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The role of special boundaries on the hydrogen embrittlement of austenitic steel
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The role of special boundaries on the hydrogen embrittlement of austenitic steel

Evan M. Kahl
Master of Science (M.S.), Drexel University
Sep 2015
DOI:
https://doi.org/10.17918/etd-7130
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Abstract

Hydrogen Austenitic stainless steel Materials Science
Because hydrogen (H) transport in a material's bulk is kinetically limited by diffusion and dislocation movement, researchers perform hydrogen embrittlement (HE) experiments at quasi-static and slower strain-rates. High strain-rate experiments limit H-diffusion and enable corroboration between experiment and atomistic modeling. This study investigates the mechanical properties of high strain-rate compression experiments and the resulting dislocation substructure at special boundaries (SB): [sigma]3s, [sigma]9s, & [sigma]27s generated through grain boundary engineering (GBE) and thought to mitigate the effects of HE. A through-thickness GBE microstructure with 61% [sigma]3s and 70% SB (length fraction) was created in commercial AISI 304L via 3 iterations of a 5% rolling reduction and 1020°C anneal while annealed control samples had 45% [sigma]3s and 48% SB. These conditions were used to prepare both 2.5x4.0x4.0 mm compression and Split-Hopkinson Pressure Bar (SHPB) specimens. Half of all samples were charged in commercially pure H2 gas for 16 days at 138 MPa and 300°C for a target H-concentration of approximately 150 wppm (0.811 at%). The team investigated the same sample area with EBSD pre- and post-deformation, tracking how microstructure evolved with strain-rate and H. Both dynamic (6E3 s-1) and quasi-static (2E-3 s-1) test samples were strained to 20%. In quasi-static tests, both H-charged annealed and GBE samples exhibited similar increases in flow stress: 420 vs. 320 MPa. A similar increase in flow stress, 610 vs. 510 MPa, for H-charged samples was observed at high strain rates with little distinction between the GBE and annealed conditions. Using the Nye Tensor, we calculated geometrically necessary dislocations (GND) from EBSD data and observed higher overall dislocation densities in the uncharged conditions; SHPB samples had higher densities than compression samples. However, analysis of bulk [sigma]3 CSL deviation indicates that H-charged samples have higher deviation post-deformation, suggesting higher accommodation of dislocations at SB. Spatial GND maps indicate that both GB character and Taylor factors influence local concentrations of dislocations at grain boundaries. These findings support the HELP mechanism and offer valuable information regarding HEDE and the apparent brittleness of [sigma]3 boundaries.

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