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Section II: Phase Diagram Evaluations

Al-Cu-Sc (Aluminum-Copper-Scandium) V. Raghavan

In the early work on this system, two isothermal sections at 500 and 450 C by [1991Kha], a partial vertical section at 20 mass% Cu by [1988Kha], and a partial vertical section and liquidus projection for Al-rich alloys by [1992Tor] were reported. Recently, [2010Bo] carried out a thermodynamic analysis of this system and compared

the calculated phase equilibria with the reported experimental data.

Binary Systems The Al-Cu phase diagram [2004Ria] depicts the following intermediate phases: CuAl2 (C16-type tetragonal,

Fig. 1 Al-Cu-Sc computed isothermal section at 500 C for Al-rich alloys [2010Bo]

Fig. 3 Al-Cu-Sc computed vertical section along 40Cu60Al2Sc98Al (mass%) join [2010Bo]

Fig. 2 Al-Cu-Sc computed vertical section at 20 mass% Cu [2010Bo]

Fig. 4 Al-Cu-Sc computed liquidus projection for Al-rich alloys [2010Bo]

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Journal of Phase Equilibria and Diffusion Vol. 31 No. 6 2010

Phase Diagram Evaluations: Section II denoted h), CuAl(HT) (g1, orthorhombic) CuAl(LT) (g2, monoclinic), Cu5Al4(HT) (f1, orthorhombic, space group Fmm2), Cu5Al4(LT) (f2, orthorhombic, space group Imm2), e1(HT) (cubic), e2(LT) (B82, Ni2In-type hexagonal), Cu3Al2 (d, rhombohedral), Cu9Al4(HT) (c0, D82, Cu5Zn8-type cubic), Cu9Al4(LT) (c1, D83, Cu9Al4-type cubic), and Cu3Al (b, bcc). The Al-Sc phase diagram [2010Bo, Massalski2] depicts the following intermetallic compounds: Al3Sc (L12, AuCu3-type cubic), Al2Sc (C15, MgCu2-type cubic), AlSc (B2, CsCl-type cubic), and AlSc2 (B82, Ni2Sntype hexagonal). The Cu-Sc phase diagram [2010Bo, Massalski2] depicts three compounds: Cu4Sc, Cu2Sc (C11b, MoSi2-type tetragonal) and CuSc (B2, CsCl-type cubic).

Computed Ternary Phase Equilibria [2010Bo] used the substitutional solution model to describe the liquid, fcc, bcc and cph phases. A ternary interaction parameter was used for the liquid phase only. The binary compounds were treated as stoichiometric phases. A ternary compound Al8-xCu4+xSc (0 £ x £ 2.6) (denoted s; ThMn12-type tetragonal) is known in this system. This ternary phase was modeled as (Al,Cu)0.6154(Al, Cu)0.3077Sc0.0769. The Al-Sc and Cu-Sc binary systems were reoptimized. The binary description of Al-Cu by [2004Wit] was adopted. The optimized interaction parameters along with those used from literature were listed [2010Bo]. The computed isothermal section for Al-rich alloys at 500 C shown in Fig. 1 was found to agree well with the experimental results of [1991Kha]. In Fig. 2 and 3, the

computed vertical sections at 20 mass% Cu and along 40Cu60Al-2Sc98Al (mass%) join are compared with the experimental results of [1988Kha] and [1992Tor] respectively. The computed liquidus projection for Al-rich alloys is shown in Fig. 4. The transition reaction U: L + Al3Sc M (Al) + s and the ternary eutectic reaction E: L M (Al) + h + s occur at 575 and 547 C respectively. The computed temperatures and compositions of the invariant reactions agreed well with the experimental values of [1992Tor].

References 1988K