High cycle fatigue of a die cast AZ91E-T4 magnesium alloy

Abstract

This study reveals the micro-mechanisms of fatigue crack nucleation and growth in a commercial high-pressure die cast automotive AZ91E-T4 Mg component. Mechanical fatigue tests were conducted under R=−1 conditions on specimens machined at different locations in the casting at total strain amplitudes ranging from 0.02% to 0.5%. Fracture surfaces of specimens that failed in the high cycle fatigue regime with lives spanning two orders of magnitude were examined using a scanning electron microscope. The difference in lives for the Mg specimens was primarily attributed to a drastic difference in nucleation site sizes, which ranged from several hundred μm’s to several mm’s. A secondary effect may include the influence of average secondary dendrite arm spacing and average grain size. At low crack tip driving forces ( K max<3.5 MPa m ) intact particles and boundaries act as barriers to fatigue crack propagation, and consequently the cracks tended to avoid the interdendritic regions and progress through the cells, leaving a fine striated pattern in this single-phase region. At high driving forces ( K max>3.5 MPa m ) fractured particles and boundary decohesion created weak paths for fatigue crack propagation, and consequently the cracks followed the interdendritic regions, leaving serrated markings as the crack progressed through this heterogeneous region. The ramifications of the results on future modeling efforts are discussed in detail.