Comparison of fatigue cracking possibility along large- and low-angle grain boundaries

Abstract

In this paper, intergranular fatigue cracking possibility along large- and low-angle grain boundaries (GBs) was compared by using a copper bicrystal with embedded grains. It was found that secondary slip lines were always activated near the large-angle GB segment owing to the plastic strain incompatibility and those slip bands couldn’t pass through the GB. However, the slip bands showed a good one-to-one correlation across the low-angle GB segment without the operation of secondary slip. If the cyclic number was high enough, fatigue cracks firstly initiated along the large-angle GB segment, but no fatigue crack could be observed along the low-angle GB segment. With the help of the electron channelling contrast (ECC) technique in SEM, the fatigued dislocation patterns near the GBs were observed. Those observations provided a direct evidence on the interactions of persistent slip bands (PSBs) with GBs. Based on the experimental results, the dependence of intergranular fatigue cracking on the interaction modes of PSBs with different kinds of GBs is discussed. A defect model (including dislocations and vacancies) leading to intergranular fatigue cracking is proposed. Because the defects carried by PSBs beside a large-angle GB are often piled-up, i.e. ‘piling-up of defects’, and make a contribution to fatigue cracking along the large-angle GB segment. On the contrary, the defects carried by PSBs can pass through the low-angle GBs and be moved into the adjacent grain. This process is defined as ‘passing-through of defects’ and cannot lead to the fatigue cracking along the low-angle GB.