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 Mg₁₇Al₁₂ particles 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 than 400-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 (∆W_t) and the number of cycles to fatigue failure (N_f), and it is expressed through a simple equation (∆W_t = 10·N_f^−0.59). This model enables fatigue life prediction of both the investigated alloys regardless of the extrusion temperature and strain amplitude.