This study investigates the effect of high current density electropulsing on the material in a rapid stress relaxation process.An AISI 1020 steel was shot-peened to induce surface compressive residual stresses in a co...This study investigates the effect of high current density electropulsing on the material in a rapid stress relaxation process.An AISI 1020 steel was shot-peened to induce surface compressive residual stresses in a controlled manner and subsequently electropulsed to investigate the changes in microstructure and defect configuration.AISI 1020 steel was chosen as it has a simple microstructure(plain ferritic)and composition with low alloying conditions.It is an appropriate material to study the effect of trans-mitting electric pulses on the microstructural defect evolution.A combination of electron-backscattered diffraction and transmission electron microscopy proved to be an effective tool in characterizing the post-electropulsing effects critically.By application of electropulsing,a reduction in the surface residual stress layer was noticed.Also,reductions in misorientation and dislocation density together with the disentan-glement of dislocations within the cold-worked layer were observed after electropulsing.Additionally,the annihilation of shot-peening-induced deformation bands beyond the residual layer depth was observed.These effects have been rationalised by taking into account the various possibilities of athermal effects of electropulsing.展开更多
The relation between the Mg treatment and ferrite grain boundaries misorientation was investigated. The orientation imaging microscopy technique based on electron backscattered diffraction technique （EBSD） was used ...The relation between the Mg treatment and ferrite grain boundaries misorientation was investigated. The orientation imaging microscopy technique based on electron backscattered diffraction technique （EBSD） was used in this work. （t was found that the addition of 0.005 wt% Mg to the steel could evidently increase the ratio of acicular ferrite crystals appearing at large angles boundaries to each other, which was attributed to the nucleation of the second-phase particles by the Mg treatment. The FBSD techniques provide a power- ful method to characterize and quantify the ferrite grain boundaries misorientation, in order to relate it to toughness.展开更多
Nowadays,thermal condition and solute field are considered as the potential dominant factors controlling competitive grain growth during directional solidification process.However,the controlling modes and critical co...Nowadays,thermal condition and solute field are considered as the potential dominant factors controlling competitive grain growth during directional solidification process.However,the controlling modes and critical conditions of competitive grain growth have been drastically debated over the past two decades.In this work,thermal condition and solute field are combined to study the competitive grain growth in the converging case by experimental observation and numerical simulation of bicrystal samples.We find the competitive grain growth is controlled by the cooperative effect of thermal condition and solute field,and the controlling modes are related to the bicrystal misorientation between favorably and unfavorably oriented grains.When the unfavorably oriented grain is low misoriented,unfavorably oriented grain dominates grain selection,and the competitive grain growth performs as solute field domination.However,with the increase of unfavorably oriented grain’s misorientation,the grain selection converts into favorably oriented grain domination,and the competitive grain growth changes to thermal condition domination.To explain these abnormal transformation phenomena,we propose a misorientation dependent thermal condition-solute field cooperative domination model and identify the critical conditions by a critical misorientation(θ_(cm)).According to dynamic equation of dendrite growth,we calculate the critical misorientationθ;to prove this model.The theoretical calculation results agree well with the experimental results.展开更多
Nickel-based single-crystal superalloys are widely used in the manufacture of aeroengine turbine vanes for their excellent high-temperature performance. Low-angle grain boundaries (LAGBs) will be generated inevitably ...Nickel-based single-crystal superalloys are widely used in the manufacture of aeroengine turbine vanes for their excellent high-temperature performance. Low-angle grain boundaries (LAGBs) will be generated inevitably during their manufacture, which are often characterized by grain boundary misorientation (GBM) and will weaken the mechanical properties of superalloys. However, the relationship between GBM and the fatigue properties of superalloys at elevated temperatures has seldom been investigated due to the difficulty in the sample preparation and experiment process. Based on six kinds of bicrystals with different tilt LAGBs made by a second-generation single-crystal superalloy, the effects of misorientation on the grain boundary microstructure and fatigue properties (980 °C) of superalloys were studied systematically in this work. It is found that, with the increase of GBM, the GB precipitates combined with the cast micropores increase monotonically, accordingly both the fatigue life and fatigue strength decrease successively. Fatigue fracture observations show that the cracks of all the bicrystals initiated from the cast micropores at GBs, and then propagated along the GBs. Therefore, the coupling effect of cast micropores and GBM on the fatigue damage mechanisms of the bicrystals are evaluated according to their hindering degrees on the piled-up dislocations. Combining with a hysteresis energy model, a quantitative fatigue strength prediction model of superalloys is established and is well verified by abundant experimental data. This study could provide guidance for fatigue performance prediction and structural design of superalloys.展开更多
In the present work the research of grain boundary (CB) energy versus angle of misorientation in fcc metals Al, Cu, An and Ni was carried out. An axis of CB misorientation is a direction [100], angle of misorientatio...In the present work the research of grain boundary (CB) energy versus angle of misorientation in fcc metals Al, Cu, An and Ni was carried out. An axis of CB misorientation is a direction [100], angle of misorientation makes from 2皍p in 23*. The interatomic interaction was opproximated by Morse' s pair semi-empirical potential. Two variants of relaxation technique were used: (1) rigid relax- ation with the change of atom quantity per a GB (vacancy relaxation ) and (2) full atomic relaxation by a molecular static method. The obtained orientation dependence has a good agreement with experi- ment. There are cusps on a curve in the range of special GB angles.The comparison of obtained curves with calculated ones in model Van der Merwe was carried out. Dependencies obtained in our investiga- tions are not smooth and have an oscillatory character. The oscillations reflect a discrete structure of a lattice.展开更多
Asymmetry in dislocation density and strain relaxation has a significant impact on device performance since it leads to anisotropic electron transport in metamorphic materials. So it is preferred to obtain metamorphic...Asymmetry in dislocation density and strain relaxation has a significant impact on device performance since it leads to anisotropic electron transport in metamorphic materials. So it is preferred to obtain metamorphic materials with symmetric properties. In this paper, we grew metamorphic Ino.27Gao.73As epilayers with symmetric low threading dislocation density and symmetric strain relaxation in two （110） directions using InA1GaAs buffer layers on 7° misoriented GaAs （001） substrates. To understand the control mechanism of symmetric properties of Ino.27Gao.73As layers by the substrate miscut angles, Ino.27Gao.73As grown on 2° and 15° misoriented substrates were also characterized as reference by atomic force microscopy, transmission electron microscopy, and high resolution triple axis x-ray diffraction. The phase separation and interaction of 60° misfit dislocations were found to be the reasons for asymmetry properties of Ino.27Gao33As grown on 2° and 15° substrates, respectively. Photoluminescence results proved that the Ino.27G°ao.73As with symmetric properties has better optical properties than the Ino.27Gao.73As with asymmetric properties at room temperature. These results imply that high quality metamorphic Ino.27Gao.73As can be achieved with controllable isotropic electron transport property.展开更多
X65, X70, and X80 belong to high grade pipeline steels. Toughness is one of the most important properties of pipeline steels when the pipeline transports the gas or oil, and the means to control toughness is very impo...X65, X70, and X80 belong to high grade pipeline steels. Toughness is one of the most important properties of pipeline steels when the pipeline transports the gas or oil, and the means to control toughness is very important for exploring even higher grade pipeline steels. We established the relationship between toughness and crystallographic parameters of high grade pipeline steels by studying the crystallographic parameters of X65, X70, and X80 using EBSD and analyzing Charpy CVN of X65, X70 and X80. The results show that the effective grain size, the frequency distribution of grain boundary misorientation and the ratio of high angle grain boundary to small angle grain boundary are important parameters. The finer the effective grain size, and the higher the frequency distribution of grain boundaries （〉 50~）, the more excellent toughness of high grade pipeline steels will be.展开更多
The fracture modes of low alloy steels and cast irons under tensile and fatigue conditions were identified by electron back-scattered diffraction（EBSD） misorientation analysis in this research. The curves of grain r...The fracture modes of low alloy steels and cast irons under tensile and fatigue conditions were identified by electron back-scattered diffraction（EBSD） misorientation analysis in this research. The curves of grain reference orientation deviation（GROD） distribution perpendicular to the fracture surface were obtained by EBSD observation, and the characteristics of each fracture mode were identified. The GROD value of the specimen fractured in tension decreases to a constant related to the elongation of corresponding specimen in the far field（farther than 5 mm away from the fracture surface）. The peak exhibits in GROD curves of two smooth specimens and a notched specimen near the fracture surface（within 5 mm away from the fracture surface）, and the formation mechanisms were discussed in detail based on the influences of specimen geometries（smooth or notched） and material toughness. The GROD value of fatigue fractured specimen is close to that at undeformed condition in the whole field, except the small area near the crack path. The loading conditions（constant stress amplitude loading or constant stress intensity factor range K loading） and the EBSD striation formation during fatigue crack propagation were also studied by EBSD observation parallel to the crack path.展开更多
Low-angle grain boundaries(LAGBs)are one of the solidification defects in single-crystal nickel-based superalloys and are detrimental to the mechanical properties.The formation of LAGBs is related to dendrite deformat...Low-angle grain boundaries(LAGBs)are one of the solidification defects in single-crystal nickel-based superalloys and are detrimental to the mechanical properties.The formation of LAGBs is related to dendrite deformation,while the mechanism has not been fully understood at the mesoscale.In this work,a model coupling dendrite growth,thermal-solutal-fluid flow,thermal stress and flow-induced dendrite deformation via cellular automaton-finite volume method and finite element method is developed to study the formation of LAGBs in single crystal superalloys.Results reveal that the bending of dendrites is primarily attributed to the thermal-solutal convection-induced dendrite deformation.The mechanical stress of dendrite deformation develops and stabilises as solidification proceeds.As the width of the mushy zone gets stable,stresses are built up and then dendritic elastoplastic bending occurs at some thin primary dendrites with the wider inter-dendritic space.There are three characteristic zones of stress distribution along the solidification direction:(i)no stress concentration in the fully solidified regions;(ii)stress developing in the primary dendrite bridging region,and(iii)stress decrease in the inter-dendritic uncontacted zone.The stresses reach maximum near the initial dendrite bridging position.The lower temperature gradients,the finer primary dendritic trunks and sudden reductions in local dendritic trunk radius jointly promote the elastoplastic deformation of the dendrites.Corresponding measures are suggested to reduce LAGBs.展开更多
Mechanism of discontinuous precipitation(DP) in AZ80 alloy was investigated by phase-orientation correlated characterization.The results show DPs nucleate by turning the original grain boundaries(GBs) as reaction fron...Mechanism of discontinuous precipitation(DP) in AZ80 alloy was investigated by phase-orientation correlated characterization.The results show DPs nucleate by turning the original grain boundaries(GBs) as reaction front(RF),and further driving the RF to realize their growth.The DPs regions retained the same orientations as their parent grains.The misorientation angle and rotation axis of RFs had strong influence on DPs nucleation.The low-angle GBs,twin boundaries(TBs) and the GBs with specific misorientation axis which are known as low energy and low mobility GBs can hardly initiate DPs.In addition,the TBs had a strong ability to inhibit the growth of DPs,but it should be noticed that the growth of DPs cannot be totally inhibited by TBs.DPs can engulf the twins when the growth direction is approximately parallel to the long axis of TBs.The inhibition behavior is related to the distribution of Al solute atoms near the RF,boundary interactions of the TBs and twin tips with the RF,and the morphology of the continuous precipitations within the twins.展开更多
Bending is a crucial deformation process in metal sheet forming.In this study,the microstructural evolution of a highly ductile Mg–Er–Zr alloy sheet was examined in various bending regions under different bending st...Bending is a crucial deformation process in metal sheet forming.In this study,the microstructural evolution of a highly ductile Mg–Er–Zr alloy sheet was examined in various bending regions under different bending strains using electron backscatter diffraction and optical microscopy.The results show that the Mg–Er–Zr extruded sheet has excellent bending properties,with a failure bending strain of 39.3%,bending yield strength,and ultimate bending strength of 75.1 MPa and 250.5 MPa,respectively.The exceptional bending properties of the Mg–Er–Zr extruded sheets are primarily due to their fine grain size and the formation of rare-earth(RE)textures resulting from Er addition.Specifically,the in-grain misorientation axes(IGMA)and the twinning behaviors in various regions of the specimen during bending were thoroughly analyzed.Due to the polarity of the tensile twins and their low activation stress,a significant number of tensile twins are activated in the compression zone to regulate plastic deformation.The addition of Er weakens the basal texture of the sheet and reduces the critical resolved shear stress difference between non-basal slip and basal slip.Consequently,in the tensile zone,the basal and non-basal slips co-operate to coordinate the plastic deformation,effectively impeding crack initiation and propagation,and thereby enhancing the bending toughness of the Mg–Er–Zr sheet.展开更多
In this work, commercially pure Zr sheets were subjected to β air cooling and then rolled to different reductions(10% and 50%)at room temperature. Microstructures of both the β-air-cooled and the rolled specimens we...In this work, commercially pure Zr sheets were subjected to β air cooling and then rolled to different reductions(10% and 50%)at room temperature. Microstructures of both the β-air-cooled and the rolled specimens were well characterized by electron channelling contrast imaging and electron backscatter diffraction techniques, with special attentions paid to their misorientation characteristics. Results show that the β-air-cooled specimen owns a Widmanst?tten structure featured by lamellar grains with typical phase transformation misorientations. The 10% rolling allows prismatic slip and tensile twinning({11-21}<11-2-6> and{10-12}<10-11>) to be activated profusely, which produce new low-angle(~3°–5°) and high-angle(~35° and ~85°) misorientation peaks, respectively. After increasing the rolling reduction to 50%, twinning is suppressed and dislocation slip becomes the dominant deformation mode, with the lamellar grains highly elongated and aligned towards the rolling direction.Meanwhile, only one strong low-angle misorientation peak related to the prismatic slip is presented in the 50%-rolled specimen,with all other peaks disappeared. Analyses on local misorientations reveal that hardly any residual strains exist in the β-air-cooled specimen, which should be related to their sufficient relaxation during slow cooling. Residual strains introduced by 10% rolling are heterogeneously distributed near grain/twin boundaries while heavier deformation(50% rolling) produces much larger residual strains pervasively existing throughout the specimen microstructure.展开更多
A high-purity Ti(HP-Ti)sheet was subjected to small strain rolling(10%reduction)with microstructural and textural characteristics examined by electron channeling contrast imaging and electron backscatter diffraction t...A high-purity Ti(HP-Ti)sheet was subjected to small strain rolling(10%reduction)with microstructural and textural characteristics examined by electron channeling contrast imaging and electron backscatter diffraction techniques.Particular attentions were paid to misorientation and textural changes aroused by twins in the rolled HP-Ti sheet.Results show that after the 10%rolling,almost all the prior equiaxed grains in the initial specimen are twinned,leading to remarkable grain refinement.The presence of two major misorientation angle peaks around 65°and 85°is ascribed to{11-22}<11-23>and{10-12}<10-11>twinning,respectively,and two minor peaks around 47°and 77°are due mainly to impingement of various variants of such twins.Distinct from earlier work,the small strain rolling is confirmed to be able to induce drastic textural changes in pure Ti sheets:largely reduced texture intensity and appearance of new textural components.This can essentially be attributed to enhanced twinning activity due to much lower impurity contents of the present material.Primary{11?22}twins are mainly responsible for the new textural component of c-axes aligned near the rolling direction with spread,while the component of caxes parallel to the normal direction is due to reorientation of secondary{10-12}twins.This study clearly demonstrates the capability of small strain rolling to effectively modify both microstructures and textures of the HP?Ti sheet and may shed some light on exploring feasible processings for such materials.展开更多
The optimized stress rupture properties of IN718 are of great interest to the aerospace industry,particularly in the presence of a notch.The deformation mechanisms for the stress rupture properties of IN718 after diff...The optimized stress rupture properties of IN718 are of great interest to the aerospace industry,particularly in the presence of a notch.The deformation mechanisms for the stress rupture properties of IN718 after different heat treatments,using electron back-scattered diffraction(EBSD)as an important tool,were investigated.The notch sensitivity of IN718 was closely associated with the distribution of theδphase.When the area fraction(A(f))of theδphase was less than 0.56%,the sample failed at the notch;while when A(f)≥2.98%,the notch sensitivity disappeared,and failure occurred on the smooth gauge.Heterogeneous deformation with strain concentrated at the grain boundaries was observed for the notch-and smooth-gauge-failed samples through EBSD analysis.Although intergranular cracking was detected in both cases,the failure mechanisms were different.Grain boundary sliding was the dominant failure mechanism for the notch-failed samples,whereas microvoid coalescence was the dominant failure mechanism for the smooth-gauge-failed samples.展开更多
Accumulative roll-bonding （ARB） was applied to Mg-Al-Zn magnesium alloy sheets to prepare ultrafine-grain microstructure. Significant grain refinement is achieved after three cycles of ARB with average grain size of...Accumulative roll-bonding （ARB） was applied to Mg-Al-Zn magnesium alloy sheets to prepare ultrafine-grain microstructure. Significant grain refinement is achieved after three cycles of ARB with average grain size of about 1.3 μm. The microstructure is characterized by nearly uniform ultrafine equiaxed microstructure without twins. The evolution of the misorientation distribution during ARB was measured by EBSD. Grain refinement can be contributed to the grain subdivision induced by severe accumulated strain, the accumulated strain enhanced concurrent dynamic recovery and recrystallization as well as the complicated distribution of interface and shear strain during ARB.展开更多
Texture and grain boundary character distribution of Cu interconnects with different line width for as-deposited and annealed conditions were measured by EBSD. All specimens appear mixed texture and （111） texture is...Texture and grain boundary character distribution of Cu interconnects with different line width for as-deposited and annealed conditions were measured by EBSD. All specimens appear mixed texture and （111） texture is the dominate component.As-deposited interconnects undergo the phenomenon of self-annealing at RT,in which some abnormally large grains are found. Lower aspect ratio of lines and anneal treatment procured larger grains and stronger （111） texture. Meanwhile, the intensity proportion of other textures with lower strain energy to （111） texture is decreased. As-deposited specimens reveal （111）（112？ and （111） （231） components, （111） （110） component appeared and （111） （112？ and （111） （231） components were developed during the annealing process. High angle boundaries are dominant in all specimens, boundaries with a misorientation of 55°-60° and ∑3 ones in higher proportion, followed by lower boundaries with a misorientation of 35°-40° and 29 boundaries. As the aspect ratio of lines and anneal treatment increase,there is a gradual in- crement in ∑3 boundaries and a decrease in ∑9 boundaries.展开更多
Synchrotron polychromatic X-ray microdiffraction（micro-XRD） was applied to study in situ deformation twinning of commercially AZ31（Mg-3Al-1Zn） strip subjected to uniaxial tension.The morphology and growth of twins...Synchrotron polychromatic X-ray microdiffraction（micro-XRD） was applied to study in situ deformation twinning of commercially AZ31（Mg-3Al-1Zn） strip subjected to uniaxial tension.The morphology and growth of twins were analyzed in situ under the load level from 64 to 73 MPa.The X-ray microdiffraction data,collected on beamline 12.3.2 at the Advanced Light Source,were then used to map an area of 396μm x 200μm within the region of interest.The experimental set-up and X-ray diffraction microscopy with a depth resolution allow the position and orientation of each illuminated grain to be determined at the submicron size.A list of parent grains sorted by crystallographic orientation were selected to examine their twinning behavior.The results depict twin variant selection,local misorientation fluctuation and mosaic spread for multi-twins within the same parent grain.As load increases,the amplitude of misorientation fluctuation along twin trace keeps increasing.This is attributable to the accumulation of geometrically necessary dislocations.展开更多
The microhardness curve trend and its relationships with microstructure and misorientation were analyzed to enhance the comprehension of the microstructure and mechanical property of micro-areas in Ti6 Al4 V laser-wel...The microhardness curve trend and its relationships with microstructure and misorientation were analyzed to enhance the comprehension of the microstructure and mechanical property of micro-areas in Ti6 Al4 V laser-welded joints with different welding speeds. The microhardness measured on the fusion line(H_m) is the highest from the weld center to the base metal. H_m increases with increasing weld width in a welded joint and increasing degree of the non-uniformity in all studied welded joints. The microhardness decreases from the weld metal to the base metal with decreasing amount of martensite α’ and increasing amount of original α phase. When the microstructure is mainly composed of martensite α’, the microhardness changes with the cooling rate, grain size of the martensite, and peak values of the fraction of misorientation angle of the martensite in a wide weld metal zone or weld center at different welding speeds, whereas the difference is small in a narrow weld metal zone.展开更多
As an advanced solid state bonding process,plastic deformation bonding(PDB)is a highly reliable metallurgical joining method that produces significant plastic deformation at the bonding interface of welded joints thro...As an advanced solid state bonding process,plastic deformation bonding(PDB)is a highly reliable metallurgical joining method that produces significant plastic deformation at the bonding interface of welded joints through thermo-mechanical coupling.In this study,PDB behavior of IN718 superalloy was systematically investigated by performing a series of isothermal compression tests at various processing conditions.It was revealed that new grains evolved in the bonding area through discontinuous dynamic recrystallization(DDRX)at 1000–1150℃.Electron backscattered diffraction(EBSD)and transmission electron microscopy(TEM)results revealed that the bonding of joints is related with interfacial grain boundary(IGB)bulging process,which is considered as a nucleation process of DRXed grain under different deformation environments.During recrystallization process,the bonded interface moved due to strain-induced boundary migration(SIBM)process.Stored energy difference(caused by accumulation of dislocations at the bonding interface)was the dominant factor for SIBM during DRX.The mechanical properties of the bonded joints were dependent upon the recrystallized microstructure and SIBM ensued during PDB.展开更多
基金supported by the National Research Foundation of Singapore,Rolls-Royce Singapore Pte.Ltd.,and Nanyang Technological University through grants#002123-00009 and #002124-00009.
文摘This study investigates the effect of high current density electropulsing on the material in a rapid stress relaxation process.An AISI 1020 steel was shot-peened to induce surface compressive residual stresses in a controlled manner and subsequently electropulsed to investigate the changes in microstructure and defect configuration.AISI 1020 steel was chosen as it has a simple microstructure(plain ferritic)and composition with low alloying conditions.It is an appropriate material to study the effect of trans-mitting electric pulses on the microstructural defect evolution.A combination of electron-backscattered diffraction and transmission electron microscopy proved to be an effective tool in characterizing the post-electropulsing effects critically.By application of electropulsing,a reduction in the surface residual stress layer was noticed.Also,reductions in misorientation and dislocation density together with the disentan-glement of dislocations within the cold-worked layer were observed after electropulsing.Additionally,the annihilation of shot-peening-induced deformation bands beyond the residual layer depth was observed.These effects have been rationalised by taking into account the various possibilities of athermal effects of electropulsing.
文摘The relation between the Mg treatment and ferrite grain boundaries misorientation was investigated. The orientation imaging microscopy technique based on electron backscattered diffraction technique （EBSD） was used in this work. （t was found that the addition of 0.005 wt% Mg to the steel could evidently increase the ratio of acicular ferrite crystals appearing at large angles boundaries to each other, which was attributed to the nucleation of the second-phase particles by the Mg treatment. The FBSD techniques provide a power- ful method to characterize and quantify the ferrite grain boundaries misorientation, in order to relate it to toughness.
基金financially supported by the Shandong Provincial Natural Science Foundation(No.ZR2020ME110)the National Natural Science Foundation of China(Nos.51331005,U1508213,51771190 and 51601102)+2 种基金the Fund of the State Key Laboratory of Solidification Processing in NWPU(Nos.SKLSP201847 and SKLSP201834)the Young Doctors Cooperation Project in Qilu University of Technology(No.2018BSHZ003)the Key Research and Development Program of Ningxia(No.2019BDE03016)。
文摘Nowadays,thermal condition and solute field are considered as the potential dominant factors controlling competitive grain growth during directional solidification process.However,the controlling modes and critical conditions of competitive grain growth have been drastically debated over the past two decades.In this work,thermal condition and solute field are combined to study the competitive grain growth in the converging case by experimental observation and numerical simulation of bicrystal samples.We find the competitive grain growth is controlled by the cooperative effect of thermal condition and solute field,and the controlling modes are related to the bicrystal misorientation between favorably and unfavorably oriented grains.When the unfavorably oriented grain is low misoriented,unfavorably oriented grain dominates grain selection,and the competitive grain growth performs as solute field domination.However,with the increase of unfavorably oriented grain’s misorientation,the grain selection converts into favorably oriented grain domination,and the competitive grain growth changes to thermal condition domination.To explain these abnormal transformation phenomena,we propose a misorientation dependent thermal condition-solute field cooperative domination model and identify the critical conditions by a critical misorientation(θ_(cm)).According to dynamic equation of dendrite growth,we calculate the critical misorientationθ;to prove this model.The theoretical calculation results agree well with the experimental results.
文摘Nickel-based single-crystal superalloys are widely used in the manufacture of aeroengine turbine vanes for their excellent high-temperature performance. Low-angle grain boundaries (LAGBs) will be generated inevitably during their manufacture, which are often characterized by grain boundary misorientation (GBM) and will weaken the mechanical properties of superalloys. However, the relationship between GBM and the fatigue properties of superalloys at elevated temperatures has seldom been investigated due to the difficulty in the sample preparation and experiment process. Based on six kinds of bicrystals with different tilt LAGBs made by a second-generation single-crystal superalloy, the effects of misorientation on the grain boundary microstructure and fatigue properties (980 °C) of superalloys were studied systematically in this work. It is found that, with the increase of GBM, the GB precipitates combined with the cast micropores increase monotonically, accordingly both the fatigue life and fatigue strength decrease successively. Fatigue fracture observations show that the cracks of all the bicrystals initiated from the cast micropores at GBs, and then propagated along the GBs. Therefore, the coupling effect of cast micropores and GBM on the fatigue damage mechanisms of the bicrystals are evaluated according to their hindering degrees on the piled-up dislocations. Combining with a hysteresis energy model, a quantitative fatigue strength prediction model of superalloys is established and is well verified by abundant experimental data. This study could provide guidance for fatigue performance prediction and structural design of superalloys.
文摘In the present work the research of grain boundary (CB) energy versus angle of misorientation in fcc metals Al, Cu, An and Ni was carried out. An axis of CB misorientation is a direction [100], angle of misorientation makes from 2皍p in 23*. The interatomic interaction was opproximated by Morse' s pair semi-empirical potential. Two variants of relaxation technique were used: (1) rigid relax- ation with the change of atom quantity per a GB (vacancy relaxation ) and (2) full atomic relaxation by a molecular static method. The obtained orientation dependence has a good agreement with experi- ment. There are cusps on a curve in the range of special GB angles.The comparison of obtained curves with calculated ones in model Van der Merwe was carried out. Dependencies obtained in our investiga- tions are not smooth and have an oscillatory character. The oscillations reflect a discrete structure of a lattice.
基金Project supported by the National Natural Science Foundation of China(Grant No.61376065)the Suzhou Science and Technology Project,China(Grant No.ZXG2013044)
文摘Asymmetry in dislocation density and strain relaxation has a significant impact on device performance since it leads to anisotropic electron transport in metamorphic materials. So it is preferred to obtain metamorphic materials with symmetric properties. In this paper, we grew metamorphic Ino.27Gao.73As epilayers with symmetric low threading dislocation density and symmetric strain relaxation in two （110） directions using InA1GaAs buffer layers on 7° misoriented GaAs （001） substrates. To understand the control mechanism of symmetric properties of Ino.27Gao.73As layers by the substrate miscut angles, Ino.27Gao.73As grown on 2° and 15° misoriented substrates were also characterized as reference by atomic force microscopy, transmission electron microscopy, and high resolution triple axis x-ray diffraction. The phase separation and interaction of 60° misfit dislocations were found to be the reasons for asymmetry properties of Ino.27Gao33As grown on 2° and 15° substrates, respectively. Photoluminescence results proved that the Ino.27G°ao.73As with symmetric properties has better optical properties than the Ino.27Gao.73As with asymmetric properties at room temperature. These results imply that high quality metamorphic Ino.27Gao.73As can be achieved with controllable isotropic electron transport property.
基金Funded by China Postdoctoral Science Foundation(No.20060390319)
文摘X65, X70, and X80 belong to high grade pipeline steels. Toughness is one of the most important properties of pipeline steels when the pipeline transports the gas or oil, and the means to control toughness is very important for exploring even higher grade pipeline steels. We established the relationship between toughness and crystallographic parameters of high grade pipeline steels by studying the crystallographic parameters of X65, X70, and X80 using EBSD and analyzing Charpy CVN of X65, X70 and X80. The results show that the effective grain size, the frequency distribution of grain boundary misorientation and the ratio of high angle grain boundary to small angle grain boundary are important parameters. The finer the effective grain size, and the higher the frequency distribution of grain boundaries （〉 50~）, the more excellent toughness of high grade pipeline steels will be.
基金financially supported by Mitsubishi Heavy Industries,Ltd.,Japanthe National Natural Science Foundation of China(Nos.11572171,11632010 and U1533134)the opening project(No.KFJJ15-12M)of State Key Laboratory of Explosion Science and Technology(Beijing Institute of Technology)
文摘The fracture modes of low alloy steels and cast irons under tensile and fatigue conditions were identified by electron back-scattered diffraction（EBSD） misorientation analysis in this research. The curves of grain reference orientation deviation（GROD） distribution perpendicular to the fracture surface were obtained by EBSD observation, and the characteristics of each fracture mode were identified. The GROD value of the specimen fractured in tension decreases to a constant related to the elongation of corresponding specimen in the far field（farther than 5 mm away from the fracture surface）. The peak exhibits in GROD curves of two smooth specimens and a notched specimen near the fracture surface（within 5 mm away from the fracture surface）, and the formation mechanisms were discussed in detail based on the influences of specimen geometries（smooth or notched） and material toughness. The GROD value of fatigue fractured specimen is close to that at undeformed condition in the whole field, except the small area near the crack path. The loading conditions（constant stress amplitude loading or constant stress intensity factor range K loading） and the EBSD striation formation during fatigue crack propagation were also studied by EBSD observation parallel to the crack path.
基金sponsored by the National Natural Science Foundation of China(Grant Nos.52074182,52304406 and U23A20612)the Natural Science Foundation of Shanghai(Grant Nos.22ZR1430700 and 23TS1401900)+1 种基金the National Science and Technology Major Project(No.2017-VII-0008-0102)Neng Ren acknowledges the Startup Fund for Young Faculty at SJTU.
文摘Low-angle grain boundaries(LAGBs)are one of the solidification defects in single-crystal nickel-based superalloys and are detrimental to the mechanical properties.The formation of LAGBs is related to dendrite deformation,while the mechanism has not been fully understood at the mesoscale.In this work,a model coupling dendrite growth,thermal-solutal-fluid flow,thermal stress and flow-induced dendrite deformation via cellular automaton-finite volume method and finite element method is developed to study the formation of LAGBs in single crystal superalloys.Results reveal that the bending of dendrites is primarily attributed to the thermal-solutal convection-induced dendrite deformation.The mechanical stress of dendrite deformation develops and stabilises as solidification proceeds.As the width of the mushy zone gets stable,stresses are built up and then dendritic elastoplastic bending occurs at some thin primary dendrites with the wider inter-dendritic space.There are three characteristic zones of stress distribution along the solidification direction:(i)no stress concentration in the fully solidified regions;(ii)stress developing in the primary dendrite bridging region,and(iii)stress decrease in the inter-dendritic uncontacted zone.The stresses reach maximum near the initial dendrite bridging position.The lower temperature gradients,the finer primary dendritic trunks and sudden reductions in local dendritic trunk radius jointly promote the elastoplastic deformation of the dendrites.Corresponding measures are suggested to reduce LAGBs.
基金supported by National Natural Science Foundation of China (52201107)Research Program of Chongqing Municipal Education Commission (KJQN202201151)Natural Science Foundation of Chongqing (CSTB2023NSCQ-MSX0067).
文摘Mechanism of discontinuous precipitation(DP) in AZ80 alloy was investigated by phase-orientation correlated characterization.The results show DPs nucleate by turning the original grain boundaries(GBs) as reaction front(RF),and further driving the RF to realize their growth.The DPs regions retained the same orientations as their parent grains.The misorientation angle and rotation axis of RFs had strong influence on DPs nucleation.The low-angle GBs,twin boundaries(TBs) and the GBs with specific misorientation axis which are known as low energy and low mobility GBs can hardly initiate DPs.In addition,the TBs had a strong ability to inhibit the growth of DPs,but it should be noticed that the growth of DPs cannot be totally inhibited by TBs.DPs can engulf the twins when the growth direction is approximately parallel to the long axis of TBs.The inhibition behavior is related to the distribution of Al solute atoms near the RF,boundary interactions of the TBs and twin tips with the RF,and the morphology of the continuous precipitations within the twins.
基金supported by the National Natural Science Foundation of China(No.52071037).
文摘Bending is a crucial deformation process in metal sheet forming.In this study,the microstructural evolution of a highly ductile Mg–Er–Zr alloy sheet was examined in various bending regions under different bending strains using electron backscatter diffraction and optical microscopy.The results show that the Mg–Er–Zr extruded sheet has excellent bending properties,with a failure bending strain of 39.3%,bending yield strength,and ultimate bending strength of 75.1 MPa and 250.5 MPa,respectively.The exceptional bending properties of the Mg–Er–Zr extruded sheets are primarily due to their fine grain size and the formation of rare-earth(RE)textures resulting from Er addition.Specifically,the in-grain misorientation axes(IGMA)and the twinning behaviors in various regions of the specimen during bending were thoroughly analyzed.Due to the polarity of the tensile twins and their low activation stress,a significant number of tensile twins are activated in the compression zone to regulate plastic deformation.The addition of Er weakens the basal texture of the sheet and reduces the critical resolved shear stress difference between non-basal slip and basal slip.Consequently,in the tensile zone,the basal and non-basal slips co-operate to coordinate the plastic deformation,effectively impeding crack initiation and propagation,and thereby enhancing the bending toughness of the Mg–Er–Zr sheet.
基金supported by the Fundamental and Cutting-Edge Research Plan of Chongqing(Grant Nos.cstc2017jcyj AX0114&cstc2016jcyj A0434)the National Natural Science Foundation of China(Grant Nos.51401040,51601075&51601165)
文摘In this work, commercially pure Zr sheets were subjected to β air cooling and then rolled to different reductions(10% and 50%)at room temperature. Microstructures of both the β-air-cooled and the rolled specimens were well characterized by electron channelling contrast imaging and electron backscatter diffraction techniques, with special attentions paid to their misorientation characteristics. Results show that the β-air-cooled specimen owns a Widmanst?tten structure featured by lamellar grains with typical phase transformation misorientations. The 10% rolling allows prismatic slip and tensile twinning({11-21}<11-2-6> and{10-12}<10-11>) to be activated profusely, which produce new low-angle(~3°–5°) and high-angle(~35° and ~85°) misorientation peaks, respectively. After increasing the rolling reduction to 50%, twinning is suppressed and dislocation slip becomes the dominant deformation mode, with the lamellar grains highly elongated and aligned towards the rolling direction.Meanwhile, only one strong low-angle misorientation peak related to the prismatic slip is presented in the 50%-rolled specimen,with all other peaks disappeared. Analyses on local misorientations reveal that hardly any residual strains exist in the β-air-cooled specimen, which should be related to their sufficient relaxation during slow cooling. Residual strains introduced by 10% rolling are heterogeneously distributed near grain/twin boundaries while heavier deformation(50% rolling) produces much larger residual strains pervasively existing throughout the specimen microstructure.
基金supported by the Fundamental and Cutting-Edge Research Plan of Chongqing (Grant No. cstc2018jcyjAX0299)the Technology Innovation and Application Demonstration Project of Chongqing (Grant No. cstc2018jszx-cyzdX0080)
文摘A high-purity Ti(HP-Ti)sheet was subjected to small strain rolling(10%reduction)with microstructural and textural characteristics examined by electron channeling contrast imaging and electron backscatter diffraction techniques.Particular attentions were paid to misorientation and textural changes aroused by twins in the rolled HP-Ti sheet.Results show that after the 10%rolling,almost all the prior equiaxed grains in the initial specimen are twinned,leading to remarkable grain refinement.The presence of two major misorientation angle peaks around 65°and 85°is ascribed to{11-22}<11-23>and{10-12}<10-11>twinning,respectively,and two minor peaks around 47°and 77°are due mainly to impingement of various variants of such twins.Distinct from earlier work,the small strain rolling is confirmed to be able to induce drastic textural changes in pure Ti sheets:largely reduced texture intensity and appearance of new textural components.This can essentially be attributed to enhanced twinning activity due to much lower impurity contents of the present material.Primary{11?22}twins are mainly responsible for the new textural component of c-axes aligned near the rolling direction with spread,while the component of caxes parallel to the normal direction is due to reorientation of secondary{10-12}twins.This study clearly demonstrates the capability of small strain rolling to effectively modify both microstructures and textures of the HP?Ti sheet and may shed some light on exploring feasible processings for such materials.
基金financial support from Taiyuan Iron&Steel(Group)Co.,Ltd.(TISCO,Grant No.KA22TAEC035)Baoshan Iron&Steel Co.,Ltd.(Baosteel,Grant No.BGTAECX0).
文摘The optimized stress rupture properties of IN718 are of great interest to the aerospace industry,particularly in the presence of a notch.The deformation mechanisms for the stress rupture properties of IN718 after different heat treatments,using electron back-scattered diffraction(EBSD)as an important tool,were investigated.The notch sensitivity of IN718 was closely associated with the distribution of theδphase.When the area fraction(A(f))of theδphase was less than 0.56%,the sample failed at the notch;while when A(f)≥2.98%,the notch sensitivity disappeared,and failure occurred on the smooth gauge.Heterogeneous deformation with strain concentrated at the grain boundaries was observed for the notch-and smooth-gauge-failed samples through EBSD analysis.Although intergranular cracking was detected in both cases,the failure mechanisms were different.Grain boundary sliding was the dominant failure mechanism for the notch-failed samples,whereas microvoid coalescence was the dominant failure mechanism for the smooth-gauge-failed samples.
基金Project (50801027) supported by the National Natural Science Foundation of ChinaProject(2007001) supported by the Public Foundation of Guangdong Key Laboratory for Advanced Metallic Materials Processing,South China University of Technology,China
文摘Accumulative roll-bonding （ARB） was applied to Mg-Al-Zn magnesium alloy sheets to prepare ultrafine-grain microstructure. Significant grain refinement is achieved after three cycles of ARB with average grain size of about 1.3 μm. The microstructure is characterized by nearly uniform ultrafine equiaxed microstructure without twins. The evolution of the misorientation distribution during ARB was measured by EBSD. Grain refinement can be contributed to the grain subdivision induced by severe accumulated strain, the accumulated strain enhanced concurrent dynamic recovery and recrystallization as well as the complicated distribution of interface and shear strain during ARB.
文摘Texture and grain boundary character distribution of Cu interconnects with different line width for as-deposited and annealed conditions were measured by EBSD. All specimens appear mixed texture and （111） texture is the dominate component.As-deposited interconnects undergo the phenomenon of self-annealing at RT,in which some abnormally large grains are found. Lower aspect ratio of lines and anneal treatment procured larger grains and stronger （111） texture. Meanwhile, the intensity proportion of other textures with lower strain energy to （111） texture is decreased. As-deposited specimens reveal （111）（112？ and （111） （231） components, （111） （110） component appeared and （111） （112？ and （111） （231） components were developed during the annealing process. High angle boundaries are dominant in all specimens, boundaries with a misorientation of 55°-60° and ∑3 ones in higher proportion, followed by lower boundaries with a misorientation of 35°-40° and 29 boundaries. As the aspect ratio of lines and anneal treatment increase,there is a gradual in- crement in ∑3 boundaries and a decrease in ∑9 boundaries.
基金Project(DP140102355)supported by the Australian Research Council(ARC)Project supported by the China Scholarship Council(CSC)
文摘Synchrotron polychromatic X-ray microdiffraction（micro-XRD） was applied to study in situ deformation twinning of commercially AZ31（Mg-3Al-1Zn） strip subjected to uniaxial tension.The morphology and growth of twins were analyzed in situ under the load level from 64 to 73 MPa.The X-ray microdiffraction data,collected on beamline 12.3.2 at the Advanced Light Source,were then used to map an area of 396μm x 200μm within the region of interest.The experimental set-up and X-ray diffraction microscopy with a depth resolution allow the position and orientation of each illuminated grain to be determined at the submicron size.A list of parent grains sorted by crystallographic orientation were selected to examine their twinning behavior.The results depict twin variant selection,local misorientation fluctuation and mosaic spread for multi-twins within the same parent grain.As load increases,the amplitude of misorientation fluctuation along twin trace keeps increasing.This is attributable to the accumulation of geometrically necessary dislocations.
基金Project(51875442)supported by the National Natural Science Foundation of China。
文摘The microhardness curve trend and its relationships with microstructure and misorientation were analyzed to enhance the comprehension of the microstructure and mechanical property of micro-areas in Ti6 Al4 V laser-welded joints with different welding speeds. The microhardness measured on the fusion line(H_m) is the highest from the weld center to the base metal. H_m increases with increasing weld width in a welded joint and increasing degree of the non-uniformity in all studied welded joints. The microhardness decreases from the weld metal to the base metal with decreasing amount of martensite α’ and increasing amount of original α phase. When the microstructure is mainly composed of martensite α’, the microhardness changes with the cooling rate, grain size of the martensite, and peak values of the fraction of misorientation angle of the martensite in a wide weld metal zone or weld center at different welding speeds, whereas the difference is small in a narrow weld metal zone.
基金the National Key Research and Development Program[grant number 2018YFA0702900]the National Natural Science Foundation of China[grant numbers U1508215,51774265]+2 种基金the National Science and Technology Major Project of China[Grant No.2019ZX06004010]the Key Program of the Chinese Academy of Sciences[Grant No.ZDRW-CN-2017-1]the CAS Interdisciplinary Innovation Team。
文摘As an advanced solid state bonding process,plastic deformation bonding(PDB)is a highly reliable metallurgical joining method that produces significant plastic deformation at the bonding interface of welded joints through thermo-mechanical coupling.In this study,PDB behavior of IN718 superalloy was systematically investigated by performing a series of isothermal compression tests at various processing conditions.It was revealed that new grains evolved in the bonding area through discontinuous dynamic recrystallization(DDRX)at 1000–1150℃.Electron backscattered diffraction(EBSD)and transmission electron microscopy(TEM)results revealed that the bonding of joints is related with interfacial grain boundary(IGB)bulging process,which is considered as a nucleation process of DRXed grain under different deformation environments.During recrystallization process,the bonded interface moved due to strain-induced boundary migration(SIBM)process.Stored energy difference(caused by accumulation of dislocations at the bonding interface)was the dominant factor for SIBM during DRX.The mechanical properties of the bonded joints were dependent upon the recrystallized microstructure and SIBM ensued during PDB.
