Thermal stability of Cu-Nb nanolamellar composites fabricated via accumulative roll bonding

In situ annealing within a neutron beam line and ex situ annealing followed by transmission electron microscopy were used to study the thermal stability of the texture, microstructure, and bi-metal interface in bulk nanolamellar Cu/Nb composites (h = 18 nm individual layer thickness) fabricated via accumulative roll bonding, a severe plastic deformation technique. Compared to the bulk single-phase constituent materials, the nanocomposite is two orders of magnitude higher in hardness and significantly more thermally stable, e.g., no observed recrystallization in Cu at temperatures as high as 85% of the melting temperature. The nanoscale h = 18 nm individual layer thickness is maintained up to 500°C, the lamellar structure thickens but is maintained up to 700°C, and recrystallization is suppressed even up to 900°C. With increasing temperature, the texture sharpens, and among the interfaces found in the starting material, the 112Cu|| 112Nb interface with a Kurdjumov-Sachs orientation relationship shows the greatest thermal stability. Our results suggest that thickening of the individual layers under heat treatment coincides with thermally driven removal of energetically unfavorable bi-metal interfaces. Thus, we uncover a temperature regime that maintains the lamellar structure but alters the interface distribution such that a single, low energy, thermally stable interface prevails.