The effects of adding boron or carbon on the microstructure and room-temperature mechanical properties of a new single-phase f.c.c. high entropy alloy Fe40.4Ni11.3Mn34.8Al7.5Cr6 are presented. ...Remarkably, 1.1at% carbon in solution not only increases the yield strength (by a factor of 2), but also increases the elongation to failure (from ~41% to 50%) and the work-hardening rate. Surprisingly, the latter increases with increasing strain up to a strain of 35%. When fine-grained (4.7µm), the C-doped HEA exhibits a yield strength of 557MPa.
•At least 1.1 at. % carbon can be dissolved in a FeNiMnAlCr HEA.•The carbon solute produces substantial increase in both strength and ductility.•The C-doped HEAs in mechanical properties are better than most advanced steels.
The effects of cold rolling followed by annealing on the mechanical properties and dislocation substructure evolution of undoped and 1.1 at. % carbon-doped Fe40.4Ni11.3Mn34.8Al7.5Cr6 high entropy ...alloys (HEAs) have been investigated. X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atom probe tomography (APT) were employed to characterize the microstructures. The as-cast HEAs were coarse-grained and single phase f.c.c., whereas the thermo-mechanical treatment caused recrystallization (to fine grain sizes) and precipitation (a B2 phase for the undoped HEA; and a B2 phase, and M23C6 and M7C3 carbides for the C-doped HEA). Carbon, which was found to have segregated to the grain boundaries using APT, retarded recrystallization. The reduction in grain size resulted in a sharp increase in strength, while the precipitation, which produced only a small increase in strength, probably accounted for the small decrease in ductility for both undoped and C-doped HEAs. For both undoped and C-doped HEAs, the smaller grain-sized material initially exhibited higher strain hardening than the coarse-grained material but showed a much lower strain hardening at large tensile strains. Wavy slip in the undoped HEAs and planar slip in C-doped HEAs were found at the early stages of deformation irrespective of grain size. At higher strains, dislocation cell structures formed in the 19 μm grain-sized undoped HEA, while microbands formed in the 23 μm grain-sized C-doped HEA. In contrast, localized dislocation clusters were found in both HEAs at the finest grain sizes (5 μm). The inhibition of grain subdivision by the grain boundaries and precipitates lead to the transformation from regular dislocation configurations consisting of dislocation-cells and microbands to irregular dislocation configurations consisting of localized dislocation clusters, which further account for the decrease in ductility. Investigation of the formation mechanism and strain hardening of dislocation cells and microbands could benefit future structural material design.
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Multi-principle component alloys (MPCAs) differ from traditional alloys in that they consist of four or more elements or components each with concentrations of 5–35 at. %. Since the first eutectic ...multi-principle component alloy (MPCA) was produced in 2008, there has been a growing number of papers on developing eutectic MPCAs as potential structural materials. Eutectic MPCAs can show high ambient temperature yield strengths that increase with decreasing interlamellar spacing, λ, according to either λ−1/2 or λ−1, similar to that observed in pearlitic steels, with a tradeoff between this increased strength and reduced tensile ductility. Ambient temperature tensile ductility has been observed in eutectic MPCAs only when one phase is f.c.c. and when the harder second phase is itself deformable. The yield strength in eutectic MPCAs has been shown to decrease with increasing temperature, and, limited data suggest that, this is related to the softening of the harder phase. Annealing of as-cast eutectic MPCAs, which are not typically at equilibrium, can produce precipitation of fine particles that further increase the strength, and which often reduce the ductility. Both thermo-mechanical processing and nitriding can increase the strengths of eutectic MPCAs by transforming the lamellar eutectic into equi-axed grains and producing fine AlN particles (in aluminum-containing MPCAs), respectively. The properties of eutectic MPCAs can largely be explained by models used for traditional alloys. While a number of different elements have been used to produce eutectic MPCAs, the design of eutectic MPCAs for structural applications should avoid the use of expensive elements like cobalt and niobium, which have often been used.
Room temperature yield strengths have been shown to increase with decreasing f.c.c. interlamellar spacing, λ, in lamellar eutectic FeNiMnAl alloys (shown right) according to either λ 1 or λ 1/2, with a resulting trade-off between increased strength and reduced tensile ductility. Display omitted
•The microstructures and mechanical properties of eutectic/eutectoid multi-principle component alloys (MPCAs) are critically reviewed.•The yield strength and interlamellar spacing, λ, of eutectic/eutectoid MPCAs obey a Hall-Petch-type relationship with either a λ−1 or λ-1/2 relationship.•Thermo-mechanical treatments transform the lamellar eutectic into equi-axed two-phase grain structures, leading to either increases or decreases in strength.•Traditional models which relate strength and ductility to the lamellae spacing of the hard and soft phases in mild steels can explain the trade-off between increased yield strength and reduced tensile ductility in eutectic MCPAs.
A systematic study of the effects of up to 1.1 at. % carbon on the mechanical properties and evolution of the dislocation substructure in a series of a high entropy alloys (HEA) based on ...Fe40.4Ni11.3Mn34.8Al7.5Cr6 is presented. Transmission electron microscopy (TEM), synchrotron X-ray diffraction (XRD) and atom probe tomography (APT) were used to show that all the alloys are single-phase f.c.c. random solid solutions. The lattice constant, determined from synchrotron XRD measurements, increases linearly with increasing carbon concentration, which leads to a linear relationship between the yield strength and the carbon concentration. The dislocation substructures, as determined by a TEM, show a transition from wavy slip to planar slip and, at higher strains, and from cell-forming structure (dislocations cells, cell blocks and dense dislocation walls) to non-cell forming structure (Taylor lattice, microbands and domain boundaries) with the addition of carbon, features related to the increase in lattice friction stress. The stacking fault energy (measured via weak-beam imaging of the separation of dislocation partials) decreases with increasing carbon content, which also contributes to the transition from wavy slip to planar slip. The formation of non-cell forming structure induced by carbon leads to a high degree of strain hardening and a substantial increase in the ultimate tensile strength. The consequent postponement of necking due to the high strain hardening, along with the plasticity accommodation arising from the formation of microbands and domain boundaries, result in an increase of ductility due to the carbon addition.
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The microstructures and mechanical properties of both undoped and carbon-doped (1.26at%) f.c.c./B2 Fe36Ni18Mn33Al13 multi-component alloys have been investigated in the as-cast, annealed and ...recrystallized states. A lamellar structure is present in the undoped alloy, while a tetragonal martensite with irregular shape is present in the carbon-doped alloy. B2-structured precipitates form upon annealing in both alloys, whose size and volume fraction increases with increasing annealing time. Lamellar coarsening also occurs during annealing of the undoped alloy. Two different thermo-mechanical treatments were applied to the carbon-doped alloy in order to decrease the grain size and disperse the martensite, which produced a significant increase in strength. The changes in yield strength are discussed in terms of the underlying strengthening mechanisms, i.e., phase boundary strengthening, grain boundary strengthening, interstitial strengthening, and precipitation strengthening. The carbon addition results in a sharp increase in ductility of the as-cast alloy, a feature ascribed to microband formation during the tensile test arising from the increase of lattice friction stress. Similar to some single-phase f.c.c. alloys, microband-induced plasticity (MBIP) effect is found to present in a two-phase multicomponent alloy in this study.
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Abstract
Deformation twinning is rarely found in bulk face-centered cubic (FCC) alloys with very high stacking fault energy (SFE) under standard loading conditions. Here, based on results from bulk ...quasi-static tensile experiments, we report deformation twinning in a micrometer grain-sized compositionally complex steel (CCS) with a very high SFE of ~79 mJ/m
2
, far above the SFE regime for twinning (<~50 mJ/m
2
) reported for FCC steels. The dual-nanoprecipitation, enabled by the compositional degrees of freedom, contributes to an ultrahigh true tensile stress up to 1.9 GPa in our CCS. The strengthening effect enhances the flow stress to reach the high critical value for the onset of mechanical twinning. The formation of nanotwins in turn enables further strain hardening and toughening mechanisms that enhance the mechanical performance. The high stress twinning effect introduces a so far untapped strengthening and toughening mechanism, for enabling the design of high SFEs alloys with improved mechanical properties.
Seasonal characteristics of atmospheric nitrous acid (HONO) were investigated with high time-resolution field measurements at an urban site of Beijing in four select months (representing four ...different seasons) from September 2015 to July 2016. The HONO concentrations displayed a pronounced seasonal profile with a maximum in autumn (2.27±1.82ppb) and a minimum in winter (1.05±0.89ppb). Significant diurnal cycles were also observed during the whole campaign. We found that the nighttime build-up of HONO was attributed to the heterogeneous conversion of NO2 on wet surface. The calculated NO2 to HONO conversion frequencies varied from 0.005h−1 in spring to 0.010h−1 in summer, with an average value of 0.008h−1. The seasonality of these conversion frequencies was closely related to the RH levels in different seasons. During daytime, large additional HONO sources were calculated. The noontime additional source was the highest in autumn 3.82ppbh−1, followed by summer 3.05ppbh−1, spring 2.63ppbh−1 and winter 1.30ppbh−1. Correlation studies between the additional HONO source and related parameters demonstrated that the controlling processes responsible for HONO daytime formation varied in different seasons, and that the photo-enhanced formation on wet surface or the photolysis of adsorbed nitric acid and nitrate could be potential HONO sources in Beijing.
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•Atmospheric HONO has been observed in Beijing over four different seasons.•Seasonality of HONO conversion frequencies was closely related to RH levels.•Daytime HONO formation was mainly controlled by the additional HONO source.•Correlation studies evidenced the key role of JNO2 for HONO formation.•Photolysis of adsorbed HNO3 and NO3− could be potential HONO source in autumn.
Measurements of speciated atmospheric mercury play a key role in identifying mercury behavior in the atmosphere. In this study, we measured speciated atmospheric mercury, including gaseous elemental ...mercury (GEM), reactive gaseous mercury (RGM), and particulate bound mercury (PBM) (<2.5 μm), in 2015 and 2016 at an urban site in Beijing, China. The mean concentrations of GEM, RGM, and PBM were 4.70 ± 3.53 ng m−3, 18.47 ± 22.27 pg m−3, and 85.18 ± 95.34 pg m−3, respectively. The concentration of PM2.5 significantly affected the distribution of reactive mercury between the gaseous and particulate phases. With the raising of PM2.5 levels, PBM concentrations increased, on the contrary, the concentrations of RGM decreased gradually. The mean concentration of PBM during air-pollution events was more than three times that during clear days. During days with air pollution, the relative humidity significantly affected the gas-particle partitioning of reactive mercury. The linear relationships between gas-particle partitioning coefficient and meteorological factors (air temperature and relative humidity) were obtained over the four seasons. The data also showed that the gas-particle partitioning coefficient of reactive mercury was related to particle composition (e.g., Cl−, BC). The data present in this paper suggested the influence of anthropogenic emissions on reactive mercury in Beijing urban. And the findings will contribute to understand the gas-particle partitioning of reactive mercury and its influencing factors with complex urban pollution.
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•Three speciated atmospheric mercury exhibited obviously seasonal and diurnal variations.•Increasing PM2.5 led to a decrease in RGM levels and an increase in PBM levels.•Temperature, relative humidity, and particle composition were important factors in determining the reactive mercury gas-particle partitioning.
PM2.5 levels influenced the RGM and PBM concentrations, and the important factors that affect the reactive mercury gas-particle partitioning were discussed.
In this work, the dynamic response at a similar high strain rate of a series of Mo-doped CoCrNi medium entropy alloys, i.e., CoCrNiMox (x = 0, 0.1, 0.2), was investigated via a split Hopkinson ...pressure bar (SHPB). We found that with the Mo amount increasing from 0 to 0.2, the yield strength nearly doubles, from ∼450 to ∼800 MPa, under such a high strain rate. Detailed microscopy investigations revealed that the strengthening from solid solution and precipitation, together with the high work hardening rate via micro-banding and nano-twinning/stacking faulting, contribute to strength increment and better work hardening ability. After dynamic compression, the sample with the more Mo was found to have the higher dislocation density and the more frequent nanotwin/stacking fault events. This suggests that Mo plays a critical role in both dislocation accumulation (planar slip) and dislocation dissociation (stacking fault energy) during the ultra-fast plastic flow, which endows the Mo0.2 alloy a potential for application in impact protection.
•Effects of Mo-alloying on dynamic deformation behavior and microstructure evolution of in CoCrNiMox MEAs were investigated.•Under a strain rate of 1000 s-1, the yield strength nearly doubles with the Mo amount increasing from 0 to 0.2.•The solid-solutioned and supersaturated Mo synergistically contribute to the better strength and work-hardening ability.
The emission of mercury (Hg) from cropland soil greatly affects the global Hg cycle. Combinations of different crop cultivars and planting densities will result in different light transmittance under ...canopies, which directly affects the solar and heat radiation flux received by the soil surface below crops. In turn, this might lead to differences in the soil–air total gaseous mercury (TGM) exchange under different cropping patterns. However, soil–air TGM exchange fluxes in croplands under differing canopies have been poorly investigated. Here, a one-year observation of TGM exchange flux was conducted for cropland soils covering five different crop cultivars and three planting densities in North China Plain using the dynamic flux chamber method. The results showed that light transmittance under the canopies was the key control on soil–air TGM exchange fluxes. High light transmittance can enhance soil TGM emission rates and increase the magnitude of diurnal variations in soil–air TGM exchange fluxes. Furthermore, we found that there were piecewise–function relationships (Peak function–constant equation) between light transmittance under the different canopies and the numbers of days after crop sowing. The soil–air TGM exchange fluxes showed a parabolic response to changes in light transmittance under the different canopies. A second-order model was established for the response relationship between soil–air TGM exchange flux and soil Hg concentration, total solar radiation above the canopy, and numbers of days after sowing. The estimated annual average soil–air TGM exchange flux was 5.46 ± 21.69 ng m−2 h−1 at corn–wheat rotation cropland with 30 cm row spacing using this second-order model. Our results might a data reference and a promising foundation for future model development of soil–air TGM exchange in croplands under different crop cultivars and planting densities.
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•Seasonal and diurnal patterns in soil–air mercury fluxes varied with different crops canopies.•Light transmittance under the canopies was the key control on soil–air mercury exchange fluxes.•A second-order model was established for different crop cultivars and planting densities soil–air mercury exchange flux.•The application of result may help to improve the accuracy of soil mercury emission estimation by satellite image inversion.
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