The debonding and fracture of Si particles during the fatigue of a cast Al-Si alloy
Constant-amplitude high-cycle fatigue tests ({sigma}{sub max} = 133 MPa, {sigma}{sub max}/{sigma}{sub y} = 0.55, and R = 0.1) were conducted on a cylindrical samples machined from a cast A356-T6 aluminum plate: The fracture surface of the sample with the smallest fatigue-crack nucleating defect was examined using a scanning electron microscope (SEM). For low crack-tip driving forces (fatigue-crack growth rates of da/dN {lt} 1 x 10{sup {minus}7} m/cycle), the authors discovered that a small semicircular surface fatigue crack propagated primarily through the Al-1% Si dendrite cells. The silicon particles in the eutectic remained intact and served as barriers at low fatigue-crack propagation rates. when the semicircular fatigue crack inevitably crossed the three-dimensional Al-Si eutectic network, it propagated primarily along the interface between the silicon particles and the Al-1% Si matrix. Furthermore, nearly all of the silicon particles were progressively debonded by the fatigue cracks propagating at low rates, with the exception of elongated particles with a major axis perpendicular to the crack plane, which were fractured. As the fatigue cracks grew with a high crack-tip driving force (fatigue-crack growth rates of da/dN {gt} 1 x 10{sup {minus}6} m/cycle), silicon particles ahead of the crack tip were fractured, and the crack subsequently propagated through the weakest distribution of prefractured particles in the Al-Si eutectic. Only small rounded silicon particles were observed to debond while the fatigue crack grew at high rates. Using fracture-surface markings and fracture mechanics, a macroscopic measure of the maximum critical driving force between particle debonding vs fracture during fatigue-crack growth was calculated to be approximately K{sub max}{sup tr} {approx} 6.0 MPa {radical}m for the present cast A356 alloy.
- Research Organization:
- Univ. of Colorado, Boulder, CO (US)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC04-94AL85000
- OSTI ID:
- 20005976
- Journal Information:
- Metallurgical and Materials Transactions. A, Physical Metallurgy and Materials Science, Vol. 30, Issue 12; Other Information: PBD: Dec 1999; ISSN 1073-5623
- Country of Publication:
- United States
- Language:
- English
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