Grain Size Stability, Superplasticity, and Internal Friction of Friction Stir Processed and Rolled Heterogeneous Al-Mg-Based Alloy with Transition Elements
摘要
The current research is devoted to microstructure evolution, recrystallization behavior and superplasticity of an Al-Mg-Fe-Ni-Mn-Cr-Zr alloy processed by different routes: hot and cold rolling, friction stir processing (FSP), and friction stir processing followed by rolling. The alloy exhibits a bimodal particle size distribution with coarse Al9FeNi and Al6(Fe,Mn) solidification-originated phases and fine Mn-, Cr-, and Zr-bearing dispersoids. Elongated deformed grains after rolling and a partially recrystallized grain structure after FSP are formed. Mechanical spectroscopy analysis helps to identify a ‘pseudo’ peak associated with recrystallization during heating and a frequency-dependent relaxation peak attributed to grain boundary relaxation during cooling for all studied treatments. Due to both particle-stimulated nucleation (PSN) and Zener pinning effects, the studied alloy exhibits a grain size of ~ 5-6 μm after elevated temperature annealing. FSP refines coarse particles from 1.0 to 0.4 μm and leads to a less homogeneous size distribution of coarse particles than hot rolling. Grain size stability and superplastic behavior directly depend on the treatment regimes. Due to a thermally stable grain structure up to the sub-solidus temperature of 540 °C, the hot and cold rolled alloy exhibits elongations to failure of ~ 300-400% at a strain rate of 1 × 10⁻2 and 600% at 2 × 10⁻3 s⁻1. A tendency toward local abnormal grain growth upon annealing above 400 °C weakens the superplastic properties and decreases elongations to ~ 250%. Cold rolling after friction stir processing improves grain structure homogeneity and grain size stability and restores superplastic properties.