Coarsening process of decomposed phases in Cu-Ni-Cr alloys

A study of the coarsening process of the decomposed phases in the Cu-34wt.%Ni-4wt.%Cr and Cu-45wt.%Ni-10wt.%Cr alloys was carried out using transmission electron microscopy. As aging progressed, the morphology of the coherent decomposed Ni-rich phase changed from cuboids to platelets aligned on the 〈100〉 Cu-rich matrix directions. Prolonged aging caused the loss of coherency between the decomposed phases and the shape of Ni-rich phase changed to ellipsoidal. The coarsening process of the decomposed phases followed the LSW theory for thermally activated growth in both alloys. The coarsening rate was faster in the symmetrical Cu-45wt.%Ni-10wt.%Cr alloy due to its higher volume fraction of precipitates. The activation energy for thermally activated growth was determined to be about 182 and 102 kJ mol ^-1 in the Cu-34wt.%Ni-4wt.%Cr and Cu-45wt.%Ni-10wt.%Cr alloys, respectively. The lower energy of the former alloy was also attributed to its higher volume fraction of precipitates. The linear variation of the matrix supersaturation Ac with time, t ^-1/3, also confirmed that the coarsening process followed the LSW theory in both alloys. The size distributions of precipitates in the Cu-Ni-Cr alloys were broader and more symmetric than that predicted by the LSW theory.