Micromechanisms of fatigue crack growth in cast aluminium piston alloys

Abstract

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 DK levels up to
a mid-DK of approx. 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 DK, the crack increasingly
seeks out Si and intermetallic particles up to a DK of approx. 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-DK. However,
crack closure appears unimportant at high DK due to the large crack openings and evidenced by the fast
crack growth rates observed.

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Identifiers

ORCID for P.A.S. Reed: orcid.org/0000-0002-2258-0347

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