Age-hardening structure and mechanism of Cu–3at%Ni–1.5 at%Si Corson alloy

Abstract

The age-hardening structure and mechanism of a Cu–3at%Ni–1.5 at%Si Corson alloy were elucidated using high-resolution scanning transmission electron microscopy. The alloy samples were solution-treated at 850 °C and reached peak strength after 6 h of aging at 450 °C. The hardening structure is composed of δ′-Ni2Si precipitates, which are slightly distorted from δ-Ni2Si (oP12, Pnma, C23, Prototype: Co2Si) by the Cu matrix to increase coherence. After identifying the structure using atomic-resolution energy dispersive X-ray spectroscopy, the electron diffraction pattern could be completely indexed. The δ′-Ni2Si precipitates have stacking faults on the (004)δ'-Ni2Si planes, which result in dark bands in the transmission electron microscopy image of the precipitates and streaks in the electron diffraction pattern. No interface-like crystal habit plane or rigid boundary was observed between the δ′-Ni2Si precipitates and the Cu matrix, indicating a diffuse boundary. The δ′-Ni2Si lattice had corresponding planes that were almost parallel and approximately equal to all the planes in a particular slip system: {111}Cu<110>Cu. From their orientation relationships, the precipitates were to be coherent with the matrix. Under the conditions of this study, the precipitates grew via Ostwald ripening and supported the peak strength via precipitation hardening with the cutting mechanism.