Microstructure and Selection of Grain Boundary Phase of Cu-Ni-Si Ternary Alloys

Cu-Ni-Si alloys with 90wt% Cu and 10wt% (Ni+Si) of different Ni/Si atomic ratios were prepared under a near-equilibrium solidification condition. The microstructures were observed by scanning electron microscope (SEM). The phase composition was identified by X-ray diffraction (XRD). The phase transformations during the solidification process were analyzed by differential scanning calorimetry (DSC) under an Ar atmosphere. Results show that the effects of Ni/Si atomic ratios on the microstructures of Cu-Ni-Si alloys can be mainly ascribed to the variation of the amount of delta-Ni2Si phase formed in the inner of alpha-Cu grains and the selection of grain boundary phase between Cu-Si and Ni-Si phases. The former is controlled by the Ni and Si concentrations in the primary alpha-Cu grains and the latter is a result of the competition between Cu-Si and Ni-Si intermetallics. With the increase of Ni/Si atomic ratio, Ni-Si intermetallics gradually win from the competition due to its lower heat of formation and the increase of Ni content in the inter-granular residual liquid. As a result, grain boundary phase changes from Cu5Si phase to Ni3Si phase gradually.