The microstructure evolution and precipitation behavior of Al-Mg-Si alloy during initial aging were studied using hardness testing, conductivity testing, differential scanning calorimetry(DSC), and high resolution tra...The microstructure evolution and precipitation behavior of Al-Mg-Si alloy during initial aging were studied using hardness testing, conductivity testing, differential scanning calorimetry(DSC), and high resolution transmission electron microscopy(HRTEM). The results show that the precipitation sequence of the Al-Mg-Si alloy during initial aging can be represented as: supersaturated solid solution → spherical Mg/Si clusters → needle-like Guinier Preston(GP) zone → β″. Clusters are completely coherent with the Al matrix. The GP zone with relatively complete independent lattice parameters that differ slightly from the Al matrix parameters, is oriented along the direction of <111>Aland lying on {111}Alplane. The strength of the Al-Mg-Si alloy is greatly enhanced by the threedimensional strain field that exists between the β″ phase and the two {200}Alplanes. After aging at 170 ℃ for 6 h, the hardness reaches the peak of 127 HV and remains for a long time. At this stage, the electrical conductivity keeps relatively stable due to the formation of coherent precipitates(Mg/Si clusters/GP zones) and the reduction in solute atom concentration in the Al matrix. The severe coarsening and decreased number density of the β″ phase during the over-aging stage result in a significant decrease in the hardness.展开更多
The solidification-precipitation behavior of Al-Mg-Si multicomponent alloys has long been an absorbing topic. Experiments were carried out to analyze the precipitation behaviors of Al-Mg-Si alloys under different heat...The solidification-precipitation behavior of Al-Mg-Si multicomponent alloys has long been an absorbing topic. Experiments were carried out to analyze the precipitation behaviors of Al-Mg-Si alloys under different heat treatments. All specimens were homogenized at 570 °C for 8 h, and then solution treated at 540 °C for 55 min. Subsequently, the specimens were age treated for different times at temperatures of 100 °C, 150 °C and 180 °C, respectively. The experimental results show that the occurrence of dispersed free zones(DFZ) is caused by the uneven distribution of dispersed phase. During the aging process, pre-β″ phases form at the initial stage and an aging temperature of 100 °C is too low to complete the transformation of pre-β″ to β″. At 150 °C, the precipitation sequence is concluded as SSSS-pre-β″-pre-β″+β″-β″-β′-β. Moreover, changes in sizes and densities of the pre-β″, β″and β′ phases during the aging process has an important influence on the evolution of microhardness and electrical resistivity. The microhardness peak value of 150 °C is similar to that of 180 °C, which is ~141 HV. While, at 100 °C, the microhardness increases slowly, and the attainable value is 127 HV up to 19 days. When the aging temperature is 100 °C, the electrical resistivity has the highest average value. When the aging temperature exceeds 100 °C, with the occurrence and growth of β″and β′, the resistivity has a distinct decrease with prolonged aging time.展开更多
The effects of austenitizing temperature(1223,1303,and 1373 K)and holding time(1-1500 s)on the microstructure,mechanical properties,and precipitation behavior of the H13 hot work die steel were investigated.The result...The effects of austenitizing temperature(1223,1303,and 1373 K)and holding time(1-1500 s)on the microstructure,mechanical properties,and precipitation behavior of the H13 hot work die steel were investigated.The results indicate a softening phenomenon when H13 steel is austenitized at 1303 K beyond 900 s and 1373 K beyond 600 s,respectively.For the sample held for 1200 s,the tensile strength is found capable of reaching up to 2.2 GPa when quenched from a temperature above 1303 K.Meanwhile,prior-austenite grain size increases with the increase in austenitizing temperature.The kinetic behavior of the precipitates(mainly MC-type carbides)in H13 steel could be elaborated through the principles set forth by the Arrhenius and Avrami equations.Finally,the comprehensive strengthening of the H13 steel was discussed in detail.The results show that the activation energy of the transformed fraction of carbides is higher than that of the diffusion process for common alloying elements(Cr,V,Mo,and Ni)found in the austenite.This suggests that it would be difficult for precipitates to dissolve into the matrix when H13 steel is austenitized at high temperatures.With the increasing austenitizing temperature,the precipitation fraction decreases,and the dislocation density increases.The dislocation strengthening is regarded as the dominant strengthening contributed to yield strength in as-quenched H13 steel.展开更多
The influence of N on the precipitation behavior,associated corrosion,and mechanical properties of S32654 were investigated by microstructural,electrochemical,and mechanical analyses.Increasing the N content results i...The influence of N on the precipitation behavior,associated corrosion,and mechanical properties of S32654 were investigated by microstructural,electrochemical,and mechanical analyses.Increasing the N content results in several alterations:(1) grain refinement,which promotes intergranular precipitation;(2) a linear increase in the driving force for Cr2 N and Mo activity,which accelerates the precipitation of intergranular Cr2 N and π phase,respectively;(3) a linear decrease in the driving force for σ phase and Cr activity,which suppresses the formation of intragranular σ phase.The total amount of precipitates first decreased and then increased with the N content increasing.Furthermore,the intergranular corrosion susceptibility depended substantially on the total amount of precipitates and also first exhibited a decreasing and then an increasing trend as the N content increased.In addition,aging precipitation caused a considerable decrement in the ultimate tensile strength(UTS) and a remarkable increment in the yield strength(YS).Both the UTS and YS always increased with N content increasing throughout the solution and aging process.Whereas the elongation was considerably sensitive to the aging treatment,it exhibited marginal variation with the N content increasing.展开更多
The rapid industrial development calls for alloys that possess higher comprehensive properties. In this study, the effect of microalloying La addition on the precipitation behavior during artificial aging as well as t...The rapid industrial development calls for alloys that possess higher comprehensive properties. In this study, the effect of microalloying La addition on the precipitation behavior during artificial aging as well as the mechanical properties and electrical conductivity of Al-Mg-Si alloys were investigated by thermal analysis, microstructural characterizations and properties tests.The results demonstrated that micro-alloying La addition does not change the whole precipitation sequence during the artificial aging of Al-Mg-Si alloys as well as the atomic structure of the precipitates. However, the higher La-vacancy binding energy as well as the strong La-Si and La-Mg interactions can decrease the solubility of Si and Mg in the Al matrix and the β″ precipitation activation energy from 89.9 to 76.7 kJ/mol, leading to the improvement of the strength and electrical conductivity of Al-Mg-Si alloys simultaneously. The microstructural features affecting the strength and electrical conductivity were theoretically discussed in terms of the La addition.展开更多
Precipitate hardening is considered as an effective method to strengthen High-and medium-entropy alloys(HEAs and MEAs),especially the recently developed Co-free HEAs/MEAs,In this work,a systematic study on precipitati...Precipitate hardening is considered as an effective method to strengthen High-and medium-entropy alloys(HEAs and MEAs),especially the recently developed Co-free HEAs/MEAs,In this work,a systematic study on precipitation and mechanical behaviors of a Co-free HEAs with dilute amount of Al addition,Ni_(47-x)Fe_(30 C)r_(12)Mn_(8) Al_(x)Ti_(3)(x=2 at.%,5 at.% and 7 at.%),is presented.Results shown that the Ni_(45)Fe_(30)Cr_(12)Mn_(8) Al_(2) Ti_(3) has a face-centered cubic(FCC)+L1_(2)+η three-phased structure.With increasing Al content,the formation of η phase is inhibited,accompanying with an enhanced formation of B2 phase,and FCC+L1_(2)+B2 three-phased structure is thus observed in alloys with x=5 and 7.The constrained lattice mismatch between FCC matrix and L1_(2) precipitates is decreased with increasing Al content,leading to an enhanced precipitation behavior of L1_(2) phase.As a result of microstructural evolution,the mechanical properties of the aged HEAs changed:the Ni_(42)Fe_(30)Cr_(12)Mn_(8) Al_5 Ti_(3) alloy exhibits a better combination of a yield strength of 661 MPa and tensile ductility of 29.7% as compared to the 2 at.% Al alloyed HEA;and addition of Al beyond 5 at.% results in an increase of strength with a large expense of ductility.We believe that the present work is helpful for developing high-performance Co-free HEAs/MEAs strengthened by nanoprecipitates via composition optimizing.展开更多
The influence of cooling rate on microstructural evolution and precipitation behavior in Ti,Ti–Nb and Ti-Mo low-carbon steels during the continuous cooling process was studied by dilatometer method,optical microscopy...The influence of cooling rate on microstructural evolution and precipitation behavior in Ti,Ti–Nb and Ti-Mo low-carbon steels during the continuous cooling process was studied by dilatometer method,optical microscopy,and transmission electron microscopy.The results indicated that austenite transformation temperature decreased with the increasing cooling rate in three steels.The addition of Nb and Mo promoted bainite and martensite transformation and improved the hardenability of steels.In addition,precipitates formed in deformed austenite and ferrite can be observed simultaneously.Deformation in the austenite non-recrystallization zone can introduce a large number of deformation bands,and then,the precipitates preferentially nucleated in these deformation bands.In the following process,randomly distributed precipitates and interphase precipitates will be formed in ferrite.The precipitates formed in deformed austenite obey Kurdjumov-Sachs orientation relationship with the matrix,while the precipitates formed in ferrite obey Baker-Nutting orientation relationship with the matrix.The addition of Nb and Mo in Ti-bearing steels decreased the precipitates size and increased the number density of precipitates and then improved the precipitation hardening.And the effect of Mo addition is more obvious than that of Nb addition.展开更多
The addition of high Ti(>0.1%) in microalloyed bainitic high strength steel was designed, and the precipitation morphology of steels with different Ti, Nb, and V contents was studied by utilizing transmission elect...The addition of high Ti(>0.1%) in microalloyed bainitic high strength steel was designed, and the precipitation morphology of steels with different Ti, Nb, and V contents was studied by utilizing transmission electron microscopy(TEM). Based on the classical nucleation-crystal growth theory and the Johnson-Mehl-Avrami equation, the precipitation thermodynamic and kinetic model of second phase particles in austenite was established in the form of(Nbx,Vy,Tiz)C, and the complex precipitation mechanism of second phase particles was emphatically studied. The experimental results show that the complex precipitation particles could be divided into two categories: the coarser particles with about 100 nm grain size and the independent complex precipitation particles in the form of(Nb,V,Ti)C with 35-50 nm grain size. The latter has a better precipitation strengthening effect, and the calculated PTT curve shows a typical "C" shape. When the deformed storage energy is 3 820 J·mol^-1, the fastest precipitation temperature of calculated PTT curve is 925℃, and the calculated result is essentially consistent with experimental values. The increase of Ti content increased the nose point temperature and expanded the range of fastest precipitation temperature.展开更多
Generally, the good combination of pre-deformation and aging can improve the mechanical strength of the Al–Cu–Li–Mg alloys. However, the effects of pre-deformation on competitive precipitation relationship and prec...Generally, the good combination of pre-deformation and aging can improve the mechanical strength of the Al–Cu–Li–Mg alloys. However, the effects of pre-deformation on competitive precipitation relationship and precipitation strengthening have not been clarified in detail in Al–Cu–Li–Mg alloys with high Mg. In the present study, the effects of pre-deformation level on the microstructure and mechanical properties of an Al–2.95 Cu–1.55 Li–0.57 Mg–0.18 Zr alloy have been investigated. It is found that the introduction of dislocation by 5% pre-deformation can facilitate the precipitation of new successive composite precipitates and T _1 precipitates along the sub-grain boundaries or dislocations and inhibit the precipitation of dispersive GPB zones which is the main precipitates of the alloys without pre-deformation. The introduction of 5% pre-deformation can enhance the mechanical properties considerably. When the pre-deformation level increases from 5 to 15%, the number density of the successive composite precipitates and T _1 precipitates increases, and the aspect ratio of T _1 precipitates decreases. The decrease in T _1 precipitate aspect ratio and the increment of the successive composite precipitates result in the reduction in precipitation strengthening. Therefore, the increase in pre-deformation level from 5 to 15% does not further improve the mechanical properties of the alloys, although the dislocation strengthening increases continuously.展开更多
The influence of magnetic field(1 T)on dislocation morphology and precipitation behaviour of ultrafinegrained(UFG)Al 7075 alloy was investigated after ageing from 90 to 200℃ via wide angle X-ray scattering(WAXS),smal...The influence of magnetic field(1 T)on dislocation morphology and precipitation behaviour of ultrafinegrained(UFG)Al 7075 alloy was investigated after ageing from 90 to 200℃ via wide angle X-ray scattering(WAXS),small angle X-ray scattering(SAXS),and transmission electron microscopy(TEM).Experimental results reveal that the improved precipitation kinetics of alloys in the thickness plane(denoted as sample II)as compared to those in the rolling plane(denoted as sample I),which arises due to a higher dislocation density(morphology of dislocation cells)of the thickness plane than that of the rolling plane(morphology of dislocations and dislocation tangles).Specifically,because of different dislocation morphologies,the magnetic field positively and negatively affects the dislocation activity in samples I and II,leading to enhanced and suppressed precipitation behaviors,respectively.Interestingly,nucleation of theηphase is facilitated in the UFG alloy at the critical temperature(140℃)because it affords a faster rate of atom diffusion and a higher dislocation density as compared to those exhibited by its coarse-grained alloy.This systematic and comprehensive study provides new insights into dislocation morphology and precipitation behaviour of the UFG 7075 Al alloy,while enabling the optimization of precipitation kinetics.展开更多
It is important to inhibit the precipitation of η phases in precipitation strengthened Fe-Ni based alloys,as they will deteriorate not only the mechanical property but also the hydrogen resistance.The present investi...It is important to inhibit the precipitation of η phases in precipitation strengthened Fe-Ni based alloys,as they will deteriorate not only the mechanical property but also the hydrogen resistance.The present investigation shows that grain boundary engineering(GBE) can retard the formation and growth of ηphase in J75 alloy.After GBE treatment with 5% cold rolling followed by annealing at 1000℃ for 1 h,the fraction of special boundaries(SBs) increases from 38.4% in conventional alloy to 77.2% and the fraction of special triple junctions increases from 10% to 74%.During 800℃ aging treatment,quite amount of cellular η phases adjacent to random grain boundary(RGB) will be found in conventional alloy,and only a few small η phases have been observed in GBE treatment alloy subjected to the same aging treatment for long time.The reason for GBE in inhibiting precipitation of η phase can be attributed to not only introducing high fraction of SBs but also breaking the connectivity of RGB networks.As nucleation and growth of η phases on SBs are difficult due to their lower Ti concentration and diffusion rate,and the disruption of RGB networks reduces supply of Ti atoms to the η phases significantly,which impedes their growth at RGB.展开更多
The precipitation behavior of different phases in a high-silicon stainless steel(6 wt%Si)during aging at 600–1050℃for24 h was investigated.The morphology,crystal structure and composition of various precipitates wer...The precipitation behavior of different phases in a high-silicon stainless steel(6 wt%Si)during aging at 600–1050℃for24 h was investigated.The morphology,crystal structure and composition of various precipitates were detailly characterized using optical microscopy,scanning electron microscopy and transmission electron microscopy.Four phases were mainly identified:χ-phase,M_6C carbides,σphase and a new type of fcc-phase.During aging at 600–900℃,the main precipitate was(Cr,Mo and Si)-richχ-phase which was directly precipitated fromγmatrix.Theχ-phase was calibrated as bcc structure with a lattice parameter of 8.90?.The peak temperature for the precipitation ofχ-phase was 800℃,and it was dissolved when aging at temperatures above 1000℃.Theσ-phase was observed only at 700℃and grew next toχ-phase.Above 700℃,a new fcc-phase was found to be precipitated along withχ-phase,with a space group of Fd3c and a lattice parameter of 12.56?.The M_6C carbides started to be precipitated at 700℃in the vicinity ofχ-phase.And its amount basically increased with the increasing of temperature.An orientation relationship between M_6C/γwas found:[100]c//[100]γ,(001)c//(001)γ,i.e.,the cube-on-cube relationship.展开更多
The microstructure,aging behavior and mechanical properties of cast Mg–3Nd–3Gd–x Zn–0.5Zr(x=0,0.5,0.8,1 wt%)alloys are investigated in this work.Zn–Zr particles with different morphologies form during solution tr...The microstructure,aging behavior and mechanical properties of cast Mg–3Nd–3Gd–x Zn–0.5Zr(x=0,0.5,0.8,1 wt%)alloys are investigated in this work.Zn–Zr particles with different morphologies form during solution treatment due to the additions of Zn.As the Zn content increases,the number density of Zn–Zr particles also increases.Microstructural comparisons of peak-aged studied alloys indicate that varying Zn additions could profoundly influence the competitive precipitation behavior.In the peak-aged Zn-free alloy,β′′phases are the key strengthening precipitates.When 0.5 wt%Zn is added,besidesβ′′precipitates,additional fineβ_(1)precipitates form.With the addition of 0.8 wt%Zn,the peak-aged 0.8Zn alloy is characterized by predominantly prismaticβ_(1)and scanty basal precipitate distributions.The enhanced precipitation ofβ_(1)should be primarily attributable to the presence of increased Zn–Zr dispersoids.When Zn content further increases to 1 wt%,the precipitation of basal precipitates is markedly enhanced.Basal precipitates andβ_(1)phases are the key strengthening precipitates in the peak-aged 1Zn alloy.Tensile tests reveal that the relatively best tensile properties are achieved in the peak-aged alloy with 0.5 wt%Zn addition,whose yield strength,ultimate tensile strength and elongation are 179 MPa,301 MPa and 5.3%,respectively.展开更多
Ti additions in Cu–Cr–Zr alloys are useful for achieving high mechanical properties.In this work,the influence of Ti contents(0.25 wt%,0.6 wt%,and 1.02 wt%)on the microstructure,mechanical,and electrical properties ...Ti additions in Cu–Cr–Zr alloys are useful for achieving high mechanical properties.In this work,the influence of Ti contents(0.25 wt%,0.6 wt%,and 1.02 wt%)on the microstructure,mechanical,and electrical properties of Cu–Cr–Zr alloys has been investigated experimentally,along with thermodynamic and kinetic calculations.The electrical conductivity decreased but the hardness/strength increased with increasing Ti content.The lower electrical conductivity is due to increased electron scattering through the solution of more Ti atoms in the Cu matrix.As for the higher hardness/strength,it is mainly owing to higher dislocation density and finer FCC-Cr precipitates.Furthermore,a model considering the size distributions of precipitates is adopted to calculate precipitation strengthening quantitatively.The calculated yield strengths are consistent with the experimental ones for the alloys.The thermodynamic and kinetic calculations reveal that increasing Ti content can facilitate the nucleation of FCC-Cr but enhance its activation energy,hence hindering the growth process.The present work study can provide an effective strategy for producing copper alloys with expected performance.展开更多
基金financially supported by the Research Foundation of Education Bureau Hunan Province,China (Grant No. 22C0598)。
文摘The microstructure evolution and precipitation behavior of Al-Mg-Si alloy during initial aging were studied using hardness testing, conductivity testing, differential scanning calorimetry(DSC), and high resolution transmission electron microscopy(HRTEM). The results show that the precipitation sequence of the Al-Mg-Si alloy during initial aging can be represented as: supersaturated solid solution → spherical Mg/Si clusters → needle-like Guinier Preston(GP) zone → β″. Clusters are completely coherent with the Al matrix. The GP zone with relatively complete independent lattice parameters that differ slightly from the Al matrix parameters, is oriented along the direction of <111>Aland lying on {111}Alplane. The strength of the Al-Mg-Si alloy is greatly enhanced by the threedimensional strain field that exists between the β″ phase and the two {200}Alplanes. After aging at 170 ℃ for 6 h, the hardness reaches the peak of 127 HV and remains for a long time. At this stage, the electrical conductivity keeps relatively stable due to the formation of coherent precipitates(Mg/Si clusters/GP zones) and the reduction in solute atom concentration in the Al matrix. The severe coarsening and decreased number density of the β″ phase during the over-aging stage result in a significant decrease in the hardness.
基金financially supported by the Natural Science Foundation of Shandong Province(ZR2016EMQ11)the Major Research and Development Program of Shandong Province(2017GGX20119),China
文摘The solidification-precipitation behavior of Al-Mg-Si multicomponent alloys has long been an absorbing topic. Experiments were carried out to analyze the precipitation behaviors of Al-Mg-Si alloys under different heat treatments. All specimens were homogenized at 570 °C for 8 h, and then solution treated at 540 °C for 55 min. Subsequently, the specimens were age treated for different times at temperatures of 100 °C, 150 °C and 180 °C, respectively. The experimental results show that the occurrence of dispersed free zones(DFZ) is caused by the uneven distribution of dispersed phase. During the aging process, pre-β″ phases form at the initial stage and an aging temperature of 100 °C is too low to complete the transformation of pre-β″ to β″. At 150 °C, the precipitation sequence is concluded as SSSS-pre-β″-pre-β″+β″-β″-β′-β. Moreover, changes in sizes and densities of the pre-β″, β″and β′ phases during the aging process has an important influence on the evolution of microhardness and electrical resistivity. The microhardness peak value of 150 °C is similar to that of 180 °C, which is ~141 HV. While, at 100 °C, the microhardness increases slowly, and the attainable value is 127 HV up to 19 days. When the aging temperature is 100 °C, the electrical resistivity has the highest average value. When the aging temperature exceeds 100 °C, with the occurrence and growth of β″and β′, the resistivity has a distinct decrease with prolonged aging time.
基金This research is financially supported by the China Scholarship Council under Grant No.201806935054the Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi,China,under Grant No.201802035.
文摘The effects of austenitizing temperature(1223,1303,and 1373 K)and holding time(1-1500 s)on the microstructure,mechanical properties,and precipitation behavior of the H13 hot work die steel were investigated.The results indicate a softening phenomenon when H13 steel is austenitized at 1303 K beyond 900 s and 1373 K beyond 600 s,respectively.For the sample held for 1200 s,the tensile strength is found capable of reaching up to 2.2 GPa when quenched from a temperature above 1303 K.Meanwhile,prior-austenite grain size increases with the increase in austenitizing temperature.The kinetic behavior of the precipitates(mainly MC-type carbides)in H13 steel could be elaborated through the principles set forth by the Arrhenius and Avrami equations.Finally,the comprehensive strengthening of the H13 steel was discussed in detail.The results show that the activation energy of the transformed fraction of carbides is higher than that of the diffusion process for common alloying elements(Cr,V,Mo,and Ni)found in the austenite.This suggests that it would be difficult for precipitates to dissolve into the matrix when H13 steel is austenitized at high temperatures.With the increasing austenitizing temperature,the precipitation fraction decreases,and the dislocation density increases.The dislocation strengthening is regarded as the dominant strengthening contributed to yield strength in as-quenched H13 steel.
基金This work was financially supported by National Natural Science Foundation of China(No.U1860204)the Fundamental Research Funds for the Central Universities(No.N172507002)the Transformation Project of Major Scientific and Technological Achievements in Shenyang(No.Z17-5-003).
文摘The influence of N on the precipitation behavior,associated corrosion,and mechanical properties of S32654 were investigated by microstructural,electrochemical,and mechanical analyses.Increasing the N content results in several alterations:(1) grain refinement,which promotes intergranular precipitation;(2) a linear increase in the driving force for Cr2 N and Mo activity,which accelerates the precipitation of intergranular Cr2 N and π phase,respectively;(3) a linear decrease in the driving force for σ phase and Cr activity,which suppresses the formation of intragranular σ phase.The total amount of precipitates first decreased and then increased with the N content increasing.Furthermore,the intergranular corrosion susceptibility depended substantially on the total amount of precipitates and also first exhibited a decreasing and then an increasing trend as the N content increased.In addition,aging precipitation caused a considerable decrement in the ultimate tensile strength(UTS) and a remarkable increment in the yield strength(YS).Both the UTS and YS always increased with N content increasing throughout the solution and aging process.Whereas the elongation was considerably sensitive to the aging treatment,it exhibited marginal variation with the N content increasing.
基金This work was supported by the Science and Technology Project of the Headquarters of State Grid Corporation of China(Grant No.5500-201924129A-0-0-00)。
文摘The rapid industrial development calls for alloys that possess higher comprehensive properties. In this study, the effect of microalloying La addition on the precipitation behavior during artificial aging as well as the mechanical properties and electrical conductivity of Al-Mg-Si alloys were investigated by thermal analysis, microstructural characterizations and properties tests.The results demonstrated that micro-alloying La addition does not change the whole precipitation sequence during the artificial aging of Al-Mg-Si alloys as well as the atomic structure of the precipitates. However, the higher La-vacancy binding energy as well as the strong La-Si and La-Mg interactions can decrease the solubility of Si and Mg in the Al matrix and the β″ precipitation activation energy from 89.9 to 76.7 kJ/mol, leading to the improvement of the strength and electrical conductivity of Al-Mg-Si alloys simultaneously. The microstructural features affecting the strength and electrical conductivity were theoretically discussed in terms of the La addition.
基金financially supported by the National Major Science and Technology Project of China (No. 2014ZX07214-002)the National Natural Science Foundation of China (No. 52001201)。
文摘Precipitate hardening is considered as an effective method to strengthen High-and medium-entropy alloys(HEAs and MEAs),especially the recently developed Co-free HEAs/MEAs,In this work,a systematic study on precipitation and mechanical behaviors of a Co-free HEAs with dilute amount of Al addition,Ni_(47-x)Fe_(30 C)r_(12)Mn_(8) Al_(x)Ti_(3)(x=2 at.%,5 at.% and 7 at.%),is presented.Results shown that the Ni_(45)Fe_(30)Cr_(12)Mn_(8) Al_(2) Ti_(3) has a face-centered cubic(FCC)+L1_(2)+η three-phased structure.With increasing Al content,the formation of η phase is inhibited,accompanying with an enhanced formation of B2 phase,and FCC+L1_(2)+B2 three-phased structure is thus observed in alloys with x=5 and 7.The constrained lattice mismatch between FCC matrix and L1_(2) precipitates is decreased with increasing Al content,leading to an enhanced precipitation behavior of L1_(2) phase.As a result of microstructural evolution,the mechanical properties of the aged HEAs changed:the Ni_(42)Fe_(30)Cr_(12)Mn_(8) Al_5 Ti_(3) alloy exhibits a better combination of a yield strength of 661 MPa and tensile ductility of 29.7% as compared to the 2 at.% Al alloyed HEA;and addition of Al beyond 5 at.% results in an increase of strength with a large expense of ductility.We believe that the present work is helpful for developing high-performance Co-free HEAs/MEAs strengthened by nanoprecipitates via composition optimizing.
基金supported by the National Natural Science Foundation of China(Grant No.52004224)the Fundamental Research Funds for the Central Universities(No.D5000200031)China Postdoctoral Science Foundation(2020M683559).
文摘The influence of cooling rate on microstructural evolution and precipitation behavior in Ti,Ti–Nb and Ti-Mo low-carbon steels during the continuous cooling process was studied by dilatometer method,optical microscopy,and transmission electron microscopy.The results indicated that austenite transformation temperature decreased with the increasing cooling rate in three steels.The addition of Nb and Mo promoted bainite and martensite transformation and improved the hardenability of steels.In addition,precipitates formed in deformed austenite and ferrite can be observed simultaneously.Deformation in the austenite non-recrystallization zone can introduce a large number of deformation bands,and then,the precipitates preferentially nucleated in these deformation bands.In the following process,randomly distributed precipitates and interphase precipitates will be formed in ferrite.The precipitates formed in deformed austenite obey Kurdjumov-Sachs orientation relationship with the matrix,while the precipitates formed in ferrite obey Baker-Nutting orientation relationship with the matrix.The addition of Nb and Mo in Ti-bearing steels decreased the precipitates size and increased the number density of precipitates and then improved the precipitation hardening.And the effect of Mo addition is more obvious than that of Nb addition.
基金Funded by the National Natural Science Foundation of China(No.U1860112)the State Key Laboratory of Marine Equipment made of Metal Material and Application(No.SKLMEAUSTL-201708 and No.SKLMEA-USTL-201703)+1 种基金the Key Project of Liaoning Education Department(No.2019FWDF03)the National Natural Science Foundation of USTL(No.2017QN11)
文摘The addition of high Ti(>0.1%) in microalloyed bainitic high strength steel was designed, and the precipitation morphology of steels with different Ti, Nb, and V contents was studied by utilizing transmission electron microscopy(TEM). Based on the classical nucleation-crystal growth theory and the Johnson-Mehl-Avrami equation, the precipitation thermodynamic and kinetic model of second phase particles in austenite was established in the form of(Nbx,Vy,Tiz)C, and the complex precipitation mechanism of second phase particles was emphatically studied. The experimental results show that the complex precipitation particles could be divided into two categories: the coarser particles with about 100 nm grain size and the independent complex precipitation particles in the form of(Nb,V,Ti)C with 35-50 nm grain size. The latter has a better precipitation strengthening effect, and the calculated PTT curve shows a typical "C" shape. When the deformed storage energy is 3 820 J·mol^-1, the fastest precipitation temperature of calculated PTT curve is 925℃, and the calculated result is essentially consistent with experimental values. The increase of Ti content increased the nose point temperature and expanded the range of fastest precipitation temperature.
基金financially supported by the National Key Research and Development Program of China (No. 2016YFB0300801)the National Natural Science Foundation of China (Nos. 51671082, 51831004, 11427806, and 51471067)。
文摘Generally, the good combination of pre-deformation and aging can improve the mechanical strength of the Al–Cu–Li–Mg alloys. However, the effects of pre-deformation on competitive precipitation relationship and precipitation strengthening have not been clarified in detail in Al–Cu–Li–Mg alloys with high Mg. In the present study, the effects of pre-deformation level on the microstructure and mechanical properties of an Al–2.95 Cu–1.55 Li–0.57 Mg–0.18 Zr alloy have been investigated. It is found that the introduction of dislocation by 5% pre-deformation can facilitate the precipitation of new successive composite precipitates and T _1 precipitates along the sub-grain boundaries or dislocations and inhibit the precipitation of dispersive GPB zones which is the main precipitates of the alloys without pre-deformation. The introduction of 5% pre-deformation can enhance the mechanical properties considerably. When the pre-deformation level increases from 5 to 15%, the number density of the successive composite precipitates and T _1 precipitates increases, and the aspect ratio of T _1 precipitates decreases. The decrease in T _1 precipitate aspect ratio and the increment of the successive composite precipitates result in the reduction in precipitation strengthening. Therefore, the increase in pre-deformation level from 5 to 15% does not further improve the mechanical properties of the alloys, although the dislocation strengthening increases continuously.
基金financially supported by the National Key Research and Development Program of China(Nos.2017YFF0210002,2016YFF0203301,2016YFF0203305,2016YFC0801903)the National Natural Science Foundation of China(No.U1537212)。
文摘The influence of magnetic field(1 T)on dislocation morphology and precipitation behaviour of ultrafinegrained(UFG)Al 7075 alloy was investigated after ageing from 90 to 200℃ via wide angle X-ray scattering(WAXS),small angle X-ray scattering(SAXS),and transmission electron microscopy(TEM).Experimental results reveal that the improved precipitation kinetics of alloys in the thickness plane(denoted as sample II)as compared to those in the rolling plane(denoted as sample I),which arises due to a higher dislocation density(morphology of dislocation cells)of the thickness plane than that of the rolling plane(morphology of dislocations and dislocation tangles).Specifically,because of different dislocation morphologies,the magnetic field positively and negatively affects the dislocation activity in samples I and II,leading to enhanced and suppressed precipitation behaviors,respectively.Interestingly,nucleation of theηphase is facilitated in the UFG alloy at the critical temperature(140℃)because it affords a faster rate of atom diffusion and a higher dislocation density as compared to those exhibited by its coarse-grained alloy.This systematic and comprehensive study provides new insights into dislocation morphology and precipitation behaviour of the UFG 7075 Al alloy,while enabling the optimization of precipitation kinetics.
基金supported by the National Natural Science Foundation of China and China Academy of Engineering Physics[No.U1730140]National Key Research and Development Program of China[Grant No.2019YFB1505201]。
文摘It is important to inhibit the precipitation of η phases in precipitation strengthened Fe-Ni based alloys,as they will deteriorate not only the mechanical property but also the hydrogen resistance.The present investigation shows that grain boundary engineering(GBE) can retard the formation and growth of ηphase in J75 alloy.After GBE treatment with 5% cold rolling followed by annealing at 1000℃ for 1 h,the fraction of special boundaries(SBs) increases from 38.4% in conventional alloy to 77.2% and the fraction of special triple junctions increases from 10% to 74%.During 800℃ aging treatment,quite amount of cellular η phases adjacent to random grain boundary(RGB) will be found in conventional alloy,and only a few small η phases have been observed in GBE treatment alloy subjected to the same aging treatment for long time.The reason for GBE in inhibiting precipitation of η phase can be attributed to not only introducing high fraction of SBs but also breaking the connectivity of RGB networks.As nucleation and growth of η phases on SBs are difficult due to their lower Ti concentration and diffusion rate,and the disruption of RGB networks reduces supply of Ti atoms to the η phases significantly,which impedes their growth at RGB.
文摘The precipitation behavior of different phases in a high-silicon stainless steel(6 wt%Si)during aging at 600–1050℃for24 h was investigated.The morphology,crystal structure and composition of various precipitates were detailly characterized using optical microscopy,scanning electron microscopy and transmission electron microscopy.Four phases were mainly identified:χ-phase,M_6C carbides,σphase and a new type of fcc-phase.During aging at 600–900℃,the main precipitate was(Cr,Mo and Si)-richχ-phase which was directly precipitated fromγmatrix.Theχ-phase was calibrated as bcc structure with a lattice parameter of 8.90?.The peak temperature for the precipitation ofχ-phase was 800℃,and it was dissolved when aging at temperatures above 1000℃.Theσ-phase was observed only at 700℃and grew next toχ-phase.Above 700℃,a new fcc-phase was found to be precipitated along withχ-phase,with a space group of Fd3c and a lattice parameter of 12.56?.The M_6C carbides started to be precipitated at 700℃in the vicinity ofχ-phase.And its amount basically increased with the increasing of temperature.An orientation relationship between M_6C/γwas found:[100]c//[100]γ,(001)c//(001)γ,i.e.,the cube-on-cube relationship.
基金financially supported by the National Natural Science Foundation of China(Nos.U2037601,51775334 and 51821001)the National Key Research&Development Program of China(No.2016YFB0701205)+3 种基金the Joint Innovation Fund of CALT and College(No.CALT2020-TS07)the Open Fund of State Key Laboratory of Advanced Forming Technology and Equipment(No.SKL2020005)the Research Program of Joint Research Center of Advanced Spaceflight Technologies(No.USCAST2020-14)。
文摘The microstructure,aging behavior and mechanical properties of cast Mg–3Nd–3Gd–x Zn–0.5Zr(x=0,0.5,0.8,1 wt%)alloys are investigated in this work.Zn–Zr particles with different morphologies form during solution treatment due to the additions of Zn.As the Zn content increases,the number density of Zn–Zr particles also increases.Microstructural comparisons of peak-aged studied alloys indicate that varying Zn additions could profoundly influence the competitive precipitation behavior.In the peak-aged Zn-free alloy,β′′phases are the key strengthening precipitates.When 0.5 wt%Zn is added,besidesβ′′precipitates,additional fineβ_(1)precipitates form.With the addition of 0.8 wt%Zn,the peak-aged 0.8Zn alloy is characterized by predominantly prismaticβ_(1)and scanty basal precipitate distributions.The enhanced precipitation ofβ_(1)should be primarily attributable to the presence of increased Zn–Zr dispersoids.When Zn content further increases to 1 wt%,the precipitation of basal precipitates is markedly enhanced.Basal precipitates andβ_(1)phases are the key strengthening precipitates in the peak-aged 1Zn alloy.Tensile tests reveal that the relatively best tensile properties are achieved in the peak-aged alloy with 0.5 wt%Zn addition,whose yield strength,ultimate tensile strength and elongation are 179 MPa,301 MPa and 5.3%,respectively.
基金the young scholars of the National Natural Science Foundation of China(Grant No.5210011609)the National Natural Science Foundation of Hunan Province(Grant No.2021JJ40749)+1 种基金Chenying Shi would like to express the support by China Scholarship Council(No.202006370260)Biaobiao Yang would like to express the support by China Scholarship Council(No.202106370122).
文摘Ti additions in Cu–Cr–Zr alloys are useful for achieving high mechanical properties.In this work,the influence of Ti contents(0.25 wt%,0.6 wt%,and 1.02 wt%)on the microstructure,mechanical,and electrical properties of Cu–Cr–Zr alloys has been investigated experimentally,along with thermodynamic and kinetic calculations.The electrical conductivity decreased but the hardness/strength increased with increasing Ti content.The lower electrical conductivity is due to increased electron scattering through the solution of more Ti atoms in the Cu matrix.As for the higher hardness/strength,it is mainly owing to higher dislocation density and finer FCC-Cr precipitates.Furthermore,a model considering the size distributions of precipitates is adopted to calculate precipitation strengthening quantitatively.The calculated yield strengths are consistent with the experimental ones for the alloys.The thermodynamic and kinetic calculations reveal that increasing Ti content can facilitate the nucleation of FCC-Cr but enhance its activation energy,hence hindering the growth process.The present work study can provide an effective strategy for producing copper alloys with expected performance.
