Thermal ageing of an Fe‒Cu alloy: Microstructural evolution and precipitation hardening

The microstructural evolution of an Fe-Cu alloy during ageing has been investigated by small-angle X-ray scattering and transmission electron microscopy (TEM). Its mechanical properties have been characterized by Vickers microhardness measurements and in situ straining TEM. Cu precipitates evolve during annealing from a small coherent bcc to a larger incoherent type. In both cases, in situ straining shows that long screw dislocations move between localised cusps owing to dislocation-precipitate interactions. In the case of incoherent precipitates, a particular three-dimensional bypassing mechanism is observed, which does not form any loop around the precipitate. On the basis of these observations, we propose a model of hardening combining the viscous motion of screws and the dislocation-precipitate interaction, which yields a monotonic decrease in stress with annealing time in both cases of particle shearing and bypassing. As a consequence, the hardness peak shown by microhardness experiments is not ascribed to a shearing-bypassing transition, but rather to an evolution of the precipitate volume fraction at short annealing times.