High thermal stability of nanostructured Al mediated by heterophase interfaces and nanotwinning

Nanostructured crystalline Al/amorphous AlN multilayer films, with a wide layer thickness (h) range from 10 to 200 nm, were prepared and exposed to high-temperature annealing from 200 to 600 °C. Microstructure of all the multilayers was highly stable under the temperature up to 400 °C. Apparent grain coarsening happened in multilayers with h > ~50 nm under 500 °C. However, the Al grains in h ≤ ~50 nm multilayers were extremely stabilized under even 500 °C annealing, showing a high thermal resistance up to about 0.8Tm (Tm: melting point of bulk pure Al). The stabilization mechanisms mainly include the constraint effect by heterophase interface and the nanotwinning in Al layers, as demonstrated by statistical measurement results and ex-situ/in-situ transmission electron microscope observations. When the annealing temperature was raised up to 600 °C, the heterophase interfaces disappeared in h ≤ ~20 nm samples but were well preserved when h > ~20 nm, revealing that the thermal failure modes were quite sensitive to h. Based on the microstructural evolution, a thermal failure map was finally developed in variation with h and annealing temperature. Furthermore, hardness of the multilayers was measured and hardening mechanism was also discussed according to the microstructural evolution after annealing.