Crystallographic character of grain boundaries resistant to hydrogen-assisted fracture in Ni-base alloy 725

Abstract

Hydrogen embrittlement (HE) causes sudden, costly failures of metal components across a wide range of industries. Yet, despite over a century of research, the physical mechanisms of HE are too poorly understood to predict HE-induced failures with confidence. We use non-destructive, synchrotron-based techniques to investigate the relationship between the crystallographic character of grain boundaries and their susceptibility to hydrogen-assisted fracture in a nickel superalloy. Our data lead us to identify a class of grain boundaries with striking resistance to hydrogen-assisted crack propagation: boundaries with low-index planes (BLIPs). BLIPs are boundaries where at least one of the neighboring grains has a low Miller index facet—{001}, {011}, or {111}—along the grain boundary plane. These boundaries deflect propagating cracks, toughening the material and improving its HE resistance. Our finding paves the way to improved predictions of HE based on the density and distribution of BLIPs in metal microstructures.

Effect of grain boundary on the crack-tip plasticity under hydrogen environment: An atomistic study

“…These results might suggest that the intragranular fracture is not favorited in HE of metals, thus support the previous experimental findings [22,40,[59][60][61][62][63], where the H-assisted fracture is usually intergranular. However, HE cannot be attributed to one simple mechanism, but is rather a collective result of H interactions with microstructures distributed on length-scales differing by orders of magnitude.…”

“…increasing the fraction of "special" GBs, which are less likely to be preferred sites for H segregation. Recent X-ray absorption tomography measurements [40] further demonstrated that the fraction of GBs with low-index planes, i.e. boundaries where at least one of the neighboring grains has a low Miller index facet -{001}, {011} or {111} along the GB plane, should be maximized to design HE-resistant materials via GB engineering.…”

Strain localisation and failure at twin-boundary complexions in nickel-based superalloys

“…Accordingly, intergranular cracking can be alleviated in many materials by introducing a high proportion of coincidence-site-lattice (CSL) boundaries 7 , 8 . In contrast, as the typical CSL boundaries preferred under the framework of GBE, coherent twin boundaries (TBs) in some Ni-based superalloys are found to be vulnerable sites for materials failure 9 – 13 . Current lack of mechanistic understanding in the TBs-related failure makes it difficult to meet the failure-intolerant demands when Ni-based superalloys are used as critical components in various environments.…”

“…Although the unexpected TBs-related failures have shown to be critical for the performance of Ni-based superalloys, the physical origin of its mechanism is still elusive. Heretofore, all studies rely on macroscale or microscale analysis 10,12,22 , without providing sufficiently detailed characterisations that can truly link the crack formation with TBs, let alone the corresponding atomic level origin. Moreover, as the main difference to pure Ni, the precipitates in superalloys (γʹ and γʺ), particularly at TBs, have not yet been considered in previous studies, which might well be the reason for the confusion around failure at TBs in Ni-based superalloys.…”

Hydrogen embrittlement through the formation of low-energy dislocation nanostructures in nanoprecipitation-strengthened steels

“…From the seminal work of Johnson ( 1 ), it has been recognized that ductility can be markedly decreased upon the formation of interfacial chemical compounds of which hydrogen can be a by-product ( 2 ). Hydrogen can then diffuse to the component bulk, producing a wealth of interactions at various scales with the microstructure fostering the formation and propagation of cracks ( 3 , 4 ). Hydrogen embrittlement (HE) is recognized to have a marked effect in emerging technologies; these include wind turbines for electricity generation, hydrogen storage, and ultralight car bodies ( 5 , 6 ).…”