CrMnFeCoNi high-entropy alloys(HEAs)exhibit an excellent combination of tensile strength and ductility at cryogenic temperatures.This study led to the introduction of a new method for the development of high-performan...CrMnFeCoNi high-entropy alloys(HEAs)exhibit an excellent combination of tensile strength and ductility at cryogenic temperatures.This study led to the introduction of a new method for the development of high-performance CrMnFeCoNi HEAs at cryogenic temperatures by jointly utilizing additive manufacturing(AM)and the addition of interstitial atoms.The interstitial oxygen present in the powder feedstock was transformed into beneficial nano-sized oxides during AM processing.The HEA nanocomposite fabricated using laser powder bed fusion(L-PBF)not only contains heterogeneous grains and substructures but also a high number density of nano-sized oxides.The tensile results revealed that the L-PBF HEA nanocomposite has superior yield strengths of 0.77 GPa and 1.15 GPa,and tensile strengths of 0.92 GPa and 1.45 GPa at 298 K and 77 K,respectively.In addition,the Charpy impact energies of the samples tested at 298 K and 77 K were measured as 176.2 J and 103.7 J,respectively.These results indicate that the L-PBF HEA nanocomposite successfully overcomes the well-known strength-toughness trade-off.The tensile deformation microstructure contained a relatively large number of deformation twins(DTs)at cryogenic temperature,a possible consequence of the decrease in the stacking fault energy with decreasing temperature.On the other hand,cracks were found to propagate along the grain boundaries at room temperature,whereas a transgranular crack was observed at cryogenic temperature in the specimens fractured as a result of the Charpy impact.展开更多
This study investigated the effect of multi-step heat treatment on the microstructure, mechanical properties and fracture behavior of thick 15 Co-12 Ni secondary hardening steel. As-quenched sample was found to have e...This study investigated the effect of multi-step heat treatment on the microstructure, mechanical properties and fracture behavior of thick 15 Co-12 Ni secondary hardening steel. As-quenched sample was found to have elongated prior austenite grain(PAG) and coarse lenticular martensitic structure. On the other hand, heat-treated sample was observed to have fine lenticular martensitic structure due to fine PAG size and a lot of nano-sized carbides. Also, after heat treatment, nano-scale reverted austenite film was formed at the martensite interfaces. The heat-treated sample showed 2.47 GPa superior tensile strength and superior elongation of about 12 %. The high strength was mainly due to fine block size and high number density of nano-sized carbides. The average value of plane strain fracture toughness(KIC) was 29.3 MPa m1/2, which indicated a good fracture toughness even with the high tensile strength. The tensile fracture surface was observed to have ductile fracture mode(cup-and-cone) and the formation of about ~1 μm ultra-fine dimples. In addition to this, nano-sized carbides were observed within the dimples.The findings suggested that the nano-sized carbide had a positive effect not only on the strength but also on the ductility of the alloy. The fractured surface after toughness test, also showed ductile fracture mode with a lot of dimples. Based on the above results, correlation among microstructural evolution,deformation and fracture mechanisms along the heat-treatment was also discussed.展开更多
Cold spray(CS)which has recently become a promising additive manufacturing(AM)technology,was used to fabricate ultra-strong pure copper.In addition,the effects of carrier gas species on the microstructural characteris...Cold spray(CS)which has recently become a promising additive manufacturing(AM)technology,was used to fabricate ultra-strong pure copper.In addition,the effects of carrier gas species on the microstructural characteristics,mechanical properties and deformation mechanisms were systemically explored.The CSAM copper manufactured with N_(2) carrier gas reveals a heterogeneous bimodal microstructure consisting of ultra-fine grains at the particle interface and relatively coarse grains in inner particles.With He carrier gas,a homogeneous grain structure consisting of ultra-fine grains in most areas was obtained.Compressive tests showed that N_(2) and He carrier gasses enabled ultra-high yield strengths of 340 and415 MPa,respectively.These values are comparable to severely plastic deformed copper,which has extremely low ductility and shape fidelity.On the other hand,both samples showed a strain-softening phenomenon that does not commonly occur at room temperature.The deformation microstructures revealed that dynamic recovery(DRV)and dynamic recrystallization(DRX)phenomena were generated despite being deformed at room temperature.Based on the above findings,the overall deformation mechanisms according to the carrier gas species in the CSAM copper manufacturing process were discussed.Furthermore,the work hardening and softening behaviors of CSAM Cu are predicted by using a constitutive equation.展开更多
High-entropy alloys(HEAs)with interstitial atoms that are produced by additive manufacturing have gained intensive interest in the materials science community because of their suitability for constructing high-strengt...High-entropy alloys(HEAs)with interstitial atoms that are produced by additive manufacturing have gained intensive interest in the materials science community because of their suitability for constructing high-strength net-shape components.Here,a strategy to additionally enhance the strength of selective laser melted carbon-containing HEAs was investigated.The as-built carbon-containing HEAs(C_(x)(Cr_(20)Mn_(20)Fe_(20)Co_(20)Ni_(20))_(100-x)(x=0.5 at.%,1.0 at.%,and 1.5 at.%))contain supersaturated carbon,and the extent of supersaturation increases as the carbon content increases.When subjected to aging treatment at 650°C for 1 h,the microstructure of the three alloys did not change at the grain scale.However,the microstructure at the sub-grain scale changed markedly,and these changes influenced the tensile properties and deformation mechanism.In particular,the tensile strength of aged 1.5C-HEA at 650°C was∼1.2 GPa at room temperature,which is higher than those reported for CrMnFeCoNi HEAs.Furthermore,the main deformation mechanism changed from deformation twinning to dislocation-mediated slip,resulting in much higher strain hardening capacity after the aging treatment.This work led to the development of an alternative promising method that involves tailoring the microstructure,to enhance the mechanical properties of additively manufactured metallic materials that contain interstitial atoms.展开更多
基金supported by a National Research Foundation of Korea(NRF)grant funded by the Korean government(MEST)(No.2019R1A2C1008904)。
文摘CrMnFeCoNi high-entropy alloys(HEAs)exhibit an excellent combination of tensile strength and ductility at cryogenic temperatures.This study led to the introduction of a new method for the development of high-performance CrMnFeCoNi HEAs at cryogenic temperatures by jointly utilizing additive manufacturing(AM)and the addition of interstitial atoms.The interstitial oxygen present in the powder feedstock was transformed into beneficial nano-sized oxides during AM processing.The HEA nanocomposite fabricated using laser powder bed fusion(L-PBF)not only contains heterogeneous grains and substructures but also a high number density of nano-sized oxides.The tensile results revealed that the L-PBF HEA nanocomposite has superior yield strengths of 0.77 GPa and 1.15 GPa,and tensile strengths of 0.92 GPa and 1.45 GPa at 298 K and 77 K,respectively.In addition,the Charpy impact energies of the samples tested at 298 K and 77 K were measured as 176.2 J and 103.7 J,respectively.These results indicate that the L-PBF HEA nanocomposite successfully overcomes the well-known strength-toughness trade-off.The tensile deformation microstructure contained a relatively large number of deformation twins(DTs)at cryogenic temperature,a possible consequence of the decrease in the stacking fault energy with decreasing temperature.On the other hand,cracks were found to propagate along the grain boundaries at room temperature,whereas a transgranular crack was observed at cryogenic temperature in the specimens fractured as a result of the Charpy impact.
基金the financial support provided by Agency for Defense Development (ADD), Republic of Korea (Project No. UE181033GD)。
文摘This study investigated the effect of multi-step heat treatment on the microstructure, mechanical properties and fracture behavior of thick 15 Co-12 Ni secondary hardening steel. As-quenched sample was found to have elongated prior austenite grain(PAG) and coarse lenticular martensitic structure. On the other hand, heat-treated sample was observed to have fine lenticular martensitic structure due to fine PAG size and a lot of nano-sized carbides. Also, after heat treatment, nano-scale reverted austenite film was formed at the martensite interfaces. The heat-treated sample showed 2.47 GPa superior tensile strength and superior elongation of about 12 %. The high strength was mainly due to fine block size and high number density of nano-sized carbides. The average value of plane strain fracture toughness(KIC) was 29.3 MPa m1/2, which indicated a good fracture toughness even with the high tensile strength. The tensile fracture surface was observed to have ductile fracture mode(cup-and-cone) and the formation of about ~1 μm ultra-fine dimples. In addition to this, nano-sized carbides were observed within the dimples.The findings suggested that the nano-sized carbide had a positive effect not only on the strength but also on the ductility of the alloy. The fractured surface after toughness test, also showed ductile fracture mode with a lot of dimples. Based on the above results, correlation among microstructural evolution,deformation and fracture mechanisms along the heat-treatment was also discussed.
基金supported by Korea Institute for Advancement of Technology(KIAT)grant funded by the Korea Government(MOTIE)(P0002007,The Competency Development Program for Industry Specialist)。
文摘Cold spray(CS)which has recently become a promising additive manufacturing(AM)technology,was used to fabricate ultra-strong pure copper.In addition,the effects of carrier gas species on the microstructural characteristics,mechanical properties and deformation mechanisms were systemically explored.The CSAM copper manufactured with N_(2) carrier gas reveals a heterogeneous bimodal microstructure consisting of ultra-fine grains at the particle interface and relatively coarse grains in inner particles.With He carrier gas,a homogeneous grain structure consisting of ultra-fine grains in most areas was obtained.Compressive tests showed that N_(2) and He carrier gasses enabled ultra-high yield strengths of 340 and415 MPa,respectively.These values are comparable to severely plastic deformed copper,which has extremely low ductility and shape fidelity.On the other hand,both samples showed a strain-softening phenomenon that does not commonly occur at room temperature.The deformation microstructures revealed that dynamic recovery(DRV)and dynamic recrystallization(DRX)phenomena were generated despite being deformed at room temperature.Based on the above findings,the overall deformation mechanisms according to the carrier gas species in the CSAM copper manufacturing process were discussed.Furthermore,the work hardening and softening behaviors of CSAM Cu are predicted by using a constitutive equation.
基金This study supported by National Research Foundation of Korea(NRF)grant funded by the Korea government(MEST)(No.2019R1A2C1008904).
文摘High-entropy alloys(HEAs)with interstitial atoms that are produced by additive manufacturing have gained intensive interest in the materials science community because of their suitability for constructing high-strength net-shape components.Here,a strategy to additionally enhance the strength of selective laser melted carbon-containing HEAs was investigated.The as-built carbon-containing HEAs(C_(x)(Cr_(20)Mn_(20)Fe_(20)Co_(20)Ni_(20))_(100-x)(x=0.5 at.%,1.0 at.%,and 1.5 at.%))contain supersaturated carbon,and the extent of supersaturation increases as the carbon content increases.When subjected to aging treatment at 650°C for 1 h,the microstructure of the three alloys did not change at the grain scale.However,the microstructure at the sub-grain scale changed markedly,and these changes influenced the tensile properties and deformation mechanism.In particular,the tensile strength of aged 1.5C-HEA at 650°C was∼1.2 GPa at room temperature,which is higher than those reported for CrMnFeCoNi HEAs.Furthermore,the main deformation mechanism changed from deformation twinning to dislocation-mediated slip,resulting in much higher strain hardening capacity after the aging treatment.This work led to the development of an alternative promising method that involves tailoring the microstructure,to enhance the mechanical properties of additively manufactured metallic materials that contain interstitial atoms.
