Evolution of dislocation structure induced by cyclic deformation in a directionally solidified cobalt base superalloy

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Cyclic deformation has been carried out on a directionally solidified cobalt base superalloy at room temperature in air under the control of different total strain amplitudes. Dislocation structures of different stages in the cyclic stress response curve were studied by TEM. Observations show that a large number of stacking faults and fault intersections are formed in the initial hardening stage, and this leads to the initial hardening of the alloy. However, formation of hexagonally close packed (hcp) zones and twins is proposed to be the cause of cyclic saturation. Models for both the transition from stacking faults to a hcp layer structure and to twins are proposed.