High pressure torsion to refine grains in pure aluminum up to saturation: mechanisms of structure evolution and their dependence on strain

Abstract

High-pressure torsion was used for the deformation processing of high-purity aluminum (4N-Al), while high-resolution electron-backscatter diffraction was used for the analysis of evolution of qualitative and quantitative microstructural characteristics. This study reveals a rather full picture of microstructure evolution in the high stacking fault energy fcc material and makes a continuous link between deformation microstructures at low, high and very high strains. Three stages of the microstructure evolution in 4N-Al at ambient temperature have been found: (i) the first stage in the range ε_eq  ≤ 1; (ii) a transition stage in the range 1 < ε_eq  ≤ 8; and (iii) a saturation stage in the range ε_eq  ≥ 8. In stages (i) and (ii), grain subdivision and typical features of deformation microstructures are found. Starting from stage (ii), formation of small equiaxed (sub)grains surrounded by high-angle boundaries (HABs) is found together with minor increase in the average subgrain size. At stage (iii), recrystallized-like microstructure mostly consisting of the dynamically stable equiaxed subgrains surrounded by HABs dominates the microstructure.

Keywords:

• high pressure torsion
• aluminum
• EBSD
• microstructure
• grain refinement

Acknowledgements

The authors of the present work gratefully appreciate Prof. Yoshikazu Todaka and Prof. Minoru Umemoto of Toyohashi University of Technology, Japan, for providing facility and support in HPT processing. The authors also acknowledge financial support from the Grant-in-Aid for scientific research from the Ministry of Education on priority areas ‘Giant straining process for advanced materials containing ultra-high density lattice defects’, as well as that on innovative area ‘Bulk nanostructured metals’, both through the Ministry of Education, Culture, Sports, Science and Technology of Japan.