THERMAL AGING 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 electro n microscopy (TEM). Its mechanical properties have been characterized by Vickers microhardness measurements and in situ straining TEM. Cu pr ecipitates evolve during annealing from a small coherent bcc to a larg er incoherent type. In both cases, in situ straining shows that long s crew dislocations move between localised cusps owing to dislocation-pr ecipitate interactions. In the case of incoherent precipitates, a part icular three-dimensional bypassing mechanism is observed, which does n ot form any loop around the precipitate. On the basis of these observa tions, we propose a model of hardening combining the viscous motion of screws and the dislocation-precipitate interaction, which yields a mo notonic decrease in stress with annealing time in both cases of partic le shearing and bypassing. As a consequence, the hardness peak shown b y microhardness experiments is not ascribed to a shearing-bypassing tr ansition, but rather to an evolution of the precipitate volume fractio n at short annealing times.