Microstructure and mechanical properties of bimodal syntactic foams with different size combination and volume fraction of alumina hollow spheres

Highlights: • Bimodal syntactic foams with combination of different size ranges are obtained. • Mixing ratios of reinforcements affect compressive properties. • Mechanical properties depend on average diameter of hollow spheres. • Mechanical properties show linear relationships with density and sphere content. • The cracks first propagate along small hollow spheres. In this study, the stir casting method was used to prepare aluminum matrix bimodal syntactic foams by mixing alumina hollow spheres of different size ranges. Alumina hollow spheres of 1.0–4.0 mm were divided into six size ranges. In bimodal syntactic foams, the hollow spheres of different size ranges were mixed in equal volume to ensure that their average diameter was almost the same, or the hollow spheres of 1.5–2.0 mm and 3.0–3.5 mm were mixed in different volume ratios. The bimodal syntactic foams under quasi-static compression first fractured along the small-sized hollow spheres, presenting brittle fracture characteristics. When mixing hollow spheres with different size ranges in equal volume, the mechanical properties of bimodal syntactic foams strongly depended on the average diameter of hollow spheres. The mechanical properties of bimodal syntactic foams with 1.5–2.0 mm and 3.0–3.5 mm hollow spheres decreased linearly with the increase of 3.0–3.5 mm hollow sphere content.