Correlation between the nanomechanical properties and microstructure of ultrafine-grained copper produced by equal channel angular pressing

Ultrafine-grained copper was manufactured by equal channel angular pressing technique. Microstructural observations performed with the aid of transmission electron microscopy and scanning force microscopy revealed that short annealings of as-pressed material in the temperature range 250-300 °C result in a duplex microstructure consisting of micrometer-sized and relatively defect-free recrystallized grains embedded in the untransformed ultrafine-grained matrix. It was found that the nanohardness of newly nucleated grains in partly recrystallized samples is very different from the nanohardness of untransformed matrix, the latter being identical with the nanohardness of as-pressed samples. Extensive pile-ups around the indents were observed in the as-pressed samples, while the pile-ups were virtually non-existent in the sample annealed at high temperature. In as-pressed samples the pile-ups relaxed with time at room temperature. Grain boundary sliding was suggested as a possible mechanism of this relaxation.