An Electrical Resistance Study of Phase Transformations in Quenched and Annealed CsCl Au-Cd Alloys

When slowly cooled from near the melting temperature, Au-47.5 at% Cd alloy transforms martensitically from a CsCl (β) to an orthorhombic structure (β′) at approximately 60°C. But, when quenched from high temperatures to temperatures just above the cubic-orthorhombic transformation temperature and subsequently cooled, this alloy transforms into another phase (β″). The β″ phase is the characteristic martensitic product of stoichiometric and near stoichiometric (Au-50% Cd) alloys. The same change in transformation behavior can also be achieved by annealing at relatively low temperatures; when slowly cooled from high temperatures to a lower temperature and then annealed for extended periods of time in the vicinity of 75°C, the transformation is from β to β″ upon further cooling. Although the result of these widely different thermal treatments is the same, the mechanisms appear to be quite different. It was also found that the quenched Au-47.5% Cd alloy exhibited the usual cubic-orthorhombic transformation behavior if the quenched-in lattice defects were annealed out (requiring about three hours at 95°C), but the prolonged annealing resulted in the β to β″ transformation.