Thermodynamic assessment of the Al–Cu–Zn system, part II: Al–Cu binary system
Introduction
The thermodynamic description of the Al–Cu system is essential for focused development of Al-based and Cu-based alloys. It is also a key binary edge system for Zn–Al–Cu ternary alloys and, as such, important for Zn-based alloys and thermodynamic databases. In the first thermodynamic description of Al–Cu system [1], four intermediate θ (Al2Cu), η (AlCu), γ (Al4Cu9) and β (Bcc) phases were considered, and the θ, η and γ phases are treated as stoichiometric phases. Later, Murray [2] improved the thermodynamic description of [1] based on a review of the experimental work on Al–Cu system by 1985. Besides these four intermetallic phases, Chen et al. [3] introduced two more stoichiometric phases, the δ1 and α2 phases, into their thermodynamic modeling. Currently, it is the thermodynamic description of the Al–Cu system [4] offered in the public COST507 database that has been most widely used and accepted in multicomponent databases. Unfortunately, only the optimized parameters were published in the COST507 book [4]. Nothing is found on the experimental data evaluation, the thermodynamic model selection and, most importantly, the comparison of calculated results with experimental data for the Al–Cu system in the COST507 publication [4] or any later publication from that author. Based on that dataset [4], Liang and Chang [5] simplified the γ1-Al4Cu9 (cP52, P-43m) phase from a three sublattice model to a single sublattice model together with the γ-Cu8Zn5 (cI52, I-43m) phase in the Cu–Zn system, thus obtaining a complete solid solution range of the γ phase in the Al–Cu–Zn ternary system. Miettinen [6] accepted all the models used as [5] and revised some parameters for their Cu–Al–Zn system. However, their thermodynamic description [6] only focused on the Cu-rich corner, and did not provide parameters for Al-rich phases. But if introducing the missing parameters from [4], then the calculated Al–Cu phase diagram in full composition range is completely off. Witusiewicz et al. [7] revised some parameters of the γ1-Al4Cu9 phase and the liquid phase of [4] in order to avoid an artifact below room temperature and the temperature dependence of the liquid enthalpy of mixing in [4].
Although several thermodynamic descriptions of the Al–Cu binary system are available in the literature [1], [2], [3], [4], [5], [6], [7], none of them provided a comprehensive comparison between results of thermodynamic calculations and available experimental data. In addition, some new experimental results after 1991, the actual year of the COST507 assessment [4] also need to be taken into consideration. It is the purpose of this study to provide this thorough thermodynamic reassessment of the Al–Cu system, which will also serve as a key binary edge system of the Al–Cu–Zn ternary.
Section snippets
Crystal structure of solid phases
Table 1 [8], [9], [10], [11] compiles the crystal structure information of all solid phases. The phase names used as Greek symbols in the text and in the TDB file of this work are provided in the first column. For convenience, the corresponding phase names used in [2], [8] are shown in column 4. The crystal structures of the solid phases are also generally accepted from the Materials Science International Team (MSIT) report [8]. Here we only present the experimental results published later than
Thermodynamic modeling
All equations for the Gibbs energy functions and the notation are the same as given in Part I of this work [46]. The Gibbs energies of the pure components are accepted from the SGTE compilation by Dinsdale in the most recent electronic version (SGTE-Unary-v5.1b, 24 November 2010, www.sgte.org), which is identical with the print version [47], except that more figures for the parameters of Al are given. The Gibbs energies for liquid, the terminal Fcc phase, denoted as (Al) and (Cu), and the
Phase diagram
Fig. 1 represents the calculated phase diagram of Al–Cu system in this work. This diagram is essential to clearly assign the current phase notation to all single phase regions of this complex system. Fig. 2 compares the calculated phase diagram with experimental data [8], [9], [10], [11]. The plotted “experimental” phase diagram is taken from the most recent and comprehensive critical review performed by Gröbner [8], and the more recent experimental data points are shown in original from [9],
Conclusion
The key improvements of this thermodynamic re-assessment compared to previous Calphad descriptions are:
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For the first time all available experimental data of the Al–Cu system have been extensively compared with calculated results.
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The model selected for the γ1 and γ2 phases partially reflects the crystal structure of the γ phase while being sufficiently simple for a realistic joining with the γ-CuZn phase in the Al–Cu–Zn ternary system.
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The γ1/γ2 transition, modeled as very narrow two-phase
Acknowledgment
This study is supported by the German Research Foundation (DFG) under Grant no. Schm 588/41.
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