Influence of modified intermetallics and Si particles on fatigue crack paths in a cast A356 Al alloy

Mechanical fatigue tests were conducted on uniaxial specimens machined from a cast A356-T6 aluminum alloy plate at total strain amplitudes ranging from 0.1 to 0.8% (R = -1). The cast alloy contains strontium-modified silicon particles (vol. fract. to approximately 6%) within an Al-Si eutectic, dispersed α intermetallic particles, Al15(Fe,Mn)3Si2 (vol. fract. to approximately 1%), and an extremely low overall volume fraction of porosity (0.01%). During the initial stages of the fatigue process, we observed that a small semicircular fatigue crack propagated almost exclusively through the Al-1% Si dendrite cells. The small crack avoided the modified silicon particles in the Al-Si eutectic and only propagated along the α intermetallics if they were directly in line with the crack plane. These growth characteristics were observed up to a maximum stress intensity factor of approximately Kmaxtr = 7.0 MPa m1/2 (maximum plastic zone size of 96 μm). When the fatigue crack propagated with a maximum crack tip driving force above 7.0 MPa m1/2 the larger fatigue crack tip process zone fractured an increased number of silicon particles and α intermetallics ahead of the crack tip, and the crack subsequently propagated preferentially through the damaged regions. As the crack tip driving force further increased, the area fraction of damaged α intermetallics and silicon particles on the fatigue fracture surfaces also increased. The final stage of failure (fast fracture) was observed to occur almost exclusively through the Al-Si eutectic regions and the α intermetallics.