Endurance limit and threshold stress intensity of die cast magnesium and aluminium alloys at elevated temperatures

High cycle fatigue properties of the high-pressure die-cast magnesium alloys AZ91 hp, AS21 hp and AE42 hp and of the aluminium alloy AlSi9Cu3 are investigated at elevated temperatures. Fatigue tests are performed at ultrasonic cyclic frequency and load ratio R=−1. Compared with ambient air environment, the S-N curves determined in warm air of 125 °C (magnesium alloys) and 150 °C (aluminium alloy) are shifted towards lower cyclic stresses. The mean endurance limits at 109 cycles of 41 MPa (AZ91 hp), 27 MPa (AS21 hp), 35 MPa (AE42 hp) and 61 MPa (AlSi9Cu3) are 70-90% of the respective stresses found in ambient air, and fracture mechanics tests delivered threshold stress intensities at elevated temperature of about 80% of the respective values at 20 °C. Fatigue cracks initiate, nearly, exclusively at casting porosity and fracture surfaces appear similar at low and elevated temperatures. Fatigue cracks may form at voids at stresses below the endurance limit, and non-propagating fatigue cracks of considerable length (greater 1 mm) are found in runnout specimens. Considering porosity as an initial crack, critical stress intensity, Kcrit is evaluated, which quantifies the sensitivity of a material to defects. Materials can sustain cyclic stresses without fatigue failure, if the cyclic stress intensity determined for their most damaging porosity is lower than Kcrit.