Al-C-Fe (Aluminum-Carbon-Iron)

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Phase Diagram Evaluations: Section II

Al-C-Fe (Aluminum-Carbon-Iron) V. Raghavan

The early literature on this ternary system was reviewed by [1987Rag], who presented for Fe-rich alloys a liquidus projection, a reaction sequence and four isothermal sections at 1250, 1100, 1000 and 25 C. Subsequently, [1991Har] computed a liquidus projection and eight isothermal sections at 2000, 1850, 1700, 1300, 1200, 1100, 1000 and 800 C. This work was updated by [1993Rag]. A second update by [2002Rag] reviewed the experimental results of [1995Pal]. More recently, [2004Oht] carried out a thermodynamic assessment of this system, which was reviewed by [2007Rag]. Most recently, [2008Con] reexamined the thermodynamic analysis and presented a new thermodynamic assessment. The new results of [2008Con] will be reviewed brieﬂy in this update.

Binary Systems In the Al-C phase diagram, Al4C3 is of ﬁxed composition and has the D71-type of rhombohedral structure. In the Fe-Al phase diagram, the solid solution c based on facecentered cubic (fcc) Fe is restricted by a loop. The solid solution based on the body-centered cubic (bcc) Fe (a) exists in both the disordered (A2) and ordered (B2 and D03) forms. The A2 ﬁ B2 transition is second order down to 600 C; below that, a two-phase ﬁeld of (A2 + B2) intervenes indicative of a ﬁrst-order transition. Apart from

Fig. 1 Al-C-Fe computed liquidus projection [2008Con]

the high temperature cubic phase e (D82, Cu5Zn8-type cubic), there are three intermediate phases in the system with restricted ranges of homogeneity: FeAl2 (triclinic), Fe2Al5 (orthorhombic) and FeAl3 or Fe4Al13 (monoclinic). The stable Fe-C phase diagram has a peritectic reaction at 1493 C, where the c (fcc) forms from L and d (bcc). At 1153 C, a eutectic reaction yields c + graphite (gr). The stable eutectoid reaction occurs at 740 C: c M a (bcc) + gr. Computed phase diagrams of the above binary systems were given by [2008Con].

Computed Ternary Phase Equilibria The phase equilibria near the c region in the computed sections of [2004Oht] were not correctly predicted, as pointed by [2007Rag]. [2008Con] reexamined the thermodynamic model parameters in order to rectify this anomaly. They carried out ab initio calculations to derive the enthalpy of formation of metastable ordered and disordered fcc phases in the Al-Fe system. The computed phase equilibria from the derived parameters showed good agreement with the experimental data of the c loop region at the Fe end and of the Fe solubility in Al at the Al-end. These ab initio parameters were fed along with experimental data in the

Fig. 2 Al-C-Fe computed isothermal section at 1200 C [2008Con]

Journal of Phase Equilibria and Diffusion Vol. 30 No. 4 2009

369

Section II: Phase Diagram Evaluations

Fig. 3 Al-C-Fe computed isothermal section at 1000 C [2008Con]

Table 1

Fig. 4 Al-C-Fe [2008Con]

computed

isothermal

section

at

800 C

Al-C-Fe invariant reaction temperatures (°C)

Invariant reaction

[2008Con]

[2004Oht]

[1995Pal]

[1991Har]

[1986Sch]

U1 : L + a M c + j U2

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