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

Coarse primary and eutectic MgSi 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 TiB nanoparticles (1%, 3%, and 5%) were added to an Mg-4Al-1.5Si alloy to refine the coarse MgSi 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 TiB/Mg-4Al-1.5Si composites were obtained. The effect of TiB content on the microstructure and mechanical properties of the composites was studied. The results showed that the average size of primary MgSi phases and α-Mg grains decreased as the TiB content raised, the dendritic primary MgSi phases were refined into polygonal shapes with smaller sizes, and the refined primary MgSi phases were uniformly distributed in the alloys after adding 1 vol.% or 3 vol.% TiB nanoparticles. As the TiB content increased, the morphology of the eutectic MgSi 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 TiB content was 5 vol.%, while the most superior ultimate tensile strength (142.4 MPa) and elongation (9.2%) were obtained when TiB 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.