A contribution to the ternary phase diagrams of Al with Co, Rh and Ir

Abstract

Phase equilibria were studied in Al–Co–Rh above 50 at.% Al at 1100 and 1000 °C and above 70 at.% Al at 900 °C; in Al–Co–Ir above 50 at.% Al at 1100 °C and above 70 at.% Al at 900 °C; in Al–Ir–Rh above 50 at.% Al at 1150 °C and above 70 at.% Al at 900 °C. Wide phase regions extended along about constant Al are common in these ternary alloy systems. A continuous ternary region was observed between isostructural Al9Co2, Al9Rh2 and Al9Ir2 (θ-phase), another region between Al5Co2 and Al5Rh2 (H-phase), and at elevated temperatures also between Al2.63Rh and Al2.75Ir (C-phase). The binary regions forming around equiatomic AlCo, AlRh and AlIr are probably connected by continuous ternary regions. The C-phase was found to extend up to at least 8 at.% Co in Al–Co–Rh and up to 10 at.% Co in Al–Co–Ir. The H-phase was found to extend up to at least 8 at.% Ir in Al–Co–Ir.The ternary extensions of m-Al13Co4 achieved ∼11 at.% Ir and ∼15 at.% Rh, and those of the Al–Co Z-phase up to ∼4 at.% Ir and ∼10 at.% Rh. O-Al13Co4 was found to dissolve up to ∼2 at. % Rh or Ir. M-Al13Co4 was not observed at ternary compositions. The Al–Rh ε-region containing both ε6 and ε16 was found to extend up to ∼8.5 at.% Co and up to ∼20 at.% Ir, while the Al–Rh V-phase was found to extend up to 5.5 at.% Co and at least 13 at.% Ir. Of the Al–Ir phases, the φ-phase was found to dissolve up to 5 at.% Rh, and the χ-phase up to at least 12 at.% Rh but only ∼5 at.% Co. Al3Ir was found to dissolve up to ∼14.5 at.% Co but very little Rh.No ternary phases were revealed in Al–Ir–Rh. A ternary E-phase (Pbma, a = 2.3555, b = 1.6497, c = 2.0035 nm) was exposed around ∼ Al77Co8Rh15. The same structure was also revealed at ∼ Al77Co10.5-16.0Ir12.5-7.0. In addition, Al–Co–Ir contains ternary phases V, ε, W and D. The Al–Co–Ir V-phase and ε-phase are structurally interrelated with the Al–Rh V-phase and ε-phase, respectively. The former is formed at ∼ Al71Co17.5Ir11.5, the latter at Al76Co7.5-10.5Ir16.5-13.5. The W-phase (Pmn21, a = 2.3736, b = 0.8153, c = 2.0757 nm for Al73Co21Ir6) occupies a region inside ∼ Al73.0-71.5Co19.0-22Ir5.0-9.0 at 1100 °C and decomposes between 1000 and 1100 °C. At 1150 °C the same compositional region belongs to a decagonal D-phase, while at 1100 °C this phase exists at ∼ Al74.5Co17.5-21.0Ir8.0-4.5 and at 900 °C around ∼ Al74.5Co19,5Ir6.0.