Effects of Sr on the microstructure and mechanical properties of in situ TiB2 reinforced A356 composite

•Remelting and diluting (RD) approach produces high quality A356-TiB2 composites.•The RD approach helps to perform Si modification by avoiding Sr–B interaction.•0.03 wt.% Sr can fully modify the eutectic Si of the RD A356-TiB2 composite.•Full modification of eutectic Si results in superior mechanical properties.

In situ A356–3 wt.% TiB2 composites were fabricated via a remelting and diluting (RD) approach, to investigate the effect of Sr on the modification of in situ A356–TiB2 composites with respect to the composite prepared by the conventional flux assisted synthesis (FAS) approach. The tensile properties of the composites were tested to evaluate the modification efficiency of Sr in different approaches. The results demonstrated that the RD composite can achieve fully modified eutectic structures than the FAS one owing to avoidance of the Sr–B interaction, which is commonly encountered in the FAS composites. The addition of Sr greatly improves the mechanical properties (especially the elongation) of thus prepared composites, only when the composites are in a fully modified state. Optimum modification of in situ A356–3 wt.% TiB2 composite was obtained with Sr addition in the range around 0.03 wt.%. The elongation of the 0.03 wt.% Sr modified RD A356–3 wt.% TiB2 composite are 6.6% and 5.6%, in as-cast and T6 states, respectively. The improvements in strength and ductility are attributed to the morphology change of Si as well as the improved melt cleanliness.