Effects of post-heat treatment on microstructure, tensile properties, and bending properties of extruded AZ80 alloy

This study investigates the effects of post-heat treatments on microstructural characteristics, uniaxial tensile properties, and three-point bending properties of an extruded AZ80 alloy. The as-extruded material (no heat treatment) has a fine recrystallized grain structure containing Mg17Al12 precipitates formed along grain boundaries. During the 10-h 400 °C treatment in Ar-SF6 (treatment T4), precipitates fully dissolved into the matrix and grain coarsening occurred with a decrease in the dislocation density. Since the as-extruded material has a higher dislocation density, it exhibited accelerated precipitation behavior during aging; consequently, its peak aging time was shorter than that of the T4 material. The peak-aged, as-extruded material (15-h at 200 °C, treatment T5) and peak-aged T4 material (30-h at 200 °C, treatment T6), i.e., T5 and T6 materials, respectively, have higher tensile strengths and lower elongations than the as-extruded and T4 materials owing to the presence of more abundant Mg17Al12 precipitates in the former two materials. The tensile yield strengths of the materials are on the order of T5 > T6 > as-extruded > T4. This order is related to the differences in grain size and precipitate amount. The bending yield strengths of the as-extruded, T4, T5, and T6 materials are linearly proportional to their tensile yield strengths and their bending formability linearly increased with the tensile elongation. The relations between the tensile and bending properties of the materials are attributed to the occurrence of macrocracking in the outer region of the bending specimen, where the tensile stress along the longitudinal direction is dominant.