The review of this system by [2005Rag1] presented a schematic liquidus projection, full isothermal sections at 1000, 800 and 600 °C, partial isothermal sections at 1200 and 497 °C and a tentative reaction sequence. Subsequently, [2009Che1] reinvestigated this system and presented a liquidus projection for Al-rich alloys, a reaction sequence and the solid-state equilibria between 900 and 600 °C. This work was briefly reviewed in an update by [2010Rag]. More recently, [2011Che] made a detailed a thermodynamic assessment of the system and computed several isothermal sections between 1300 and 800 °C and a liquidus projection. The phase equilibria in the Al-rich region were investigated by [2009Rus].

Binary Systems

The Al-Cr phase diagram was calculated by [2009Che2]. The intermediate phases in the system are: CrAl7 (V7Al45-type monoclinic, denoted θ), Cr2Al11 (CrAl5-type monoclinic, denoted η), CrAl4 (hexagonal, P63/mmc, denoted μ), CrAl3 (ν), Cr2Al (MoSi2-type tetragonal) and an unconfirmed low-temperature phase X at 75 at.% Cr. Between ∼30 and ∼41 at.% Cr, the high-temperature phase denoted γ1 (Cu5Zn8-type cubic) transforms on cooling via a second-order transition to γ2 (Cr5Al8-type rhombohedral). The Al-Ti phase diagram [2006Sch] has the following intermediate phases: Ti3Al (D019, Ni3Sn-type hexagonal, denoted α2), TiAl (L10, AuCu-type tetragonal, denoted γ), TiAl2 (HfGa2-type tetragonal), TiAl3(HT) (D022-type tetragonal), and TiAl3(LT) (tetragonal, space group I4/mmm). In a new assessment, [2008Wit] included two previously-known compounds: Ti2+x Al5−x (tetragonal, space group P4/ mmm, denoted ζ) and Ti3Al5 (Ti3Ga5-type tetragonal, space group P4/mbm). These were excluded by [2006Sch] in their assessed diagram. A second-order transition of (βTi) \(\leftrightarrow\,B2\) suggested in the data reviewed by [2005Rag2] appears to be supported by recent results. The Cr-Ti phase diagram [2002Gho] depicts a continuous bcc solid solution between βTi and Cr. The only intermediate phase of this system Cr2Ti exists in all three Laves modifications. At 1370 °C, γCr2Ti (C14, MgZn2-type hexagonal) forms congruently at 66 at.% Cr from the bcc phase. It transforms at 1270 °C to βCr2Ti (C36, MgNi2-type hexagonal), which decomposes eutectoidally at 801 °C. αCr2Ti (C15, MgCu2-type cubic) forms peritectoidally at 1220 °C and is stable at low temperatures.

Computed Ternary Phase Equilibria

[2011Che] adopted the thermodynamic descriptions of the binary systems from [2009Che2] (Al-Cr), [2008Wit] (Al-Ti), and [2002Gho] (Cr-Ti). The literature experimental data on the ternary system were reviewed and the basis for selection of data as inputs for optimization was outlined. The liquid, bcc and cph phases were modeled as substitutional solutions. The binary intermetallic compounds and the Laves phases were described with appropriate sublattice models, with provision for ternary solubility. The binary compounds in the Al-rich region of the Al-Cr system with ≤25 at.% Cr were assumed to have no Ti solubility. The B2 phase was extrapolated from the binary description of [2008Wit] in the Al-Ti system. The ternary phase with the L12 structure (denoted here as L12 and as τ by [2010Rag]) was modeled as a TiAl3-based intermetallic, in which Cr atoms occupy both Al and Ti sites [2011Che]. Three sublattices were used: (Al,Cr,Ti)0.25(Al,Cr)0.08(Al,Cr,Ti)0.68. The interaction parameters obtained by optimization were listed.

Isothermal sections were computed by [2011Che] at 800, 900, 1000, 1150, 1200 and 1300 °C. Comparison with the corresponding experimental information showed satisfactory agreement. Here, the computed sections at 800, 1000, 1200 and 1300 °C are shown in Fig. 1-4. At 800 °C (Fig. 1), the C14 Laves phase and the ordered B2 phase are stable only in the ternary region. The ternary phase L12 is present. At 1000 °C (Fig. 2) and at 900 °C (figure not shown here), the B2 region is contiguous with the (βTi) region, separated by a second-order transition boundary. At 1200 °C (Fig. 4) and 1300 °C (Fig. 5), the computed sections are compared with the experimental results of [2000Kai]. The B2 phase is not seen in the composition range depicted in these sections, although it is expected along the Al-Ti side [2008Wit]. [2011Che] calculated a liquidus projection (not shown here), which is in reasonable agreement with the experimental findings of [2009Che1].

Fig. 1
figure 1

Al-Cr-Ti computed isothermal section at 800 °C [2011Che]

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Fig. 2
figure 2

Al-Cr-Ti computed isothermal section at 1000 °C [2011Che]

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Fig. 3
figure 3

Al-Cr-Ti computed partial isothermal section at 1200 °C [2011Che]

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Fig. 4
figure 4

Al-Cr-Ti computed partial isothermal section at 1300 °C [2011Che]

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Fig. 5
figure 5

Al-Cr-Ti partial isothermal section at 497 °C in the Al-rich region [2009Rus]

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In a parallel thermodynamic assessment, [2011Wan] incorporated the presence of the B2 phase and recomputed the ternary phase equilibria, using the database of [2005Zha] for the other phases present in the system. Recalculation of the Al-Cr binary phase diagram by [2011Wan] indicated the presence of a very narrow region of B2, consistent with the experimental findings of [1999Hel]. The computed liquidus projection and the isothermal sections at 1000, 800 and 600 °C by [2011Wan] agree with the assessed data of [2005Rag].

With starting metals of 99.999% Al, 99.98% Cr and 99.5% Ti, [2009Rus] arc-melted 20 Al-rich ternary alloys containing up to 13 at.% Cr and 22.5 at.% Ti. The alloys were annealed at 497 °C for 1000 h and quenched in water. The phase equilibria were studied with optical microscopy, x-ray powder diffraction, electron probe microanalysis and differential thermal analysis at a heating/cooling rate of 25 °C/min. The partial isothermal section constructed by [2009Rus] in the Al-rich region at 497 °C (770 K) is shown in Fig. 5. TiAl3(LT) forms tie-lines with CrAl7 and Cr2Al11. From the thermal analysis data, two vertical sections were constructed by [2009Rus] at a Cr:Ti atom ratio of 3:1 and along the TiAl3-CrAl7 join. The vertical section at Cr:Ti atom ratio of 3:1 is shown in Fig. 6. The ternary eutectic reaction E: L \(\leftrightarrow\) (Al) + TiAl3(LT) + CrAl7 is at 617 °C (890 K) [2009Rus].

Fig. 6
figure 6

Al-Cr-Ti vertical section at Cr:Ti atom ratio of 3:1 [2009Rus]

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Since this review was completed, a recent (third) thermodynamic assessment of this ternary system by [2011Cup] has come to the attention of this reviewer.