Strengthening effects from Mg and Ni in Al-mischmetal eutectic alloys: Insights from microstructures and Bayesian analysis

This study investigates, both experimentally and via machine-learning modeling, the strengthening mechanisms and effects of Mg and Ni additions to Al-MM (mischmetal, a sustainable Ce+La+Nd mixture to replace Ce) alloys, aiming to develop creep- and coarsening-resistant Al-MM-Mg-Ni alloys with a focus on twin eutectic co-solidification microstructures. Based on near-eutectic Al-9MM (wt%) alloy, the Mg and Ni additions introduce solid-solution and eutectic strengthening, respectively. Ternary hypo-eutectic Al-9MM-0.25/0.5/0.75Mg variants improve hardness up to 592 MPa. Ternary Al-9MM-2/4Ni display hypo-eutectic microstructures. Al-9MM-2Ni shows separated growth of Al₁₁MM₃ and Al₃Ni eutectic phases, while Al-9MM-4Ni features finely intertwined Al₁₁MM₃-Al₃Ni fibers from co-solidification. The hyper-eutectic Al-9MM-5Ni contains primary Al₃Ni particles alongside the intertwined fibers, raising the hardness to 739 MPa. Finally, quaternary Al-9MM-0.5/0.75Mg-4/4.5Ni alloys maintain hypo-eutectic microstructures with a significant increase of hardness to 929 MPa. The series Al-9MM, Al-9MM-0.75Mg, Al-9MM-4Ni, and Al-9MM-4.5Ni-0.75Mg exhibits increasing tensile yield strengths of 70, 83, 88, and 105 MPa, with decreasing ductility of 12.0, 5.8, 2.0, and 0.8 %. All Al-9MM-Mg-Ni alloys exhibit excellent hardness retention and coarsening resistance after thermal exposure at 300 or 350°C for up to 11 weeks. A machine-learning model accurately predicts the alloy's hardness evolution under thermal exposure. Feature analysis quantitively shows the strengthening impact of MM, Ni, and Mg addition and demonstrate enhanced strengthening retention, under thermal exposure, of Al₁₁MM₃ over Al₃Ni, alongside the beneficial effects of Mg homogenization. At 300 °C, Al-9MM-Mg-Ni alloys demonstrate higher creep resistance than most precipitation-strengthened Al-Sc-Zr-based alloys and solid-solution-strengthened Al-Mg alloys, with Al-9MM-4Ni as the best performer.