Carbon Segregation in CoCrFeMnNi High‐Entropy Alloy Driven by High‐Pressure Torsion at Room and Cryogenic Temperatures

Herein, a CoCrFeMnNi high-entropy alloy with reduced Cr content and with the addition of 2 at% C interstitial is processed via high-pressure torsion (HPT) under 6.5 GPa by three turns at room and cryogenic temperatures. The microstructure is investigated by transmission electron microscopy (TEM) and atom probe tomography (APT). The results indicate that C atoms segregate at the boundaries of the nanograins in the sample processed at room temperature, while the sample processed at cryogenic temperature does not show any notable segregations of carbon

Mechanical Properties and Thermal Stability of Nanocrystalline High-entropy Alloys

High entropy alloys (HEAs) have been the subject of numerous investigations during past 20 years. Various alloy systems have been explored to identify HEA systems with improved property combinations, leading to an extraordinary growth of this field. Equiatomic single face-centered cubic (FCC) structured CoCrFeMnNi alloy, also known as Cantor alloy, has attracted increased attention in the past decades largely because of its excellent mechanical properties. The most remarkable feature of this alloy is the superior combination of ductility and strength at cryogenic temperatures in comparison with that at room temperature, especially in a fine-grained state. Generally, CoCrFeMnNi alloy exhibits a dramatic ductility and strong work hardening performance at cryogenic temperature but lacks comparable strength. To improve the strength, CoCrFeMnNi alloys with reduced Cr content and with the addition of different contents of carbon as interstitial impurity were synthesized for following study. High-pressure torsion (HPT) process, as the most effective severe plastic deformation (SPD) method, was performed to obtain nanocrystalline HEAs. Meanwhile, the evolution of microstructure and hardness was investigated during HPT process. Subsequently, the mechanical properties were analyzed in specimen with saturated microstructure. Based on electron microscopy characterization, the microstructure and composition fluctuation were investigated on the nanocrystalline HEAs. The results indicated that carbon interstitial alloying significantly promoted the grain refinement, dislocation density increase, improvement of yield strength and the carbon segregation at the grain boundaries. Herein, the mechanism of the grain fragmentation, deformation behavior and strengthening and fracture mechanisms were discussed in the following chapters. The C segregation behavior during HPT at room and cryogenic temperature were studied in detail. The results of this work could be a good reference for the production of high strength HEAs using SPD methods. Post deformation annealing has been used in SPD processed alloys to gain comprehensive performance avoiding the brittle fracture. Hence, the exploration of thermal stability of the C alloyed nanocrystalline HEAs is essential to promote the improvement of the mechanical properties. Using electron microscopy, the elemental segregation, nucleation of precipitates, decomposition of matrix phase decomposition and grain growth were illustrated after annealing at different temperature. The results suggested that the single FCC phase nanocrystalline HEA is thermally stable up to 400 °C. Significant co-segregation of alloy constituent elements and precipitation occur from 500 to 600 °C. New phases such as CoFe B2 phase, NiMn FCC phase and M7C3 carbides formed during the annealing at the medium temperature interval from 500 to 600 °C. The development of the precipitation process and the effect of precipitates on the mechanical properties are unveiled in following chapters. Consequently, the results in the present work optimized the current inference on the thermal stability of nanocrystalline HEAs and proposed a theoretical model for the precipitation process

Evolution of iron-rich intermetallics and its effect on the mechanical properties of Al–Cu–Mn–Fe–Si alloys after thermal exposure and high-temperature tensile testing

Si addition is commonly used to modify the iron-rich intermetallics in Al–Cu–Mn–Fe alloys, which is beneficial to increasing the use of recycled aluminum. Most of the available research has focused on the effect of Si content on the room-temperature mechanical properties of Al–Cu–Mn–Fe alloys. To expand the application of Al–Cu–Mn–Fe–Si alloys as light heat-resistant structural components in the automotive and aerospace industries, it is of great importance to investigate the evolution of iron-rich intermetallics and its effect on the fracture behavior of Al–Cu–Mn–Fe–Si alloys after thermal exposure and high-temperature tensile testing. In this work, the evolution of iron-rich intermetallics and the high-temperature mechanical properties of heat-treated Al-6.5Cu-0.6Mn-0.5Fe alloys with different Si contents after thermal exposure and high-temperature tensile testing were assessed by tensile tests, image analysis, scanning electron microscopy, X-Ray diffraction, transmission electron microscopy, and atomic probe tomography. The results indicate that the Al-6.5Cu-0.6Mn-0.5Fe alloys with 0.1Si and 0.5Si additions have excellent and stable high-temperature mechanical properties after long thermal exposure, which are better than those of most heat-resistant Al alloys. The high performance of the high-temperature mechanical properties is attributed to the high heat resistance of secondary intermetallics and precipitated particles. The addition of Si is detrimental to the strength of Al-6.5Cu-0.6Mn-0.5Fe alloys after long thermal exposure. This can be attributed to the solid-state phase transformation of iron-rich intermetallics from α-Fe to β-Fe, which results in the increase of needle-like Fe-rich phases and Si particles, the agglomeration of secondary intermetallics, and the consumption of Al$_{2}$Cu phases

Influence of carbon on the mechanical behavior and microstructure evolution of CoCrFeMnNi processed by high pressure torsion

In this study, a Cantor type high entropy alloys with the addition of C interstitials (, 0, 0.5 and 2 at.%) were processed via high pressure torsion (HPT) under 6.5 GPa by 0.5, 1 and 3 turns at room temperature. The microstructures and mechanical properties of samples before and following HPT were investigated. In all compositions studied, HPT deformation led to a dramatic grain size refinement down to a nanoscale range and also resulted in a considerable increase in dislocation densit

Electromagnetic Field Analysis of Operation Space of the Switch in the Substation Based on the Time Domain Integration Method

In this paper, the approximate calculation formula is used to calculate the horizontal electric field from lightning which has arbitrary current, with the help of time domain integral equation method solving the lightning induced over voltages on overhead lines. In this paper, the time domain integral equations of horizontal electric field is established. Taking the conductor axis current as the variable, the pulse function is used as the basis function, and the discretization equation is used in space and time. Using time stepping algorithm to solve the resulting linear equations. The calculation process of this method is simple, and it is proved that the method is consistent with the conclusion of the relevant literature

Electromagnetic Field Analysis of Operation Space of the Switch in the Substation Based on the Time Domain Integration Method

In this paper, the approximate calculation formula is used to calculate the horizontal electric field from lightning which has arbitrary current, with the help of time domain integral equation method solving the lightning induced over voltages on overhead lines. In this paper, the time domain integral equations of horizontal electric field is established. Taking the conductor axis current as the variable, the pulse function is used as the basis function, and the discretization equation is used in space and time. Using time stepping algorithm to solve the resulting linear equations. The calculation process of this method is simple, and it is proved that the method is consistent with the conclusion of the relevant literature

Endophytic fungi from a pharmaceutical plant, Camptotheca acuminata: isolation, identification and bioactivity

About 174 endophytic fungi were isolated from the pharmaceutical plant, Camptotheca acuminata. Of the 18 taxa obtained, non-sporulating fungi (48.9%), Alternaria (12.6%), Phomopsis (6.9%), Sporidesmium (6.3%), Paecilomyces (4.6%) and Fusarium (4.6%) were dominant. ITS rDNA assay indicated that most of the non-sporulating fungi belonged to the Pyrenomycetes and Loculoascomycetes ascomycetes or their anamorph Coelomycetes. The results of the bioactivity test showed that 27.6% of the endophytic fungi displayed inhibition against more than one indicator microorganism. 4.0% and 2.3% of the endophytic fungi showed cytotoxicity and protease inhibition, respectively. The endophytic fungi with bioactivities were distributed in more than 12 taxa including non-sporulating fungi, which are reliable sources for bioactive agents