Fatigue crack growth behavior of nanocrystalline copper with grain size less than 250 nm is studied and compared to its microcrystalline counterpart. The nanocrystalline samples are prepared by processing commercially pure Cu using the equal channel angular extrusion (ECAE) process. The microcrystalline samples are prepared from the same Cu material and normalized at 500°Cfor 2.5 hours. All samples are tested as per the ASTM E647 standard at room temperature under tension-tension loading, and data over a wide range of stress intensity levels is presented. A cross-over in the behavior of two materials has been observed -showing that the nanocrystalline copper has a higher fatigue crack growth resistance over the middle Paris regime, but a lower fatigue crack growth resistance in the near threshold regime. To explain the cross-over behavior, fractography and microhardness measurements in the plastic zone are conducted. These results show evidence of softening due to grain coalescence and growth in recovery and grain coalescence in the plastic zone for nanocrystalline Cu and work hardening for microcrystalline Cu. The formation of oxide particles on the fracture surface in the threshold region of microcrystalline Cu are observed that cause crack closure, resulting in a lower effective ΔK (ΔK_(cff)).
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