The microstructures of a thermally sprayed and heat treated Al–20 wt.%Sn–3 wt.%Si alloy

High velocity oxy-fuel thermal spraying has been used to produce deposits of an Al–20 wt.%Sn–3 wt.%Si alloy approximately 300 μm thick on a steel substrate. The microstructures of the as-sprayed and annealed deposits were investigated by X-ray diffraction, scanning and transmission electron microscopy. The as-sprayed material comprised, mainly, elongated splats which were fully molten at the point of impact along with a small proportion of particles which had not been fully melted in the spray. The microstructure was composed of an α-Al matrix, a bimodal distribution of β-Sn and a small fraction of nanoscale Si precipitates. In partially molten splats, Sn formed as micron-sized particles. However, in fully molten splats the high cooling rate from the melt produced a dispersion of Sn particles, 20–100 nm in size, in the Al matrix. This is explained by the existence of a metastable liquid-phase miscibility gap in the Al–Sn phase diagram. Much of the Si, in the as-sprayed material, was retained in solid solution in the α-Al and the small amount of precipitate which formed appeared nano-sized. Following annealing at 300 °C for 1 h, the dispersed Sn particles coarsened and Si precipitated from the supersaturated α-Al as particles 50–200 nm in size.