Thermal stability of twins and strengthening mechanisms in differently oriented epitaxial nanotwinned Ag films

Sputter-deposited epitaxial (111) and (110) Ag films have high-density nanotwins with respective twin boundary orientations perpendicular and angled to the growth direction. Twin density in as-deposited (111) Ag films is much greater than in (110) films, leading to higher hardness in the (111) films. Annealing up to 800 degrees C (homologous temperature of 0.85 T-m) leads to increased twin thickness, although the average twin thickness remains < 100 nm in both systems. Twinned volume fraction falls dramatically in annealed (110) films but remains constant at similar to 50% in (111) films. The mechanisms leading to the elimination of nanotwins in (110) films and their remarkable stability in (111) films at elevated temperatures are discussed. Coarsening and elimination of twins result in hardness reduction after annealing. The variety of microstructures achieved via annealing allows for the introduction of a strengthening model considering both twin and grain boundaries.