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In this work, a copper–brass hetero-laminate with dual-heterostructured interfaces was fabricated by diffusion welding in combination with cold deformation and annealing. These dual-heterostructured interfaces possess the characteristics of both gradient interface and sharp interface. The evolutions of geometrically necessary dislocations (GNDs) and local strains near these dual-heterostructured interfaces were investigated by in-situ and quasi in-situ tests. By employing a constitutive model, the evolutions of GND density and the HDI synergistic strengthening mechanism were systematically explored. The results demonstrated that the distribution of GNDs and local strains near the coarse/fine-grained interface exhibited a gradient feature, without any obvious concentrations of GNDs and strains, which can be attributed to the transition effect of the gradient interface. Furthermore, the density of sample-scale GNDs accumulated near the coarse/fine-grained interface was found to be at least one order of magnitude lower than that of grain-scale GNDs accumulated near grain boundaries. Both the constitutive model and experimental results confirmed that grain-scale GNDs serve as the primary origins of significant HDI stress, rather than the sample-scale GNDs. • GND density and local strains distribute in gradient manner near the coarse/fine-grained interface. • Numerous grain-scale GNDs accumulate around GBs to accommodate the strain gradient between adjacent grains. • A small number of sample-scale GNDs accumulate around the coarse/fine-grained interface. • HDI strengthening originates largely from the accumulation of grain-scale GNDs, rather than the sample-scale GNDs. [ABSTRACT FROM AUTHOR]

• The WNZ re-precipitates η′ and η particles and forms the PFZ in the vicinity of grain boundaries. • Intergranular microcracks initiate from the WNZ due to the local strain incompatibility. • Low-cycle fatigue failure occurs at the WNZ instead of the LHZ. High-temperature ratcheting fatigue behavior of a friction stir welding (FSW) Al-Zn-Mg-Cu alloy was investigated. The substructured grains in the weld nugget zone (WNZ) undergo continuous dynamic recrystallization (CDRX) during ratcheting fatigue. The WNZ re-precipitates intragranular η′ and η particles and forms the precipitate free zone (PFZ) in the vicinity of the grain boundaries. At high mean stress or stress amplitude, ratcheting failure with obvious necking takes place due to large ratcheting strain. At low stress levels, intergranular fatigue microcracks initiate from grain boundaries of the WNZ due to the local strain incompatibility between the PFZ and grain interiors. The fatigue microcracks coalesce along grain boundaries and propagate via typical striation formation mechanisms. Low-cycle fatigue failure occurs at the WNZ instead of the low hardness zone (LHZ). [ABSTRACT FROM AUTHOR]

During tensile deformation, nanostructured (NS) metals often fail soon after yielding by forming a localized shear band. Here we report the observation of high density of shear bands that are homogeneously dispersed in the NS layer of a gradient Ni sample. These shear bands were nucleated at early elastic/plastic strain stage, reached number saturation at ∼3% strain, and remained arrested by the central coarse-grained (CG) matrix during the entire plastic deformation, resulting in a uniform tensile plasticity comparable to that of CG matrix. The formation of dispersed shear bands was promoted by the elastic/plastic interaction between NS surface layer and CG matrix, and affected by the surface roughness and the hardness variation in the NS surface layer. The width of shear bands remained constant, but the intensity of strain accumulation increased almost linearly with applied tensile strain, suggesting a stable shear banding process. Microstructure examination revealed that the strain in shear bands was accommodated by mechanically driven grain boundary migration and grain coarsening. These results clarify the fundamental questions: why/how does the NS layer supported by CG matrix achieve large uniform elongation? Moreover, the findings demonstrate the possibility of activating dispersed stable shear bands by synthesizing gradient architecture for optimized mechanical performances, i.e., a new strategy for evading the strength-ductility tradeoff in NS metals. Image 1 • Questions 'why/how does the NS layer supported by CG matrix achieve large uniform elongation?' are clarified. • Dense shear bands are homogeneously dispersed in the NS layer of gradient Ni. • Shear bands evolved stably during deformation, resulting in large uniform elongation. • Elastic/plastic interaction between NS and CG layers promoted the nucleation of shear bands. • Strain in shear bands is accommodated by mechanically driven grain coarsening. [ABSTRACT FROM AUTHOR]

• An optimum grain size (d o p t i m u m) on the order of a few micrometers exist ubiquitously for strength-ductility synergy, at which the strain energy density limit reaches a maximum while maintaining high yield strength. • Theoretical models on the grain size-dependence of uniform elongation and ultimate strength are developed, which accurately predict the d o p t i m u m. • The d o p t i m u m ≈ 2 l G b a r , where l G b a r ≈ k H P R 3 / 2 2 M G b is the characteristic width of grain boundary affected region. • The d o p t i m u m is the critical grain size having the strongest intragranular plastic strain gradient effect. • The d o p t i m u m is the limiting dimension that allows toughening by grain refinement. A strength-ductility trade-off usually occurs when grains are refined to increase strength. A question arises on if there exists a grain size for the best strength-ductility combination, i.e., with the highest possible strain energy density limit and strength simultaneously. This issue is crucial for guiding the design of strong and tough structural materials. Here we reveal an optimum grain size (d o p t i m u m) on the order of a few micrometers, at which the strain energy density limit, estimated as the product of strength and uniform elongation, reaches a maximum while maintaining reasonably high yield strength. The d o p t i m u m is found to exist in a series of single-phase FCC, BCC and HCP materials, indicating it as a universal phenomenon. Theoretical models on the grain size-dependence of uniform elongation and ultimate strength are developed by considering dislocation accumulation in grain boundary affected region (Gbar) and grain interior based on the classical Kocks-Mecking-Estrin model. Combined with the Hall-Petch relationship, the models accurately predict the d o p t i m u m. Importantly, the models disclose this d o p t i m u m to be close to twice of the characteristic width of Gbar (l G b a r), suggesting that it is exactly at or near the critical grain size with the strongest intragranular strain gradient effects. [Display omitted] [ABSTRACT FROM AUTHOR]

Due to its characteristic of low stress brittle fracture, hydrogen embrittlement (HE) is a great challenge for the alloys exposed to hydrogen-containing environments, threatening the safety and integrity of structural components. The physical and chemical status of the interfaces, among which grain boundary (GB), twin boundary (TB), and matrix/nano-precipitate interfaces (coherent, semi-coherent, and incoherent) are the representative ones, play a crucial role in determining the HE susceptibility of materials. Hence, this study mainly reviews recent progress in the interaction between hydrogen and these interfaces, i.e., 1) hydrogen-GB interaction (dominant HE mechanisms, crystallographic features of hydrogen-assisted intergranular cracking, and the strategies for resisting HE through GB segregation and GB engineering); 2) hydrogen-TB interaction (the effect of deformation/pre-existing twins on HE susceptibility, four types of TB-related cracking mechanisms, and the improvement of HE-tolerance by the control of pre-twins, gradient-twins, and twin orientations); and 3) hydrogen-precipitate interaction (hydrogen capacity, hydrogen trapping sites, hydrogen activation energy, and the effect of nano-precipitates on HE of alloys). The correlation between HE susceptibility, active HE mechanisms and their synergy (HELP + HEDE model), and three types of interfaces have been comprehensively summarized and discussed. Also, the strategies for the improvement of HE resistance are proposed in terms of the control of these microstructural interfaces in metallic alloys. [Display omitted] • Effect of the interfaces on hydrogen embrittlement (HE) of alloys is reviewed. • Hydrogen-assisted interface cracking mechanisms of alloys is summarized. • Strategies for resisting HE are proposed through the control of the interfaces. • Semi-coherent precipitates are suggested to balance strength and HE-resistance. [ABSTRACT FROM AUTHOR]

• Inherent ductile propagation is attributed to stacking faults in lower monocrystal. • Twist grain boundary can significantly reduce the ductility of crack propagation. • Contact ratio and driving force affect stresses for cleavage and stacking nucleation. • Time-dependent factor is introduced to explain variations of critical stresses. • Contact ratio and large driving force have little effect on final ductile level. Due to alternative exchange between single and double-teeth meshings, the upper part of gear tooth is subjected to dynamic tensile stress whose growth rate presents rectangular fluctuation. This work investigates the atomistic scale behaviors of intergranular crack propagation along twist grain boundary in body-centered cubic (bcc) iron under dynamic tensile stress. The effects of driving force and contact ratio are fully discussed. Results show that only stacking faults with face-centered cubic (fcc) atoms can be formed in lower monocrystal portion. Edge dislocations in upper monocrystal portion are suppressed by intergranular crack cleavage. Critical stresses for stacking fault nucleation and intergranular crack cleavage vary with driving force and contact ratio. By calculating actual stress intensity factor at crack tip, variations of critical stresses are found to be attributed to the variations of time-dependent factors. Although critical stresses vary with driving force and contact ratio, the effects of these factors on the growths of crack length and plastic zone are not obvious in the early stage of intergranular crack propagation. Accumulated plastic strain energy before intergranular crack cleavage is independent of driving force and contact ratio. Departing from the early stage, the growth rates of crack length and plastic zone increase significantly with an increase in driving force or a decrease in contact ratio. However, the final ductile level of intergranular crack propagation cannot vary with contact ratio and large driving force. By applying dynamic load, this work can be used to reveal the atomistic scale mechanism of gear failure. The results can provide a good reference for gear safety design. [ABSTRACT FROM AUTHOR]

[Display omitted] • Special grain boundary can be eliminated by twinning and dislocation activities. • All twinning bands are formed by atomic slip along ordinary twinning direction. • All dislocations are nucleated by atomic slip along anti-twinning direction. • Grain boundary elimination enhances resistance to intergranular fracture. • Conditions for grain boundary elimination are proposed to improve grain boundary design. Grain boundary structures with high resistance to intergranular fracture are the target of grain boundary design. In this work, grain boundary elimination in front of crack tip is observed in two special bcc iron bicrystals through molecular dynamics simulations under mode I loading. Grain boundary elimination depends on crack advance direction and enhances resistance to intergranular fracture. Direction-dependent elimination leads to directional anisotropy of intergranular crack propagation. By analytical analysis and molecular dynamics simulation, grain boundary elimination is found to be attributed to the activities of twinning and dislocation. All twinning bands are formed by atomic slip along ordinary twinning direction, but all dislocations are nucleated by atomic slip along anti-twinning direction. Mechanisms of twinning formation and dislocation nucleation are revealed by calculating energy barriers of atomic slip and shear stress field. According to the mechanism of grain boundary elimination, conditions for grain boundary elimination are proposed to find all special grain boundaries. Results show that twist grain boundaries cannot be eliminated under mode I loading, while tilt grain boundaries with axes of 〈 110 〉 can. This work provides a good reference for grain boundary design. [ABSTRACT FROM AUTHOR]

Effect of heat treatment on hydrogen-assisted fracture behavior of PH13-8Mo steel was investigated. The results show that hydrogen embrittlement susceptibility of the specimens aged at 600 °C is the lowest compared with that of the specimens aged at 430 °C and 540 °C at the same aging time. With an increase in aging time, the susceptibility to hydrogen embrittlement increases for specimens aged at 430 °C while it decreases for specimens aged at 540 °C and 600 °C. Hydrogen-assisted cracks initiate and propagate along lath and prior austenite grain boundaries. In addition, the quantitative relationship between hydrogen embrittlement index and fracture mode is given. [ABSTRACT FROM AUTHOR]

The effect of hydrogen on mechanical properties and failure mechanism of a CoCrFeMnNi high-entropy alloy was evaluated by in/ex-situ tensile tests and electron backscatter diffraction. The results indicate that yield strength first decreases and then increases as hydrogen charging time increases, which is attributed to the competition between the hydrogen-induced softening effect and the hydrogen-enhanced twinning formation effect. The hydrogen-uncharged sample shows the micro-void coalescence failure mechanism, whereas hydrogen-assisted cracking of the alloy initiates from grain boundaries and slip bands caused by plasticity-mediated decohesion mechanism. Crystallographic analysis demonstrates that {110}//ND grains and {001}//ND-{111}//ND grain boundaries are vulnerable to hydrogen embrittlement. • Variation of yield strength of the HEA depends on hydrogen charging time. • Hydrogen-assisted cracking in the HEA initiates from grain boundaries and slip bands. • {110}//ND grain and {001}//ND-{111}//ND grain boundary are vulnerable to hydrogen damage. • A strategy for designing HE-resistance HEAs is proposed. [ABSTRACT FROM AUTHOR]