On the Al-Al₁₁Ce₃ Eutectic Transformation in Aluminum-Cerium Binary Alloys

The L ↔ Al + Al₁₁Ce₃ technologically important eutectic transformation in Al-Ce binary alloys, containing from 5 to 20 wt.% Ce and ranging from hypo- to hypereutectic compositions, was examined along with the microstructure and properties of its solidified product. A combination of thermal analysis and metallography determined the coordinates of the eutectic point at 644.5 ± 0.6 °C and 10.6 wt.% Ce, clarifying the existing literature ambiguity. Despite the high entropy of melting of the Al₁₁Ce₃ phase, in hypoeutectic alloys the eutectic was dominated by the regular morphology of periodically arranged lamellae, typical for non-faceted systems. In the lamellar eutectic, however, the faceting of Al₁₁Ce₃ was identified at the atomic scale. In contrast, for hypereutectic compositions, the Al₁₁Ce₃ eutectic phase exhibited complex morphology, influenced by the proeutectic Al₁₁Ce₃ phase. The Al₁₁Ce₃ eutectic phase lost its coherency with Al; it was deduced that a partial coherency was present only at early stages of lamellae growth. The orientation relationships between the Al₁₁Ce₃ and Al in the eutectic structure, leading to partial coherency, were determined to be _Al ║ _Al11Ce3 with _Al ║ _Al11Ce3 and _Al ║ _Al11Ce3 with _Al ║ _Al11Ce3. The Al₁₁Ce₃ phase with a hardness of 350 HV and Al matrix having 35 HV in their eutectic arrangement formed in situ composite, with the former playing a role of reinforcement. However, the coarse and mostly incoherent Al₁₁Ce₃ eutectic phase provided limited strengthening and the Al-Ce alloy consisting of 100% eutectic reached at room temperature a yield stress of just about 70 MPa.