Near‐threshold fatigue crack growth properties of wrought magnesium alloy AZ61 in ambient air,dry air,and vacuum

Environmental influences on near‐threshold fatigue crack growth in wrought magnesium alloy AZ61 were investigated. Fatigue tests were performed in ambient (humid) air, dry air, vacuum, and dry nitrogen gas at 19 kHz cycling frequency and load ratio R = ?1. Threshold stress intensity factor amplitudes, K_th, determined for limiting growth rates below 5 × 10^?13 m/cycle were 1.1 MPam^1/2 in ambient air and 1.2 MPam^1/2 in dry air. A much higher K_th of 1.9 MPam^1/2 was measured in vacuum and dry nitrogen gas. This suggests oxygen to be the most detrimental constituent of ambient air that increases near‐threshold crack propagation rates and decreases K_th. The deleterious effect of humidity is comparatively small. Corrosive influences are effective at ultrasonic cycling frequency for growth rates below approximately 3 × 10^?9 m/cycle. The crack propagation curves in ambient and dry air show a plateau‐like regime where the fracture mode changes from purely ductile to a mixed ductile and brittle mode. In vacuum and dry nitrogen gas, a ductile crack path is found for all investigated crack growth rates.