Abstract
The grain boundary reactions in Au-Ag-Pd-Cu alloys containing 0-40 mass% Au were investigated by electrical resistivity measurements, hardness tests, optical microscopic observations, and X-ray diffraction.
These alloys were quenched from 1073 K and aged at 573, 623, 673, and 773 K. They showed a single phase (fcc) after the solution treatments. The grain boundary reaction after the aging treatments resulted in the formation of the nodule consisting of Ag-rich solid solution, α2 (fcc), and PdCu superlattice, β (CsCl type).
The main results obtained are as follows.
(1) Retardation of the grain boundary reaction and increasing of the interlamellar spacing with increasing Au content were considered due to the fact that the nucleation of the nodule is difficult as the aging temperature approaches the phase boundary temperature which becomes lower with increasing Au content.
(2) Internal strain resulting from continous precipitation become an effective driving force for the nucleation of the nodule because the nodule is also formed in the precipitate free zone.
(3) The area fraction of the nodule fits to the Johnson-Mehl equation, the nucleation sites of nodule being the edge and plane of the grain boundary.
(4) The estimated values of activation energy for the nodule formation are 110-170 kJ/mol.
