Deformation-induced concurrent formation of 9R phase and twins in a nanograined aluminum alloy

Twins and 9R phase (a structure containing a high density of stacking faults) have played important roles in simultaneously improving strength and strain hardening capacity of low-to-medium stacking fault energy (SFE, smaller than 50 mJ/m ² ) metallic materials. Due to the high SFE (166 mJ/m ² ) in pure face-centered-cubic Al and its insensitivity to most soluble alloying elements, concurrent introduction of deformation twins and 9R phase into an originally twin-free Al alloy is intuitively impossible. Here, we propose a strategy to simultaneously produce deformation twins and 9R structure in an Al-Mg alloy by nanostructuring grain sizes and alloying with Y. We demonstrate that plastic deformation of the nanograined Y-doped Al-Mg alloy can concurrently drive the dissociation of incoherent twin boundaries and direct emission of grain boundary Shockley partial dislocations, enabling the development of two different 9R structures in the nanograined Al-Mg-Y alloy. Our strategy combining grain size nanostructuring and SFE tuning presents a new microstructure design in strengthening bulk Al alloys by planar defects of deformation twins and 9R structure and/or stacking faults.