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To investigate the effects of hydrogen (H) on the mechanical properties of 316L austenitic stainless steel (316L) additively manufactured using laser-powder bed fusion (L-PBF), tensile, nanoindentation, and micropillar compression tests were performed on samples in both the as-built (AB) and H-charged (HC) conditions. Microstructural characterization revealed that the electrochemical H charging exerted marginal effects on grains, phases, and overall dislocation densities of L-PBF 316L. However, the dislocation networks inherent to L-PBF 316L appear to disentangle upon H charging and are distributed more homogeneously through the matrix. Mechanical test results showed that H charging results in a marginal reduction in hardness and strength, indicating H-induced softening, possibly due to the elastic shielding effects of H that weaken the interactions between dislocations. The elastic shielding effect and the enhanced slip planarity, facilitate the H accumulation along slip bands and twins, promote H-enhanced localized plasticity, resulting cracking when a critical H concentration is reached. This, in turn, aids in H embrittlement mechanism that results in a significant loss in ductility upon H charging. A detailed discussion of all these micromechanisms is presented. [Display omitted] [ABSTRACT FROM AUTHOR]

• A novel method to estimate hydrogen diffusivity via nanoindentation was proposed. • CoCrNi MEA shows 0.7 times higher hydrogen diffusivity than SS316L. • The high Ni content in CoCrNi MEA enhances the hydrogen diffusivity. • Hydrogen-vacancy interaction caused hardening-softening competition in CoCrNi MEA. Hydrogen diffusivity is one of the important factors affecting the susceptibility to hydrogen embrittlement of metals and alloys. Here, we proposed a novel approach to estimate hydrogen diffusivity in FCC alloys by performing nanoindentation experiments with continuous stiffness measurement. In consideration of the plastic zone size around the indentation, the relationship between hydrogen-induced hardness change and the depth from the sample surface was established for two typical FCC alloys—316L stainless steel (SS) and CoCrNi medium-entropy alloy (MEA)—subjected to hydrogen-charging and subsequent room-temperature aging. By combining with the estimation of the through-thickness distribution of hydrogen concentration, hydrogen diffusivity values were successfully determined, which agreed well with the literature data. It was revealed that CoCrNi MEA exhibited a hydrogen diffusivity ∼0.68 times higher than that in SS316L, and that hydrogen-induced softening upon long-term aging (at room temperature) was observed in CoCrNi but not in SS316L. The underlying mechanisms for the distinct hydrogen-related phenomena in CoCrNi MEA were elucidated based on the density functional theory calculations of hydrogen solution energy and vacancy formation energy. [ABSTRACT FROM AUTHOR]

The influence of hydrogen on the mechanical behavior of the CoCrFeMnNi high-entropy alloy (HEA) was examined through tensile and nanoindentation experiments on specimens hydrogenated via gaseous and electrochemical methods. Results show that the HEA's resistance to gaseous hydrogen embrittlement is better than that of two representative austenitic stainless steels, in spite of the fact that it absorbs a larger amount of hydrogen than the two steels. Reasons for this were discussed in terms of hydrogen-enhanced localized plasticity mechanism and the critical amount of hydrogen required for it. These were further substantiated by additional experiments on electrochemically charged specimens. [ABSTRACT FROM AUTHOR]

[Display omitted] The significant strengthening of selectively laser melted (SLM) 304L stainless steel that occurs due to high-pressure torsion (HPT) is examined by recourse to detailed nanomechanical and microstructural characterization. In the as-built alloy, dislocation hardening is the main strengthening mechanism. After HPT, however, the synergistic combination grain refinement and martensitic transformation dominate the strength of the alloy. In the nanocrystalline regime, the dominance of the grain boundary-mediated plasticity retards the martensite-induced hardening. [ABSTRACT FROM AUTHOR]

Hydrogen embrittlement in 304L austenitic stainless steel fabricated by laser powder-bed-fusion (LPBF) was investigated and compared to conventionally produced 304L samples with two different processing histories; casting plus annealing (CA) and CA plus thermomechanical treatment (CA-TMT). Interestingly, no significant difference in the amount of deformation-induced α′ martensite between the LPBF and CA-TMT samples was observed, suggesting that the solidification substructure in the LPBF sample enhanced the strength without promoting the harmful hydrogen embrittlement effect. These results are discussed in terms of the chemical inhomogeneity, hydrogen-assisted cracking behavior, and hydrogen diffusion and trapping in the present 304L samples. [ABSTRACT FROM AUTHOR]

This work focused on the mechanical properties and serration-involved deformation behavior of advanced alloys at 15 K. Evolution of stacking faults and ε -martensite improved the mechanical performance of CoCrNi alloys, and significant strain-induced martensite transformation of DED-SS316L led to superior strength and strain hardening. A magnitude in stress drop was governed by dislocation density, phase type, and lattice defects, irrespective of processing method. FCC {200} notably was influenced recovery behavior after stress drop, and the contribution of strain energy density by serration on tensile toughness was the greatest for HR-CoCrNi. [Display omitted] [ABSTRACT FROM AUTHOR]

The influence of charging method on hydrogen (H) distribution and the resultant nanomechanical behavior of CoCrFeMnNi high-entropy alloy was examined and compared with another face-centered cubic structured alloy, an austenitic stainless steel. Through thermal desorption spectroscopy measurement and theoretical analysis, it was revealed that electrochemical (E-) charging induces steep gradient of H concentration near the surface while H was homogenously distributed after gaseous (G-) charging. Nanoindentation results show significant hardening in E-charged alloys while the hardness of G-charged alloys remains invariant. These differences were rationalized in terms of the nature of H distributions induced by different charging methods. Unlabelled Image [ABSTRACT FROM AUTHOR]

We studied the solid flow characteristics of a multistage circulating moving bed reactor manufactured to produce high purity H2 using a chemical-looping process at high temperature. The reactor was constructed of stainless steel 304 and comprised an inclined fuel reactor ((bottom: 0.07 m, top: 0.16 m) × 0.06 × 1 m3), a steam reactor (0.16 × 0.06 × 1.4 m3), a riser (0.03 × 0.06 × 3.8 m3), two loop-seals (0.03 × 0.06 m2), and a cyclone. Zirconia beads (d p = 186 m, ρ p = 3720 kg/m3, U mf = 4.95 × 10−2 m/s, Geldart classification B Group) were used as the bed material. To distribute compressed air, a bubble cap was used as the distributor. Solid mass flux appeared at 6.2–56.4 kg/m2s and the solid mean residence time appeared 92.5–889.3 s in the steam reactor and 75.3–717.7 s in the fuel reactor. Solid mass flux increased with increasing inlet gas velocity into the loop-seal, temperature and the bed height of the steam reactor. However, the solid mean residence time decreased with increasing inlet gas velocity into the loop-seal and the bed height of the steam reactor. [ABSTRACT FROM AUTHOR]

The effect of hydrogen (H) charging on the nanoindentation response of a selective laser melted (SLM) 316L austenitic stainless steel was investigated and compared with its conventionally manufactured (CM) counterpart. Results show that the hardness increment in the SLM samples due to H charging is relatively smaller. Thermal desorption spectroscopy analysis suggests that the charged SLM alloy has not only a smaller H content but a lower apparent H diffusivity in comparison to the CM alloy. This was attributed to the ultrafine solidification cell structure in the SLM alloy. Through the low-load nanoindentation experiments and forward-scattered electron imaging analysis, statistical distributions of the hardness of the cell walls and interiors were assessed. The cell walls, consisting of high-density dislocations with segregated elements, were relatively insensitive to H charging than the cell interiors. These results are discussed in terms of the apparent H solubility and diffusivity in the SLM alloy. Image, graphical abstract [ABSTRACT FROM AUTHOR]