Effect of existing form of alloying elements on the microhardness of Al–Si–Cu–Ni–Mg piston alloy

Effect of alloying elements including Si, Cu, Ni and Mg with different existing forms on the microhardness of Al–12Si–4Cu–2Ni–2.6Mg (in wt.%) piston alloy was investigated. It is found that microhardness of the alloy reaches to 1.88 GPa when all the Cu, Ni, Mg and 3.63 wt.% Si dissolved in the α-Al matrix, and increases to 2.02 GPa while only the solid-solved Si precipitates to form nanoparticles. The precipitated Si has greater contribution to the alloyʹs microhardness than the solid-solved Si in the same level. Furthermore, the precipitation of intermetallics containing Cu, Ni and Mg that disperses in the Al-matrix can further improve the microhardness to 2.09 GPa. The hardening efficiency of the intermetallics precipitation of Cu, Mg and Ni on microhardness is less than that of the nanoscale Si precipitates under the same mass fraction. For Cu, Mg and Ni alloying elements, whether dissolving in α-Al matrix or precipitating as nanoparticles, there is no evident difference on improving microhardness of the tested alloy.