Cyclic stress–strain response of ultrafine grained copper

The present study reports on the cyclic stress–strain response of ultrafine grained copper obtained by equal channel angular extrusion (ECAE). Fatigue behavior of material subjected to between 8 and 16 ECAE passes was studied both in constant amplitude and incremental step tests. Transmission electron microscopy was employed to shed light on the microstructural evolution. Samples that had been extruded by an optimized ECAE route (16E) displayed stable cyclic stress–strain response. Independent of the actual cyclic loading condition, all the ECAE processed material demonstrated near perfect Masing behavior indicating that the ECAE routes selected resulted in microstructures stable against fatigue-induced changes. This is in contrast to copper with conventional grain sizes. An additional heat treatment that resulted in a bimodal microstructure eliminated the differences in cyclic response that had resulted from the various ECAE routes applied. Still, the bimodal structure proved to be cyclically stable in strain-controlled tests conducted at room temperature. Preliminary studies from an in-situ tests conducted in a scanning electron microscope demonstrated that non-optimized ECAE routes will lead to rapid cyclic instability, severe damage localization and premature fatigue failure.