Effect of calcium oxide particle size on microstructure and properties of AZ91 Mg alloy

•CaO particles with different sizes show different reactions in AZ91 alloy melts.•Submicro-CaO particles fully undergo in-situ chemical reactions and refine the grain size significantly.•Nano-CaO particles produce the same effects as micro-CaO, because of the insufficient reaction of CaO and the matrix.•The AZ91 alloy with submicro-CaO particles exhibit excellent mechanical properties and corrosion resistance.