A comprehensive study of twinning phenomena in low and high stacking fault energy metals

Nanotwinned (nt) metals are materials that consist of several Σ3 twin boundaries (TBs) located within multiple grains in the overall microstructure, where the distance between any two consecutive TBs is within the range of 1 nm to 100 nm. In general, nt metals have attractive mechanical, thermal, electrical and/or chemical properties. The aforementioned properties of nt metals can be engineered based on the modification of two nanostructural features: (i) the density of TBs in the material, and (ii) the average distance between TBs or the twin thickness. These favorable and tunable properties of nt metals have encouraged extensive research during the last decade, specifically on fabrication methods that control nanostructural features. Nevertheless, the synthesis of nt metals has been limited due to stacking fault energy (SFE) restrictions, where many studies focus only on low SFE materials (< 45 mJ/m²) that are prone to twinning. To date, no studies have shown explicit control of the nanostructural features in nt metals. This dissertation discusses two main points: (i) the synthesis of nt metals with low, intermediate, and high SFEs by using magnetron sputtering; and (ii) the control of nanostructural features in nt metals with low and intermediate SFEs by adjusting the sputtering parameters. The general results are summarized as follows: 1) a study in low SFE materials (6 − 45 mJ/m²) was conducted by synthesizing thick Cu and Cu-Al films (film thickness ~4 μm). It was revealed that under identical sputtering conditions the twin thickness increased as the SFE increased. Contrary to theoretical predictions, the experimental results indicated that the twin thickness can be increased by using higher deposition rates; 2) a comprehensive study on Cu and Cu alloy films (film thickness >15 μm) elucidated three main mechanisms to describe twin nucleation and TB mobility, where the interplay of the mechanisms granted control of the twin thickness in nt metals. The results from this study were summarized in a protean contour zone map that can be used as a versatile guide to synthesize fully nt metals with tailored twin thicknesses in materials with a wide range of stacking fault energies (6 − 60 mJ/m²); 3) a study in high SFE metals (SFE > 125 mJ/m²) was conducted by synthesizing Ni, Al, and Al alloys thick films (film thickness >10 μm). Several characterization techniques were used to investigate the nature of twin boundaries in high SFE materials. The results provide an alternative perspective on the evaluation of twin boundaries, where new directions can be explored on the synthesis of nt metals with high SFEs. Overall, this dissertation contributes towards expanding the working space of nt metals, builds on the characterization of twin boundaries, and provides a guide to tune nanostructural features of nt metals for potential future applications.