Effect of TiB₂ Nanoparticle Content on the Microstructure and Mechanical Properties of TiB₂/Mg-4Al-1.5Si Composites

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.