Effect of morphological changes of eutectic Si particles on the ambient and high temperature tensile properties of Zr containing Al–Si alloys

Abstract

In the present study, the morphological evolution of eutectic Si particles subjected to extended thermal modification and chemical treatment has been investigated for Zr containing A354 and A356 alloys along with its effect on the mechanical properties. Thermal modification of silicon particles proved to be more effective in the Sr-modified alloys rather than their Sr-free counterparts. The evolution of silicon particles during extended solution treatments followed the same trends and sequences for non-modified and Sr-modified 354- and 356-type alloys, at different evolution rates. The coarsening of eutectic Si particles occurred through particle coalescence and Ostwald ripening mechanisms. While both mechanisms were active at the same time, however, they operated independently and additively. The pinholes observed in the silicon particles derive from the impression or imprint left behind from the collision of small particles with, and their diffusion into, larger particles. With respect to the tensile test data obtained at room temperature, solution heat treatment improved the UTS and ductility values of both Sr-modified and non-modified alloys in the first 100h of the treatment followed by reduction in the values as a result of the morphological changes in the Si particles; however, YS values remained almost unchanged. Morphological changes in the Si particles had a very limited effect on the high temperature tensile properties and, surprisingly, this limited effect extended to the ductility values as well.