The thermal stability and mechanical properties of a gradient-nanograined structure(GNS)CoCrNi medium entropy alloy(MEA)processed by ultrasonic surface rolling were studied by using isothermal/isochronal annealing tes...The thermal stability and mechanical properties of a gradient-nanograined structure(GNS)CoCrNi medium entropy alloy(MEA)processed by ultrasonic surface rolling were studied by using isothermal/isochronal annealing tests combined with quasi-in-situ electron backscatter diffraction(EBSD)characterization and Vickers micro-hardness(HV)measurements.A layer by layer high-throughput investigation method was used to quantitatively study the grain growth kinetics and grain boundary evolution with different initial grain sizes,which could effectively save specimen and time costs.The grain nucleation and growth,as well as shrink and disappearance process throughΣ3 coincidence site lattice boundary migration with slightly lattice rotation during annealing were directly revealed.The layer by layer grain growth kinetics and calculated activation energy indicate that the thermal stability of nanograined top surface layer is relatively higher than that of nano-twined subsurface layer for the gradient CoCrNi MEA processed by ultrasonic surface rolling.Further analysis show that the grain boundary relaxation and dynamic recrystallization of the topmost nano-grains led to the decrease of grain boundary energy,thus improving their thermal stability.The present work provided theoretical basis for the application of CoCrNi MEA at high temperatures.Moreover,the high-throughput method on the investigation of grain stability by using gradient structure can be easily extended to other materials and it is of great significance for understanding the microstructural evolution of gradient materials.展开更多
Coherent detection measures both the amplitude and phase of pulsed terahertz(THz)waves simultaneously,forming the foundation for THz time-domain spectroscopy(THz-TDS).This technique has become increasingly prominent i...Coherent detection measures both the amplitude and phase of pulsed terahertz(THz)waves simultaneously,forming the foundation for THz time-domain spectroscopy(THz-TDS).This technique has become increasingly prominent in the fields of physics and materials science,allowing researchers to investigate the dynamic properties of various dielectric materials within the 0.1 to 10 THz frequency range,which is previously a challenging spectrum to access.This paper reviews recent advancements and the challenges faced by commonly used coherent detectors in THz-TDS.Our discussion emphasizes the potential for new discoveries in THz photonics and highlights the crucial role of coherent detection in the study of laser-matter interactions.展开更多
We demonstrate three-dimensional tomographic imaging vising a Fresnel lens with broadband terahertz pulses. Objects at various locations along the beam propagation path are uniquely imaged on the same imaging plane us...We demonstrate three-dimensional tomographic imaging vising a Fresnel lens with broadband terahertz pulses. Objects at various locations along the beam propagation path are uniquely imaged on the same imaging plane using a Fresnel lens with different frequencies of the imaging beam. This procedure allows the reconstruction of an object's tomographic contrast image by assembling the frequency-dependent images.展开更多
Matters are generally classified within four states:solid,liquid,gas,and plasma.Three of the four states of matter(solid,gas,and plasma)have been used for THz wave generation with short laser pulse excitation for deca...Matters are generally classified within four states:solid,liquid,gas,and plasma.Three of the four states of matter(solid,gas,and plasma)have been used for THz wave generation with short laser pulse excitation for decades,including the recent vigorous development of THz photonics in gases(air plasma).However,the demonstration of THz generation from liquids was conspicuously absent.It is well known that water,the most common liquid,is a strong absorber in the far infrared range.Therefore,liquid water has historically been sworn off as a source for THz radiation.Recently,broadband THz wave generation from a flowing liquid target has been experimentally demonstrated through laser-induced microplasma.The liquid target as the THz source presents unique properties.Specifically,liquids have the comparable material density to that of solids,meaning that laser pulses over a certain area will interact with three orders more molecules than an equivalent cross-section of gases.In contrast with solid targets,the fluidity of liquid allows every laser pulse to interact with a fresh area on the target,meaning that material damage or degradation is not an issue with the high-repetition rate intense laser pulses.These make liquids very promising candidates for the investigation of high-energy-density plasma,as well as the possibility of being the next generation of THz sources.展开更多
基金financial supports from the National Natural Science Foundation of China(Nos.51725503,52105144 and 52005185)Shanghai Super Postdoctoral Incentive Plan(No.2020134)Postdoctoral Fellowship for Research in Japan(FY2020 P20350)by the Japan Society for the Promotion of Science(JSPS)。
文摘The thermal stability and mechanical properties of a gradient-nanograined structure(GNS)CoCrNi medium entropy alloy(MEA)processed by ultrasonic surface rolling were studied by using isothermal/isochronal annealing tests combined with quasi-in-situ electron backscatter diffraction(EBSD)characterization and Vickers micro-hardness(HV)measurements.A layer by layer high-throughput investigation method was used to quantitatively study the grain growth kinetics and grain boundary evolution with different initial grain sizes,which could effectively save specimen and time costs.The grain nucleation and growth,as well as shrink and disappearance process throughΣ3 coincidence site lattice boundary migration with slightly lattice rotation during annealing were directly revealed.The layer by layer grain growth kinetics and calculated activation energy indicate that the thermal stability of nanograined top surface layer is relatively higher than that of nano-twined subsurface layer for the gradient CoCrNi MEA processed by ultrasonic surface rolling.Further analysis show that the grain boundary relaxation and dynamic recrystallization of the topmost nano-grains led to the decrease of grain boundary energy,thus improving their thermal stability.The present work provided theoretical basis for the application of CoCrNi MEA at high temperatures.Moreover,the high-throughput method on the investigation of grain stability by using gradient structure can be easily extended to other materials and it is of great significance for understanding the microstructural evolution of gradient materials.
基金supported by grants from the National Natural Science Foundation of China(12074272)the R&D Program of Beijing Municipal Education Commission(KZ20231002825)the Youth Beijing Scholar Program administered by the Beijing Government.
文摘Coherent detection measures both the amplitude and phase of pulsed terahertz(THz)waves simultaneously,forming the foundation for THz time-domain spectroscopy(THz-TDS).This technique has become increasingly prominent in the fields of physics and materials science,allowing researchers to investigate the dynamic properties of various dielectric materials within the 0.1 to 10 THz frequency range,which is previously a challenging spectrum to access.This paper reviews recent advancements and the challenges faced by commonly used coherent detectors in THz-TDS.Our discussion emphasizes the potential for new discoveries in THz photonics and highlights the crucial role of coherent detection in the study of laser-matter interactions.
文摘We demonstrate three-dimensional tomographic imaging vising a Fresnel lens with broadband terahertz pulses. Objects at various locations along the beam propagation path are uniquely imaged on the same imaging plane using a Fresnel lens with different frequencies of the imaging beam. This procedure allows the reconstruction of an object's tomographic contrast image by assembling the frequency-dependent images.
基金supported by the Army Research Office(Grant no.W911NF-17-1-0428)Air Force Office of Scientific Research(Grant no.FA9550-18-1-0357),and National Science Foundation(Grant no.ECCS1916068)+2 种基金A.Tcypkin and S.Kozlov are supported by the Russian Science Foundation(Grant no.19-12-00097).L.Zhang and C.Zhang are supported by the Beijing Natural Science Foundation(Grant no.JQ18015)the National Natural Science Foundation of China(Grant no.12074272).
文摘Matters are generally classified within four states:solid,liquid,gas,and plasma.Three of the four states of matter(solid,gas,and plasma)have been used for THz wave generation with short laser pulse excitation for decades,including the recent vigorous development of THz photonics in gases(air plasma).However,the demonstration of THz generation from liquids was conspicuously absent.It is well known that water,the most common liquid,is a strong absorber in the far infrared range.Therefore,liquid water has historically been sworn off as a source for THz radiation.Recently,broadband THz wave generation from a flowing liquid target has been experimentally demonstrated through laser-induced microplasma.The liquid target as the THz source presents unique properties.Specifically,liquids have the comparable material density to that of solids,meaning that laser pulses over a certain area will interact with three orders more molecules than an equivalent cross-section of gases.In contrast with solid targets,the fluidity of liquid allows every laser pulse to interact with a fresh area on the target,meaning that material damage or degradation is not an issue with the high-repetition rate intense laser pulses.These make liquids very promising candidates for the investigation of high-energy-density plasma,as well as the possibility of being the next generation of THz sources.
