Correlation between unsteady-state solidification thermal parameters and microstructural growth of Zn–8 mass% Al and Zn–8 mass% Al–XBi tribological alloys

Abstract

In the present study, directionally solidification experiments are performed with Zn–8 mass% Al and Zn–8 mass% Al–X mass% Bi (X = 1.5, 2.3 and 3.0 mass%) alloys using a water-cooled solidification system, which permits a wide range of solidification cooling rates to be investigated in a single experiment. The microstructural phases of the ternary alloys are shown to be quite similar to those of the binary alloy, with equiaxed dendrites immersed in a matrix formed by fibers or lamellae of the Al/Zn eutectoid product, with the difference that the ternary alloys have Bi droplets disseminated into the matrix and dendrite branches. The dendrites, lamellae and fibers evolve from a refined microstructure at regions closer to the water-cooled bottom of the casting, to increasingly coarser microstructures toward the top. Experimental expressions relating the secondary dendritic arm spacing, the spacing between lamellae and the fiber spacing to solidification thermal parameters (growth and cooling rates) are derived. For the alloys having higher Bi content, a bimodal distribution of Bi droplets was shown to occur, with small droplets, consisting of lamellae and fibers, disseminated into the matrix and larger droplets between the dendritic branches.