Loss of stress equivalence in the strain-rate sensitivity of flow stress in fine-grain polycrystalline copper

The strain-rate sensitivity of flow stress (3.7 x 10(-3) s(-1) reversible a rrow 3.7 x 10(-4) s(-1)) in fine-grain polycrystalline copper has been stud ied over the entire stress-strain curve at room temperature. It is observed that the yield stress varies with mean grain diameter (32 to 59 mu m) in a ccord with the Hall-Fetch relation, whereas ultimate tensile stress and fra cture stress are independent of the grain size. The strain-rate sensitivity aa of a given flow stress a depends on the grain size; it increases as gra in size is increased from 32 to 59 mu m. For a given grain size, the Cottre ll-Stokes ratio Delta sigma/sigma and the product of flow stress a and asso ciated activation volume V-sigma remain constant in the entire stress range 50 to 210 MPa studied. Values of both Delta sigma/sigma and V(sigma)sigma depend on the grain size. The observations have been accounted for within t he framework of Feltham's single barrier model of plastic flow.