Effect of Coarse Eutectic-Originated Particles on the Microstructure and Properties of the Friction Stir-Processed Al-Mg-Zr-Sc-Based Alloys

Friction stir processing (FSP) is a promising technique to refine grains and improve the mechanical properties of metallic materials. The current work compares a FSP-ed Al-Mg-Sc-Zr-alloy with fine nanoscale L1 2 -structured Al 3 (Sc,Zr) precipitates and an alloy modified by 0.9 wt.%Fe-0.9 wt.%Ni containing both fine Al 3 (Sc,Zr) and coarse Al 9 FeNi-phase particles of solidification origin. The influences of both secondary-phase particles and FSP parameters on the microstructure, mechanical properties, and superplasticity of the alloys were examined. During FSP, the temperature increased to ~ 520°C, a mean size of the Al 3 (Sc,Zr) precipitates increased from 12 nm to 31 nm, whereas a size of the Al 9 FeNi particles decreased from 1.9 ± 0.1 µm to 1.1 ± 0.1 µm and exhibited a uniform distribution in the matrix. The reference alloy with fine Al 3 (Sc,Zr) precipitates exhibited a non-uniform partially recrystallized grain structure, whereas the modified alloy with both fine Al 3 (Sc,Zr) and coarse Al 9 FeNi-particles exhibited equiaxed fine grains after FSP. In the stirred zone of the reference and modified alloys, the average grain size was 3.3 ± 0.1 µm and 4.0 ± 0.3 µm, respectively. As a result, the modified alloy exhibited an improvement in ultimate tensile strength and elongation-to-fracture compared with the reference alloy by 22% and 40%, respectively. The fine Al 3 (Sc,Zr) and coarse Al 9 FeNi particles and their uniform distribution after FSP provided effective grain refinement by Zener pinning and particle-stimulated nucleation mechanisms resulting in the modified alloy's high strain rate superplasticity.