Purple Plague Revisited

Intermetallic formation in gold-aluminum systems has been studied at 200°C to 460°C using butt-welded diffusion couples. The predominant intermetallic phase was found to be Au5Al2, but all five equilibrium phases of the system were present in the couples after annealing at long times. The kinetics of the process were measured and the activation energy for overall intermetallic formation was found to be 15.9 kcal/mol. The effect of silicon on intermetallic formation was also investigated. It was found that while silicon had little effect on the growth rate of AuAl, Au2Al, Au5Al2, and Au4Al, it substantially increased the amount of purple phase in the couples. An electron microprobe traverse indicated that this purple phase contained 6 wt% Si. The relative tensile strengths of the intermetallics were determined indirectly by measuring the tensile properties of the diffused couples. It was found that all the intermetallics were stronger than gold or aluminum. However, appreciable voiding occurred in Au5Al2 after long times above 300°C and in AuAl2 after long times above 400°C weakening these phases catastrophically. The voiding was greatly accelerated by intermittent aging.