Title

Elucidating the mechanistic influence of strengthening precipitate morphology on hydrogen-assisted cracking in a Ni-Cr superalloy

Conference Dates

July 15-20, 2018

Abstract

Recent studies on Ni and Fe-based alloys utilizing transmission electron microscopy (TEM) of the near-fracture surface region have revealed a systematic refinement in the dislocation cell structure of hydrogen-charged specimens relative to non-charged specimens. These observations suggest that hydrogen-induced localization of deformation may provide an important contribution to the microscale processes by which hydrogen degrades the performance of structural metals. Moreover, this finding implies that microstructural features which affect deformation processes could be tuned so as to increase an alloy’s intrinsic resistance to hydrogen-induced degradation. For example, given the dominant influence of strengthening precipitates on deformation processes in precipitation-hardened alloys, it is hypothesized that such particles could be optimized to mitigate this hydrogen-induced localized deformation. However, studies which evaluate the role of precipitate morphology on these localized deformation processes in the context of hydrogen-induced cracking are limited in number and often complicated by the simultaneous influence of other microstructural features.

This study seeks to remedy this knowledge gap by examining the role of strengthening precipitate morphology on the hydrogen environment-assisted cracking susceptibility of a model precipitation-hardened alloy. First, Monel K-500 was systematically heat-treated so as to produce strengthening precipitate (γ’ (Ni3Al)) sizes which result in distinct global slip behavior. Evaluated heat treatments corresponded to the non-aged (no γ’; wavy slip), under-aged (small γ’; planar slip via particle shearing), peak-aged (medium γ’; mixed), and over-aged (large γ’; wavy slip via Orowan looping) conditions. TEM of each heat treatment condition confirmed the presence of the targeted precipitate/slip morphologies; the absence of strengthening precipitates in the non-aged alloy was also corroborated by X-ray diffraction experiments. Second, the effect of precipitate morphology on HEAC metrics was assessed through slow-rising stress intensity (K) testing in dry N2 gas and 0.6 M NaCl solutions under applied potentials of -1000, -1100, and -1200 mVSCE. Testing revealed a systematically decreased HEAC susceptibility for the non-aged and over-aged conditions relative to the under-aged and peak-aged conditions, suggesting that HEAC susceptibility is increased in alloys which exhibit planar slip. Third, a detailed microstructural characterization effort was completed to ensure that this difference in HEAC behavior is predominantly attributable to the variations in precipitate morphology. Microstructural features which may vary with applied isothermal ageing heat treatment are (1) grain size, (2) grain boundary character, (3) grain boundary impurity concentrations, and (4) precipitate morphology. Electron backscatter diffraction (EBSD) showed minimal changes in grain size and grain boundary character distribution between the four tested conditions, confirming that such variables are not responsible for the observed variations in HEAC susceptibility. Auger electron spectroscopy (AES) revealed a systematic enhancement in the grain boundary sulfur concentration with increased ageing time. However, the increased resistance of the over-aged alloy relative to the under-aged and peak-aged conditions implies that grain boundary sulfur concentration is not responsible for the measured differences in HEAC susceptibility.

Taken together, these results demonstrate the potent role of precipitate strengthening morphology on HEAC susceptibility, suggesting that such microstructural features can be optimized to improve an alloy’s intrinsic resistance to HEAC. Variations in precipitate morphology are expected to affect HEAC through changes in hydrogen trapping behavior and bulk slip morphology. Given that hydrogen trapping in Monel K-500 was previously shown to vary marginally with precipitate character, these results imply a governing role of bulk slip morphology (e.g. planar versus wavy slip). This observed dependence of HEAC susceptibility on bulk slip morphology motivates the re-examination of micromechanical models of HEAC metrics which assume a specific slip geometry. Possible model modifications to account for these new results are discussed and future experiments to further explore the mechanistic implications of this dependence on bulk slip morphology are proposed.

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