Phase Decomposition during Aging for Cu-Ni-Cr Alloys

Abstract

A study of phase decomposition in the Cu-34% Ni-4%Cr and Cu-45% Ni-10%Cr alloys was carried out in order to analyze the kinetics and morphology of phases. Samples were solution treated, quenched and aged at 600, 700 and 800°C for different times. The solution treated and aged samples were characterized by X-ray diffraction analysis, analytical transmission electron microscopy and field ion microscopy. The X-ray diffraction results indicated the presence of band sides on the 200 diffraction peak and an increase in its intensity with aging time. The TEM microanalysis showed the chemical composition of the decomposed phases increased with aging time. These facts suggest that the phase decomposition took place by the mechanism of spinodal decomposition. The decomposed phases are coherent and aligned on the (100) directions during the first steps of aging, which promote the increase in hardness. The coherency between the decomposed phases disappeared with prolonged agings, which facilitates the overaging, The morphology of decomposed phases changed from equiaxial to cuboids and then to an elongated plate oriented on the (100) directions with the increase in aging time, The equilibrium phases corresponded to a mixture of Cu-rich and Cr-rich phases. Their compositions showed a good agreement with those predicted by the miscihility gap in the equilibrium Cu-Ni-Cr phase diagram. The chromium-richer alloy showed the fastest kinetics of phase decomposition and coarsening, as well as the highest maximum of hardness.