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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 ºC for 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 (ΔKeff).