Tensile properties at room and elevated temperatures and high-cycle fatigue properties of extruded AZ80 and TAZ711 alloys

The microstructure, tensile properties at room and elevated temperatures, and high-cycle fatigue behavior of an extruded Mg-7Sn-1Al-1Zn (TAZ711) alloy were investigated in comparison with those of an Mg-8Al-0.5Zn (AZ80) alloy extruded under the same conditions. The extruded AZ80 alloy exhibited a fully recrystallized structure, whereas the extruded TAZ711 alloy had a bimodal structure consisting of coarse non-recrystallized grains and fine recrystallized grains. The TAZ711 alloy had smaller recrystallized grains and a larger amount of fine dynamic precipitates than the AZ80 alloy. Both alloys showed the typical texture of extruded Mg alloys in which the basal planes of the grains are arranged parallel to the extrusion direction, but the texture intensity of the TAZ711 alloy was greater than that of the AZ80 alloy. The yield strength of the TAZ711 alloy was higher than that of the AZ80 alloy at room temperature because of the finer grain size, stronger texture intensity, and greater amount of fine precipitates in the TAZ711 alloy. However, at an elevated temperature of 150 °C, the yield strength of the TAZ711 alloy was lower than that of the AZ80 alloy because the climb-controlled dislocation creep, grain boundary sliding, and diffusional creep behaviors were more easily generated in the TAZ711 alloy owing to its finer grain structure. As the applied tensile strain rate was reduced, the decrease in the strength of the TAZ711 alloy was greater than that of the AZ80 alloy owing to the higher strain rate sensitivity of the former alloy. Although the yield strength of the TAZ711 alloy at room temperature was higher than that of the AZ80 alloy, the TAZ711 alloy exhibited lower high-cycle fatigue resistance, which can be attributed to the occurrence of fatigue cracks at the twins formed in the coarse non-recrystallized grains remaining in the TAZ711 alloy.