| dc.description.abstract | Light vehicle engine pistons have traditionally been cast from near eutectic Al-Si cast
alloys due to several favourable functional and processing attributes. The
increasingly demanding engine performance requirements have necessitated the
need for the development of multicomponent alloys with high alloy content and
highly complex microstructure. In this regard, recent trends in new piston alloy
development have been to increase the level of various alloying elements such as Cu,
Ni and even Si. However, low Si compositions of ∼7 wt% Si and ∼0.7 wt% Si have also
been proposed largely due to observations that the large blocky primary Si particles
found in the near-eutectic alloys are potent fatigue crack initiators. Nonetheless,
previous research on these low Si piston alloys has demonstrated that their fatigue
performance is significantly impaired by porosity which increases with decreasing Si
content. With improved processing techniques, porosity can be reduced to levels that
make it impotent in fatigue failure processes. The aim of this work was therefore to
characterise the microstructure and fatigue micromechanisms of the low Si piston
alloys after hot isostatic pressing (hipping) to reduce porosity. This was achieved
using a combination of various imaging tools and fatigue testing to establish the role
of microstructure on initiation and growth of fatigue cracks.
It has been demonstrated using X-ray microtomography that hipping significantly
reduces porosity, especially in the 0.7 wt% Si alloy, while the intermetallic structures
remain largely unaffected. The eutectic Si particles in the 7 wt% Si alloy are however
transformed from a fine fibrous interconnected structure to coarse, spheroidised and
discrete particles. Hipping has also been observed to improve the fatigue
performance of the 0.7 wt% Si alloy due to the significant reduction in porosity.
Fatigue crack initiation has been observed to occur mainly at intermetallic particles
in both alloys after hipping and, consistent with previous work, the most frequent
crack initiating phase is found to be Al9
FeNi.
Analysis of short fatigue crack growth profiles has shown that intermetallics and
eutectic Si particles preferentially debond, thus providing a weak path for crack
propagation along their interfaces with the α-Al matrix. However, grain boundaries as
well as these hard particles have also been shown to frequently act as effective
barriers to crack growth. On the other hand, long fatigue crack growth analysis has
shown that fatigue cracks tend to avoid Si and/or intermetallic particles at low ΔK
levels (up to ΔK∼7 MPa√m). At higher levels of ΔK, the cracks increasingly seek out
these hard particles up to a ΔK of ∼9 MPa√m after which the crack preferentially
propagates through them. It has also been observed that crack interaction with
intermetallics causes significant crack deflection which may result in roughness
related closure mechanisms to be activated. | en |
