Effects of grain size and stacking fault energy on fatigue-crack-propagation thresholds in Cu-Al aluminium alloys

Fatigue-crack propagation rates (d _a /d _N ) and fatiguecrack ‘thresholds’ ∆K_th have been studied in copper alloys (Cu-2Al, Cu-4Al, and Cu-8Al) of various grain sizes. Propagation rates are slower in fine-grained material and ∆K_th appears to vary in a linear manner with (grain diameter)^−1/2, in much the same manner as the monotonic yield stress. These effects are more pronounced in alloys of higher aluminium content. The scatter in values of propagation rate increases with grain size, because crack growth is dependent on the orientations of the individual grains, and there are insufficient grains along the macroscopic crack front, in very coarse-grained material, to produce an averaging effect.