Length scale effect on the thermal stability of nanoscale Cu/Ag multilayers

The annealing induced grain growth (GG) and heterogeneous interface evolution of Cu/Ag multilayers with individual layer thickness (h) varying from 5 to 50 nm were investigated by transmission electron microscopy (TEM). The results demonstrate that the thermal stability of Cu/Ag multilayers exhibits strong length scale dependence. For samples with h<20 nm, the heterogeneous interfaces completely disappear when the annealing temperature exceeds 200 °C. However, the temperature for stable layered structure can reach 300 °C as the h≥20 nm, where the interfaces remain remarkably intact. The existence of a large number of grain boundaries (GBs) decrease the stability of multilayers, while more heterogeneous interfaces contribute to resisting atomic diffusion, inhibiting GG. The equilibrium is achieved by a competitive process between GBs diffusion and heterogeneous interfaces resistance. Moreover, the formation of annealing twins in multilayer also significantly improve the microstructural stability.