).Experimental characterization and molecular dynamics(MD)simulations were used to reveal the mechanism of the thermal stability of the grain boundary(GB)and dislocation in high-temperature treatment at 1000 and 1200℃.At high temperatures,short-range uphill diffusion occurs within the banded B2/BCC phase,forming the dispersed B2/BCC phase with higher(Al,Ni)content.This extends the stability of the banded B2/BCC phase and ensures high-strain hardening.Additionally,the long-range diffusion of Al atoms from the banded B2/BCC into the FCC matrix utilizes GBs as rapid channels at high temperatures.This process stabilizes GBs by reducing their cohesive energy and maintaining the nailing effect of the B2/BCC phase at GBs.Furthermore,after high-temperature treatment,dislocations within the FCC matrix exhibit a relatively high-density level,and many dislocations are generated within the B2/BCC regions subsequent to phase transition.This is attributed to the geometrically necessary dislocation(GND)generation caused by lattice distortion stemming from variations in Al content in the FCC matrix and lattice shrinkage induced by the phase transformation.As a result,the mechanical properties exhibit remarkable resistance to softening compared to traditional L-PBFed single FCC phase alloys.In terms of tensile properties at room temperature,after treatment at 1000℃/1 h,ultimate tensile strength(UTS)increased from 797 to 873 MPa.Even after 10 h at 1200℃,the UTS retained 86%of its original value.In terms of tensile properties at high temperature,compared to the L-PBFed FeCoCrNi alloy,the alloys prepared in this work exhibit an increase in yield strength(YS)by approximately 100 MPa under the same temperature conditions.This work can provide a new perspective for improving the thermal stability of L-PBFed alloys.展开更多
IN617B nickel-base superalloy is considered as a good candidate material in 700℃advanced ultrasupercritical coal-fired power plants.The effect of Ta addition on solidification microstructure and element segregation o...IN617B nickel-base superalloy is considered as a good candidate material in 700℃advanced ultrasupercritical coal-fired power plants.The effect of Ta addition on solidification microstructure and element segregation of IN617B alloy was investigated by OM,SEM,TEM,EDS,EPMA and thermodynamic calculation.The results showed that the solidification microstructure exhibited a dendritic segregation pattern with many primary carbides distributed in interdendritic regions,such as network M_(6)C,lath M_(23)C_(6) and granular Ti(C,N).The addition of Ta promoted the precipitation of Ta-rich MC significantly inhibiting the precipitation of M_(6)C and M_(23)C_(6),and reduced the segregation degree of Al,Mo and Ti alloying elements.The addition of Ta decreased the melting temperature of MC carbide,but did not impact the solidification path,that was,L→γmatrix→MC or Ti(C,N)→M_(6)C→M_(23)C_(6),where MC and Ti(C,N)tended to form symbiotic microstructure with M_(6)C.This study will provide theoretical basis and data support for the alloy optimization and casting structure control of IN617B nickel-based superalloy.展开更多
In aluminum-copper alloy, the segregation has a severe bad effect on the alloying degree, strength and corrosion resistance. A deeper understanding of element segregation behavior will have a great signif icance on th...In aluminum-copper alloy, the segregation has a severe bad effect on the alloying degree, strength and corrosion resistance. A deeper understanding of element segregation behavior will have a great signif icance on the prevention of segregation. In the study, the element segregation behavior of ZL205 A aluminum-copper alloy was investigated by examining isothermally solidifi ed samples using scanning electron microscopy and energy dispersive spectroscopy. The calculated results of segregation coeffi cients show that Cu and Mn are negative segregation elements; while Ti, V and Zr are positive segregation elements. The sequence of element segregation degree from the greatest to the least in ZL205 A alloy is Cu, Mn, V, Ti, Zr and Al. The density of residual liquid is expected to increase with a decrease in the quenching temperature ranging from 630 ℃ to 550 ℃. The calculated results conf irm that the quenching temperature has an insignif icant effect on the liquid density; and the variation of density is mainly due to element segregation. Consequently, segregations of Al, Cu and Mn lead to an increase in density, but Ti, V and Zr present the opposite effect. The contribution of each element to the variation of the liquid density was analyzed. The sequence of contributions of alloying elements to the variation of total liquid density is Cu>Al>Mn>V>Ti>Zr.展开更多
A new insight into the microstructural stability was proposed in Ni-based single crystal superalloys with Ru addition,and the element segregation behavior atγ/γ′interface was investigated by three-dimensional atom ...A new insight into the microstructural stability was proposed in Ni-based single crystal superalloys with Ru addition,and the element segregation behavior atγ/γ′interface was investigated by three-dimensional atom probe technology(3D-APT).After standard heat treatment,it was found that Ru addition barely altered the element partitioning coefficient betweenγmatrix andγ′phase,and no element-segregation layer was observed atγ/γ′interface.During the heat exposure at 1100°C,Ru addition obviously promoted the rafting of theγ′precipitates and inhibited the precipitation of topological close-packed(TCP)phases.It was more important that an element-segregation layer containing Re,Co,and Cr was formed in theγmatrix close to theγ/γ′interface due to an“uphill diffusion”effect,and its concentration was obviously reduced after Ru addition.Finally,the microstructural stability based on the element segregation behavior atγ/γ′interface was discussed.This element-segregation layer increased theγ/γ′interfacial energy by increasing the absolute value of the lattice misfit ofγ/γ′interface to promote the rafting of theγ′precipitates after Ru addition.On the other hand,the decrease of the segregation concentration of Re,Co,and Cr elements as TCP phase-forming elements near theγ/γ′interface due to a“reverse partitioning”effect inhibits the precipitation of TCP phases in Ni-based single crystal superalloys after Ru addition.展开更多
Bimetallic alloys could form three typical structures including solid solution,heterostructure,and intermetallic compound,depending on the interactions between identical and different atoms.Although the trend can be p...Bimetallic alloys could form three typical structures including solid solution,heterostructure,and intermetallic compound,depending on the interactions between identical and different atoms.Although the trend can be predicted by the types of binary phase diagram,different synthetic protocols will trap the system in various kinetic intermediates among the three typical structures.Herein,we studied the phase evolution and elemental segregation in the alloy nanoparticles of immiscible Pd-Ru before and after thermal annealing.By developing an analysis method of local element segregation(LES)based on the energy dispersive spectroscopy(EDS)mapping signals,we were able to quantify the mixing of Pd and Ru atoms during the gradual phase transition from face-centered cubic(fcc)to hexagonal close packed(hcp).Density functional theory was also applied to calculate the energies of all possible PdRu4 structures(93 fcc models and 267 hcp models),which helps to rationalize the phase transition and element segregation.The annealing process also leads to the change of the electronic structure,which further influences the performance in the electrocatalytic hydrogen evolution reaction.The highest activity of PdRu4-400 was largely attributed to the proper interface between the Pd-rich fcc phase and Ru-rich hcp phase,as revolved by the above methods.展开更多
To reduce microsegregation,a series of homogenization treatments were carried out on a Ni-Co based superalloy prepared through directional solidification(DS).The element segregation characteristics and microstructural...To reduce microsegregation,a series of homogenization treatments were carried out on a Ni-Co based superalloy prepared through directional solidification(DS).The element segregation characteristics and microstructural evolution were investigated by optical microscopy(OM),scanning electron microscopy(SEM),and electron probe microanalysis(EPMA).The results show that the elements are non-uniformly distributed in the solidified superalloy,in which W and Ti have the greatest tendency of microsegregation.Furthermore,severe microsegregation leads to complicated precipitations,includingη-Ni_(3) Ti and eutectic(γ+γ’).EPMA results show that Al and Mo are uniformly distributed between the eutectic(γ+γ’)andγmatrix,whereas Ti is segregated in the eutectic(γ+γ’)andηphases.The positive segregation element Ti,which is continuously rejected into the remaining liquid duringγmatrix solidification,promotes the formation of eutectic(γ+γ’)and the transformation of theηphase.According to the homogenization effect,the optimal single-stage homogenization process of this alloy is 1180℃for 2 h because of the sufficient diffusion segregation of the elements.In the present study,a kinetic diffusion model was built to reflect the degree of element segregation during homogenization,and the diffusion coefficients of W and Ti were estimated.展开更多
Here we first report the fully abundant rare earth(RE)-based nanocrystalline multi-component(Ce,La,Y)-Fe-B alloys containing no critical RE elements of Nd,Pr,Dy,and Tb by melt-spinning technique.The roles of La and Y ...Here we first report the fully abundant rare earth(RE)-based nanocrystalline multi-component(Ce,La,Y)-Fe-B alloys containing no critical RE elements of Nd,Pr,Dy,and Tb by melt-spinning technique.The roles of La and Y substitutions for Ce have been fully understood.La plays a positive role on both thermal stability and room-temperature(RT)magnetic properties.The enhanced coercivity H_(cj)by partial substitution of La is attributed to the increases of anisotropy field H_A and the formation of continuously distributed grain boundaries resulting from the suppre s sion of CeFe_(2)phase.Although Y substitution is not benefit for H_(cj),both remanent polarization J_r and thermal stability have been effectively improved since Y_(2)Fe_(14)B shows relatively high saturation magnetization M_s and a positive temperature coefficient of HA over a certain temperature range.In addition,RE element segregation has been confirmed,La prefers to enter into the grain boundaries than Ce and Y prefers to remain in the 2:14:1 phase.Based on these understanding,a series of melt-spun(Ce,La,Y)-Fe-B alloys have been designed.A relatively good combination of magnetic properties with maximum energy product(BH)_(max)=7.4 MGOe,H_(Cj)=400 kA/m,and J_r=0.63 T has been obtained in[(Ce_(0.8)La_(0.2))_(0.7)Y_(0.3)]_(17)Fe_(78)B_6 alloy,together with high Curie temperature(T_c=488 K)and low temperature coefficients of remanence(α=-0.255%/K)and coercivity(β=-0.246%/K).展开更多
The second phase dissolution and elements migration behavior of a nickel-based single crystal superalloy during solution heat treatment with direct current were investigated for simplifying and shortening the solution...The second phase dissolution and elements migration behavior of a nickel-based single crystal superalloy during solution heat treatment with direct current were investigated for simplifying and shortening the solution heat treatment of the Ni-based single crystal superalloy.The results showed that the electric current solution heat treatment improved microstructural homogenization as well as the distribution of alloying elements,especially for the refractory metal W and Mo.The microsegregation ratios for Mo and W after electric current solution heat treatment at 1230℃for 4 h are near those without electric current at 1250℃for 4 h.The electric current accelerated theγ′phase dissolution process,and theγ′phase could be completely dissolved at a lower treatment temperature or within a shorter treatment time under electric current solution heat treatment with direct current.A microcosmic current model was proposed to analyze the effect of the electric current on the solution heat treatment of the Ni-based single crystal superalloy.展开更多
The effect of Cr/Mn segregation on the abnormal banded structure of high carbon bearing steel was studied by reheating and hot rolling.With the use of an optical microscope, scanning electron microscope, transmission ...The effect of Cr/Mn segregation on the abnormal banded structure of high carbon bearing steel was studied by reheating and hot rolling.With the use of an optical microscope, scanning electron microscope, transmission electron microscope, and electron probe microanalyzer, the segregation characteristics of alloying elements in cast billet and their relationship with hot-rolled plate banded structure were revealed.The formation causes of an abnormal banded structure and the elimination methods were analyzed.Results indicate the serious positive segregation of C, Cr, and Mn alloy elements in the billet.Even distribution of Cr/Mn elements could not be achieved after 10 h of heat preservation at 1200℃, and the spacing of the element aggregation area increased, but the segregation index of alloy elements decreased.Obvious alloying element segregation characteristics are present in the banded structure of the hot-rolled plate.This distinct white band is composed of martensitic phases.The formation of this abnormal pearlite–martensite banded structure is due to the interaction between the undercooled austenite transformation behavior of hot-rolled metal and the segregation of its alloying elements.Under the air cooling after rolling, controlling the segregation index of alloy elements can reduce or eliminate the abnormal banded structure.展开更多
In this study,aberration-corrected scanning transmission electron microscopy coupled with electron energy-loss spectroscopy(STEM-EELS)was used to investigate the atomistic structure and chemical com-position of true t...In this study,aberration-corrected scanning transmission electron microscopy coupled with electron energy-loss spectroscopy(STEM-EELS)was used to investigate the atomistic structure and chemical com-position of true twin and order twin boundaries in ferromagneticτ-MnAl-C.True twins and order twins were distinguished based on the diffraction patterns using TEM.No elemental segregation was observed at the coherent true twin boundary but some Mn enrichment within a region of about 1.5-2 nm was found at the incoherent true twin boundary.A transition region with Mn enrichment about 4-6 nm wide was found at the order twin boundary.A carbon cluster with a size of around 5 nm was also found at the twin boundary.Micromagnetic simulations were conducted to study the effect of this chemical seg-regation at twin interfaces on the magnetic properties.The results showed that the coercivity tends to increase with increasing structural and chemical disorder at the interface.展开更多
Through thermodynamic calculations and microstructural characterization,the effect of niobium(Nb)content on the solidifica-tion characteristics of Alloy 625 Plus was systematically investigated.Subsequently,the effect...Through thermodynamic calculations and microstructural characterization,the effect of niobium(Nb)content on the solidifica-tion characteristics of Alloy 625 Plus was systematically investigated.Subsequently,the effect of Nb content on hot deformation behaviorwas examined through hot compression experiments.The results indicated that increasing the Nb content lowers the liquidus temperatureof the alloy by 51℃,producing a denser solidification microstructure.The secondary dendrite arm spacing(SDAS)of the alloy decreasesfrom 39.09 to 22.61μm.Increasing the Nb content alleviates element segregation but increases interdendritic precipitates,increasing theirarea fraction from 0.15% to 5.82%.These precipitates are primarily composed of large Laves,δ,η,and γ″phases,and trace amounts of Nb C.The shapes of these precipitates change from small chunks to large elongated forms.No significant change in the type or amount ofinclusions within the alloy is detected.The inclusions are predominantly individual Al_(2)O_(3) and TiN,as well as Al_(2)O_(3)/Ti N composite inclu-sions.Samples with varying Nb contents underwent hot compression deformation at a true strain of 0.69,a strain rate of 0.5 s^(-1),and a de-formation temperature of 1150℃.Increasing the Nb content also elevates the peak stress observed in the flow curves.However,alloyswith higher Nb content exhibit more pronounced recrystallization softening effects.The Laves phase precipitates do not completely redis-solve during hot deformation and are stretched to elongated shapes.The high-strain energy storage increases the recrystallization fractionfrom 32.4% to 95.5%,significantly enhancing the degree of recrystallization and producing a more uniform deformation microstructure.This effect is primarily attributed to the addition of Nb,which refines the initial grains of the alloy,enhances the solid solution strengthen-ing of the matrix,and improves the induction of particle-stimulated nucleation.展开更多
High Nbβ/γ-TiAl(HNBG)intermetallics and Ni-based superalloy(IN718)were diffusion-bonded using pure Ti foil interlayer under pulse current.The microstructure,element segregation,and mechanical properties of HNBG/Ti/I...High Nbβ/γ-TiAl(HNBG)intermetallics and Ni-based superalloy(IN718)were diffusion-bonded using pure Ti foil interlayer under pulse current.The microstructure,element segregation,and mechanical properties of HNBG/Ti/IN718 joint were investigated.The effect of Ti interlayer on microstructure and mechanical properties of the joint was discussed.The typical microstructure of HNBG/Ti/IN718 joint was HNBG//β/B2,τ_(3)-NiAl_(3)Ti_(2)//α_(2)-Ti_(3)Al//α-Ti+δ-NiTi_(2),β-Ti//δ-NiTi_(2)//β2-(Ni,Fe)Ti//Cr/Fe-richη-Ni_(3)Ti,η-Ni_(3)Ti,α-Cr,δ-Ni_(3)Nb//η-Ni_(3)Ti,γ-Ni,δ-Ni_(3)Nb//IN718.The gaps and Kirkendall voids exhibited a gradual disappearance with increasing bonding temperature.The mechanism of Cr,Fe and Nb elements segregation was that NiTi phase hindered the diffusion of them.The nano-indentation results demonstrated that diffusion zones on IN718 alloy side had higher hardness.The maximum shear strength of the joint(326 MPa)was achieved at bonding parameters of 850℃,20 min and 10 MPa.The fracture occurred in Zones IV and V,and the fracture modes were brittle fracture and cleavage fracture.The introduction of Ti interlayer resulted in improved microstructure and enhanced bonding strength of the joint.展开更多
A low-carbon,low-cost,and high-efficient method was reported for remarkably improving corrosion resistance of C–Mn structural steel by weak deoxidation.The results showed that,with the total oxygen content(wOT)increa...A low-carbon,low-cost,and high-efficient method was reported for remarkably improving corrosion resistance of C–Mn structural steel by weak deoxidation.The results showed that,with the total oxygen content(wOT)increasing in the tested steel from 41×10^(−6)to 195×10^(−6),both the degree of element segregation and the level of banded microstructure weakened,presenting the lower potential difference between pearlite(P)and ferrite(F),and then smaller galvanic corrosion driving force,and thus effectively improving general corrosion properties.In addition,with wOT growing up,the number and size of inclusions increased,and the shape also changed from long chain or small particle to large particle ball with typical mosaic structure,which could effectively inhibit the preferential dissolution of local component due to multiple complex interfaces,and correspondingly suppress the pitting susceptibility.However,the impact toughness at low temperature of the tested steel reduced with wOT increasing,and then,taking the mechanical properties and corrosion resistance all into account,160×10^(−6) was the optimal oxygen content within the present scope.展开更多
To satisfy the demands of modern society for high-energy–density sulfide-based all-solid-state lithium batteries(ASSLBs),Ni-rich cathode materials have gained much attention for their high capacity and energy density...To satisfy the demands of modern society for high-energy–density sulfide-based all-solid-state lithium batteries(ASSLBs),Ni-rich cathode materials have gained much attention for their high capacity and energy density.However,their practical deployment is hindered by accelerated interface degradation and capacity decay originating from surface oxygen release and lattice oxygen activation during prolonged cycling.In this study,Ti_(x)NbB_((1−x))C_(2)was successfully coated on the surface of LiNi_(0.94)Co_(0.05)Mn_(0.01)O_(2).Density functional theory(DFT)calculations first elucidate a“point-to-point”anchoring mechanism where each surface oxygen atom coordinates with single species(Ti/Nb/B)offered by Ti_(x)NbB_((1−x))C_(2),which forms robust O–M bonds and sustain a stable interface structure.The electron energy loss spectroscopy(EELS)reveals the segregation of Ti/Nb toward subsurface layers during cycling,creating an optimized lattice oxygen coordination environment and suppressing oxygen activation.The dual oxygen stabilization mechanism dramatically improves the reversibility of phase transition and the structural stability of the Ni-rich cathode materials.Moreover,Ti_(x)NbB_((1−x))C_(2)as the protective layer decreases mechanical strain and suppresses the parasitic reactions.Consequently,the engineered cathode delivers 91%capacity retention after 1000 cycles at 0.3 C,suggesting excellent cycling stability.The research delivers a new design philosophy for the coating layer that can stabilize surface oxygen.Furthermore,the atomistic understanding of the structure–property relationship of the Ni-rich cathode materials provides valuable guidance for the future design of new cathode materials with superior structural stability in ASSLBs.展开更多
The effect of melt superheating treatment on the solidification microstructure and mechanical properties of theγ'phase precipitation-strengthened K424 superalloy was investigated.Differential scanning calorimetry...The effect of melt superheating treatment on the solidification microstructure and mechanical properties of theγ'phase precipitation-strengthened K424 superalloy was investigated.Differential scanning calorimetry(DSC)experiments were conducted to explore the influence of melt treatment temperature on the undercooling of the superalloy.Additionally,pouring experiments were carried out to assess how alterations in both the temperature and duration of melt treatment impacted the grain size,secondary dendrite arm spacing(SDAS),elemental segregation,and mechanical properties of the alloy.Metallographic analysis,scanning electron microscopy,energy dispersive spectroscopy(EDS)and Thermo-Calc software were employed for microstructure characterization.The test specimens were subjected to tensile testing at room temperature and stress rupture testing at 975℃ under 196 MPa.The findings reveal that appropriate melt treatment conditions result in decreased grain size,refined SDAS,minimized elemental segregation,and significant improvements in mechanical properties.Specifically,the study demonstrates that a melt treatment at 1,650℃ for 5 min results in the smallest average grain size of 949μm and the smallest SDAS of 25.38μm.Furthermore,the room temperature tensile properties and creep resistance are notably affected by the melt treatment parameters.It is shown that specific melt treatment conditions,such as holding at 1,650℃ for 5 min,result in superior room temperature strength and extended stress rupture life of the K424 superalloy,while a balance between strength and stability is achieved at 1,600℃ with a holding time of 10 min.These findings offer guidance for optimizing the melt treatment parameters for the K424 superalloy,laying a foundation for further investigations.展开更多
Upon approaching 850℃,the GH4151 alloy exhibits diminished high-temperature strength,primarily attributed to the disruption of γ′phase coherence at elevated temperatures,which reduces its strengthening contribution...Upon approaching 850℃,the GH4151 alloy exhibits diminished high-temperature strength,primarily attributed to the disruption of γ′phase coherence at elevated temperatures,which reduces its strengthening contribution.Tantalum(Ta)additions enhance the stability of the γ′phase but introduce processing challenges,including pronounced solidification segregation,the formation of secondary phases,and increased susceptibility to cracking during processing.The influence of Ta content on elemental segregation,solidification microstructure,phase precipitation kinetics,and hot deformation behaviour in GH4151 was systematically investigated.Processing windows derived from the dynamic materials model(DMM)and microstructural evolution under varying thermomechanical conditions are further examined.Key findings reveal the severe Ta segregation(segregation coefficient K≈1.608);Ta promotes γ/γ′eutectic and η phase formation,increasing γ′phase volume fraction from 54% to approximately 63%;and increased Ta content elevates flow stress and progressively narrows the DMM-defined processing window;optimized thermomechanical processing parameters(elevated temperatures and strain rates)enhance recrystallization kinetics and hot workability,thereby mitigating cracking propensity.展开更多
The developing of large size superalloy vacuum induction melting(VIM)ingots is limited owing to hot cracking,The hot cracking behavior of the large size GH4742 superalloy VIM ingot was investigated via experiment and ...The developing of large size superalloy vacuum induction melting(VIM)ingots is limited owing to hot cracking,The hot cracking behavior of the large size GH4742 superalloy VIM ingot was investigated via experiment and simulation.The microstructure was examined by optical microscopy,and element segregation was investigated by electron probe microanalysis.The solidification temperature range and yield strength at high temperature(YSHT)were calculated by JMatPro software.The results show that the variations of microstructure and element segregation in different locations are caused by different cooling rates.Moreover,the larger secondary dendrite arm spacing and serious element segregation of Nb accelerate hot cracking of the VIM ingot.In addition,the solidification temperature range is wider,and YSHT is lower in center than at edge of the ingot.Therefore,the hot cracking susceptibility is the highest in the center of the GH4742 superalloy VIM ingot.The critical criterion of element segregation for hot cracking is that the partition coeffcient of Nb should be larger than 0.5.展开更多
The hot deformation behavior of Mn18Cr18N and Mn18Cr18N+Ce high nitrogen austenitic stainless steels at 1173-1473 K and 0.01-1 s^(-1) were investigated by thermal compression tests.The influence mechanism of Ce on the...The hot deformation behavior of Mn18Cr18N and Mn18Cr18N+Ce high nitrogen austenitic stainless steels at 1173-1473 K and 0.01-1 s^(-1) were investigated by thermal compression tests.The influence mechanism of Ce on the hot deformation behavior was analyzed by Ce-containing inclusions and segregation of Ce.The results show that after the addition of Ce,large,angular,hard,and brittle inclusions(TiN-Al_(2)O_(3),TiN,and Al_(2)O_(3)) can be modified to fine and dispersed Ce-containing inclusions(Ce-Al-O-S and TiN-Ce-Al-O-S).During the solidification,Ce-containing inclusions can be used as heterogeneous nucleation particles to refine as-cast grains.During the hot deformation,Ce-containing inclusions can pin dislocation movement and grain boundary migration,induce dynamic recrystallization(DRX)nucleation,and avoid the formation and propagation of micro cracks and gaps.In addition,during the solidification,Ce atoms enrich at the front of solid-li-quid interface,resulting in composition supercooling and refining the secondary dendrites.Similarly,during the hot deformation,Ce atoms tend to segregate at the boundaries of DRX grains,inhibiting the growth of grains.Under the synergistic effect of Ce-containing inclusions and Ce segregation,although the hot deformation resistance and hot deformation activation energy are improved,DRX is more likely to occur and the size of DRX grains is significantly refined,and the problem of hot deformation cracking can be alleviated.Finally,the microhardness of the samples was measured.The results show that compared with as-cast samples,the microhardness of hot-deformed samples increases signific-antly,and with the increase of DRX degree,the microhardness decreases continuously.In addition,Ce can affect the microhardness of Mn18Cr18N steel by affecting as-cast and hot deformation microstructures.展开更多
Using SEM and EDS some particles containing La and P were discovered in low oxygen and sulfur purity steel with lanthanum. In these particles low melting point elements such as As and Sb also were detected. According ...Using SEM and EDS some particles containing La and P were discovered in low oxygen and sulfur purity steel with lanthanum. In these particles low melting point elements such as As and Sb also were detected. According to the result of simulative calculation of segregation, it is shown that La and P enrich so strongly in the process of solidification that their concentrations can satisfy thermodynamic condition to forme LaP compound or meet the requirements to forme eutectic phase in the last stage of solidification. Therefore, the particles are concluded as LaP compound or eutectics, which exist on grain boundary mainly.展开更多
The infamous type Ⅳ failure within the fine-grained heat-affected zone (FGHAZ) in G115 steel weldments seriously threatens the safe operation of ultra-supercritical (USC) power plants.In this work,the traditional the...The infamous type Ⅳ failure within the fine-grained heat-affected zone (FGHAZ) in G115 steel weldments seriously threatens the safe operation of ultra-supercritical (USC) power plants.In this work,the traditional thermo-mechanical treatment was modified via the replacement of hot-rolling with cold rolling,i.e.,normalizing,cold rolling,and tempering (NCT),which was developed to improve the creep strength of the FGHAZ in G115 steel weldments.The NCT treatment effectively promoted the dissolution of preformed M_(23)C_(6)particles and relieved the boundary segregation of C and Cr during welding thermal cycling,which accelerated the dispersed reprecipitation of M_(23)C_(6) particles within the fresh reaustenitized grains during post-weld heat treatment.In addition,the precipitation of Cu-rich phases and MX particles was promoted evidently due to the deformation-induced dislocations.As a result,the interacting actions between precipitates,dislocations,and boundaries during creep were reinforced considerably.Following this strategy,the creep rupture life of the FGHAZ in G115 steel weldments can be prolonged by 18.6%,which can further push the application of G115 steel in USC power plants.展开更多
基金supported by the National Natural Science Foundation of China(No.52475371)the Key Research and Development Program of Shandong Province,China(No.2021ZLGX01)the Shandong Natural Science Foundation of China(No.ZR2020ZD05).
文摘A method is proposed to enhance the thermal stability of laser-powder bed fusion fabricated(L-PBFed)FeCoCrNi alloy by introducing Al element segregation through in-situ alloying.The introduced Al segregation exists in two forms of B2/BCC phases,one in banded shape within the FCC matrix and the other as particles at grain boundaries(GBs).Experimental characterization and molecular dynamics(MD)simulations were used to reveal the mechanism of the thermal stability of the grain boundary(GB)and dislocation in high-temperature treatment at 1000 and 1200℃.At high temperatures,short-range uphill diffusion occurs within the banded B2/BCC phase,forming the dispersed B2/BCC phase with higher(Al,Ni)content.This extends the stability of the banded B2/BCC phase and ensures high-strain hardening.Additionally,the long-range diffusion of Al atoms from the banded B2/BCC into the FCC matrix utilizes GBs as rapid channels at high temperatures.This process stabilizes GBs by reducing their cohesive energy and maintaining the nailing effect of the B2/BCC phase at GBs.Furthermore,after high-temperature treatment,dislocations within the FCC matrix exhibit a relatively high-density level,and many dislocations are generated within the B2/BCC regions subsequent to phase transition.This is attributed to the geometrically necessary dislocation(GND)generation caused by lattice distortion stemming from variations in Al content in the FCC matrix and lattice shrinkage induced by the phase transformation.As a result,the mechanical properties exhibit remarkable resistance to softening compared to traditional L-PBFed single FCC phase alloys.In terms of tensile properties at room temperature,after treatment at 1000℃/1 h,ultimate tensile strength(UTS)increased from 797 to 873 MPa.Even after 10 h at 1200℃,the UTS retained 86%of its original value.In terms of tensile properties at high temperature,compared to the L-PBFed FeCoCrNi alloy,the alloys prepared in this work exhibit an increase in yield strength(YS)by approximately 100 MPa under the same temperature conditions.This work can provide a new perspective for improving the thermal stability of L-PBFed alloys.
文摘IN617B nickel-base superalloy is considered as a good candidate material in 700℃advanced ultrasupercritical coal-fired power plants.The effect of Ta addition on solidification microstructure and element segregation of IN617B alloy was investigated by OM,SEM,TEM,EDS,EPMA and thermodynamic calculation.The results showed that the solidification microstructure exhibited a dendritic segregation pattern with many primary carbides distributed in interdendritic regions,such as network M_(6)C,lath M_(23)C_(6) and granular Ti(C,N).The addition of Ta promoted the precipitation of Ta-rich MC significantly inhibiting the precipitation of M_(6)C and M_(23)C_(6),and reduced the segregation degree of Al,Mo and Ti alloying elements.The addition of Ta decreased the melting temperature of MC carbide,but did not impact the solidification path,that was,L→γmatrix→MC or Ti(C,N)→M_(6)C→M_(23)C_(6),where MC and Ti(C,N)tended to form symbiotic microstructure with M_(6)C.This study will provide theoretical basis and data support for the alloy optimization and casting structure control of IN617B nickel-based superalloy.
基金financially supported by the National Natural Science Foundation of China(Grant No.51375391)
文摘In aluminum-copper alloy, the segregation has a severe bad effect on the alloying degree, strength and corrosion resistance. A deeper understanding of element segregation behavior will have a great signif icance on the prevention of segregation. In the study, the element segregation behavior of ZL205 A aluminum-copper alloy was investigated by examining isothermally solidifi ed samples using scanning electron microscopy and energy dispersive spectroscopy. The calculated results of segregation coeffi cients show that Cu and Mn are negative segregation elements; while Ti, V and Zr are positive segregation elements. The sequence of element segregation degree from the greatest to the least in ZL205 A alloy is Cu, Mn, V, Ti, Zr and Al. The density of residual liquid is expected to increase with a decrease in the quenching temperature ranging from 630 ℃ to 550 ℃. The calculated results conf irm that the quenching temperature has an insignif icant effect on the liquid density; and the variation of density is mainly due to element segregation. Consequently, segregations of Al, Cu and Mn lead to an increase in density, but Ti, V and Zr present the opposite effect. The contribution of each element to the variation of the liquid density was analyzed. The sequence of contributions of alloying elements to the variation of total liquid density is Cu>Al>Mn>V>Ti>Zr.
基金funded by the National Science and Technol-ogy Major Project(Nos.2017-VI-0002-0072,2019-VI-0020-0135)the National Natural Science Foundation of China(Nos.51771148,52071263,52031012)+2 种基金the Key Research and Development Program of Shaanxi Province(Nos.2020ZDLGY13-02,2023-YBGY-432)the Natural Science Basic Research Plan in Shaanxi Province of China(No.2021JC-13)the Research Fund of the State Key Labora-tory of Solidification Processing(NPU),China(No.2021-QZ-03).
文摘A new insight into the microstructural stability was proposed in Ni-based single crystal superalloys with Ru addition,and the element segregation behavior atγ/γ′interface was investigated by three-dimensional atom probe technology(3D-APT).After standard heat treatment,it was found that Ru addition barely altered the element partitioning coefficient betweenγmatrix andγ′phase,and no element-segregation layer was observed atγ/γ′interface.During the heat exposure at 1100°C,Ru addition obviously promoted the rafting of theγ′precipitates and inhibited the precipitation of topological close-packed(TCP)phases.It was more important that an element-segregation layer containing Re,Co,and Cr was formed in theγmatrix close to theγ/γ′interface due to an“uphill diffusion”effect,and its concentration was obviously reduced after Ru addition.Finally,the microstructural stability based on the element segregation behavior atγ/γ′interface was discussed.This element-segregation layer increased theγ/γ′interfacial energy by increasing the absolute value of the lattice misfit ofγ/γ′interface to promote the rafting of theγ′precipitates after Ru addition.On the other hand,the decrease of the segregation concentration of Re,Co,and Cr elements as TCP phase-forming elements near theγ/γ′interface due to a“reverse partitioning”effect inhibits the precipitation of TCP phases in Ni-based single crystal superalloys after Ru addition.
基金This work was financially supported by the National Natural Science Foundation of Tianjin,China(No.22175127)Institute of Energy,Hefei Comprehensive National Science Center(No.19KZS207).
文摘Bimetallic alloys could form three typical structures including solid solution,heterostructure,and intermetallic compound,depending on the interactions between identical and different atoms.Although the trend can be predicted by the types of binary phase diagram,different synthetic protocols will trap the system in various kinetic intermediates among the three typical structures.Herein,we studied the phase evolution and elemental segregation in the alloy nanoparticles of immiscible Pd-Ru before and after thermal annealing.By developing an analysis method of local element segregation(LES)based on the energy dispersive spectroscopy(EDS)mapping signals,we were able to quantify the mixing of Pd and Ru atoms during the gradual phase transition from face-centered cubic(fcc)to hexagonal close packed(hcp).Density functional theory was also applied to calculate the energies of all possible PdRu4 structures(93 fcc models and 267 hcp models),which helps to rationalize the phase transition and element segregation.The annealing process also leads to the change of the electronic structure,which further influences the performance in the electrocatalytic hydrogen evolution reaction.The highest activity of PdRu4-400 was largely attributed to the proper interface between the Pd-rich fcc phase and Ru-rich hcp phase,as revolved by the above methods.
基金financially supported by the National Key R&D Program of China(Nos.2019YFA0705304 and 2017YFA0700703)the National Natural Science Foundation of China(No.51671189)+1 种基金Innovation Program of Institute of Metal Research,China Academy of Science(No.2021-PY09)the Doctoral Scientific Research Foundation of Liaoning Province(No.2020-BS-007)。
文摘To reduce microsegregation,a series of homogenization treatments were carried out on a Ni-Co based superalloy prepared through directional solidification(DS).The element segregation characteristics and microstructural evolution were investigated by optical microscopy(OM),scanning electron microscopy(SEM),and electron probe microanalysis(EPMA).The results show that the elements are non-uniformly distributed in the solidified superalloy,in which W and Ti have the greatest tendency of microsegregation.Furthermore,severe microsegregation leads to complicated precipitations,includingη-Ni_(3) Ti and eutectic(γ+γ’).EPMA results show that Al and Mo are uniformly distributed between the eutectic(γ+γ’)andγmatrix,whereas Ti is segregated in the eutectic(γ+γ’)andηphases.The positive segregation element Ti,which is continuously rejected into the remaining liquid duringγmatrix solidification,promotes the formation of eutectic(γ+γ’)and the transformation of theηphase.According to the homogenization effect,the optimal single-stage homogenization process of this alloy is 1180℃for 2 h because of the sufficient diffusion segregation of the elements.In the present study,a kinetic diffusion model was built to reflect the degree of element segregation during homogenization,and the diffusion coefficients of W and Ti were estimated.
基金This work was financially supported by the National Natural Science Foundation of China(Nos.51774146 and 52071143).
文摘Here we first report the fully abundant rare earth(RE)-based nanocrystalline multi-component(Ce,La,Y)-Fe-B alloys containing no critical RE elements of Nd,Pr,Dy,and Tb by melt-spinning technique.The roles of La and Y substitutions for Ce have been fully understood.La plays a positive role on both thermal stability and room-temperature(RT)magnetic properties.The enhanced coercivity H_(cj)by partial substitution of La is attributed to the increases of anisotropy field H_A and the formation of continuously distributed grain boundaries resulting from the suppre s sion of CeFe_(2)phase.Although Y substitution is not benefit for H_(cj),both remanent polarization J_r and thermal stability have been effectively improved since Y_(2)Fe_(14)B shows relatively high saturation magnetization M_s and a positive temperature coefficient of HA over a certain temperature range.In addition,RE element segregation has been confirmed,La prefers to enter into the grain boundaries than Ce and Y prefers to remain in the 2:14:1 phase.Based on these understanding,a series of melt-spun(Ce,La,Y)-Fe-B alloys have been designed.A relatively good combination of magnetic properties with maximum energy product(BH)_(max)=7.4 MGOe,H_(Cj)=400 kA/m,and J_r=0.63 T has been obtained in[(Ce_(0.8)La_(0.2))_(0.7)Y_(0.3)]_(17)Fe_(78)B_6 alloy,together with high Curie temperature(T_c=488 K)and low temperature coefficients of remanence(α=-0.255%/K)and coercivity(β=-0.246%/K).
基金The authors gratefully acknowledge the financial support by the National Key Research and Development Program(No.2018YFA0702900)National Science and Technology Major Project(No.J2019-VII-0002-0142)the National Natural Science Foundation of China(No.51831007).
文摘The second phase dissolution and elements migration behavior of a nickel-based single crystal superalloy during solution heat treatment with direct current were investigated for simplifying and shortening the solution heat treatment of the Ni-based single crystal superalloy.The results showed that the electric current solution heat treatment improved microstructural homogenization as well as the distribution of alloying elements,especially for the refractory metal W and Mo.The microsegregation ratios for Mo and W after electric current solution heat treatment at 1230℃for 4 h are near those without electric current at 1250℃for 4 h.The electric current accelerated theγ′phase dissolution process,and theγ′phase could be completely dissolved at a lower treatment temperature or within a shorter treatment time under electric current solution heat treatment with direct current.A microcosmic current model was proposed to analyze the effect of the electric current on the solution heat treatment of the Ni-based single crystal superalloy.
文摘The effect of Cr/Mn segregation on the abnormal banded structure of high carbon bearing steel was studied by reheating and hot rolling.With the use of an optical microscope, scanning electron microscope, transmission electron microscope, and electron probe microanalyzer, the segregation characteristics of alloying elements in cast billet and their relationship with hot-rolled plate banded structure were revealed.The formation causes of an abnormal banded structure and the elimination methods were analyzed.Results indicate the serious positive segregation of C, Cr, and Mn alloy elements in the billet.Even distribution of Cr/Mn elements could not be achieved after 10 h of heat preservation at 1200℃, and the spacing of the element aggregation area increased, but the segregation index of alloy elements decreased.Obvious alloying element segregation characteristics are present in the banded structure of the hot-rolled plate.This distinct white band is composed of martensitic phases.The formation of this abnormal pearlite–martensite banded structure is due to the interaction between the undercooled austenite transformation behavior of hot-rolled metal and the segregation of its alloying elements.Under the air cooling after rolling, controlling the segregation index of alloy elements can reduce or eliminate the abnormal banded structure.
基金funded by the Deutsche Forschungsgemein-schaft(DFG,German Research Foundation,No.326646134)the Austrian Science Fund(FWF,No.Ⅰ3288-N36).
文摘In this study,aberration-corrected scanning transmission electron microscopy coupled with electron energy-loss spectroscopy(STEM-EELS)was used to investigate the atomistic structure and chemical com-position of true twin and order twin boundaries in ferromagneticτ-MnAl-C.True twins and order twins were distinguished based on the diffraction patterns using TEM.No elemental segregation was observed at the coherent true twin boundary but some Mn enrichment within a region of about 1.5-2 nm was found at the incoherent true twin boundary.A transition region with Mn enrichment about 4-6 nm wide was found at the order twin boundary.A carbon cluster with a size of around 5 nm was also found at the twin boundary.Micromagnetic simulations were conducted to study the effect of this chemical seg-regation at twin interfaces on the magnetic properties.The results showed that the coercivity tends to increase with increasing structural and chemical disorder at the interface.
基金the financial support from the National Natural Science Foundation of China(No.52174303)the Program of Introducing Talents of Disciplineto Universities,China(No.B21001)the Joint Program of Science and Technology Plans in Liaoning Province,China(No.2023JH2/101700302t)。
文摘Through thermodynamic calculations and microstructural characterization,the effect of niobium(Nb)content on the solidifica-tion characteristics of Alloy 625 Plus was systematically investigated.Subsequently,the effect of Nb content on hot deformation behaviorwas examined through hot compression experiments.The results indicated that increasing the Nb content lowers the liquidus temperatureof the alloy by 51℃,producing a denser solidification microstructure.The secondary dendrite arm spacing(SDAS)of the alloy decreasesfrom 39.09 to 22.61μm.Increasing the Nb content alleviates element segregation but increases interdendritic precipitates,increasing theirarea fraction from 0.15% to 5.82%.These precipitates are primarily composed of large Laves,δ,η,and γ″phases,and trace amounts of Nb C.The shapes of these precipitates change from small chunks to large elongated forms.No significant change in the type or amount ofinclusions within the alloy is detected.The inclusions are predominantly individual Al_(2)O_(3) and TiN,as well as Al_(2)O_(3)/Ti N composite inclu-sions.Samples with varying Nb contents underwent hot compression deformation at a true strain of 0.69,a strain rate of 0.5 s^(-1),and a de-formation temperature of 1150℃.Increasing the Nb content also elevates the peak stress observed in the flow curves.However,alloyswith higher Nb content exhibit more pronounced recrystallization softening effects.The Laves phase precipitates do not completely redis-solve during hot deformation and are stretched to elongated shapes.The high-strain energy storage increases the recrystallization fractionfrom 32.4% to 95.5%,significantly enhancing the degree of recrystallization and producing a more uniform deformation microstructure.This effect is primarily attributed to the addition of Nb,which refines the initial grains of the alloy,enhances the solid solution strengthen-ing of the matrix,and improves the induction of particle-stimulated nucleation.
基金supported by the National Natural Science Foundation of China(Nos.52071021,51871012)Beijing Natural Science Foundation,China(No.2162024)+1 种基金the Fundamental Research Funds for the Central Universities,China(No.FRF-GF-20-20B)the National Program on Key Basic Research Project of China(No.2011CB605502).
文摘High Nbβ/γ-TiAl(HNBG)intermetallics and Ni-based superalloy(IN718)were diffusion-bonded using pure Ti foil interlayer under pulse current.The microstructure,element segregation,and mechanical properties of HNBG/Ti/IN718 joint were investigated.The effect of Ti interlayer on microstructure and mechanical properties of the joint was discussed.The typical microstructure of HNBG/Ti/IN718 joint was HNBG//β/B2,τ_(3)-NiAl_(3)Ti_(2)//α_(2)-Ti_(3)Al//α-Ti+δ-NiTi_(2),β-Ti//δ-NiTi_(2)//β2-(Ni,Fe)Ti//Cr/Fe-richη-Ni_(3)Ti,η-Ni_(3)Ti,α-Cr,δ-Ni_(3)Nb//η-Ni_(3)Ti,γ-Ni,δ-Ni_(3)Nb//IN718.The gaps and Kirkendall voids exhibited a gradual disappearance with increasing bonding temperature.The mechanism of Cr,Fe and Nb elements segregation was that NiTi phase hindered the diffusion of them.The nano-indentation results demonstrated that diffusion zones on IN718 alloy side had higher hardness.The maximum shear strength of the joint(326 MPa)was achieved at bonding parameters of 850℃,20 min and 10 MPa.The fracture occurred in Zones IV and V,and the fracture modes were brittle fracture and cleavage fracture.The introduction of Ti interlayer resulted in improved microstructure and enhanced bonding strength of the joint.
基金supported by the National Natural Science Foundation of China(No.U21A20113)the Natural Science Foundation of Hubei Province of China(No.2021CFA023).
文摘A low-carbon,low-cost,and high-efficient method was reported for remarkably improving corrosion resistance of C–Mn structural steel by weak deoxidation.The results showed that,with the total oxygen content(wOT)increasing in the tested steel from 41×10^(−6)to 195×10^(−6),both the degree of element segregation and the level of banded microstructure weakened,presenting the lower potential difference between pearlite(P)and ferrite(F),and then smaller galvanic corrosion driving force,and thus effectively improving general corrosion properties.In addition,with wOT growing up,the number and size of inclusions increased,and the shape also changed from long chain or small particle to large particle ball with typical mosaic structure,which could effectively inhibit the preferential dissolution of local component due to multiple complex interfaces,and correspondingly suppress the pitting susceptibility.However,the impact toughness at low temperature of the tested steel reduced with wOT increasing,and then,taking the mechanical properties and corrosion resistance all into account,160×10^(−6) was the optimal oxygen content within the present scope.
基金supported by the National Natural Science Foundation of China(21203008,21975025,12274025,22372008,and 22179007)Hainan Province Science and Technology Special Fund(ZDYF2021SHFZ232 and ZDYF2023GXJS022)Hainan Province Postdoctoral Science Foundation(300333)。
文摘To satisfy the demands of modern society for high-energy–density sulfide-based all-solid-state lithium batteries(ASSLBs),Ni-rich cathode materials have gained much attention for their high capacity and energy density.However,their practical deployment is hindered by accelerated interface degradation and capacity decay originating from surface oxygen release and lattice oxygen activation during prolonged cycling.In this study,Ti_(x)NbB_((1−x))C_(2)was successfully coated on the surface of LiNi_(0.94)Co_(0.05)Mn_(0.01)O_(2).Density functional theory(DFT)calculations first elucidate a“point-to-point”anchoring mechanism where each surface oxygen atom coordinates with single species(Ti/Nb/B)offered by Ti_(x)NbB_((1−x))C_(2),which forms robust O–M bonds and sustain a stable interface structure.The electron energy loss spectroscopy(EELS)reveals the segregation of Ti/Nb toward subsurface layers during cycling,creating an optimized lattice oxygen coordination environment and suppressing oxygen activation.The dual oxygen stabilization mechanism dramatically improves the reversibility of phase transition and the structural stability of the Ni-rich cathode materials.Moreover,Ti_(x)NbB_((1−x))C_(2)as the protective layer decreases mechanical strain and suppresses the parasitic reactions.Consequently,the engineered cathode delivers 91%capacity retention after 1000 cycles at 0.3 C,suggesting excellent cycling stability.The research delivers a new design philosophy for the coating layer that can stabilize surface oxygen.Furthermore,the atomistic understanding of the structure–property relationship of the Ni-rich cathode materials provides valuable guidance for the future design of new cathode materials with superior structural stability in ASSLBs.
基金financially supported by the Natural Science Foundation Joint Fund of Liaoning Province,China(No.2023-MSLH-342).
文摘The effect of melt superheating treatment on the solidification microstructure and mechanical properties of theγ'phase precipitation-strengthened K424 superalloy was investigated.Differential scanning calorimetry(DSC)experiments were conducted to explore the influence of melt treatment temperature on the undercooling of the superalloy.Additionally,pouring experiments were carried out to assess how alterations in both the temperature and duration of melt treatment impacted the grain size,secondary dendrite arm spacing(SDAS),elemental segregation,and mechanical properties of the alloy.Metallographic analysis,scanning electron microscopy,energy dispersive spectroscopy(EDS)and Thermo-Calc software were employed for microstructure characterization.The test specimens were subjected to tensile testing at room temperature and stress rupture testing at 975℃ under 196 MPa.The findings reveal that appropriate melt treatment conditions result in decreased grain size,refined SDAS,minimized elemental segregation,and significant improvements in mechanical properties.Specifically,the study demonstrates that a melt treatment at 1,650℃ for 5 min results in the smallest average grain size of 949μm and the smallest SDAS of 25.38μm.Furthermore,the room temperature tensile properties and creep resistance are notably affected by the melt treatment parameters.It is shown that specific melt treatment conditions,such as holding at 1,650℃ for 5 min,result in superior room temperature strength and extended stress rupture life of the K424 superalloy,while a balance between strength and stability is achieved at 1,600℃ with a holding time of 10 min.These findings offer guidance for optimizing the melt treatment parameters for the K424 superalloy,laying a foundation for further investigations.
基金supported by the National Science and Technology Major Project(No.J2019-VI-0006-0120)the National Natural Science Foundation of China(No.52074092).
文摘Upon approaching 850℃,the GH4151 alloy exhibits diminished high-temperature strength,primarily attributed to the disruption of γ′phase coherence at elevated temperatures,which reduces its strengthening contribution.Tantalum(Ta)additions enhance the stability of the γ′phase but introduce processing challenges,including pronounced solidification segregation,the formation of secondary phases,and increased susceptibility to cracking during processing.The influence of Ta content on elemental segregation,solidification microstructure,phase precipitation kinetics,and hot deformation behaviour in GH4151 was systematically investigated.Processing windows derived from the dynamic materials model(DMM)and microstructural evolution under varying thermomechanical conditions are further examined.Key findings reveal the severe Ta segregation(segregation coefficient K≈1.608);Ta promotes γ/γ′eutectic and η phase formation,increasing γ′phase volume fraction from 54% to approximately 63%;and increased Ta content elevates flow stress and progressively narrows the DMM-defined processing window;optimized thermomechanical processing parameters(elevated temperatures and strain rates)enhance recrystallization kinetics and hot workability,thereby mitigating cracking propensity.
基金supported by the National Natural Science Foundation of China(Nos.Ui708253 and 51571052)。
文摘The developing of large size superalloy vacuum induction melting(VIM)ingots is limited owing to hot cracking,The hot cracking behavior of the large size GH4742 superalloy VIM ingot was investigated via experiment and simulation.The microstructure was examined by optical microscopy,and element segregation was investigated by electron probe microanalysis.The solidification temperature range and yield strength at high temperature(YSHT)were calculated by JMatPro software.The results show that the variations of microstructure and element segregation in different locations are caused by different cooling rates.Moreover,the larger secondary dendrite arm spacing and serious element segregation of Nb accelerate hot cracking of the VIM ingot.In addition,the solidification temperature range is wider,and YSHT is lower in center than at edge of the ingot.Therefore,the hot cracking susceptibility is the highest in the center of the GH4742 superalloy VIM ingot.The critical criterion of element segregation for hot cracking is that the partition coeffcient of Nb should be larger than 0.5.
基金supported by the National Natural Science Foundation of China(No.51874084)the Fundamental Research Funds for the Central Universities(No.2125026)。
文摘The hot deformation behavior of Mn18Cr18N and Mn18Cr18N+Ce high nitrogen austenitic stainless steels at 1173-1473 K and 0.01-1 s^(-1) were investigated by thermal compression tests.The influence mechanism of Ce on the hot deformation behavior was analyzed by Ce-containing inclusions and segregation of Ce.The results show that after the addition of Ce,large,angular,hard,and brittle inclusions(TiN-Al_(2)O_(3),TiN,and Al_(2)O_(3)) can be modified to fine and dispersed Ce-containing inclusions(Ce-Al-O-S and TiN-Ce-Al-O-S).During the solidification,Ce-containing inclusions can be used as heterogeneous nucleation particles to refine as-cast grains.During the hot deformation,Ce-containing inclusions can pin dislocation movement and grain boundary migration,induce dynamic recrystallization(DRX)nucleation,and avoid the formation and propagation of micro cracks and gaps.In addition,during the solidification,Ce atoms enrich at the front of solid-li-quid interface,resulting in composition supercooling and refining the secondary dendrites.Similarly,during the hot deformation,Ce atoms tend to segregate at the boundaries of DRX grains,inhibiting the growth of grains.Under the synergistic effect of Ce-containing inclusions and Ce segregation,although the hot deformation resistance and hot deformation activation energy are improved,DRX is more likely to occur and the size of DRX grains is significantly refined,and the problem of hot deformation cracking can be alleviated.Finally,the microhardness of the samples was measured.The results show that compared with as-cast samples,the microhardness of hot-deformed samples increases signific-antly,and with the increase of DRX degree,the microhardness decreases continuously.In addition,Ce can affect the microhardness of Mn18Cr18N steel by affecting as-cast and hot deformation microstructures.
基金Project supported by the Ministry of Science and Technology of China (2002BA315A-5)
文摘Using SEM and EDS some particles containing La and P were discovered in low oxygen and sulfur purity steel with lanthanum. In these particles low melting point elements such as As and Sb also were detected. According to the result of simulative calculation of segregation, it is shown that La and P enrich so strongly in the process of solidification that their concentrations can satisfy thermodynamic condition to forme LaP compound or meet the requirements to forme eutectic phase in the last stage of solidification. Therefore, the particles are concluded as LaP compound or eutectics, which exist on grain boundary mainly.
基金financially supported by the National Key R&D Program of China(No.2022YFB3705300)the National Natural Science Foundation of China(Nos.U1960204 and 51974199)the Postdoctoral Fellowship Program of CPSF(No.GZB20230515)。
文摘The infamous type Ⅳ failure within the fine-grained heat-affected zone (FGHAZ) in G115 steel weldments seriously threatens the safe operation of ultra-supercritical (USC) power plants.In this work,the traditional thermo-mechanical treatment was modified via the replacement of hot-rolling with cold rolling,i.e.,normalizing,cold rolling,and tempering (NCT),which was developed to improve the creep strength of the FGHAZ in G115 steel weldments.The NCT treatment effectively promoted the dissolution of preformed M_(23)C_(6)particles and relieved the boundary segregation of C and Cr during welding thermal cycling,which accelerated the dispersed reprecipitation of M_(23)C_(6) particles within the fresh reaustenitized grains during post-weld heat treatment.In addition,the precipitation of Cu-rich phases and MX particles was promoted evidently due to the deformation-induced dislocations.As a result,the interacting actions between precipitates,dislocations,and boundaries during creep were reinforced considerably.Following this strategy,the creep rupture life of the FGHAZ in G115 steel weldments can be prolonged by 18.6%,which can further push the application of G115 steel in USC power plants.
