Improving the mechanical properties in an aluminum alloy at high temperatures should acknowledge several factors which are related principally to a decrease in the strength of the metal with increasing temperatures. Most applications of Al-Si casting alloys are generally used at temperatures of no more than 230°C. To improve the strength of the alloys under high temperature conditions, a microstructure containing thermally stable and coarsening resistant dispersoids is required. Nickel leads to the formation of Al3Ni and Al9FeNi in the presence of iron, while zirconium forms Al3Zri. These intermetallics improve the high temperature strength of Al-Si alloys, depending on the shape of the intermetallic particles, their volume fraction and the contiguity with the eutectic Si. The present work aims to investigate the effects of individual and combined additions of nickel and zirconium on the microstructure and strength of the cast Al-Si alloy, namely 354, at high temperatures. The cast alloys were given a solutionizing treatment followed by artificial aging at 190°C for 2 hr. High temperature tensile tests were conducted at various temperatures from 25°C to 300°C. Optical microscopy and electron probe micro-analyzer were used to study the microstructure of different intermetallic phases formed. The fractographic observations of fracture surface were analysed by SEM to understand the fracture mechanism. The results revealed that the intermetallics phases of (Al,Si)3(Zr,Ti), Al3CuNi and Al9NiFe are the main feature in the microstructures of alloys with Zr and Ni additions. The results also indicated that the tensile strength of alloy decreases with an increase in temperature. The combined addition of 0.2 wt% Zr and 0.2 wt% Ni leads to a 30% increase in the tensile properties at 300°C compared to the base alloy. Zr and Ni bearing phases played a vital role in the fracture mechanism of the alloys studied. Tensile strength of 354 alloy with additions of Ni and Zr are decreasing with increase of temperatures. The ultimate tensile strength and yield strength of alloys containing Ni- and Zr-bearing phases are higher than that of alloy for all testing temperatures.


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