Micromechanisms of fatigue crack growth in cast aluminium piston alloys

The fatigue crack growth behaviour in as-cast and hot isostatically pressed (HIP) model cast aluminium piston alloys with hypoeutectic Si compositions of 6.9 wt% and 0.67 wt% has been investigated. The HIP alloys showed slightly improved fatigue crack growth resistance. Analysis of the crack path profiles and fracture surfaces showed that the crack tends to avoid Si and intermetallic particles at low Delta K levels up to a mid-Delta K of similar to 7 MPa root m. However, some particles do fail ahead of the crack tip to facilitate crack advance due to the interconnected microstructure of these alloys. At higher levels of Delta K, the crack increasingly seeks out Si and intermetallic particles up to a Delta K of similar to 9 MPa root m after which the crack preferentially propagates through intermetallic particles in the 0.67 wt%Si alloy or Si and intermetallics in the 6.9 wt%Si alloys. It was also observed that crack interaction with intermetallics caused crack deflections that led to roughness-induced crack closure and possibly oxide-induced crack closure at low to mid-Delta K. However, crack closure appears unimportant at high Delta K due to the large crack openings and evidenced by the fast crack growth rates observed. (C) 2011 Elsevier Ltd. All rights reserved.