Microstructure characteristics of a rapidly solidified Al-Si-Ti-Pb alloy

The microstructure and annealing behavior of a rapidly solidified Al-5.10Si-1.00Ti-0.70Pb (wt.%) alloy have been examined. As the cooling rates were increased from 10 to 106K/s, the dendritic structures of α-Al were replaced in order by cellular and microcellular structures; the interdendritic or intercellular phases changed from prolonged sheets into discrete rounded particles and from several phases enriched in Si, Ti, Pb, Fe, or P into a single second Pb phase. At the cooling rate of 106K/s, approximately 5.0 wt.% Si and 1.0 wt.% Ti were trapped in the α-Al microcells, whereas only the dispersed Pb particles were observed at the cell boundaries and had an epitaxial cube-cube orientation relation with α-Al matrix. The as-solidified microstructure obtained at the cooling rate of 106K/s has been found to be stable up to 450 °C for 5 h. After annealing at 450 °C for 100 h, a high volume fraction of finely dispersed phases was observed. These dispersed phases are thought to be very important in raising the room- and elevated-temperature performance and wear-resistance capability of the alloy. The precipitation characteristics are thought to result mainly from the effect of the element Ti.