Deformation and microstructure-independent Cottrell–Stokes ratio in commercial Al alloys

Cottrell–Stokes-type experiments are performed with AA6022, a heat treatable commercial Al alloy, at different stages of precipitation. It is shown that the ratio of the flow stress at given temperature and that extrapolated to 0 K, measured at given material state, is independent of the strain and of the precipitation state. The ratio depends only on temperature and strain rate. However, when probed using strain rate jump experiments, the Cottrell–Stokes law appears not to be fulfilled in any of these materials, and the strain rate sensitivity parameter depends on the precipitation state. A model based on the interaction of dislocations with populations of obstacles of various types is used to provide an interpretation of the Cottrell–Stokes law. The model indicates that as the dislocation velocity increases, the effective Cottrell–Stokes ratio in systems with various obstacle compositions takes values in a narrow range close to the critical value of 1 (i.e. “microstructure” insensitivity). Conversely, the model suggests that the Cottrell–Stokes ratio should become more sensitive to the microstructure under creep conditions.