Improved mechanical properties, grain refinement, and modification of Mg-Si in situ composites by yttrium addition and adjusting cooling rate during solidification

Recently, Mg-based composites have gained more attention in response to the weak strength and wear properties of Mg alloys. The in-situ fabrication of composites via a chemical reaction between elements in the melt enhances the distribution of particles by preventing agglomeration, improves the wettability of the particles, the process is more cost effective, and superior mechanical properties can be achieved. One of the most widely used in situ formed particles is Mg2Si, which can be easily produced by a chemical reaction between Mg and Si elements in the melt. In this regard, the hypoeutectic compositions result in the formation of α–Mg and the Mg2Si/α–Mg eutectic constituent; while in the microstructure of the hypereutectic compositions, the coarse primary Mg2Si particles are present beside the Mg2Si/α–Mg eutectic constituent. The composites containing Mg2Si particles exhibited inadequate room-temperature strength, due to the formation of large dendritic-like primary Mg2Si particles and also flake-like eutectics that act as crack nucleation sites. Adding modifier elements such as Ca, Sr, Ce, Gd, and Y and increasing the cooling rate during solidification could change the morphologies of primary and eutectic Mg2Si phases toward improvement of the mechanical properties. Among the modification elements, the addition of Y at micro-alloying levels has shown promising results for modification of Mg2Si in Mg-Si and Al-Mg-Si alloys. However, the effect of Y addition and adjusting cooling rate on the modification of Mg2Si phase and also grain refinement of α–Mg needs to be systematically investigated. Accordingly, in the present work, pure Mg, Mg–0.6Si, Mg–0.6Si–0.4Y, Mg–3Si, and Mg–3Si–0.4Y were cast into a mold with variable cross sections to adjust the cooling rate during solidification. It was revealed that the addition of 0.6 wt% Si to Mg led to the grain refinement and appearance of Mg2Si/α–Mg eutectic constituent. The addition of 0.4 wt% Y to Mg–0.6Si resulted in the modification of flake-like eutectic Mg2Si to a more round shape and resulted further grain refinement of α–Mg matrix with improved hardness, yield stress (YS), ultimate tensile strength (UTS), and elongation to failure. For the hypereutectic composite (Mg–3Si), coarse primary Mg2Si were observed, resulting in the deterioration of mechanical properties compared to the hypoeutectic composite (Mg–0.6Si). Addition of 0.4 wt% Y to Mg–3Si resulted in the modification of both primary and eutectic Mg2Si and led to the improvement of mechanical properties. Accordingly, the present work revealed that the addition of Y at micro-alloying level and increasing cooling rate during solidification is a viable approach for improvement of final properties of Mg–Si composites.