A novel high nitrogen medium-entropy alloy CrCoNiN, which had higher strength and slightly lower ductility than CrCoNi alloy, was successfully manufactured by pressurized metallurgy.The microstructure and corrosion be...A novel high nitrogen medium-entropy alloy CrCoNiN, which had higher strength and slightly lower ductility than CrCoNi alloy, was successfully manufactured by pressurized metallurgy.The microstructure and corrosion behaviour were investigated by microscopic, electrochemical and spectroscopic methods. The results indicated that nitrogen existed in the form of Cr2N precipitates and uniformly distributed N atoms, and nitrogen alloying significantly refined the grain size. Besides, nitrogen enriched on the outmost surface of passive film and metal/film interface as ammonia （NH3 and NH4^＋） and CrN, respectively. The significant improvement of corrosion resistance of CrCoNiN was attributed to the lower metastahle pitting susceptibility together with thicker, less defective and more compact passive film.展开更多
Gradient structure(GS)possesses a typical trans-scale grain hierarchy with varying internal plastic stability,and the mutual plastic accommodation plays a crucial role in its superior strength-ductility combination.Us...Gradient structure(GS)possesses a typical trans-scale grain hierarchy with varying internal plastic stability,and the mutual plastic accommodation plays a crucial role in its superior strength-ductility combination.Using the in-situ synchrotron X-ray diffraction(XRD)during tensile loading,we measured lattice strains sequentially from the nanostructured(NS)surface layer to the central coarsegrained(CG)layer to elucidate when and how plastic accommodation occurs and evolves within the GS,along with their roles in plastic deformation and strain hardening.Throughout the tensile deformation,two types of plastic incompatibility occur in the GS.One is an extended elastoplastic transition due to layer-by-layer yielding.The other is strain localization and softening in the NS layer,in contrast with the stable plastic deformation in the CG layer.Plastic accommodation thus occurs concurrently and manifests as both an inter-layer and intra-layer change of stress state throughout tensile deformation.This produces different micromechanical responses between layers.Specifically,the NS layer initially experiences strain hardening followed by an elastoplastic deformation.The hetero-deformation induced hardening,along with forest hardening,facilitates a sustainable tensile strain in the NS layer,comparable to that in the CG layer.展开更多
Structures with single gradient and dual gradients have been designed and fabricated in an Al_(0.5)Cr_(0.9)FeNi_(2.5)V_(0.2) medium entropy alloy.Structures with dual gradients(with increasing grain size and a decreas...Structures with single gradient and dual gradients have been designed and fabricated in an Al_(0.5)Cr_(0.9)FeNi_(2.5)V_(0.2) medium entropy alloy.Structures with dual gradients(with increasing grain size and a decreasing volume fraction of nanoprecipitates from the surface to the center)were observed to show much better dynamic shear properties compared to both structures with single grainsize gradient and coarse-grained structures with homogeneously distributed nanoprecipitates.Thus,the dual gradients have a synergetic strengthening/toughening effect as compared to the sole effect of a single gradient and the sole precipitation effect.Initiation of the adiabatic shear band(ASB)is delayed and propagation of ASB is slowed down in structures with dual gradients compared to structures with single gradients,resulting in better dynamic shear properties.A higher magnitude of strain gradient and higher density of geometrically necessary dislocations are induced in the structures with dual gradients,resulting in extra strain hardening.Higher density dislocations,stacking faults,and Lomer-Cottrell locks can be accumulated by the interactions between these defects and B2/L1_(2) precipitates,due to the higher volume fraction of nanoprecipitates in the surface layer of the structures with dual gradients,which could retard the early strain localization in the surface layer for better dynamic shear properties.展开更多
Deformation-induced hcp nano-lamellae with various widths and interspacings were observed in the Co Cr Ni medium-entropy alloy(MEA)under high strain rate and cryogenic temperature in the present study.Higher hardness ...Deformation-induced hcp nano-lamellae with various widths and interspacings were observed in the Co Cr Ni medium-entropy alloy(MEA)under high strain rate and cryogenic temperature in the present study.Higher hardness was found in the cryogenic-deformed samples compared to the room temperature-deformed samples without hcp phase.Then,size effects of embedded hcp nano-lamellae on the tensile behaviors in the fcc Co Cr Ni MEA were investigated by molecular dynamics simulations.The overall strengthening was found to have two components:phase strengthening and extra interface strengthening,and the interface strengthening was observed to be always stronger than the phase strengthening.Both overall strengthening and interface strengthening were found to increase with increasing width and decreasing interspacing of embedded hcp nano-lamellae.The samples with small spaced hcp nano-lamellae are even stronger than the pure hard hcp phase due to the extra interface strengthening.The samples with larger width of embedded hcp nano-lamellae can provide stronger resistance for dislocation slip and transmission.Nanotwins were observed to be formed in the embedded hcp nano-lamellae.Higher density of phase boundaries and newly formed twin boundaries can provide more barriers for dislocation glide in the other slip systems,resulting in higher strength for samples with smaller interspacing.展开更多
A new medium-Mn steel was designed to achieve unprecedented tensile properties,with a yield strength beyond 1.1 GPa and a uniform elongation over 50%.The tensile behavior shows a heterogeneous deforma-tion feature,whi...A new medium-Mn steel was designed to achieve unprecedented tensile properties,with a yield strength beyond 1.1 GPa and a uniform elongation over 50%.The tensile behavior shows a heterogeneous deforma-tion feature,which displays a yield drop followed by a large Lüders band strain and several Portevin-Le Châtelier bands.Multiple strain hardening mechanisms for excellent tensile properties were revealed.Firstly,non-uniform martensite transformation occurs only within a localized deformation band,and ini-tiation and propagation of every localized deformation band need only a small amount of martensite transformation,which can provide a persistent and complete transformation-induced-plasticity effect dur-ing a large strain range.Secondly,geometrically necessary dislocations induced from macroscopic strain gradient at the front of localized deformation band and microscopic strain gradient among various phases provide strong heter-deformation-induced hardening.Lastly,martensite formed by displacive shear trans-formation can inherently generate a high density of mobile screw dislocations,and interstitial C atoms segregated at phase boundaries and enriched in austenite play a vital role in the dislocation multipli-cation due to the dynamic strain aging effect,and these two effects provide a high density of mobile dislocations for strong strain hardening.展开更多
基金supported by the National Natural Science Foundation of China(Grant Nos.51434004,U1435205,51774074)the Transformation Project of Major Scientific and Technological Achievements in Shenyang(Grant No.Z17-5-003)
文摘A novel high nitrogen medium-entropy alloy CrCoNiN, which had higher strength and slightly lower ductility than CrCoNi alloy, was successfully manufactured by pressurized metallurgy.The microstructure and corrosion behaviour were investigated by microscopic, electrochemical and spectroscopic methods. The results indicated that nitrogen existed in the form of Cr2N precipitates and uniformly distributed N atoms, and nitrogen alloying significantly refined the grain size. Besides, nitrogen enriched on the outmost surface of passive film and metal/film interface as ammonia （NH3 and NH4^＋） and CrN, respectively. The significant improvement of corrosion resistance of CrCoNiN was attributed to the lower metastahle pitting susceptibility together with thicker, less defective and more compact passive film.
基金supported by the Ministry of Science and Technology, China (2017YFA0204402)NSFC Basic Science Center Program for “Multiscale Problems in Nonlinear Mechanics” (11988102)+1 种基金the National Natural Science Foundation of China (11972350 and 11890680)the Chinese Academy of Sciences (XDB22040503)
文摘Gradient structure(GS)possesses a typical trans-scale grain hierarchy with varying internal plastic stability,and the mutual plastic accommodation plays a crucial role in its superior strength-ductility combination.Using the in-situ synchrotron X-ray diffraction(XRD)during tensile loading,we measured lattice strains sequentially from the nanostructured(NS)surface layer to the central coarsegrained(CG)layer to elucidate when and how plastic accommodation occurs and evolves within the GS,along with their roles in plastic deformation and strain hardening.Throughout the tensile deformation,two types of plastic incompatibility occur in the GS.One is an extended elastoplastic transition due to layer-by-layer yielding.The other is strain localization and softening in the NS layer,in contrast with the stable plastic deformation in the CG layer.Plastic accommodation thus occurs concurrently and manifests as both an inter-layer and intra-layer change of stress state throughout tensile deformation.This produces different micromechanical responses between layers.Specifically,the NS layer initially experiences strain hardening followed by an elastoplastic deformation.The hetero-deformation induced hardening,along with forest hardening,facilitates a sustainable tensile strain in the NS layer,comparable to that in the CG layer.
基金supported by the National Natural Science Foundation of China (Nos.52192591,12202459,and 11790293)the NSFC Basic Science Center Program for“Multiscale Problems in Nonlinear Mechanics” (No.11988102)the fellowship of China Postdoctoral Science Foundation (No.2021M703292).
文摘Structures with single gradient and dual gradients have been designed and fabricated in an Al_(0.5)Cr_(0.9)FeNi_(2.5)V_(0.2) medium entropy alloy.Structures with dual gradients(with increasing grain size and a decreasing volume fraction of nanoprecipitates from the surface to the center)were observed to show much better dynamic shear properties compared to both structures with single grainsize gradient and coarse-grained structures with homogeneously distributed nanoprecipitates.Thus,the dual gradients have a synergetic strengthening/toughening effect as compared to the sole effect of a single gradient and the sole precipitation effect.Initiation of the adiabatic shear band(ASB)is delayed and propagation of ASB is slowed down in structures with dual gradients compared to structures with single gradients,resulting in better dynamic shear properties.A higher magnitude of strain gradient and higher density of geometrically necessary dislocations are induced in the structures with dual gradients,resulting in extra strain hardening.Higher density dislocations,stacking faults,and Lomer-Cottrell locks can be accumulated by the interactions between these defects and B2/L1_(2) precipitates,due to the higher volume fraction of nanoprecipitates in the surface layer of the structures with dual gradients,which could retard the early strain localization in the surface layer for better dynamic shear properties.
基金supported by the National Key R&D Program of China[grant numbers 2017YFB0202802,2017YFA0204402 and 2019YFA02099-01/-02]the NSFC Basic Science Center Program for“Multiscale Problems in Nonlinear Mechanics”[grant number 11988102]+1 种基金the National Natural Science Foundation of China[grant numbers 11672313,11790293 and 52071326]the Strategic Priority Research Program of the Chinese Academy of Sciences[grant number XDB22040503]。
文摘Deformation-induced hcp nano-lamellae with various widths and interspacings were observed in the Co Cr Ni medium-entropy alloy(MEA)under high strain rate and cryogenic temperature in the present study.Higher hardness was found in the cryogenic-deformed samples compared to the room temperature-deformed samples without hcp phase.Then,size effects of embedded hcp nano-lamellae on the tensile behaviors in the fcc Co Cr Ni MEA were investigated by molecular dynamics simulations.The overall strengthening was found to have two components:phase strengthening and extra interface strengthening,and the interface strengthening was observed to be always stronger than the phase strengthening.Both overall strengthening and interface strengthening were found to increase with increasing width and decreasing interspacing of embedded hcp nano-lamellae.The samples with small spaced hcp nano-lamellae are even stronger than the pure hard hcp phase due to the extra interface strengthening.The samples with larger width of embedded hcp nano-lamellae can provide stronger resistance for dislocation slip and transmission.Nanotwins were observed to be formed in the embedded hcp nano-lamellae.Higher density of phase boundaries and newly formed twin boundaries can provide more barriers for dislocation glide in the other slip systems,resulting in higher strength for samples with smaller interspacing.
基金supported by the National Key R&D Pro-gram of China(No.2017YFA0204402)the NSFC Basic Science Cen-ter Program for“Multiscale Problems in Nonlinear Mechanics”(No.11988102)the National Natural Science Foundation of China(Nos.11790293 and 52192591).
文摘A new medium-Mn steel was designed to achieve unprecedented tensile properties,with a yield strength beyond 1.1 GPa and a uniform elongation over 50%.The tensile behavior shows a heterogeneous deforma-tion feature,which displays a yield drop followed by a large Lüders band strain and several Portevin-Le Châtelier bands.Multiple strain hardening mechanisms for excellent tensile properties were revealed.Firstly,non-uniform martensite transformation occurs only within a localized deformation band,and ini-tiation and propagation of every localized deformation band need only a small amount of martensite transformation,which can provide a persistent and complete transformation-induced-plasticity effect dur-ing a large strain range.Secondly,geometrically necessary dislocations induced from macroscopic strain gradient at the front of localized deformation band and microscopic strain gradient among various phases provide strong heter-deformation-induced hardening.Lastly,martensite formed by displacive shear trans-formation can inherently generate a high density of mobile screw dislocations,and interstitial C atoms segregated at phase boundaries and enriched in austenite play a vital role in the dislocation multipli-cation due to the dynamic strain aging effect,and these two effects provide a high density of mobile dislocations for strong strain hardening.
