Abstract The mechanical behaviour of multiphased materials strongly depends on the topology of the hard phase. An extreme case is found when both the soft and hard phases percolate through the material. From a topological point of view, contrary to the case of isolated reinforcing particles, a crack nucleated in the hard phase can propagate in this phase through the entire crystal. From an renergetic point of view however, this propagation requires plastic flow in the soft phase, which controls crack propagation. This situation is illustrated by the example of an aged austenoferritic duplex steel, in which a crack nucleated in brittle ferrite readily passes round austenite ligaments, whose plastic stretching controls the crack propagation. Final failure is considered to occur when crack size and average separation between crack nucleation sites become of the same order of magnitude, giving an excellent estimate of ductility.