Microstructural characteristics and low-cycle fatigue properties of AZ91 and AZ91–Ca–Y alloys extruded at different temperatures

The commercial AZ91 alloy and nonflammable SEN9(AZ91–0.3Ca–0.2Y, wt%) alloy are extruded at 300 °C and 400 °C. Their microstructure, tensile and compressive properties, and low-cycle fatigue(LCF) properties are investigated, with particular focus on the influence of the extrusion temperature. In the AZ91 and SEN9 materials extruded at 300 °C(300-materials), numerous fine Mg17Al12particles are inhomogeneously distributed owing to localized dynamic precipitation during extrusion, unlike those extruded at 400 °C(400-materials).These fine particles suppress the coarsening of recrystallized grains, decreasing the average grain size of 300-materials. Although the four extruded materials have considerably different microstructures, the difference in their tensile yield strengths is insignificant because strong grain-boundary hardening and precipitation hardening effects in 300-materials are offset almost completely by a strong texture hardening effect in 400-materials. However, owing to their finer grains and weaker texture, 300-materials have higher compressive yield strengths than400-materials. During the LCF tests, {10–12} twinning is activated at lower stresses in 400-materials than in 300-materials. Because the fatigue damage accumulated per cycle is smaller in 400-materials, they have longer fatigue lives than those of 300-materials. A fatigue life prediction model for the investigated materials is established on the basis of the relationship between the total strain energy density(ΔWt)and the number of cycles to fatigue failure(Nf), and it is expressed through a simple equation(ΔWt= 10·Nf-0.59). This model enables fatigue life prediction of both the investigated alloys regardless of the extrusion temperature and strain amplitude.