Although the rate-dependence of metals has been widely researched,the deformation mechanism under small-scale impact conditions lacked exploration and in-depth understanding.Using quasi-static nanoin-dentation(strain ...Although the rate-dependence of metals has been widely researched,the deformation mechanism under small-scale impact conditions lacked exploration and in-depth understanding.Using quasi-static nanoin-dentation(strain rate,SR,<1 s^(-1))and high strain-rate nano-impact(SR>10^(3)s^(-1))with a pyramidal Berkovich tip,this study investigates the influence of SR on the deformation response of an aluminium single crystal(110).The underlying microstructural variance was analyzed using on-axis TKD and TEM.The results show that the impact deformation involves great elastic recovery and different substructural characteristics.In contrast to the uniform sub-grain substructure with medium and high-angle grain boundaries formed during quasi-static indentation,the substructure formed under impact has a more heterogeneous nature including microbands near the surface and sub-grains underneath with dominant low-angle grain boundaries.The significant change in substructure for the impact deformation comes from suppressed thermally activated dislocation motion,leading to the conversion of dislocation glide from wave-like(quasi-static)to planar regime(impacting),and the insufficient rearrangement of geomet-rically necessary dislocations(GNDs).The heterogeneous microstructure develops due to the competition between high strain-rate-induced planar-slip and strain gradient-induced GND rearrangement,as well as the uneven distribution of SR and strain gradient.Moreover,the underlying incipient mechanism of the microband is proposed,in which successive primary dislocations are nucleated at the surface and glide perpendicular to flanks to pile up.Finally,the influence of SR on indentation size effects is discussed.展开更多
The mechanism of the shear band formation in the high cold rolled BCC metal is analyzed. Based on the plastic deformation theory, the shear distribution in the deformed grain is calculated by using the Taylor constrai...The mechanism of the shear band formation in the high cold rolled BCC metal is analyzed. Based on the plastic deformation theory, the shear distribution in the deformed grain is calculated by using the Taylor constraint model and the Bishop ＆ Hill maximum work principle. Results show that when the rolling direction （RD） is parallel to a certain direction of a grain, the large localized shear occurs on one slip plane, thus generating microbands in the grain because of the high localized shear strain. The angle between the RD and the shear band is about 30°. The plate-like structure of the microband is formed because of the dislocation double cross slip. The transmission electron microscope （TEM） observation of the microband in the cold rolled BCC metal confirms the formation mechanism of the microband.展开更多
Beijing Shihua Cave is located at the northeastern part of the granite fringed Paleozoic to Mesozoic synclinal belt in Fangshan,which is face to face with the Peking Man(Homo erectus pekinensis) Relic Site.The stratig...Beijing Shihua Cave is located at the northeastern part of the granite fringed Paleozoic to Mesozoic synclinal belt in Fangshan,which is face to face with the Peking Man(Homo erectus pekinensis) Relic Site.The stratigraphy sequence is composed of Majiagou Formation limestone formed before 460 Ma in Middle Ordovician.The large forms of cave began to展开更多
Metallic alloys with high strength and large ductility are required for extreme structural applications.However,the achievement of ultrahigh strength often results in a substantially decreased ductility.Here,we report...Metallic alloys with high strength and large ductility are required for extreme structural applications.However,the achievement of ultrahigh strength often results in a substantially decreased ductility.Here,we report a strategy to achieve the strength-ductility synergy by tailoring the alloy composition to control the local stacking fault energy(SFE)of the face-centered-cubic(fcc)matrix in an L1_(2)-strengthened superlattice alloy.As a proof of concept,based on the thermodynamic calculations,we developed a non-equiatomic CoCrNi_(2)(Al_(0.2)Nb_(0.2))alloy using phase separation to create a near-equiatomic low SFE disordered CoCrNi medium-entropy alloy matrix with in situ formed high-content coherent Ni_(3)(Al,Nb)-type ordered nanoprecipitates(∼12 nm).The alloy achieves a high tensile strength up to 1.6 GPa and a uniform ductility of 33%.The low SFE of the fcc matrix promotes the formation of nanotwins and parallel microbands during plastic deformation which could remarkably enhance the strain hardening capacity.This work provides a strategy for developing ultrahigh-strength alloys with large uniform ductility.展开更多
Grey microstructure of microbanding in thin sections of stalagmite T9501 from Shihua Cave, Beijing was studied. The types of grey and the morphologic features of organic material were classified. The signification of ...Grey microstructure of microbanding in thin sections of stalagmite T9501 from Shihua Cave, Beijing was studied. The types of grey and the morphologic features of organic material were classified. The signification of microbanding and annual layer was discussed. The standard distinguishing the false annual layer and the multi-year layer is set up, which provides the theoretic fundaments for paleoclimatic study.展开更多
基金This work was supported by the National Natural Science Foun-dation of China(No.52371058)the Ministry of Science and Tech-nology of the People’s Republic of China(No.2019YFE0191500)the State Key Laboratory for Mechanical Behavior of Materials(No.20222406).
文摘Although the rate-dependence of metals has been widely researched,the deformation mechanism under small-scale impact conditions lacked exploration and in-depth understanding.Using quasi-static nanoin-dentation(strain rate,SR,<1 s^(-1))and high strain-rate nano-impact(SR>10^(3)s^(-1))with a pyramidal Berkovich tip,this study investigates the influence of SR on the deformation response of an aluminium single crystal(110).The underlying microstructural variance was analyzed using on-axis TKD and TEM.The results show that the impact deformation involves great elastic recovery and different substructural characteristics.In contrast to the uniform sub-grain substructure with medium and high-angle grain boundaries formed during quasi-static indentation,the substructure formed under impact has a more heterogeneous nature including microbands near the surface and sub-grains underneath with dominant low-angle grain boundaries.The significant change in substructure for the impact deformation comes from suppressed thermally activated dislocation motion,leading to the conversion of dislocation glide from wave-like(quasi-static)to planar regime(impacting),and the insufficient rearrangement of geomet-rically necessary dislocations(GNDs).The heterogeneous microstructure develops due to the competition between high strain-rate-induced planar-slip and strain gradient-induced GND rearrangement,as well as the uneven distribution of SR and strain gradient.Moreover,the underlying incipient mechanism of the microband is proposed,in which successive primary dislocations are nucleated at the surface and glide perpendicular to flanks to pile up.Finally,the influence of SR on indentation size effects is discussed.
文摘The mechanism of the shear band formation in the high cold rolled BCC metal is analyzed. Based on the plastic deformation theory, the shear distribution in the deformed grain is calculated by using the Taylor constraint model and the Bishop ＆ Hill maximum work principle. Results show that when the rolling direction （RD） is parallel to a certain direction of a grain, the large localized shear occurs on one slip plane, thus generating microbands in the grain because of the high localized shear strain. The angle between the RD and the shear band is about 30°. The plate-like structure of the microband is formed because of the dislocation double cross slip. The transmission electron microscope （TEM） observation of the microband in the cold rolled BCC metal confirms the formation mechanism of the microband.
文摘Beijing Shihua Cave is located at the northeastern part of the granite fringed Paleozoic to Mesozoic synclinal belt in Fangshan,which is face to face with the Peking Man(Homo erectus pekinensis) Relic Site.The stratigraphy sequence is composed of Majiagou Formation limestone formed before 460 Ma in Middle Ordovician.The large forms of cave began to
基金The authors also thank the Microscope and Imaging Center at Southern University of Science and Technology,China.This work was financially supported by the National Natural Science Foundation of China(52122102)Guangdong Innovative&Entrepreneurial Research Team Program(2016ZT06C279)APT research was conducted at the Inter-University 3D APT Unit of City University of Hong Kong(CityU),which is supported by the CityU grant(9360161).
文摘Metallic alloys with high strength and large ductility are required for extreme structural applications.However,the achievement of ultrahigh strength often results in a substantially decreased ductility.Here,we report a strategy to achieve the strength-ductility synergy by tailoring the alloy composition to control the local stacking fault energy(SFE)of the face-centered-cubic(fcc)matrix in an L1_(2)-strengthened superlattice alloy.As a proof of concept,based on the thermodynamic calculations,we developed a non-equiatomic CoCrNi_(2)(Al_(0.2)Nb_(0.2))alloy using phase separation to create a near-equiatomic low SFE disordered CoCrNi medium-entropy alloy matrix with in situ formed high-content coherent Ni_(3)(Al,Nb)-type ordered nanoprecipitates(∼12 nm).The alloy achieves a high tensile strength up to 1.6 GPa and a uniform ductility of 33%.The low SFE of the fcc matrix promotes the formation of nanotwins and parallel microbands during plastic deformation which could remarkably enhance the strain hardening capacity.This work provides a strategy for developing ultrahigh-strength alloys with large uniform ductility.
文摘Grey microstructure of microbanding in thin sections of stalagmite T9501 from Shihua Cave, Beijing was studied. The types of grey and the morphologic features of organic material were classified. The signification of microbanding and annual layer was discussed. The standard distinguishing the false annual layer and the multi-year layer is set up, which provides the theoretic fundaments for paleoclimatic study.
