Effects of grain refinement due to severe plastic deformation on the growth behavior of small cracks in copper

To clarify the effects of grain refinement on the growth behavior of small surface cracks, fatigue tests on round bar specimens were conducted on coarse-grained (CG) copper and ultrafine-grained (UFG) copper processed with equal channel angular pressing. The growth behavior of a small crack was monitored using a plastic replication technique. The difference in the growth behavior tendency between the CG and UFG copper occurred at an extremely low fatigue crack growth rate (FCGR) of dl/dN < 10−6 mm/cycle. For the UFG copper, the FCGR temporarily dropped at around dl/dN = 3 × 10−7 mm/cycle and then gradually recovered with subsequent cycling. After the FCGR exceeded dl/dN = 10−6 mm/cycle, it was nearly proportional to the crack length. On the other hand, the FCGR of the CG copper showed no temporary drop but rather increased steadily with cycling. To understand the reason for the temporary decrease of the FCGR of UFG copper, a crack growth model based on the reversible plastic zone size at a crack tip and the related microstructural factors were developed. In addition, FCGR evaluation was discussed by applying σanl (σa, nominal stress amplitude; l, crack length; n, material constant) to both materials.

► The difference in the growth behavior tendency between CG and UFG copper occurred at an extremely low CGR (dl/dN < 10−6 mm/c).
► A quantitative model describing the crack growth mechanism was developed based on the RPZ size.
► The changes in the CGR and morphological features of the fracture surface were successfully explained by this model.
► The CGR of small cracks growing with dl/dN > 10−6 mm/c could be estimated by σanl, (n = 7.5 for CG, n = 4.4 for UFG).