Grain interior precipitation and related lamellar forming grain boundary reaction in Ag–Pd–Cu–Au–Zn alloy

Grain interior precipitation and related lamellar forming grain boundary reaction in an Ag–Pd–Cu–Au–Zn alloy were examined by characterising the hardening behaviour, phase transformations, changes in microstructure and changes in element distribution during aging. The solution treated specimen was composed of the Ag–Au rich α₁ matrix and the CuPd β₁ particle-like structures of various sizes containing Zn. The α₁ phase decomposition into α₂ and α₃ in the grain interior accompanied the microstructural changes into fine tweed-like structure containing lattice strains in the interphase boundaries, resulting in hardening in the early stage of aging process. The increased lattice strains in the grain interior induced the grain boundary reaction, stimulating the development of coarse lamellar structure composed of the α₂ and α₃ phases. The increase in volume fraction of the lamellar structure accompanied the decrease in hardness by consuming the dense interphase boundaries of the fine tweed-like structure.