Thermodynamic descriptions of the Cu–Zn system

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Sinn-wen Chena) Department of Chemical Engineering, National Tsing Hua University, Hsin-Chu 300, Taiwan (Received 17 July 2007; accepted 8 October 2007)

Cu–Zn is an important binary alloy system. In the interested temperature range from 300 to 1500 K, there are eight phases, liquid, Cu, ␤, ␤⬘, ␥, ␦, ⑀, and Zn phases. The thermodynamic descriptions of the Cu–Zn system are reassessed using the CALPHAD method. A new description of liquid phase and simplified description of body-centered cubic (bcc) phase are proposed. Good agreement has been found among the calculated thermodynamic properties, phase diagram, and the experimental information.

I. INTRODUCTION

Cu–Zn is an important binary alloy system. Copperrich Cu–Zn-based alloys are primary structural materials.1,2 Phase relationships of the Cu–Zn system at the Cu-rich side had been examined extensively very early due to the industrial interests of brasses.1,2 Even though not as critical as in the Cu industry, the Cu–Zn binary system is important for the aluminum industry as well, because the Al-rich Al–Mg–Cu–Zn alloys are the base alloys for almost all the commercial aluminum alloys.3,4 The Cu–Zn system has received attention again recently because the Sn-rich Sn–Cu–Zn-based alloys are promising Pb-free solders.5,6 Understanding the interfacial reactions between Cu and Zn is valuable for the electronic products as well, since Cu is frequently used as wetting layers for under-bump metallurgy (UBM) and conduction lines and Zn is an promising ingredient element of Pb-free solders.5,6 It is well known that good thermodynamic descriptions of a binary system are crucial for both the understanding of the binary system itself and its use for building thermodynamic descriptions of higherorder systems.7 This study thus reassesses the liquid phase and proposes a simplified description of the bodycentered cubic (bcc) phase in the Cu–Zn binary system, primarily due to the recent interests of electronic applications. II. PHASE EQUILIBRIA DATA

The first phase diagram of Cu–Zn system was published in 1913.1 Based on the available experimental a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0035 258 J. Mater. Res., Vol. 23, No. 1, Jan 2008 http://journals.cambridge.org Downloaded: 07 Mar 2015

data,1,2,8–10 Hansen11 proposed the first assessed Cu–Zn phase diagram in 1936. Although there are some modifications,12,13 there are no dramatic changes of the phase boundaries of the Cu–Zn phase diagram proposed by Hansen.11 There are eight solution phases in the system: liquid, Cu, ␤, ␤⬘, ␥, ␦, ⑀, and Zn.13 The crystal structures of the solid phases are: Cu—FCC_A1, ␤—BCC_A2, ␤⬘—BCC_B2, ␥—D82 BCC, ␦—BCC_A2, ⑀—HCP_A3, and Zn—HCP_Zn, and they are listed in Table I. The most recent assessment by Massalski et al.13 summarized that there are six invariant reactions: five peritectics and one eutectoid. All the data concerning invariant reactions used for the assessment are shown in Table II. Order–disorder transformation of the ␤⬘–␤ phase has be

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