Effects of B on microstructure and mechanical properties of Mg-7Al-0.4Zn-0.2Mn alloy were investigated.It was found the as-cast microstructure and average grain size are remarkably refined with trace B addition

exhibiting more strongly refining effects with increase of the B content.The average grain size was decreased from about 140μm without B alloy to 40μm with 0.15% B addition.It was concluded that the grain size was refined due to the high melt point compound AlB

2

with HCP crystal structure serving as the heterogeneous nuclei site.The mechanical properties of the as-cast alloys are increased with trace B addition.The microhardness

tensile strength and yield strength of the alloy with 0.15% B addition were increased by 13.1%

19.5% and 22.0%

respectively

comparing with those of the alloy without B addition

and the energy to fracture is 2.3 times than that of the alloy without B addition.Elongation of the alloy with 0.10% B content is increased by 21.6% comparing with that of as-cast alloy.