The crystallographic and morphological evolution

The crystallographic and morphological evolution of the strengthening precipitates in Cu–Ni–Si alloys

T.Hu,J.H.Chen ⇑,J.Z.Liu,Z.R.Liu,C.L.Wu ⇑

Center for High-Resolution Electron Microscopy,College of Materials Science and Engineering,

Hunan University,Changsha,Hunan 410082,China

Received 14October 2012;accepted 26October 2012

Available online 1December 2012

Abstract

High-resolution transmission electron microscopy and first-principles energy calculations reveal that,upon formation,the hardening precipitates in Cu–Ni–Si alloys are unchanged d -Ni 2Si nanocrystals.However,their crystallographic and morphological features evolve during the precipitation process.It is shown that,in terms of crystallographic orientation relationships,there are basically two types of d -Ni 2Si precipitates in the alloys,referred to as d 1-Ni 2Si and d 2-Ni 2Si respectively.In the early stages of aging (including peak aging),the precipitates are small and belong to the d 1type,with the following orientation relationship with the Cu matrix:[010]d ||[110]Cu and (001)d ||(001)Cu .In the late stages,the precipitates are clearly larger and become the d 2type,with the orientation relationship:[010]d ||[110]Cu and approximately ð301Þd kð1 11ÞCu .Governed by the minimization of its overall energy,a developing d precipitate has to evolve from an almond-like d 1particle with a low-index coherent habit plane to a French baguette bread-slice-shaped d 2particle that has a high-index broad interface.This evolution is found to be in excellent agreement with predictions provided by the invariant line theory.Intermediate stages exist for a particle to accomplish such an evolution,leading to many different crystallographic and morpho-logical appearances of these d -Ni 2Si particles being observed in the alloys.Ó2012Acta Materialia Inc.Published by Elsevier Ltd.All rights reserved.

Keywords:Copper alloys;Precipitation;High-resolution electron microscopy;First-principles calculations;Invariant line

1.Introduction

Cu–Ni–Si alloys are important industrial materials that are widely used for electrical connectors and lead frames,owing to their high electrical conductivity combined with high strength.They can be manufactured with relatively easy processes.They are typical of Cu-based alloy systems with a precipitation hardening effect,due to the formation of Ni–Si precipitates in the Cu matrix upon heating.Understanding the precipitation characteristics,such as the structure,morphology,crystallographic orientation relationship (OR)and evolution,of the strengthening pre-cipitates in these alloys is of crucial importance for making

improvements in the synthetic processes,and has therefore long been a serious subject for study.

However,there is still debate about the structures of the precipitates in age-hardened Cu–Ni–Si alloys.The harden-ing precipitates in the alloys were first identified as the d -Ni 2Si phase according to a quasi-binary section of the ternary Cu–Ni–Si phase diagram [1,2].However,this was later refuted following the identification of the precipitates as the c -Ni 5Si 2phase [3]and the b -Ni 3Si phase [4],respec-tively.Further,the precipitates were found to be an unknown orthorhombic intermetallic nickel–silicon com-pound with a lattice parameter of about four times that for copper [5].Nonetheless,a number of studies support the existence of d -Ni 2Si precipitates in the alloys,though other types of precipitates,such as b -Ni 3Si,(Cu,Ni)3Si,and even Si-rich and Si-poor regions,might coexist [6–13],especially in the early stages of aging [13].

1359-6454/$36.00Ó2012Acta Materialia Inc.Published by Elsevier Ltd.All rights reserved./10.1016/j.actamat.2012.10.031

⇑Corresponding authors.Tel.:+8673188664009(J.H.Chen);tel.:+86

73188664010(C.L.Wu).

E-mail addresses:jhchen123@ (J.H.Chen),cuilan-wu@ (C.L.Wu).

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Available online at

Acta Materialia 61(2013)

1210–1219