Stress driven creep deformation and cavitation damage in pure copper

The stress dependence of creep deformation and cavitation damage in pure copper at 250 degrees C under uniaxial loading is studied using a flat hourglass test specimen under uniaxial tensile load. In-situ digital image correlation (DIC) is used to monitor time dependent surface creep deformation, ex-situ small angle neutron scattering (SANS) applied to measure volumetric cavitation damage, and scanning electron microscopy used for surface characterisation. A self-consistent discolation model is successfully applied to explain the full field multi-stress creep deformation behaviour measured by DIC. Through approximating a range of shaped cavities with a model distribution of spherical voids, a minimum stable cavity nucleation diameter range of 600 to 1200 angstrom, depending on the applied stress level, is clearly observed in the SANS results. This finding supports the validity of the classical surface energy/work balance expression defining the minimum stable cavity size. All cavities observed in interrupted life samples were facetted in nature. The SANS data imply continuous cavity nucleation and growth throughout creep life, with a nucleation rate at stresses less than 100 MPa linearly related to the creep rate. This is in accordance with the double ledge grain boundary sliding nucleation model of Sandstrom and Wu [1].