New mechanisms and approaches in design and heat treatment of Al-Cu-Mn-Zr(-Sn, Si) based casting alloys aiming to increase heat resistance via formation of α(AlMnSi) phase dispersoids assisted by θʹ precipitates

The new ACLSZ alloy with the composition (wt.%) Al-5.6Cu-0.48La-0.44Mn-0.2Zr-0.1Sn-0.21Si was subjected in the as-cast state to high-temperature aging at 300 °C for 3 h (without preliminary solid solution heat treatment) and further stabilization annealing at 350 °C for up to 200 h. Comprehensive structural analysis revealed the presence of the previously undescribed intermetallic Al₄LaSi(Cu,Mn)₂ compound with a tetragonal P4/mmm structure (tP8/19). The hardness of the alloy after aging and stabilization annealing reaches ∼91 HV after a 100 h exposure and then remains almost unchanged. Transmission electron microscopy (TEM) study showed the formation of classical plate-like precipitates corresponding to the θʹ-phase after aging. After long-term annealing at 350 °C for up to 200 h, fine rounded or slightly elongated crystals of the α(AlMnSi) phase with a size of 30–50 nm and a high number density are found in association with θʹ-phase platelets. The data obtained allowed one to develop new mechanisms for the improvement of the heat resistance of the Al-Cu-Mn-Zr based alloys: (i) formation of plate-like θʹ-phase precipitates through high-temperature aging. At the same time, the aluminum solid solution must be supersaturated with Mn; (ii) at the second sub-stage the θʹ-phase crystals should be further stabilized through annealing due to the combined segregation of Zr, Si and Mn atoms at the θʹ/(Al) interfaces; (iii) during further annealing, the assisted formation of the fine α(AlMnSi) phase dispersoids with high number density and high heat resistance takes place directly at the θʹ-phase precipitates.