Fatigue crack growth mechanisms at the microstructure scale in Al–Si–Mg cast alloys: Mechanisms in the near-threshold regime

The near-threshold crack growth behavior (ΔKth/Region I) of hypoeutectic and eutectic Al–Si–Mg cast alloys was investigated for both long and short cracks. To isolate and establish the mechanistic contributions of microstructural features (dendritic α-Al matrix/eutectic phases/Mg–Si strengthening precipitates), alloys with various Si content/morphology, grain size level, and matrix strength were studied; the effect of secondary dendrite arm spacing was also assessed. The mechanisms in Paris-Region II and Region III will be presented in a subsequent publication. The long-crack threshold behavior of the materials is attributed to combined effects of microstructure/roughness-induced and residual stress (introduced during heat treatment)-induced closure mechanisms. The threshold behavior of small cracks is explained through closure-unrelated mechanisms and barrier effects of the characteristic microstructural features of each alloy. The fatigue crack growth tests were conducted on compact tension and corner crack tension specimens under constant stress ratio, R = 0.1, in ambient conditions.