A one-parameter approach to determining the critical conditions for the initiation of dynamic recrystallization

The onset of dynamic recrystallization is treated in terms of a model based on the principles of irreversible thermodynamics. In the analysis, the initiation of dynamic recrystallization is governed by both energetic and kinetic critical conditions. The former requires that the energy stored during a given deformation schedule attain its maximum, while the latter requires the dissipative processes associated with deformation to decelerate to a critical level. The kinetic critical condition leads to a minimum value of \-partial derivative theta/partial derivative sigma\ (theta is the conventional strain hardening rate) when the critical state is attained and to the appearance of an inflection point in the theta/sigma curve. In a range where the strain rate sensitivity is constant, the onset of dynamic recrystallization corresponds to a constant value of Gamma = partial derivative ln sigma/partial derivative epsilon. The initiation of dynamic recrystallization has much in common with the beginning of flow localization. These conclusions of the model are verified using experimental data obtained during the high temperature compression of nickel and of a type 305 austenitic stainless steel.