A review of underlying reasons for intergranular cracking for a variety of failure modes and materials and examples of case histories. (June 2019) - Lynch, Stan
Abstract: The reasons why cracking and corrosion can occur preferentially along grain boundaries in a wide variety of metallic materials are discussed, along with case histories of failures involving intergranular fracture and corrosion. Failure modes discussed include fast fracture, fatigue (in inert and embrittling environments), liquid-metal embrittlement, hydrogen embrittlement, stress corrosion cracking, and corrosion fatigue, in materials such as steels, Al alloys, Ni alloys, and Cu alloys. The features sometimes observed on grain-boundary facets (e.g. fatigue striations, crack-arrest markings, slip lines, steps/microfacets, nano−/micro-scale dimples, second-phase particles, and deposits), and how they are formed, are described, as such knowledge helps in diagnosing fracture modes and causes. Reasons for intergranular cracking in the cases considered include (i) preferential precipitation at grain boundaries (sometimes with an associated precipitate-free zone), (ii) segregation of metalloid impurities, hydrogen, or some alloying elements, (iii) preferential adsorption of embrittling environmental species (such as some metal atoms and hydrogen) at intersections of grain boundaries with surfaces/crack tips, and (iv) locally anodic regions at or adjacent to grain boundaries. Some lessons for failure-analysis and prevention in general, illustrated by the case histories, are then summarised. Highlights: Intergranular fracture and corrosion occur for a wide variety of reasons. Fractographic features on intergranular facets, and their formation, are described. Basic aspects of grain boundaries, e.g. phase transitions, are reviewed. Nine case histories involving intergranular fracture/corrosion are summarised. Some general lessons for failure analysis and prevention are highlighted.