Effects of quenching temperature on the structure, segregation, and properties of the AM4.5Kd + 0.2 wt.% La alloy after artificial aging

Abstract

The identification of structural components in the AM4.5Kd + 0.2 wt.% La alloy, subjected to quenching at different temperatures (535–605 °C) and artificial aging at 155 °C for 4 h, was conducted through electron microscopy and XRD. An increase in the quenching temperature from 535 to 605 °C promotes the enlargement of structural components, including the α-solid solution, various aluminides, and eutectics. We observed that the base metal is not homogeneous in its chemical composition, consisting of two types of solid solutions: α1 and α2. The Cu and Mn solubility in the α2-solid solution is higher than in the α1-solid solution. As the quenching temperature increases to tq = 605 °C, the copper content in the α1-solid solution decreases. In contrast, the copper content in the α2-solid solution follows a curve with two maxima at 545 °С (4.5 at.%) and 585 °С (8.7 at.%). The Mn content in the α1-solid solution decreases sharply to the 545 °С quenching temperature and remains relatively constant up to tq = 605 °С (0.2 at.%). The Mn content in the α2-solid solution follows a curve with its maximum at tq = 545 °С (4.3 at.% Mn). Subsequent temperature rise results in a sharp drop in Mn content from 1.0 at.% at t = 565 °С to 0.3 at.% at 605 °С. Hence, the max solubility of Cu and Mn in the α2-solid solution occurs at 545 °C. At 585 °С, only an elevated Cu content (~8.7 at.%) was observed. Aluminides of alloying elements with different stoichiometries crystallize at different quenching temperatures, with complex AlxTiyLazCuvCdw and AlxCuyMnzCdv alloyed aluminides being most commonly found. ncreasing the quenching temperature to 535–545 °С results in higher hardness of the AM4.5Kd + 0.2 wt.% of La alloy, reaching 98–104 HB, with subsequent decrease to 60 HB as the quenching temperature reaches 605 °С. The hardness of the unhardened alloy is 60 HB. The optimal quenching temperature for the AM4.5Kd + 0.2 wt.% of La alloy is in the range of 535–545 °С. This temperature corresponds to the highest hardness of the alloy and the microhardness of the aluminide.