Microstructure and precipitation kinetics in a Cu-7.5Ni-5Sn alloy

TEM was employed to study the microstructure and transformation kinetics of a Cu-7.5Ni-5Sn (wt.%) alloy. Four different kinds of transformation products were observed in the Cu-7.5Ni-5Sn alloy: a modulated structure resulting from spinodal decomposition; a D0 sub 22 ordered structure; grain boundary and intragranular gamma (D0 sub 3 ) precipitates; and discontinuous gamma precipitates. Therefore, the corresponding TTT diagram has four C-curves. To the authors' knowledge, this investigation is the first study of transformation kinetics for this alloy; thus, the TTT diagram determined here provides valuable information for the general understanding of the precipitation kinetics in this alloy. Spinodal decomposition took place before D0 sub 22 ordering in this alloy. The coherent spinodal temperature was identified to be approx440plus/minus10 deg C. No L1 sub 2 ordered phase was found in the time and temperature regions examined. From the present investigation, the following three observations would be useful for directing the heat treatment schedules of the Cu-7.5Ni-5Sn alloy: (a) precipitation hardening treatment should be performed at temperatures < 450 deg C; (b) the fastest precipitation hardening is at 350 deg C, where the nose of the C-curve for spinodal decomposition and D0 sub 22 ordering is situated, thus the shortest annealing time is required to reach good hardness and strength; and (c) this alloy is not very susceptible to overaging.