Plasticity of Nanotwinned Aluminum and Aluminum Alloy

Abstract

Nanotwinned (nt) metals have been intensively studied and has shown unique mechanical properties, including high strength and high ductility. Although twins can be introduced into face-centered-cubic (fcc) metals by annealing (annealing twins), deformation (deformation twins) and growth (growth twins), most of these twinned metals have low stacking fault energy (SFE). The twinnability of fcc metals remains largely controlled by their SFE. Consequently, the high SFE of Al typically prohibits the formation of twins in aluminum (Al). This dissertation focuses on the introduction of several innovative strategies that can introduce high density growth twins in Al and Al alloys and study the influence of twinnability on strengthening and plastic deformation of these twinned alloys.

The growth twins were observed in a polycrystalline Al thin film fabricated by magnetron sputtering. And the twin formation mechanism was discussed in a thermodynamic view. Then, we show that high-density twin boundaries can be introduced in Al films by tailoring the texture of the films without any seed layers. Transmission Kikuchi diffraction and transmission electron microscopy studies on (111), (110) and (112) textured Al films. Epitaxial Al (112) film has the highest density of ITBs, because the twin variants (335) and (535) are separated by Al (102) islands, promoting the formation of ITBs. The smaller domain size can thus be achieved by introducing HAGBs into the twinned bicrystal structure to inhibit the abnormal growth of single variant. Furthermore, twin boundaries in Al appear to be stronger barriers to dislocations than conventional high angle grain boundaries. Besides tailoring the twin structure by changing the growth orientation, alloying has been used in an Al matrix. The high strength epitaxial AlMg alloy has been fabricated with a high density twinned structure. The strong ITB barriers play an important role to strengthen the film. Combined with the solid-solution strengthening, the calculated flow stress correlated well with the experimental data.

The knowledge derived from this study may facilitate the design of high-strength, light-weight, and ductile Al alloys.