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In this study, we describe the effect of the main alloying elements Ni, Cu and Mg on the mechanical properties of near eutectic and hypoeutectic Al-Si foundry alloys at 250uC after a long term exposure to test temperature. Systematic compositional variations illustrate the significant hardening effect of secondary precipitates such as Al 2 Cu and Mg 2 Si. It is also shown that the strength is increased by the addition of Ni, albeit only to a certain level, depending on the fraction of eutectic phase in the alloy. The alloys are considered as coarse two-phase systems, where a hardening effect is caused by load transfer to the harder phase, which requires a certain contiguity of the latter. This paper discusses the individual influences of Ni, Cu and Mg on the high temperature strength and describes a potentially adverse effect of the combinations Cu/Ni and Cu/Mg.

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In order to investigate the effect of Ni on the high-temperature strength of Al-Si cast alloys, tensile properties of hypoeutectic and eutectic alloys were determined at 250 °C after long-term annealing at test temperature. LOM- and SEM-analysis revealed the existence of Al3Ni-phases in close contact to eutectic Si. It was shown that the strength can be increased by the addition of Ni, though just to a certain level, depending on the fraction of eutectic phase in the alloy. The alloys were considered as a coarse two-phase system where a hardening effect is caused by load transfer to the harder phase, which requires a certain connectivity/contiguity of the latter. The paper describes the extent of contiguity of the eutectic as well as the hard silicon and Al3Ni-phases within the eutectic, and discusses their contribution to an enhanced strength of Al-Si alloys at elevated temperatures.

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