Spinodal decomposition, ordering transformation, and discontinuous precipitation in a Cu–15Ni–8Sn alloy

Abstract

Transmission electron microscopy (TEM) was employed to study the microstructure and transformation kinetics of a Cu–15Ni–8Sn (wt%) alloy. Five different transformation products were observed: a modulated structure resulting from spinodal decomposition; a DO 22 ordered structure; an L1 2 ordered structure; grain boundary and intragranular γ (DO 3) precipitates; and discontinuous γ precipitates. It was found that spinodal decomposition occurred during the early stage of decomposition below ∼500°C. DO 22 ordering appeared after spinodal decomposition, and L1 2 ordering appeared after DO 22 ordering. Discontinuous precipitation took place at a very late stage of the decomposition process. A time–temperature transformation (TTT) diagram was established on the basis of extensive TEM characterization. Five separate C-curves were found for the five transformation products. This TTT diagram is very helpful in clarifying the confusion and inconsistency in the literature concerning the precipitation process in the alloy. This work is one of the first experimental studies which report that spinodal decomposition can take place before ordering. It also provides evidence for the coexistence of both DO 22 and L1 2 long range order (LRO) structures in the alloy.