Microstructure and Fatigue Behaviour of Mg2Si-Dispersed Magnesium Alloys Produced by Solid-State Synthesis

This paper describes the microstructure and fatigue behaviour of a newly developed magnesium (Mg) alloy that was produced by solid-state synthesis using Mg alloy (AZ31) powder and SiO2 powder as starting materials. Two different powder sizes of AZ31 were evaluated. SiO2 reacted with AZ31 to form Mg2Si when heated at 753K and subsequently extrusion was applied to produce bars of 24mm diameter at the same temperature. It was found that the microstructure of the produced alloys, Mg2Si-dispersed Mg alloys, consisted of particles such as Mg2Si or MgO dispersed within the equiaxed-grain matrix structure and large particles tended to be present in the material using coarse powder. The mechanical properties of Mg2Si-dispersed Mg alloys decreased compared with a conventional extruded AZ31 alloy, in which the material using fine powder had slightly higher tensile strength and ductility than the counterpart. Mg2Si-dispersed Mg alloys also showed lower fatigue strength than the extruded AZ31 alloy and the fatigue strength of the material using coarse powder decreased significantly. Fatigue cracks always initiated from large particles immediately after cyclic loading was applied and subsequent small crack growth was faster than the extruded AZ31 alloy. Therefore, it was concluded that the decrease of the fatigue strength of Mg2Si-dispersed Mg alloys was attributed to the premature crack initiation from particles and faster small crack growth, and the observed powder size dependence of fatigue strength was due to difference in the size of the particle from which the crack initiated.