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

Strain localization and hydrogen-related fracture in martensitic steels investigated by combined digital image correlation and electron backscatter diffraction

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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

Effective grain size refinement of an Fe-24Ni-0.3C metastable austenitic steel by a modified two-step cold rolling and annealing process utilizing the deformation-induced martensitic transformation and its reverse transformation

Metastable austenitic steels having ultrafine grained (UFG) microstructures can be fabricated by conventional cold rolling and annealing processes by utilizing the deformation-induced martensitic transformation during cold rolling and its reverse transformation to austenite upon annealing. However, such processes are not applicable when the austenite has high mechanical stability against deformation-induced martensitic transformation, since there is no sufficient amount of martensite formed during cold rolling. In the present study, a two-step cold rolling and annealing process was applied to an Fe-24Ni-0.3C metastable austenitic steel having high mechanical stability. Prior to the cold rolling, a repetitive subzero treatment and reverse annealing treatment were applied. Such a treatment dramatically decreased the mechanical stability of the austenite and greatly accelerated the formation of deformation-induced martensite during the following cold rolling processes. As a result, the grain refinement was significantly promoted, and a fully recrystallized specimen with a mean austenite grain size of 0.5 μm was successfully fabricated, which exhibited both high strength and high ductility

Change of Deformation Mechanisms Leading to High Strength and Large Ductility in Mg-Zn-Zr-Ca Alloy with Fully Recrystallized Ultrafine Grained Microstructures

Recently, we have found that fully recrystallized ultrafine-grained (UFG) microstructures could be realized in a commercial precipitation-hardened Magnesium (Mg) alloy. The UFG specimens exhibited high strength and large ductility under tensile test, but underlying mechanisms for good mechanical properties remained unclear. In this study, we have carried out systematic observations of deformation microstructures for revealing the influence of grain size on the change of dominant deformation modes. We found that plastic deformation of conventionally coarse-grained specimen was predominated by {0001} slip and {10–12} twinning, and the quick decrease of work-hardening rate was mainly due to the early saturation of deformation twins. For the UFG specimens, {10–12} twinning was dramatically suppressed, while non-basal slip systems containing component of Burgers vector were activated, which contributed significantly to the enhanced work-hardening rate leading to high strength and large ductility. It was clarified by this study that limited ductility of hexagonal Mg alloys could be overcome by activating unusual slip systems ( dislocations) in fully recrystallized UFG microstructures

Three-dimensional crack propagation behavior in hydrogen-related fracture of high-strength martensitic steel

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Mesoscopic nature of serration behavior in high-Mn austenitic steel

セレーション挙動の解明 --高強度・高延性を示す高Mn鋼の変形の本質に迫る--. 京都大学プレスリリース. 2020-12-25.We have thoroughly clarified the mesoscopic nature of serration behavior in a high-Mn austenitic steel in connection with its characteristic localized deformation. A typical high-Mn steel, Fe-22Mn-0.6C (wt. %), with a face centered cubic (FCC) single-phase structure was used in the present study. After 4 cycles of repeated cold-rolling and annealing process, a specimen with a fully recrystallized microstructure having a mean grain size of 2.0 μm was obtained. The specimen was tensile tested at room temperature at an initial strain rate of 8.3 × 10−4 s−1, during which the digital image correlation (DIC) technique was applied for analyzing local strain and strain-rate distributions in the specimen. Obtained results indicated that a unique strain localization behavior characterized by the formation, propagation and annihilation of deformation localized bands, so-called Portevin–Le Chatelier (PLC) bands, determined the global mechanical response appearing as serration on the stress-strain curve. In addition, the in-situ synchrotron XRD diffraction during the tensile test was utilized to understand what was happening in the material with respect to the PLC banding. Lattice strain of (200) plane nearly perpendicular to the tensile direction dropped when every PLC band passed through the beam position, which indicated a stress relaxation occurred inside the PLC band. At the same time, the dislocation density increased drastically when the PLC band passed the beam position, which described that the material was plastically deformed and work-hardened mostly within the PLC band. All the results obtained consistently explained the serration behavior in a mesoscopic scale. The serration behavior on the stress-strain curve totally corresponded to the formation, propagation and annihilation of the PLC band in the 22Mn-0.6C steel, and the localized deformation, i.e., the PLC banding, governed the characteristic strain hardening of the material