Very high cycle fatigue of wrought magnesium alloy AZ61

Abstract

Wrought magnesium alloys show a high strength to weight ratio, which makes them potentially attractive for load bearing components in automotive applications. Structural parts may be subjected to very high numbers of load cycles, and therefore the very high cycle fatigue properties of these materials are of great interest. Fatigue tests in laboratory air were carried out on wrought magnesium alloy AZ61 using ultrasonic fatigue testing equipment. The fatigue strength at 109 cycles is 98 MPa, which is 32% of the tensile strength. Fatigue cracks are predominantly initiated from slip bands at the surface. Near threshold fatigue crack growth was studied in ambient air and vacuum and showed a dominant influence of the environment. The threshold stress intensity amplitude at load ratio R=-1 is Ka,th = 1.1 MPam1/2 in air and 1.9 MPam1/2 in vacuum. Crack propagation as small as 10-12 m/cycle could be documented in vacuum. Lowest growth rates observed in ambient air were about 10-10 m/cycle. In vacuum, a transgranular and ductile crack path is found for all investigated crack growth rates. Corrosive processes induced by ambient air cause a transition from ductile to brittle failure in the near threshold regime.