Effect of annealing on percolating porosity in ultrafine-grained copper produced by equal channel angular pressing

Percolating porosity as a specific type of deformation-induced was discovered in ultrafine-grained (UFG) Cu produced by equal channel angular pressing (Ribbe et al., Phys Rev Lett 2009;102:165501). The stability of this defect type against annealing under various conditions is investigated for UFG Cu of different purity levels. The porosity is found to withstand the annealing treatments up to 1073 K for several hours in purified Ar atmosphere, despite significant microstructure transformation. Annealing at 1313 K in Ar removes the percolating porosity, as do relatively short heat treatments at 427 K in a hydrogen-containing atmosphere. Quasi-hydrostatic pressure applied at moderate temperatures, e.g. 1 GPa at 423 K, eliminates the percolating porosity, too. A model of porosity evolution, which accounts for the experimental findings, is suggested.