Coarse primary and eutectic Mg2Si phases were generally precipitated in Mg-Al-Si alloys during solidification at a low cooling rate, which tends to deteriorate the strength and ductility of magnesium alloys due to stress concentration. Different volume fractions of TiB2 nanoparticles (1%, 3%, and 5%) were added to an Mg-4Al-1.5Si alloy to refine the coarse Mg2Si phases based on a heterogeneous nucleation mechanism. The nanoparticles were incorporated and dispersed in the molten Mg alloys and by using semi-solid stirring followed by ultrasonic treatment (SSUT), and TiB2/Mg-4Al-1.5Si composites were obtained. The effect of TiB2 content on the microstructure and mechanical properties of the composites was studied. The results showed that the average size of primary Mg2Si phases and α-Mg grains decreased as the TiB2 content raised, the dendritic primary Mg2Si phases were refined into polygonal shapes with smaller sizes, and the refined primary Mg2Si phases were uniformly distributed in the alloys after adding 1 vol.% or 3 vol.% TiB2 nanoparticles. As the TiB2 content increased, the morphology of the eutectic Mg2Si phases was modified from coarse Chinese characters to short rod or fine dot shapes. Vickers hardness and yield strength of the composites reached a maximum (153 HV and 90.9 MPa, respectively) when TiB2 content was 5 vol.%, while the most superior ultimate tensile strength (142.4 MPa) and elongation (9.2%) were obtained when TiB2 content was 3 vol.%, which were improved by 173.2%, 31.5%, 69.8%, and 187.5%, respectively compared with the Mg-4Al-1.5Si alloys.
Keywords: Mg matrix composites; Mg2Si phases; TiB2; mechanical properties; microstructure.