Previous work shows that a maximal recovery strain of 8.4% after being bent at room temperature is realized in annealed cast polycrystalline Fe–Mn–Si‐based alloys because of the reduced annealing ...twin boundaries. Theoretically, higher recovery strains can be expected at cryogenic deformation by reducing thermal activation. To confirm this idea, martensitic transformation behaviors and the recovery strains after being bent at 77 K are investigated in the cast Fe–(20–26)Mn–5.5Si–8.5Cr–5Ni alloys. Herein, larger recovery strains as high as ≈10.0% are realized in an annealed cast Fe–23Mn–5.5Si–8.5Cr–5Ni alloy after being bent at 77 K. The reduced thermal activation together with suppressed formation of thermal‐induced martensite by optimizing Mn content can account for the larger recovery strain. In these results, a new avenue is opened up for further enhancement of the recovery strain in Fe–Mn–Si‐based shape memory alloys.
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
An improved genetic algorithm is employed to optimize the structure of (C60)N (N≤25) fullerene clusters with the lowest energy. First, crossover with variable precision, realized by introducing the ...hamming distance, is developed to provide a faster search mechanism. Second, the bit string mutation and feedback mutation are incorporated to maintain the diversity in the population. The interaction between C60 molecules is described by the Pacheco and Ramalho potential derived from first-principles calculations. We compare the performance of the Improved GA (IGA) with that of the Standard GA (SGA). The numerical and graphical results verify that the proposed approach is faster and more robust than the SGA. The second finite differential of the total energy shows that the (C60)N clusters with N=7, 13, 22 are particularly stable. Performance with the lowest energy is achieved in this work.
Development of high‐efficiency and low‐cost electrocatalysts as the platinum substitute for the oxygen reduction reaction (ORR) is of significance for electrochemical energy conversion and storage ...devices, such as fuel cells and rechargeable batteries. Here we report graphene‐based nitrogen‐coordinated Fe−Co dual‐atom catalysts (referred to as FeCoN6) with markedly enhanced ORR activity by ligand‐modification. By density functional theory calculations, we thoroughly investigated the ORR activity of three preferred FeCoN6 isomers (denoted as FeCoN6‐I, FeCoN6‐II, and FeCoN6‐III) and their complexes with the ligands of ORR intermediates such as *O, *OH, and *O2. Our results reveal that these ligands cause the apparent shift of d‐orbitals of metal atoms toward the Fermi level to modulate the adsorption strength for reaction intermediates, thereby significantly improving the ORR activity of FeCoN6 complexes. Notably, FeCoN6‐I(OH) and FeCoN6‐I(O2) with the lowest overpotential of ∼0.23 V possessed the best ORR activity, which are superior to pristine FeCoN6 and available Pt catalysts. These results not only build on the fundamental understanding of the catalytic mechanism of ligand modified dual‐atom catalysts but also provide a new strategy to develop highly efficient ORR electrocatalysts by ligand‐modification engineering.
Inside the intermediates: The reaction intermediates of *O, *OH, and *O2 can be stably adsorbed on FeCoN6 to bring in new active centers by forming FeCoN6(O), FeCoN6(OH) and FeCoN6(O2) complexes and to boost the ORR activity. Especially, the overpotentials of FeCoN6‐I(OH) and FeCoN6‐I(O2) are as low as ∼0.23 V, showing the extraordinary ORR activity. This study provides a new strategy to developing high‐efficiency ORR electrocatalysts by ligand modification.
Full text
Available for:
FZAB, GIS, IJS, IZUM, KILJ, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK
In order to clarify the relationship among grain size, annealing twins and the shape memory effect in Fe-Mn-Si based shape memory alloys, the Fe-21.63Mn-5.60Si-9.32Cr-5.38Ni (weight %) alloy with a ...grain size ranging from 48.9 m-253.6 m was obtained by adjusting the heating temperature or heating time after 20% cold-rolling. The densities of grain boundaries and annealing twins increase with a decrease in grain size, whereas the volume fraction and width of stress-induced martensite after 9% deformation at Ms + 10 K decrease. This result indicates that grain refinement raises the constraint effects of grain boundaries and annealing twins upon martensitic transformation. In this case, the ability to suppress the plastic deformation and facilitate the stress-induced martensite transformation deteriorates after grain refinement owing to the enhancement of the constraint effects. It is demonstrated by the result that the difference at Ms + 10 K between the critical stress for plastic yielding and that for inducing martensitic transformation is smaller for the specimen with a grain size of 48.9 m than for the specimen with a grain size of 253.6 m. Therefore, the shape memory effect declined by decreasing the grain size.
In this study, it is proposed that coarsening austenitic grains is a key criterion for achieving giant recovery strains in polycrystalline Fe-Mn-Si based shape memory alloys. In order to verify the ...hypothesis, the relationship between recovery strains and austenitic grain-sizes in cast and processed Fe-Mn-Si based shape memory alloys was investigated. The recovery strain of cast Fe-19Mn-5.5Si-9Cr-4.5Ni alloy with the coarse austenitic grains of 652µm reached 7.7% while the recovery strain of one with the relatively small austenitic grains of 382µm was only 5.4%. Moreover, a recovery strain of 5.9%, which is the highest previously published value for solution-treated processed Fe-Mn-Si based shape memory alloys, was obtained by coarsening the austenitic grains through only solution treatment at 1483K for 360min in a processed Fe-17Mn-5.5Si-9Cr-5.5Ni-0.12C alloy. However, its recovery strain was still 5.9% after thermo-mechanical treatment consisting of 10% tensile strain at room temperature and annealing at 1073K for 30min. This happens because annealing twins play a negative role, refining the austenitic grains, limiting the recovery strains to below 6%. In summary, coarse austenitic grains enable the achievement large recovery strains by two mechanisms. Firstly, the grains are bigger, and consequently there are fewer grain boundaries, and thus their suppressive effects of grain boundaries on stress-induced ε martensitic transformation is reduced. Secondly, coarse austenitic grains are advantageous to introduce ε martensite with single orientation and reduce the collisions of different martensite colonies, especially when the deformation strain is large. As such, the ceiling of recovery strains is dependent on the austenitic grain-sizes.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Nanotwins introduced by cryogenic pre-deformation at the optimum temperature can remarkably improve both strength and ductility. In the present study, the effects of temperature on the tensile ...properties, stacking faults and mechanical twinning of Fe-17Mn-1.0Si-1.1C steel were systematically investigated to introduce mechanical nanotwins. With decreasing temperature, more stacking faults and thinner mechanical twins are produced in the solution-treated steel; however, the product of strength and ductility of the solution-treated steel is firstly increased and then decreased, achieving the greatest (about 125 GPa ▪ %) at −40°C. For the nanotwinned steel prepared by cryogenic pre-deformation, there exists an optimum pre-deformation temperature for the greatest strength-ductility combination. After introducing mechanical nanotwins at −40°C, the nanotwinned steel obtains the greatest strength-ductility combination of yield strength (1215 MPa), ultimate tensile strength (1521 MPa) and ductility (45.2%).
•Both strength and ductility are improved in nanotwinned steel.•The optimum temperature to prepare nanotwinned steel is −40°C.•σ0.2 of the nanotwinned steel prepared at −40°C is ∼1215 MPa, with a ductility of 45.2%.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Our previous work had shown that shape memory effect (SME) of FeMnSiCrNi alloys can be remarkably improved by aligning Cr23C6 particles through introducing directional interfaces of austenite and ...deformation-induced martensite at room temperature before ageing. In present paper, we investigated the effects of ageing after pre-deformation at 203 K and room temperature on the precipitation of aligned Cr23C6 particles in an Fe15.3Mn5.8Si8.5Cr4Ni0.2C alloy to further improve the SME. The results showed that regardless of the deformation temperature, the SME improvement by the ageing was obviously higher after pre-deformation by 12% than 5% due to more aligned Cr23C6 particles. The ageing did not improve the SME more obviously after 12% pre-deformation at 203 K than room temperature although the number of directional interfaces between the austenite and martensite was higher. This abnormality could be rationalized by the more dislocations after pre-deformation at room temperature and concurrent comparable number and size of aligned Cr23C6 particles.
•The improvement of shape memory effect (SME) by ageing after pre-deformation at different temperatures was obviously higher than direct ageing.•The number and size of carbides were keys for realizing a good SME instead of their volume fraction.•The ageing of longer time would deteriorate the SME regardless of the direct ageing and the ageing after pre-deformation.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
We propose that high strain-hardening in face-centered cubic (FCC) crystals containing interstitial atoms could be achieved through the formation of nanoscale hexagonal close-packed (HCP) martensite ...with severe lattice distortions. Our experimental results showed that carbon atoms could more effectively prevent the HCP martensite from thickening and result in the formation of nano-HCP martensite. This caused a higher strain-hardening rate in FeMnSiC steel with heavy carbon than in FeMnSiCrNi steel with trace carbon though the HCP martensite transformation kinetics was almost the same in these two steels.
Display omitted
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
PDF
