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

•Simultaneously enhanced hardness and electrical conductivity in a Cu–Ni–Si alloy by addition of Cobalt.•The precipitation evolution of Cu–Ni–Si and Cu–Ni–Co–Si aged at 500 for different time are investigated in detail.•The existence of spinodal decomposition has been confirmed in the Cu–Ni–Si alloy, while is not found in the Cu–Ni–Co–Si alloy.•The formation process of Ni2Si and (Ni, Co)2Si phases during aging process are characterized at the atomic scale using 3DAP.