Dependence of intergranular fatigue cracking on the interactions of persistent slip bands with grain boundaries

Intergranular fatigue cracking mechanisms in various copper crystals with different grain boundaries (GBs) were systematically investigated and summarized. In the present investigation, the GBs are classified into three types, i.e. (1) random large-angle GBs parallel, perpendicular or tilting to the stress axis in various copper bicrystals; (II) low-angle GBs parallel or perpendicular to the stress axis in copper columnar crystals; (III) large-angle Sigma19b GB in a special [(4) over bar 1520]/[18 (2) over bar7]copper bicrystal. The slip planes of the adjacent crystals are coplanar across the later two types of GBs, but the slip directions of the two component grains are different beside the Sigma19b GB. With the help of electron channeling contrast (ECC) technique in scanning electron microscopy (SEM), fatigue cracks and the interactions of dislocations with the GBs in all the fatigued crystals were observed and revealed. The results show that all the large-angle GBs (type I) in copper bicrystals always become the preferential sites to initiate fatigue cracks, independent of the interaction angle between the GB plane and the stress axis. This intergranular fatigue cracking mechanism can be attributed to the piling-up of dislocations at the large-angle GBs. For the columnar crystals containing low-angle GBs (type II), it is observed that persistent slip bands (PSBs), which transfer through low-angle GBs continuously, are the preferential sites for the nucleation of fatigue cracks. However, fatigue cracks were never observed at the low-angle GBs, no matter whether they were perpendicular or parallel to the stress axis. The non-cracking behavior of the low-angle GBs can be explained by the continuity of the dislocations, which led to the disappearance of piling-up of dislocations. For the Sigma19b GB (type III), it is found that the favorable fatigue cracking mechanism is still intergranular type in comparison with PSB cracking even though the two component grains have a coplanar slip system. The corresponding GB cracking mechanism should be attributed to the difference in the slip directions between two component grains, which only allows for partial passing through of dislocations across the Sigma19b GB. Based on the results above, it is suggested that intergranular fatigue cracking strongly depends on the interactions of PSBs with GBs in fatigued crystals, rather than the GB structure itself. Among all the GBs, only the low-angle GBs are intrinsically strong to resist the nucleation of fatigue cracks under cyclic loading. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.