Ultrafine-grain structure formation in an Al-Mg-Sc alloy during warm ECAP

Microstructural evolution taking place during equal-channel angular pressing was studied in a commercial Al-6Mg-0.3Sc alloy at 523 K (250 C) (∼0.5T _m). The structural changes are mainly associated with development of microshear bands (MSBs) that are continuously formed by strain accumulation and microstructural heterogeneities in each pass, which result in fragmentation of coarse original grains. New ultrafine grains (UFGs) with moderate-to-high angle boundary misorientations are concurrently evolved in the interiors of MSBs accompanied by rigid body rotation at medium-to-large strains. Such strain-induced grain refinement process occurs very slowly and incompletely in the present heavily alloyed Al alloy, leading to formation of a mixed microstructure, i.e., the UFGs in colony and some weakly misoriented fragments of original grains. The microstructure evolved at ε ≈ 12 is characterized by a bimodal crystallite size distribution with two peaks at d ₁ ≈ 0.2 to 0.3 μm and d ₂ ≈ 0.6 to 0.7 μm, and the fraction of high angle boundaries of about 0.35 ± 0.05. The main factors promoting dynamic formation of UFGs and the effects of thermal processes on it during severe plastic deformation are discussed in detail.