Direct observation of local chemical ordering in a few nanometer range in CoCrNi medium-entropy alloy by atom probe tomography and its impact on mechanical properties

ミディアムエントロピー合金の局所規則構造の原子レベル観察に成功 --新しい高強度合金の設計・開発に期待--. 京都大学プレスリリース. 2021-08-30.Local chemical ordering in CoCrNi medium-entropy alloy (MEA) was directly observed by the use of atom probe tomography. It was found that the densities of Cr, Co, and Ni were almost the same along the [111] direction, while those along the [001] direction were modulated to take a slightly enhanced and depleted value alternately within approximately 10 atomic layers corresponding to about 2 nm. The degree of modulation of Co and Ni was stronger than that of Cr. It was suggested that Cr-rich {001} atomic layers and (Ni + Co)-rich {001} layers tended to align mutually in the face-centered-cubic CoCrNi solid solution alloy. The mechanical properties of the MEA was found not to be affected by the presence of the local chemical ordering

Effect of hydrogen on evolution of deformation microstructure in low-carbon steel with ferrite microstructure

In this study, the deformation microstructure of hydrogen-charged ferritic-pearlitic 2Mn-0.1C steel was characterized using SEM-BSE, SEM-EBSD, TEM, and neutron diffraction. The microscopic mechanism of hydrogen-related quasi-cleavage fracture along the {011} planes was also discussed. It was found that hydrogen increased the relative velocity of screw dislocations to edge dislocations, leading to a tangled dislocation morphology, even at the initial stage of deformation (e = 3%). In addition, the density of screw dislocations at the later stage of deformation (e = 20%) increased in the presence of hydrogen. Based on the experimental results, it is proposed that a high density of vacancies accumulated along {011} slip planes by jog-dragging of screw dislocations, and coalescence of the accumulated vacancies led to the hydrogen-related quasi-cleavage fracture along the {011} slip planes

Crystallographic analysis of fatigue fracture initiation in 8Ni-0.1C martensitic steel

The present paper investigated characteristics of fatigue fracture behavior, particularly initiation stage of fatigue fracture (Stage I), in an as-quenched martensitic steel from microstructural and crystallographic points of view. The detailed crystallographic orientation analysis using EBSD revealed that block boundaries in lath martensite structure were the most preferential initiation sites for fatigue cracks. We found that incompatibility of plastic strains between adjacent blocks was the origin for the formation of initial fatigue cracks at block boundaries. Moreover, plastic deformation along {0 1 1} slip planes also played an important role on the transgranular crack propagation

Transcript levels of the nuclear-encoded respiratory genes in rice decrease by oxygen deprivation: evidence for involvement of calcium in expression of the alternative oxidase 1a gene

AbstractWe investigated the effect of oxygen on the expressions of respiratory genes encoded in the nuclear and mitochondrial genomes of rice (Oryza sativa L.). Hypoxic treatment decreased the transcript levels of nuclear-encoded, but not mitochondrial-encoded respiratory genes. The effects of ruthenium red (an inhibitor of Ca2+ fluxes from organelles) and/or CaCl2 on plants under hypoxic conditions suggested that Ca2+ is a physiological transducer of a low-oxygen signaling pathway for expression of the alternative oxidase 1a gene (AOX1a), but not for expressions of genes involved in the cytochrome respiratory pathway, in rice

Comparative Analysis of Plastic Flow and Grain Refinement in Pure Aluminium Subjected to Simple Shear-Based Severe Plastic Deformation Processing

In the present work, effects of loading scheme and strain reversal on structure and hardness evolution have been studied by using high pressure torsion (HPT) and twist extrusion (TE) techniques. High purity aluminium (99.99%) was processed at room temperature up to a maximum total equivalent strain of ε[max] ≈ 8 by TE, and HPT in monotonic and reversal deformation modes with strain increment Δε[max] = 1. Minimum subgrain sizes reached in this study were 1.6 µm for TE and 1.1 µm for HPT. It was revealed that microstructural change with straining was a common consequence of severe plastic deformation (SPD) processing and was not affected significantly by the loading scheme. Among the SPD methods used in this study, HPT in monotonic regime produced the smallest grain size, while the most homogeneous microstructure was obtained by TE due to specific vortex-like flow field imposed by the tool geometry

Significant Bauschinger effect and back stress strengthening in an ultrafine grained pure aluminum fabricated by severe plastic deformation process

Bauschinger test in uniaxial tension-compression mode was carried out for the first time on the pure Al specimens having homogeneous ultra-fine grained (UFG) microstructures fabricated by equal-channel angular pressing (ECAP) and subsequent annealing processes. Significant Bauschinger stress (transient softening), Bauschinger energy parameter and their strong dependences on the tensile plastic pre-strain at the very early stage of the tensile deformation were measured in the UFG specimens, in sharp contrast to their coarse-grained (CG) counterpart. The grain size dependence of the Bauschinger effect in pure Al was qualitatively discussed in terms of the back stress arising from the formation of dislocation pile-up against the grain boundary during plastic deformation

Global understanding of deformation behavior in CoCrFeMnNi high entropy alloy under high-strain torsion deformation at a wide range of elevated temperatures

A number of recent studies have investigated deformation behavior of CoCrFeMnNi (Cantor) alloy at elevated temperatures by using plastic deformation to relatively small strains such as tensile testing. Therefore, little has been known about the deformation behavior of this typical FCC high-entropy alloy (HEA) in case that the material is subjected to ultra-high strains at various temperatures. In the present study, the equi-atomic CoCrFeMnNi HEA was successfully deformed over a wide range of strains (von Mises equivalent strains (ε) of 1∼5.5) by torsion at various temperatures ranging from 25 °C to 1100 °C. Deformation twinning was extensively activated at moderate to high strains (ε ≥ 1) and even found in the deformation at elevated temperatures as high as 600 °C where deformation twinning is not normally expected in Cantor alloy. The HEA showed outstanding deformability and the highest strains to fracture reached 4.0∼5.5 at low temperatures below 400 °C. The excellent deformability was attributed to the extensive twin activities including the formation of twin bundles and thin nanotwins as well as microbands formation. However, localized shear deformation that was promoted by the high straining at low temperatures could negatively affect the deformability. The heavy deformation led to a significant reduction of the grain sizes down to 50 nm∼150 nm. A sudden shortage of ductility occurred at intermediate temperatures, where small strains to fracture (1.2∼1.3) were realized at 600 °C∼700 °C. The embrittlement was accompanied by the formation of micro-voids at grain boundaries and intergranular fracture. The susceptibility to the embrittlement was caused by the precipitation of Cr-rich σ-phases at grain boundaries. Dynamic recrystallization (DRX) of the FCC matrix led to an accelerated softening at high temperatures above 600 °C. Nucleation and growth of DRX grains in Cantor alloy were not fundamentally different from those in conventional FCC alloys. This study gives an insight into the microstructure evolution and mechanical response in Cantor alloy under shear deformation over a wide range of strains and temperatures

Improvement of resistance against hydrogen embrittlement by increasing carbon segregationat prior austenite grain boundary in low-carbon martensitic steels

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Improvement of resistance against hydrogen embrittlement by controlling carbon segregation at prior austenite grain boundary in 3Mn-0.2C martensitic steels

This study challenged to improve the resistance against hydrogen embrittlement by increasing the concentration of carbon segregated at prior austenite grain boundary (PAGB), XPAGB, in low-carbon martensitic steels. The specimens with/without carbon segregation treatment (Non-seg and Seg specimens, respectively) had almost the same microstructure, other than higher XPAGB in the Seg specimen. While the uncharged Non-seg and Seg specimens exhibited similar mechanical properties, the maximum stress of the hydrogen-charged specimen was much higher in the Seg specimen than that in the Non-seg specimen even when diffusible hydrogen contents were almost the same. In addition, the fraction of intergranular fracture surface was much smaller in the Seg specimen. Based on these results, we conclude that the segregated carbon suppressed the accumulation of hydrogen around PAGB by site competition and increased cohesive energy of PAGB, leading to the significantly improved resistance against hydrogen-related intergranular fracture

In-situ observation of twinning and detwinning in AZ31 alloy

Twinning and detwinning behavior of a commercial AZ31 magnesium alloy during cyclic compression–tension deformation with a total strain amplitude of 4% (±2%) was evaluated using the complementary techniques of in-situ neutron diffraction, identical area electron backscatter diffraction, and transmission electron microscopy. In-situ neutron diffraction demonstrates that the compressive deformation was dominated by twin nucleation, twin growth, and basal slip, while detwinning dominated the unloading of compressive stresses and subsequent tension stage. With increasing number of cycles from one to eight: the volume fraction of twins at -2% strain gradually increased from 26.3% to 43.5%; the residual twins were present after 2% tension stage and their volume fraction increased from zero to 3.7% as well as a significant increase in their number; and the twinning spread from coarse grains to fine grains involving more grains for twinning. The increase in volume fraction and number of residual twins led to a transition from twin nucleation to twin growth, resulting in a decrease in yield strength of compression deformation with increasing cycles. A large number of -component dislocations observed in twins and the detwinned regions were attributed to the dislocation transmutation during the twinning and detwinning. The accumulation of barriers including twin boundaries and various types of dislocations enhanced the interactions of migrating twin boundary with these barriers during twinning and detwinning, which is considered to be the origin for increasing the work hardening rate in cyclic deformation of the AZ31 alloy