The effect of lithium on structure formation during crystallization of cast aluminum matrix composite materials

The article presents the results of the study of the structure of cast aluminum matrix composite materials reinforced with Mg2Si particles (15 and 25% (mass.) and modified with lithium (from 0.05 to 0.3% (mass.)). The effect of lithium on the crystallization process, dispersion, morphology, and particle distribution of the reinforcing phase is studied by metallographic analysis and thermodynamic modeling. It is found that the addition of lithium leads to a significant reduction in the size of Mg2Si particles. In the range of 0.05–0.15% (mass.) Li, the proportion of Mg2Si particles in the 1200×970 μm image area increases slightly. However, if the modifier content is increased from 0.15 to 0.2% (mass.) an almost 4.5-fold increase in their total number is recorded (from 724 to 3319 units). Further increase of lithium concentration to 0.3% (mass.) does not lead to a pronounced increase in the number of particles. If a lithium content is 0.3% (mass.) the minimum particle size (5.93 μm for 15%(mass.) Mg2Si and 10.2 μm for 25% (mass.) Mg2Si) and their most uniform distribution (v ≈ 0.41) are achieved. According to thermodynamic modeling by the Sheil-Gulliver method, lithium alters the sequence of phase transformations in the Al – Mg2Si system, contributing to the formation of AlLiSi ternary phase, presumably inhibiting the growth of primary Mg2Si crystals. The obtained results can form the basis for optimizing the technology of synthesis of composites in terms of targeted effects on the processes of formation of their structure, which will expand the potential areas of industrial application of cast aluminum-matrix composites with reinforcing phases of crystallization origin.  The research is funded by the grant of the Russian Science Foundation No. 20-19-00687-P, https://rscf.ru/project/23-19-45019/.