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hase Diagram Evaluations: Section II

Al-Co-Sc (Aluminum-Cobalt-Scandium) V. Raghavan

This ternary system was investigated by [2001Buk], who presented a partial isothermal section at 600
C and two vertical sections along the ScAl3 -Co2 Al9 and ScAl3 Al9 Co3 Sc2 joins. This system depicts three ternary compounds.

diagram [Massalski2] depicts the following compounds: Co2 Sc (C15; MgCu2 -type cubic), CoSc (B2, CsCl-type cubic), CoSc2 (C16, CuAl2 -type tetragonal), and CoSc3 (CoSc3 -type orthorhombic).

Ternary Phases Binary Systems The Al-rich region of the Al-Co phase diagram was investigated by [1996God], who reported three modifications of Co4 Al13 ; all occurring in a narrow range of composition between 24 and 24.7 at.% Co. The other phases on the Al-rich side are: Co2 Al9 ðD8d -type monoclinic), CoAl3 ðD011 ; Fe3C-type orthorhombic), and Co2 Al5 (D811 -type hexagonal). On the Co-rich side, CoAl (B2, CsCl-type cubic) has a wide range of homogeneity from 48 to 78.5 at.% Co. The Al-Sc phase diagram [Massalski2] depicts the following intermediate compounds: ScAl3 (L12 ; AuCu3 -type cubic), ScAl2 (C15; MgCu2 -type cubic), ScAl (B2, CsCl-type cubic) and Sc2 AlðB82 ; Ni2 In-type hexagonal). The Co-Sc phase

Fig. 1

Three ternary compounds exist in this system: AlCoSc (denoted s1 here) has an extended homogeneity range from 33 to 45 at.% Al and from 28 to 37 at.% Sc. It has the C14, MgZn2 -type hexagonal structure, with the lattice parameters of a ¼ 0:511 nm and c ¼ 0:823 nm at the composition 30Al-37Co-33Sc (at.%) [2001Buk]. It forms from the melt at 1370
C: The second compound Al15 Co8 Sc6 (denoted s2 here) has the D8a ; Mn23 Th6 -type cubic structure with the lattice parameter of a ¼ 1:2094 nm at 55Al-23Co-22Sc [2001Buk]. A third compound Al9 Co3 Sc2 (denoted s3 here) has the Ga9 Co3 Y2 -type of orthorhombic structure (space group Cmcm), with a ¼ 1:2550 nm; b ¼ 0:7317 nm and c ¼ 0:9084 nm; with a small homogeneity range [1987Mar, 2001Buk].

Al-Co-Sc isothermal section at 600
C in the Al-rich region [2001Buk]

Journal of Phase Equilibria and Diffusion Vol. 30 No. 2 2009

179

Section II: Phase Diagram Evaluations

Fig. 2 Al-Co-Sc vertical section along the ScAl3 -Co2 Al9 join [2001Buk]

Ternary Phase Equilibria With starting metals of 99.999% Al, 99.6% Co, and 99.6% Sc, [2001Buk] arc-melted under Ar atm about 20 ternary alloys. The alloys were annealed at 600
C for 1 month and quenched in water. The phase equilibria were studied with x-ray powder diffraction, metallography, hardness measurements, and differential thermal analysis at a heating rate of 80
C per min. The compositions of coexisting phases were measured by electron probe microanalysis and listed. The isothermal section at 600
C constructed by [2001Buk] is redrawn in Fig. 1. All three ternary phases are present. The compounds s2 and s3 are shown in Fig. 1 with a small homogeneity region around the stoichiometric composition. These locations are slightly different from those shown by [2001Buk] in their figure. The homogeneity range of s1 is as indi