Near-microscopic grain boundary facilitates fatigue crack propagation in a polycrystalline Al–Zn–Mg–Cu alloy

In present study, grain characteristics with sizes within 10–30 ​μm were fabricated from a same Al–Zn–Mg–Cu alloy, FCP behaviors of the alloys with small grain (SG alloy), medium grain (MG alloy) and large grain (LG alloy) were investigated and related fatigue fracture morphology was analyzed. With the enhancement of stress intensity factor range (ΔK), the alloy with larger grains possessed faster FCP rate, which were successively arranged as SG alloy ​> ​MG alloy ​> ​LG alloy at initial stage while turned to LG alloy ​> ​MG alloy ​> ​SG alloy at final stage for the stable expanding region. Except for conventional characteristics of striations, tearing ridges, secondary cracks, second phases, voids and dimples, more prominent grain boundary features appeared on fracture surface, especially for MG and LG alloy. The calculation of cyclic plastic zone (CPZ) sizes proved that grain boundary participated and promoted FCP behavior when CPZ covered grain and grain boundary together.