Ultrafine-Grained Al-Mg-Sc Alloy via Friction-Stir Processing

Friction-stir processing (FSP) of twin-roll cast (TRC) Al-Mg-Sc alloy resulted into ultrafine-grained microstructure. The alloy was processed in as-received and aged (563 K [290 °C], 22 hours) conditions and at three different tool rotation rates: 800, 400, and 325 rpm. The microstructural features were characterized using electron backscattered diffraction (EBSD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The grain size varied from 0.89 μm to 0.39 μm depending on the processing and initial thermo-mechanical conditions of the alloy. The TRC alloy processed at 325 rpm in aged condition had all the grains less than 1 μm, and 95 pct of grains had high-angle grain boundaries (HAGBs). In all the cases, the fraction of HAGBs were more than 80 pct. The variation of misorientation angle distribution was similar to the theoretical MacKenzie distribution for cubic crystal materials. Grain size analysis at different sections and locations on the transverse section of the dynamically recrystallized zone showed a homogeneous and equiaxed microstructure. The average dispersoid (Al₃(Sc,Zr)) size was ~8.0 nm in diameter obtained using high-resolution TEM. Grain size reduction was observed with increase in Zener–Hollomon parameter. It was shown that under the current microstructural and deformation conditions, dynamic recrystallization via particle-stimulated nucleation might not be possible during FSP.