We investigated the grain refinement effect on the resistance to hydrogen embrittlement in an equiatomic CoCrFeMnNi high-entropy alloy. In a 100 MPa hydrogen gas pre-charging condition, grain ...refinement to 1.9 μm improved the hydrogen embrittlement resistance in terms of the strength-elongation balance. In particular, the tensile strength of the grain refined high-entropy alloy did not decrease by hydrogen uptake. However, when the σ phase formed at a relatively low temperature, both the strength and elongation decreased due to hydrogen charging.
•Grain refinement below 2 μm was achieved by cold rolling and annealing in the HEA.•Hydrogen embrittlement resistance was evaluated by 100 MPa-H gas pre-charging.•Grain refinement improved hydrogen embrittlement resistance.•σ phase formation increases hydrogen embrittlement susceptibility.•γ/σ interface cracking occurs when hydrogen is introduced.
Fatigue failures create enormous risks for all engineered structures, as well as for human lives, motivating large safety factors in design and, thus, inefficient use of resources. Inspired by the ...excellent fracture toughness of bone, we explored the fatigue resistance in metastability-assisted multiphase steels. We show here that when steel microstructures are hierarchical and laminated, similar to the substructure of bone, superior crack resistance can be realized. Our results reveal that tuning the interface structure, distribution, and phase stability to simultaneously activate multiple micromechanisms that resist crack propagation is key for the observed leap in mechanical response. The exceptional properties enabled by this strategy provide guidance for all fatigue-resistant alloy design efforts.
In this study, the effects of austempering treatment on the microstructure, mechanical properties, and fracture mechanisms of Fe-0.4C-1.5Si-1.5Mn transformation-induced plasticity (TRIP)-aided ...bainitic ferrite (TBF) steel were investigated. When the austempering time increased from 0 to 3600 s, the volume fraction of retained austenite and its carbon concentration initially increased and then decreased. The increase in carbon concentration during austempering may be attributed to the formation of fine bainitic ferrite, representing the matrix and in turn the formation of retained austenite with a film-type morphology. When the austempering time was increased from 0 to 1000 s, the TBF steel showed excellent mechanical properties such as high yield and tensile strengths, and uniform and total elongations. Meanwhile, void density increased significantly in the initial stages and then slightly decreased; in contrast, the trend observed for mean crack length was opposite to that of void density leading to the change in the fracture mode of the TBF steels from intergranular to quasi-cleavage and dimple fractures. It could be inferred that the observed improvement in mechanical properties, especially the good ductility of TBF steels, at the optimal austempering time is due to the suppression of crack propagation and crack blunting to form voids owing to the effective deformation-induced transformation of metastable retained austenite.
•Effect of autempering times of TRIP-aided steel was investigated.•Microstructural and fracture behaviors at different autempering times were presented.•Transformation of metastable retained austenite improved work-hardening rate.•Transformation of retained austenite promoted crack blunting to form voids.
Intergranular fracture is the primary fracture mode when a significant amount of hydrogen is introduced into lath martensitic steels. When the introduced hydrogen content is reduced, the contribution ...of plasticity in the intergranular fracture is significant. Specifically, the crack path is still along prior austenite grain boundaries, but there are some signs of activation of the ductile mechanism (
i.e.,
the tear ridges) on the fracture surface. The plasticity-assisted fracture mode is classified as an intergranular-like fracture. To characterize the difference between intergranular and intergranular-like fractures, these two hydrogen-assisted fracture modes were analyzed in terms of crack path, crack morphology, and local plasticity evolutions. Intergranular fracture occurred in the macroscopically elastic regime with a significant portion of smooth regions on the fracture surface. The intergranular crack growth was discontinuous at the grain size scale, and the preferential microstructure preventing the crack growth was triple junctions of the prior austenite grain boundary. For intergranular-like fracture, plasticity significantly evolved during crack growth, which caused nano-voids in the vicinity of the crack tip. Coalescence of the crack and nano-voids resulted in intergranular-like crack growth along prior austenite grain boundaries. The coalescence with nano-voids caused tear ridges on the intergranular-like fracture surface.
Elastoplastic phenomena, such as plastic deformation and failure, are multi-scale, deformation-path-dependent, and mechanical-field-sensitive problems associated with metals. Accordingly, ...visualization of the microstructural deformation path under a specific mechanical field is challenging for the elucidation of elastoplastic phenomena mechanisms. To overcome this problem, a dislocation-resolved in-situ technique for deformation under mechanically controllable conditions is required. Thus, we attempted to apply electron channeling contrast imaging (ECCI) under tensile loading, which enabled the detection of lattice defect motions and the evolution of elastic strain fields in bulk specimens. Here, we presented the suitability of ECCI as an in-situ technique with dislocation-detectable spatial resolution. In particular, the following ECCI-visualized plasticity-related phenomena were observed: (1) pre-deformation-induced residual stress and its disappearance via subsequent reloading, (2) heterogeneous dislocation motion during plastic relaxation, and (3) planar surface relief formation via loading to a higher stress.
The effects of mechanical loading on hydrogen uptake were investigated by thermal desorption experiments under various combinations of hydrogen charging and loading conditions. The diffusible ...hydrogen content increased with increasing elastic and plastic strains. The increase in diffusible hydrogen content per elastic strain was larger than that per plastic strain.
•Hydrogen embrittlement resistance in austenitic steels requires compositional complexity.•ε-martensite has higher resistance to hydrogen embrittlement than ά-martensite.•The low diffusivity of ...ε-martensite can be a reason for the superior hydrogen-resistance.•Ductility of ε-martensite is another key for developing ε-stability-based alloy design.•The ε-stability-based alloy design strategy is compatible to high-entropy concept.
The main factors affecting resistance to hydrogen-assisted cracking are hydrogen diffusivity and local ductility. In this context, we note fcc (γ) to hcp (ε) martensitic transformation, instead of γ to bcc (ά) martensitic transformation. The γ-ε martensitic transformation decreases the local hydrogen diffusivity, which thereby can increase strength without critical deterioration of hydrogen embrittlement resistance. Furthermore, ε-martensite in a high-entropy alloy is extraordinary ductile. Consequently, the metastable high-entropy alloys showed lower fatigue crack growth rates under a hydrogen effect compared with those of conventional metastable austenitic steels such as type 304.
This paper presents an overview of the recent works on dynamic strain aging (DSA) of Fe–Mn–C austenitic steels including Hadfield and twinning-induced plasticity (TWIP) steels. First, a model of the ...DSA mechanism and its controlling factors are briefly explained in terms of Mn–C coupling and dislocation separation. Then, we introduce the effects of DSA on mechanical properties such as work hardening capability, uniform elongation, post-uniform elongation, and fatigue strength. Specifically, we note (1) the pinning effect on extended dislocation for the work hardening, (2) the Poretvin–Le Chatelier banding effect on damage evolution for the tensile elongation, and (3) the crack tip hardening/softening effect on crack resistance for the fatigue strength. We believe that this overview will help in designing advanced high-strength steels with superior ductility and fatigue resistance.
In the present work, we clarify the fracture mechanisms in a dual-phase (DP) steel consisting of ∼75% martensite and exhibiting tensile strength of 1.5 GPa. Generally, the DP steels are considered ...resistant to brittle fracture due to the crack-arresting behavior of the ferrite. However, contrary to conventional wisdom, we report a predominant brittle fracture in the DP steel and clarify the associated damage mechanisms. Crack initiation occurs via martensite cracking along the prior austenite grain boundaries. The crack propagation in ferrite primarily occurs via {100} brittle cleavage cracking. Occasional void coalescence is also observed in ferrite. The brittle fracture observed in martensite is a mixed mode fracture consisting of both intergranular fracture and transgranular cleavage cracking along {110} planes. Furthermore, it is observed that the morphology of the martensite crack (sharp and blunt crack) influences ferrite cleavage cracking. The ferrite damage mechanisms associated with the sharp and blunt crack have been elucidated. Finally, the micro mechanism for the occasional ductile ferrite fracture has been explained.
Hydrogen embrittlement was observed in an Fe–18Mn–1.2C (wt.%) steel. The tensile ductility was drastically reduced by hydrogen charging during tensile testing. The fracture mode was mainly ...intergranular fracture, though transgranular fracture was also partially observed. The transgranular fracture occurred parallel to the primary and secondary deformation twin boundaries, as confirmed by electron backscattering diffraction analysis and orientation-optimized electron channeling contrast imaging. The microstructural observations indicate that cracks are initiated at grain boundaries and twin boundaries.
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