Effect of Ni and/or Si Contents on Age–Precipitation in Cu–Ni–Si Alloys

Abstract

Cu–Ni–Si alloys, so–called the Corson alloys, have been used for electronic devices such as semiconductor lead frames which have excellent electrical conductivity and strength. It is well known that Cu–Ni–Si alloys are strengthened by fine precipitates of δ–Ni₂Si（orthorhombic crystal lattice）during aging treatment. There are many reports about mechanical properties of this alloy system, however, a few papers are dealt with crystal structure evolution of both matrix and precipitates using high resolution transmission electron microscope （HRTEM）.

In this work, we prepared 3 types of Cu–Ni–Si alloys, the base（Cu–5.3 mol％ Ni–2.7 mol％ Si）, excess Ni and excess Si alloys, and the aging behavior and microstructure of those 3 alloys were investigated to understand the effect of ratio of Ni and Si（Ni/Si）added above the solvus of Ni₂Si in the Cu–Ni–Si phase diagram on the types of intermetallic compounds formed during solidification and precipitates formed by aging, and on the hardness. In as quenched samples, 2 types of intermetallic compounds were remained in the 3 alloys, namely, large ones more than 5 μm, and finer ones less than 5 μm. They are mostly identified as δ–Ni₂Si by scanning electron microscope–wave length dispersive X–ray spectroscopy（SEM–WDS）technique. In aged samples, many fine precipitates were confirmed by TEM and HRTEM observation, and they are mostly identified as δ–Ni₂Si in the base and excess Si alloys. The existence of β–Ni₃Si was also pointed out in the excess Ni alloy because of the higher ratio of Ni/Si＝4. It was suggested that the difference in hardening behavior of 3 alloys were caused by these precipitates.