Loss of Stress Equivalence in the Strain-Rate Sensitivity of Flow Stress in Fine-Grain Polycrystalline Copper

Abstract

The strain-rate sensitivity of flow stress (3.7 × 10 -3 s -1 ⇄ 3.7 × 10 -4 s -1 ) in fine-grain polycrystalline copper has been studied over the entire stress-strain curve at room temperature. It is observed that the yield stress varies with mean grain diameter (32 to 59 μm) in accord with the Hall-Petch relation, whereas ultimate tensile stress and fracture stress are independent of the grain size. The strain-rate sensitivity Δσ of a given flow stress σ depends on the grain size; it increases as grain size is increased from 32 to 59 μm. For a given grain size, the Cottrell-Stokes ratio Δσ/σ and the product of flow stress σ and associated activation volume V σ remain constant in the entire stress range 50 to 210 MPa studied. Values of both Δσ/σ and V σ σ depend on the grain size. The observations have been accounted for within the framework of Feltham's single barrier model of plastic flow.