Simultaneously enhanced hardness and electrical conductivity in a Cu–Ni–Si alloy by addition of Cobalt

Abstract The effects of Co on the physical properties and precipitation evolution of a Cu–Ni–Si alloy are investigated via hardness and electrical conductivity testing, transmission electron microscopy and a quantitative three-dimensional atom probe. The precipitation sequence of Cu–Ni–Si alloy aging at 500 °C can be expressed as supersaturated solid solution → solute clusters → GP zones → Ni2Si phases. In the early aging stage, the formation of atomic clusters is accompanied by the occurrence of spinodal decomposition. The addition of Co accelerates the redistribution and agglomeration of solute atoms, resulting in the nucleation of (Ni, Co)2Si phases directly from the supersaturated state, which further hinders atomic cluster formation and spinodal decomposition. The Ni2Si and (Ni, Co)2Si phases, which are divided into a region II containing pure Ni2Si and the (Ni, Co)2Si phase and an unstable region I containing a small amount of Cu atoms, show the same crystal structure and orientation relationship with the matrix. During the aging process, the range of region I is gradually replaced by the core stable region II and accompanied by the growth of the total thickness of the precipitated phase. This remarkable age effect with Co addition can be attributed to the enhanced number of (Ni, Co)2Si precipitates, which simultaneously increases the strength and electrical conductivity.