Mg-Al-Zn-Ca-Y (SEN) alloys have been recently developed by adding small amounts of Ca and Y to commercial Mg-Al-Zn (AZ series) alloys. These alloys possess superior corrosion and ignition resistances to their commercial AZ series counterparts. Here, commercial AZ91 (Mg-9Al-0.8Zn, wt%) and developed SEN9 (Mg-9Al-0.8Zn-0.3Ca-0.2Y, wt%) alloys are extruded under the same conditions, and the microstructure, tensile properties, and high-cycle fatigue properties of the extruded alloys are compared. The extruded SEN9 alloy has a smaller average grain size and higher microstructural homogeneity than the extruded AZ91 alloy because the Al-2 Y, Al-2 Ca, and Al8Mn4 Y particles in the homogenized SEN9 billet promote dynamic recrystallization during extrusion. Despite their different microstructures, the two alloys possess similar tensile strengths because the strong precipitation hardening in the extruded AZ91 alloy is offset by strong grain-boundary hardening in the extruded SEN9 alloy. However, the extruded SEN9 alloy exhibits higher tensile elongation because deformation twinning is suppressed by the finer grains. The fatigue strength of the extruded SEN9 alloy (100 MPa) is slightly lower than that of the extruded AZ91 alloy (110 MPa). For the extruded AZ91 alloy, fatigue cracks initiate on the surface in all specimens, whereas for the extruded SEN9 alloy, fatigue cracks initiate in an Al2Ca or Al2Y particle present on the subsurface in some specimens, especially at low stress amplitudes. The Al2Ca and Al2Y particles are larger than the Mg17Al12 precipitates, and considerably harder than the matrix. Consequently, local stress is highly concentrated in these particles during cyclic loading, which eventually causes premature fatigue cracking and decreased fatigue resistance.
