The bimodal effect of La on the microstructures and mechanical properties of in-situ A356–TiB2 composites

Highlights: • The addition of La refines the grains and modifies the Si into fibrous particles. • The optimum amount of La for α-Al grain refinement is 0.1 wt.%. • The optimum amount of La for eutectic Si modification is 0.8 wt.%. • Adding La allows the elongation in fracture to be nearly doubled. • Tensile properties show a bimodal distribution peaking at 0.1 wt.% and 0.8 wt.% La. - Abstract: In this work, in-situ A356–2.5 wt.% TiB₂ composites were fabricated via a re-melting and diluting (RD) approach. The effects of lanthanum (La) on the microstructures and mechanical properties of the composites were investigated. The results demonstrated that the addition of La can, simultaneously, refine the second dendrite arm spacing (SDAS) of the α-Al grains and modify the coarse lamellar eutectic Si into fine fibrous particles. The optimum addition levels for refinement and modification lay at 0.1 wt.% and 0.8 wt.%, respectively, due to the distinct mechanisms between the two. Since the resultant mechanical properties are a combined function of refinement and modification, the tensile properties of the composites show a bimodal distribution with the first peak value at 0.1 wt.% and the latter one at 0.8 wt.% La, in accordance with the optimum refinement and modification, respectively.