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

John P Hanson,Akbar Bagri,Jonathan Lind,Peter Kenesei,Robert M Suter,Silvija Gradečak,Michael J Demkowicz

doi:10.1038/s41467-018-05549-y

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.

Highlights

Hydrogen embrittlement (HE) causes sudden, costly failures of metal components across a wide range of industries
Our investigation of crack morphology in a H-embrittled sample of alloy 725 identified ten crack deflection events (CDEs): instances where a hightoughness grain boundaries (GBs) deflected the crack onto a tortuous, meandering path
By analyzing the crystallographic character of the ten hightoughness boundaries, we found that nine of them fit our definition of a boundaries with low-index planes (BLIPs), i.e., one where at least one of the grain facets that meet at the boundary have Miller indices within the {001}, {011}, or {111} families

Summary

Introduction

Hydrogen embrittlement (HE) causes sudden, costly failures of metal components across a wide range of industries. 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. While it is known that H-assisted fracture is intergranular in many metals[20,39,40,41,42,43], current knowledge is not sufficient to predict which GBs are most or least susceptible to HE, hindering microstructure-informed lifetime predictions. GBs are these alloys’ weakest links, the microstructural features that are most susceptible to HE Understanding their deformation and fracture behavior in the presence of H is key to better lifetime predictions and to the design of HE-resistant microstructures. Previous investigations of GB HE, do not account for the variability of GB properties—and GB–H interactions—arising from GB crystallographic character

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