In the slightly deformed Al-Mg-Si alloys,dislocation-induced precipitates are frequently observed,and they usually line up,forming sophisticated precipitation microstructures.Using atomic-resolution electron microscop...In the slightly deformed Al-Mg-Si alloys,dislocation-induced precipitates are frequently observed,and they usually line up,forming sophisticated precipitation microstructures.Using atomic-resolution electron microscopy in association with hardness measurements,we systematically investigated these precipitates in relation to the age-hardening responses of the alloys.Our study reveals that the majority of dislocation-induced complex precipitates are actually short-range ordered while long-range disordered polycrystalline precipitates and multiphase composite precipitates,including polycrystalline U2 precipitates,B’/U2,B’-2/U2,B’/B’-2/U2 and’/U2 composite precipitates.It is suggested that the formation of these complex precipitates is mainly owing to a high nucleation rate and rapid growth of different precipitate phases parallel to the associated dislocation lines.Since dislocation-induced precipitates consume more Mg than Si from the matrix and have a high formation kinetics,they will have different impacts on the matrix precipitation in different types of Al-Mg-Si alloys.Our results further demonstrate that for the"normally-β"-hardened"alloy,their formation leads to a coarser precipitate microstructure in the matrix,whereas for the"normally-β’-hardened"alloy,their formation reverses the precipitation pathway in the matrix,resulting in a reduced age-hardening potential of the former alloy and an improved age-hardening potential of the latter alloy.展开更多
The roles of Zn content and thermo-mechanical treatment in affecting microstructures and mechanical properties of Mg-x%Zn-1%Mn（mass fraction,x=4,5,6,7,8,9） wrought Mg alloys were investigated.The microstructure was ...The roles of Zn content and thermo-mechanical treatment in affecting microstructures and mechanical properties of Mg-x%Zn-1%Mn（mass fraction,x=4,5,6,7,8,9） wrought Mg alloys were investigated.The microstructure was extremely refined by dynamic recrystallization（DRC） during extrusion.With increasing Zn content,the DRC grains tended to grow up,at the same time,more second phase streamlines would be present,which restricted the further growing.During solution treatment,the DRC grains would rapidly grow up;however,higher Zn content could hinder the grain boundary expanding,which results in finer ultimate grains.MgZn2 dispersoid particles which are coherent with the matrix would precipitate from the supersaturated solid solution during the one-step aging process,and nano-sized GP zones formed during the pre-aging stage of the two-step aging provide a huge amount of effective nuclei for the MgZn2 phases formed in the second stage,which makes the MgZn2 particles much finer and more dispersed.The mechanical properties of as-extruded samples were not so sensitive to the variation of Zn content,the tensile strength fluctuates between 300 and 320 MPa,and the elongation maintains a high value between 11% and 14%.The strength of aged samples rises as a parabolic curve with increasing Zn content,specifically,the tensile strength of one-step aged samples rises from 278 to 374 MPa,and that of two-step aged ones rises from 284 to 378 MPa,yet the elongation of all aged samples is below 8%.When Zn content exceeds its solid solution limit in Mg-Zn system（6.2%,mass fraction）,the strength rises slowly but the elongation deteriorates sharply,so a Mg-Zn-Mn alloy with 6% Zn possesses the best mechanical properties,that is,the tensile strengths after one-and two-step aging are 352 and 366 MPa,respectively,and the corresponding elongations are 7.98% and 5.2%,respectively.展开更多
Al-Cu binary alloys are important and interesting industry materials.Up to date,the formation mechanisms of the key strengthening precipitates,namedθ’-phase,in the alloys are still controversial.Here,we report that ...Al-Cu binary alloys are important and interesting industry materials.Up to date,the formation mechanisms of the key strengthening precipitates,namedθ’-phase,in the alloys are still controversial.Here,we report that for non-deformed bulk Al-Cu alloys theθ’-phase actually has its own direct precursors that can form only at elevated aging temperature(>ca.200℃).These high-temperature precursors have the same plate-like morphology as theθ’-phase precipitates but rather different structures.Atomicresolution imaging reveals that they have a tetragonal structure with a=0.405 nm and c=1.213 nm,and an average composition of Al_(5-x)Cu_(1+x)(0≤x<1),being fully coherent with the Al-lattice.This precursor phase may initiate with a composition of Al5 Cu and evolve locally towards Al_(4)Cu_(2)in composition,eventually leading to a consequent structural transformation into theθ’-phase(Al4 Cu2=Al2 Cu).There are evidences that because of their genetic links in structure,such a high-temperature precursor may transform to theθ’-phase without having to change their morphology and interface structure.Our study reveals a well-defined and previously hidden precipitation scenario for theθ’-phase to form in Al-Cu alloys at an elevated aging temperature.展开更多
The effects of trace elements Cd and Sn on precipitation process of Al-Si-Cu-Mg cast alloys were investigated in the present research.It is shown that the addition of Cd and Sn not only increases remarkably the aging ...The effects of trace elements Cd and Sn on precipitation process of Al-Si-Cu-Mg cast alloys were investigated in the present research.It is shown that the addition of Cd and Sn not only increases remarkably the aging peak hardness and reduces the time to reach aging peak,but also eliminates the double-aging-peak phenomenon which appears in Al-Si-Cu-Mg alloys.In Al-Si-Cu-Mg alloys the first aging peak corresponds to GP zones(especially GPⅡ) ,and the second one is caused by metastable phases.The obvious time interval of transition from GPⅡ to metastable phases associates with the double-aging-peak phenomenon.The results of DSC and TEM show that Cd/Sn elements suppress the formation of GPⅠzone,stimulate the formation of θ",θ' and θ phases,and then shorten remarkably the temperature intervals of each exothermic peak.Because the transition interval between GPⅡzone and metastable phases is shortened by Cd/Sn in Al-Si-Cu-Mg cast alloys,θ' phase coexists with θ" phase in matrix of ageing peak condition,which causes effective hardening on the alloys,and at the same time,eliminates the double-aging-peak phenomenon.展开更多
基金supported by the National Key Research and Development Program of China(No.2016YFB0300801)the National Natural Science Foundation of China(Nos.51831004,11427806,51671082,51471067).
文摘In the slightly deformed Al-Mg-Si alloys,dislocation-induced precipitates are frequently observed,and they usually line up,forming sophisticated precipitation microstructures.Using atomic-resolution electron microscopy in association with hardness measurements,we systematically investigated these precipitates in relation to the age-hardening responses of the alloys.Our study reveals that the majority of dislocation-induced complex precipitates are actually short-range ordered while long-range disordered polycrystalline precipitates and multiphase composite precipitates,including polycrystalline U2 precipitates,B’/U2,B’-2/U2,B’/B’-2/U2 and’/U2 composite precipitates.It is suggested that the formation of these complex precipitates is mainly owing to a high nucleation rate and rapid growth of different precipitate phases parallel to the associated dislocation lines.Since dislocation-induced precipitates consume more Mg than Si from the matrix and have a high formation kinetics,they will have different impacts on the matrix precipitation in different types of Al-Mg-Si alloys.Our results further demonstrate that for the"normally-β"-hardened"alloy,their formation leads to a coarser precipitate microstructure in the matrix,whereas for the"normally-β’-hardened"alloy,their formation reverses the precipitation pathway in the matrix,resulting in a reduced age-hardening potential of the former alloy and an improved age-hardening potential of the latter alloy.
基金Project(2007CB613700)supported by the National Basic Research Program of ChinaProject(2007BAG06B04)supported by the National Science and Technology Pillar Program During the 11th Five-Year Plan Period,China+1 种基金Project(50725413)supported by the National Natural Science Foundation of ChinaProject(CDJXS10132202)supported by the Fundamental Research Funds for the Central Universities,China
文摘The roles of Zn content and thermo-mechanical treatment in affecting microstructures and mechanical properties of Mg-x%Zn-1%Mn（mass fraction,x=4,5,6,7,8,9） wrought Mg alloys were investigated.The microstructure was extremely refined by dynamic recrystallization（DRC） during extrusion.With increasing Zn content,the DRC grains tended to grow up,at the same time,more second phase streamlines would be present,which restricted the further growing.During solution treatment,the DRC grains would rapidly grow up;however,higher Zn content could hinder the grain boundary expanding,which results in finer ultimate grains.MgZn2 dispersoid particles which are coherent with the matrix would precipitate from the supersaturated solid solution during the one-step aging process,and nano-sized GP zones formed during the pre-aging stage of the two-step aging provide a huge amount of effective nuclei for the MgZn2 phases formed in the second stage,which makes the MgZn2 particles much finer and more dispersed.The mechanical properties of as-extruded samples were not so sensitive to the variation of Zn content,the tensile strength fluctuates between 300 and 320 MPa,and the elongation maintains a high value between 11% and 14%.The strength of aged samples rises as a parabolic curve with increasing Zn content,specifically,the tensile strength of one-step aged samples rises from 278 to 374 MPa,and that of two-step aged ones rises from 284 to 378 MPa,yet the elongation of all aged samples is below 8%.When Zn content exceeds its solid solution limit in Mg-Zn system（6.2%,mass fraction）,the strength rises slowly but the elongation deteriorates sharply,so a Mg-Zn-Mn alloy with 6% Zn possesses the best mechanical properties,that is,the tensile strengths after one-and two-step aging are 352 and 366 MPa,respectively,and the corresponding elongations are 7.98% and 5.2%,respectively.
基金the National Natural Science Foundation of China(Nos.51831004,51801060,51671082,11427806,51471067)the National Key Research and Development Program of China(No.2016YFB0300801)。
文摘Al-Cu binary alloys are important and interesting industry materials.Up to date,the formation mechanisms of the key strengthening precipitates,namedθ’-phase,in the alloys are still controversial.Here,we report that for non-deformed bulk Al-Cu alloys theθ’-phase actually has its own direct precursors that can form only at elevated aging temperature(>ca.200℃).These high-temperature precursors have the same plate-like morphology as theθ’-phase precipitates but rather different structures.Atomicresolution imaging reveals that they have a tetragonal structure with a=0.405 nm and c=1.213 nm,and an average composition of Al_(5-x)Cu_(1+x)(0≤x<1),being fully coherent with the Al-lattice.This precursor phase may initiate with a composition of Al5 Cu and evolve locally towards Al_(4)Cu_(2)in composition,eventually leading to a consequent structural transformation into theθ’-phase(Al4 Cu2=Al2 Cu).There are evidences that because of their genetic links in structure,such a high-temperature precursor may transform to theθ’-phase without having to change their morphology and interface structure.Our study reveals a well-defined and previously hidden precipitation scenario for theθ’-phase to form in Al-Cu alloys at an elevated aging temperature.
文摘The effects of trace elements Cd and Sn on precipitation process of Al-Si-Cu-Mg cast alloys were investigated in the present research.It is shown that the addition of Cd and Sn not only increases remarkably the aging peak hardness and reduces the time to reach aging peak,but also eliminates the double-aging-peak phenomenon which appears in Al-Si-Cu-Mg alloys.In Al-Si-Cu-Mg alloys the first aging peak corresponds to GP zones(especially GPⅡ) ,and the second one is caused by metastable phases.The obvious time interval of transition from GPⅡ to metastable phases associates with the double-aging-peak phenomenon.The results of DSC and TEM show that Cd/Sn elements suppress the formation of GPⅠzone,stimulate the formation of θ",θ' and θ phases,and then shorten remarkably the temperature intervals of each exothermic peak.Because the transition interval between GPⅡzone and metastable phases is shortened by Cd/Sn in Al-Si-Cu-Mg cast alloys,θ' phase coexists with θ" phase in matrix of ageing peak condition,which causes effective hardening on the alloys,and at the same time,eliminates the double-aging-peak phenomenon.
