Grain boundaries(GBs)are often known as intergranular cracking sources in alloys at high temperatures,resulting in limited high-temperature strength and ductility.Here,we propose a GB-dual-carbide(de-noted as GB-DC)st...Grain boundaries(GBs)are often known as intergranular cracking sources in alloys at high temperatures,resulting in limited high-temperature strength and ductility.Here,we propose a GB-dual-carbide(de-noted as GB-DC)strengthening strategy and have developed a high-performance(NiCoFeCr)_(99)Nb0.5 C_(0.5) high-entropy alloy(HEA)with exceptional strength-ductility synergy at 1073 K.Chain-like coherent M23 C6 carbides have been successfully introduced at GBs and remain a cube parallel crystallographic orientation with the face-centered cubic(FCC)matrix during deformation.Nano-scale NbC particles are distributed alternatively between M_(23)C_(6) carbides and inhibit their coarsening.Both strength and duc-tility of the GB-DC HEA increase dramatically at strain rates ranging from 10^(−4) to 10^(−2) s^(−1) at 1073 K,compared with those of the single-phase NiCoFeCr HEA.Specifically,yield strength of 142 MPa,ultimate tensile strength of 283 MPa,and elongation of 34%were obtained,which are twice that of the refer-ence NiCoFeCr HEA(82 MPa,172 MPa,and 18%,respectively).EBSD investigations demonstrated that chain-like carbides enhance the GB cohesion at high temperature,and TEM analysis revealed that dislo-cations can go through the coherent phase boundaries(CPBs)and activate dipoles inner M_(23)C_(6) carbides,which weakened the stress concentration in GBs.This substantially reduces the critical stress for dislo-cation generation and transmission to a stress level lower than that required for intergranular fracture.Theoretical estimation suggests that carbides result in a much higher activation energy(∼510 kJ/mol)for GB sliding and a rather low interface energy(∼101 mJ/m^(2))compared with the GB energy(1000 mJ/m^(2)),which rationalizes the enhanced GB cohesion by carbides.展开更多
The Cr Fe Co Ni high-entropy alloy(HEA)exhibits excellent mechanical properties at lower temperatures due to its low stacking-fault energy,however,its medium-and high-temperature strengths are still insufficient.In co...The Cr Fe Co Ni high-entropy alloy(HEA)exhibits excellent mechanical properties at lower temperatures due to its low stacking-fault energy,however,its medium-and high-temperature strengths are still insufficient.In consideration of the potential diversified applications,more strengthening approaches except for the previously proposed L12 phase hardening deserve further exploration due to its rapid coarsening tendency at high temperatures.Here,we achieved significant high-temperature strengthening of the cast Cr Fe Co Ni HEA by in-situ precipitation of highly thermostable carbides.Alloys with 0.5 at.%and 1 at.%niobium and carbon were prepared by simple casting processes,i.e.drop cast,solute solution and aging.A highly thermostable microstructure was formed,which comprises very coarse grains accompanied with extensive thermostable carbide precipitates embedded,including submicrometer coherent Nb C particles in grain interiors and intergranular coherent M_(23)C_(6)carbides.This high thermostability of microstructure,which is beneficial for the high-temperature loading,is ascribed to the synergy of lacking growth driving force and Zenner pinning effect by the carbides.Tensile properties tested at 673,873 and1073 K show that the yield strength and ultimate tensile strength are significantly increased by Nb/C doping,along with the elongation escalation at higher temperatures.The strengthening is mainly due to the precipitation hardening of carbide particles.展开更多
An improved smoke-wire flow visualization technique using a large capacitor as the power source was proposed. Electric current discharged from capacitors was used to heat a fine metal wire suspended in the flow field....An improved smoke-wire flow visualization technique using a large capacitor as the power source was proposed. Electric current discharged from capacitors was used to heat a fine metal wire suspended in the flow field. The oil droplets attached to the wire were vaporized and smoke filaments followed the flow motions. A digital camera was used to record the images of the smoke filaments. The actions of discharging and camera shutter were triggered by signals from a microcontroller to ensure the accurate timing. Clear images of the streaklines were captured at a freestream velocity up to 12.9 m/s, much higher than the limits of the existing techniques.展开更多
基金sponsored by the fund of National Natural Sci-ence Foundation of China(Grant No.52371028 and 52271097)the State Key Laboratory of Solidification Processing in NPU(Grant No.SKLSP202401)the Fundamental Research Funds for the Central Universities,HUST(No.2023JYCXJJ016).
文摘Grain boundaries(GBs)are often known as intergranular cracking sources in alloys at high temperatures,resulting in limited high-temperature strength and ductility.Here,we propose a GB-dual-carbide(de-noted as GB-DC)strengthening strategy and have developed a high-performance(NiCoFeCr)_(99)Nb0.5 C_(0.5) high-entropy alloy(HEA)with exceptional strength-ductility synergy at 1073 K.Chain-like coherent M23 C6 carbides have been successfully introduced at GBs and remain a cube parallel crystallographic orientation with the face-centered cubic(FCC)matrix during deformation.Nano-scale NbC particles are distributed alternatively between M_(23)C_(6) carbides and inhibit their coarsening.Both strength and duc-tility of the GB-DC HEA increase dramatically at strain rates ranging from 10^(−4) to 10^(−2) s^(−1) at 1073 K,compared with those of the single-phase NiCoFeCr HEA.Specifically,yield strength of 142 MPa,ultimate tensile strength of 283 MPa,and elongation of 34%were obtained,which are twice that of the refer-ence NiCoFeCr HEA(82 MPa,172 MPa,and 18%,respectively).EBSD investigations demonstrated that chain-like carbides enhance the GB cohesion at high temperature,and TEM analysis revealed that dislo-cations can go through the coherent phase boundaries(CPBs)and activate dipoles inner M_(23)C_(6) carbides,which weakened the stress concentration in GBs.This substantially reduces the critical stress for dislo-cation generation and transmission to a stress level lower than that required for intergranular fracture.Theoretical estimation suggests that carbides result in a much higher activation energy(∼510 kJ/mol)for GB sliding and a rather low interface energy(∼101 mJ/m^(2))compared with the GB energy(1000 mJ/m^(2)),which rationalizes the enhanced GB cohesion by carbides.
基金financially supported by the National Nature Science Foundation of China(Nos.51971099 and 11805171)。
文摘The Cr Fe Co Ni high-entropy alloy(HEA)exhibits excellent mechanical properties at lower temperatures due to its low stacking-fault energy,however,its medium-and high-temperature strengths are still insufficient.In consideration of the potential diversified applications,more strengthening approaches except for the previously proposed L12 phase hardening deserve further exploration due to its rapid coarsening tendency at high temperatures.Here,we achieved significant high-temperature strengthening of the cast Cr Fe Co Ni HEA by in-situ precipitation of highly thermostable carbides.Alloys with 0.5 at.%and 1 at.%niobium and carbon were prepared by simple casting processes,i.e.drop cast,solute solution and aging.A highly thermostable microstructure was formed,which comprises very coarse grains accompanied with extensive thermostable carbide precipitates embedded,including submicrometer coherent Nb C particles in grain interiors and intergranular coherent M_(23)C_(6)carbides.This high thermostability of microstructure,which is beneficial for the high-temperature loading,is ascribed to the synergy of lacking growth driving force and Zenner pinning effect by the carbides.Tensile properties tested at 673,873 and1073 K show that the yield strength and ultimate tensile strength are significantly increased by Nb/C doping,along with the elongation escalation at higher temperatures.The strengthening is mainly due to the precipitation hardening of carbide particles.
基金supported by the National Natural Science Foundation of China (91752101 and 11572078)
文摘An improved smoke-wire flow visualization technique using a large capacitor as the power source was proposed. Electric current discharged from capacitors was used to heat a fine metal wire suspended in the flow field. The oil droplets attached to the wire were vaporized and smoke filaments followed the flow motions. A digital camera was used to record the images of the smoke filaments. The actions of discharging and camera shutter were triggered by signals from a microcontroller to ensure the accurate timing. Clear images of the streaklines were captured at a freestream velocity up to 12.9 m/s, much higher than the limits of the existing techniques.
