Grain Boundary Reaction in a Cu-10 mass%Ni-8 mass%Sn Alloy

The characteristics of the grain boundary reaction in a Cu-10 mass%Ni-8 mass%Sn alloy aged isothermally at 623 to 773 K after water quenching from 1123 K were investigated by optical and electron microscopic observations, hardness measurements and the X-ray diffraction methed. Results obtained are as follows:(1) The incubation periods were observed in the nodule growth curves, namely the nodule began to grow after the hardness of the matrix nearly reached a maximum value, and then engulfed the matrix entirely.(2) The pearlitic structure which consisted of the equilibrium α phase and the equilibrium γ phase [(Cu·Ni)3Sn] was observed in the nodule from the period of nodule formation, and the shape of the γ phase scarecely changed with progress of aging. The lattice parameter of the γ phase of an ordered fcc structure was 0.5940 nm.(3) The secondary pearlitic nodule containing γ precipitates larger than those in the primary nodule was observed on the prolonged aging after the completion of the primary grain boundary reaction.(4) The area fraction of the nodule, X, can be represented by Johnson-Mehl's equation: X=1−exp(−btn), where t is the aging time and b and n are constants, and values 2.2∼2.6 were obtained as n.(5) The value of apparent activation energy for nodule growth was about 120 kJ/mol.