Microstructure Formation of Al-Based Alloys in the Presence of Rare Earth Metals (Ce, La) and Mn under High Cooling Rate

This study explores the influence of chemical composition and cooling rate on the microstructural formation of Al-based alloys, predominantly alloyed with Mn and Ce. Approximately 1 kg of the alloys with varying alloying element content was prepared in a protective resistance furnace. The solidified alloys were then sectioned into 100 g pieces, remelted, and cast into massive copper molds, resulting in samples with different thicknesses ranging from 0.3 to 6 mm. The cooling rates were precisely determined using numerical simulation, ranging from 10³ to 10⁴ K/s. The experimental results demonstrate that, at moderately high cooling rates (above 10³ K/s), the primary intermetallics precipitated were identified as Al₂₀Mn₂Ce. However, at lower cooling rates, particularly when the Mn and Ce content were high, the formation of the “τ”-phase, Al₈Mn₄Ce, was observed and characterized by its detrimental dendrite-like morphology. Interestingly, as the cooling rate approached 10⁵ K/s and in the presence of an optimal chemical composition (with sufficiently high Mn and Ce content), an amorphous structure formed at the outer surface of the samples. This amorphous structure exhibits intriguing potential for certain applications due to its unique properties. The desirable microstructure of bulk samples consists of submicron intermetallic compounds embedded in a boundary-free Al-matrix, which is promising for enhancing material strength and performance. This investigation sheds light on the crucial role of chemical composition and cooling rate in controlling the microstructural features of Al-based alloys, thereby providing valuable insights for the development of advanced materials with tailored properties for specific industrial applications. The role of rare earth metals (Ce and La) as poisoning and GFA (glass-forming ability) contributing elements is clarified. The Widmanstätten structure, appeared in the presence of high content of Mn, is discovered.