Application of the in situ TEM deformation technique to observe how “clean” and doped grain boundaries respond to local stress concentrations

Intergranular fracture, grain-boundary structure, and dislocation-density interactions in FCC bicrystals

A dislocation-density based crystalline plasticity (DCP) and nonlinear finite element (FE) analysis were used to predict, and fundamentally understand how and why fracture nucleation and propagation are related to the interrelated microstructural mechanisms of dislocation-density pileups, GB structure, orientation, and total and partial dislocation density interactions within and adjacent for a random low angle grain boundary (LAGB) and a random high angle GB (HAGB). The GB orientations and structures were obtained from micropillar experiments, such that LAGBs and the HAGBs can be accurately represented and used for the modeling predictions. The normal stress, density of pileups, and dislocation-density accumulation along and within the GB were higher for the low angle GB bicrystal. These interrelated phenomena delineate how fracture for high angle GBs nucleate and propagate at lower nominal strains than the lower angle GB bicrystal case. These predictions underscore how fundamental mechanisms can be identified and used to understand how failure nucleates and propagates for different GB structures and orientations.

In situ TEM studies on hydrogen-related issues: hydrogen storage, hydrogen embrittlement, fuel cells and electrolysis

Hydrogen is attracting attention as an energy carrier for realizing a low-carbon society, because it can directly convert the energy obtained from chemical reactions into electrical energy without carbon dioxide emissions. This paper presents in situ transmission electron microscopy (TEM) observations related to hydrogen storage in metal and metal hydrides, hydrogen embrittlement of metallic materials used for storing and transporting hydrogen in containers and pipes, and fuel cells and water electrolysis using metal catalysts and oxides as electrode materials. All of these processes are important for practical applications of hydrogen. Numerous in situ TEM studies have revealed the microscopic structural changes when hydrogen reacts with the materials, when hydrogen is solidly dissolved in the materials and during the operation of the material. This review is expected to facilitate further development of TEM operando observations of hydrogen-related materials.

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

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.