The role of nonequilibrium grain boundaries in deformation and fracture of nanocrystalline Fe-Cr

• Nanocrystalline grains through severe plastic deformation show unconventional mechanical behaviour. • Higher strength, ductility and crack growth resistance for smaller grains. • Micromechanical spectroscopy suggests metastable defects as origin. • Distributed grain boundary dislocations enhance the failure tolerance. Nanocrystalline materials are considered as great candidates for structural applications due to increased yield onset. However, this comes often at the cost of low ductility and fracture toughness. While such a tradeoff is common also for classical coarse-grained materials, the underlying deformation processes are fundamentally different in the nanocrystalline case, where the larger number of grain boundaries plays a major role for nucleation and accommodation of dislocation plasticity. In this study an equiatomic Fe-Cr model material system processed by severe plastic deformation has been investigated by micromechanical testing methods, revealing that although the initial state has the smallest grain size and highest yield strength (2182 MPa vs. 1980 MPa), it still shows the highest ductility (8.9% vs. 1.7%) and failure tolerance (stable vs. unstable crack extension). These counterintuitive results originate from the nonequilibrium grain boundary states synthesized during severe plastic deformation, which can be considered as a pathway for more failure tolerant design.