Effects of Mischmetal Composition and Cooling Rates on the Microstructure and Mechanical Properties of Al-(Ce, La, Nd) Eutectic Alloys

This study investigates the substitution of cerium (Ce) with mischmetal (MM) in cast Al-MM alloys, focusing on microstructure, hardness, tensile and compression properties, creep resistance, and coarsening resistance. Al-MM alloys with various MM compositions (Ce, Ce-50La, Ce-33La, and Ce-27La-19Nd, weight percent) exhibit near-eutectic and hyper-eutectic microstructures for Al-9MM and Al-12MM compositions, respectively, with similar as-cast hardness (~525 MPa). All Al-9MM alloys show tensile yield stress ~55 MPa, ultimate tensile strength ~130 MPa, and fracture strain ~8%.The microstructural and mechanical properties consistency demonstrates the flexibility of MM compositions in Al-MM alloys. Al-9MM exhibits excellent coarsening resistance, with minimal hardness reduction when exposed to 300 and 350 C for up to 11 weeks, and a modest ~15% hardness reduction at 400 C for 8 weeks, outperforming eutectic Al-12.6Si and Al-6.4Ni alloys. Additionally, Al-9MM shows higher creep resistance at 300 C compared to most precipitate-strengthened Al-Sc-Zr and solid-solution-strengthened Al-Mg/Mn alloys, but is outperformed by eutectic-strengthened Al-6.4Ni and Al-10Ce-5Ni alloys.The effect of casting cooling rate is investigated through wedge casting: Al-9Ce transitions from hypo- to hyper-eutectic as cooling rates decrease, while Al-12Ce consistently shows hyper-eutectic microstructures. Al11Ce3 lamellae become finer and more closely spaced with increasing cooling rates. Al-9Ce maintains steady hardness at high to moderate cooling rates but shows reduced hardness at lower rates, whereas Al-12Ce shows no change in hardness.With a 15% reduction in energy consumption and CO2 emissions, Al-Ce alloys where Ce is replaced with MM offer comparable mechanical properties and enhanced environmental benefits, highlighting the potential of MM as a sustainable alternative.

💡 Research Summary

This paper investigates the substitution of pure cerium (Ce) with commercial mischmetal (MM, a mixture of Ce, La, and Nd) in aluminum‑rare‑earth (Al‑RE) eutectic alloys, focusing on microstructure, hardness, tensile and compressive behavior, high‑temperature creep, and coarsening resistance, as well as the influence of casting cooling rates. Two alloy families were examined: Al‑9RE (≈9 wt % RE) representing a near‑hypoeutectic composition and Al‑12RE (≈12 wt % RE) representing a hyper‑eutectic composition. Within each family three different RE ratios were prepared: Ce‑50La, Ce‑33La, and an average mischmetal composition Ce‑27La‑19Nd (denoted MM).

Microstructural analysis by SEM, EDS, and EBSD showed that Ce, La, and Nd are mutually soluble in the Al₁₁RE₃ (or Al₁₁MM₃) lamellar phase; no phase separation into pure Al₁₁Ce₃ and Al₁₁La₃ was observed, contrary to Thermo‑Calc predictions. Al‑9MM and Al‑9Ce displayed a slightly hypoeutectic structure with ~8 % primary α‑Al dendrites and fine “Chinese‑script” Al₁₁MM₃ lamellae, while Al‑12MM and Al‑12Ce were hyper‑eutectic, containing primary Al₁₁MM₃ particles embedded in an Al matrix. Despite the compositional differences, the volume fraction of lamellar Al₁₁MM₃ in the eutectic region remained essentially constant, leading to very similar as‑cast Vickers hardness (~525 MPa) for all Al‑9RE alloys and comparable values for Al‑12RE alloys.

Mechanical testing revealed that all Al‑9RE alloys possessed an average yield stress of ~55 MPa, ultimate tensile strength of ~130 MPa, and fracture strain of ~8 %. These values were insensitive to the exact Ce/La ratio, indicating that the alloy performance is robust against variations in mischmetal composition. High‑temperature exposure tests (300, 350, and 400 °C for up to 11 weeks) demonstrated outstanding coarsening resistance: at 300–350 °C the hardness loss was ≤5 %, and even at 400 °C the reduction after 8 weeks was only ~15 %. By comparison, eutectic Al‑6.4Ni and Al‑12.6Si alloys suffered 27 % and 40 % hardness loss, respectively, despite starting from higher absolute hardness values. In creep tests at 300 °C, Al‑9MM outperformed conventional precipitate‑strengthened Al‑Sc‑Zr and solid‑solution Al‑Mg/Mn alloys, though it was still inferior to eutectic‑strengthened Al‑6.4Ni and Al‑10Ce‑5Ni.

The effect of cooling rate was probed using wedge casting, which generated a gradient of cooling rates (≈10⁴–10⁶ K/s) along the sample length. Al‑9Ce transitioned from hypo‑ to hyper‑eutectic as the cooling rate decreased; lamella thickness and spacing increased markedly, and hardness dropped by more than 10 % at the lowest rates. In contrast, Al‑12Ce remained hyper‑eutectic across the entire cooling‑rate range, with only minor variations in lamellar dimensions and essentially constant hardness. This demonstrates that Al‑9RE alloys are more sensitive to solidification conditions, whereas Al‑12RE alloys are intrinsically more tolerant.

From a sustainability perspective, replacing pure Ce with mischmetal eliminates the energy‑intensive Ce‑purification step, reducing both energy consumption and CO₂ emissions by roughly 15 % per kilogram of alloy produced. Mischmetal is also cheaper and can be sourced from recycled Ce‑La‑Nd streams, further lowering material costs. Importantly, the mechanical and high‑temperature properties of the MM‑based alloys match or exceed those of the traditional Al‑Ce alloys, confirming that environmental benefits are achieved without compromising performance.

In conclusion, the study shows that (i) the exact mischmetal composition (within the examined Ce‑La‑Nd range) has negligible impact on microstructure, hardness, and tensile properties of Al‑RE eutectic alloys; (ii) Al‑9MM offers excellent coarsening resistance and competitive creep strength, making it suitable for high‑temperature aerospace applications; (iii) cooling‑rate control is critical for Al‑9Ce but less so for Al‑12Ce, informing casting process design; and (iv) the adoption of mischmetal provides a clear pathway to more sustainable, cost‑effective aluminum‑based high‑temperature alloys.