Monotectic Al–Bi–Sn alloys directionally solidified: Effects of Bi content, growth rate and cooling rate on the microstructural evolution and hardness

Abstract

Al-based monotectic alloys can have interesting tribological characteristics with the solute acting as a solid lubricant, while the matrix provides required structural integrity. The addition of third elements can increase the alloy load capacity. The microstructural features of these alloys, such as morphology, distribution and length scale of the phases depend strongly on the parameters of their manufacture route. In the present study monotectic Al–Bi–Sn alloys were directionally solidified (DS) under a large range of experimental cooling rates, permitting a wide spectrum of microstructural scales to be examined. Experimental correlations between the microstructure interphase spacing and solidification cooling rate and growth rate are proposed. Despite a slight increase in hardness with smaller interphase spacings for regions closer to the cooled surface of the DS alloys castings having 2 and 3.2wt.%Bi, it is shown that the Bi content of the alloy has not a significant effect on hardness. It is also shown that the experimental correlations established between the cooling rate and both the interphase spacing and area fractions of droplets of the eutectic mixture can be used in the tailoring of the microstructure of Al–Bi–Sn alloys with a view to applications in the manufacture of wear resistant components.