| The effects of Mn substitution for Ni on the decomposition behavior of the supersaturated α and properties in Cu-x%Ni-y%Mn(x＋y＝9)-6%Sn alloys were investigated. When the alloys with less than 2 wt% Mn were aged below 350℃, the supersaturated α was decomposed in three stages. The spinodal decomposition occurred in the first stage and the metastable γ` phase with D0_(22) type superlattice nucleated from the modulated structure in the second stage. In the final stage of transformation, the γ` phase decomposed into the equilibrium γ phase. For the alloys with more than 3 wt% Mn, however, the spinodal decomposition and the γ` were not observed at all the aging temperatures. All the alloys were found to strengthen exhibiting very high tensile strength larger than 1,000 MPa by aging at the temperatures below 350℃. However, the strengthening mechanisms were changed with the amount of Mn substitution for Ni. The less than 2 wt% Mn alloys were strengthened by the spinodal decomposition and γ` formation, while the more than 3 wt% Mn alloys were strengthened by the precipitation of Ni₂MnSn phase. When the specimens were aged at above 500℃, the discontinuous precipitate consisting of α and γ formed at the initial stage of transformation in all the alloys examined. All the alloys were not strengthened by aging at temperatures above 450℃. In all of the alloys examined, the electrical resistivity was decreased by aging. With increasing the amount of Mn substitution for Ni, the formation of discontinuous precipitate accelerated and the electrical resistivity increased. The maximum amount of Mn substitution for Ni to ensure the spinodal decomposition in the Cu-xNi-yMn(x＋y＝9)-6Sn alloys appears to be about 2 wt%. |
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