Migration of Geometrically Necessary Dislocations in Microstructurally-Sensitive Short Crack Growth of FCC and HCP Alloys

In metallic systems, short crack growth usually occurs with the interplay among slip activation, lattice rotation, and geometrically necessary dislocations (GNDs). Understanding the microstructurally sensitive short crack growth behaviour is essential for the rational prediction of fracture. In this study, by combining in-situ observation and multi-scale simulations, two representative alloys with face-centred cubic (FCC) and hexagonal close-packed (HCP) lattice structures are employed to elucidate the GND migration behaviour. Short crack propagation in FCC single-crystal (SX) alloy occurs along primary slip planes, while short crack in HCP Zr alloy follows prismatic slip planes with low Schmid factors. During growth, single slip is progressively activated in FCC, whereas crack-tip plasticity in HCP promotes dual-slip activation. Experimental observations reveal that GND hotspots migrate with the advancing crack tip in both alloys, exhibiting non-monotonic density increases and confirming that GNDs solely carry strain gradients/lattice curvature in the current deformed configurations. Crystal plasticity finite element simulations show persistent stress and plastic strain localisation at the crack tip in FCC SX alloy, with pronounced stored energy density accumulation at the crack wake. Complementary discrete dislocation plasticity simulations demonstrate that dual prismatic slip in HCP alloys enhances local plastic strain incompatibility, leading to strong GND localisation. These findings provide mechanistic insights into the interplay among slip, lattice rotation, GND evolution, and energy dissipation during short crack growth in distinct crystal structures. This study delivers new mechanistic insight that supports accurate modelling of short-crack growth rate and propagation path. • Short cracks propagate along primary slip in FCC SX alloys, but along low-Schmid-factor prismatic slip in HCP Zr alloys, with distinct secondary/dual-slip activation mechanisms. • GND migration with advancing short cracks is experimentally observed in both alloys, evidenced by moving GND peaks and non-monotonic GND evolution. • CPFE simulations show persistent crack-tip localisation in FCC SX alloys, while stored energy density preferentially accumulates in the crack wake. • DDP simulations demonstrate that dual prismatic slip under high stress triaxiality in HCP Zr alloys intensifies GND localisation via enhanced strain incompatibility.