Crack initiation and propagation dominated by strain localization in quasi-single crystal and poly-crystalline of a Ni-based complex concentrated alloy

Cracking is the precursor of the final fracture for most engineering materials, often affected by various microstructural characteristics including grain orientation, boundary, precipitation, etc. In this study, the fracture behaviour of a typical Ni-based complex concentrated alloy (CCA) was systematically investigated. These CCAs were fabricated using fine-grain casting and directional-solidification processes, which consists of quasi-single crystal (QSC) and poly crystalline structures respectively. Quasi-static tensile test results combined with morphology characterizations and crystal plasticity analyses suggest that microcracks tend to nucleate at the matrix/carbide interfaces in QSC and then propagate along the maximum slip direction at the crack tips. The crack path showed a distinctive zig-zag shape, beneficial for consuming the crack propagation energy. In consideration of carbide morphology, script-typed carbides were found to be very effective in hindering dislocation movements, resulting in admirable work hardening ability. In polycrystalline, however, microcracks are commonly found near the carbides that precipitate at grain boundaries, then developed quickly to connect with each other, leading to premature intergranular fracture. Grain boundary carbides were detrimental to mechanical performance. Considering the large amounts of investigations on second phase and precipitation, these new findings about carbide-related crack behaviours could be extended to other second phase hardening materials, providing new routes to design high-performance alloys by controlling the precipitation morphology and position.