Invariant characterizing thermally activated plastic flow
Keywords:thermal activation, strain rate sensitivity, plastic flow, Cottrell-Stokes law, Al alloys
Thermal activation leads to a reduction of the flow stress below the athermal value (the zero Kelvin flow stress). The ratio between the flow stress at a given temperature and the athermal stress (at given material structure) is known as the Cottrell-Stokes ratio – a fundamental parameter describing thermal activation. This ratio is independent of strain in pure metals. This observation represents the CottrellStokes law. Here it is shown that the ratio is also independent of the annealing state of the material, i.e. an invariant of the deformation and microstructure in Al alloys. A model of thermally activated dislocation motion across fields of obstacles is used to investigate the physics of the process. It is also used to discuss whether the validity of the Cottrell-Stokes law can be determined based on strain rate jump experiments and the Haasen plot, as usual in the current practice.
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