The Role of Interfaces in Evolution of Structure and Thermal Stability of Cu-Nb Composite Processed by High-Pressure Torsion

摘要

The structure and thermal stability of Cu-18Nb multicore composite fabricated by repeated cold-drawing of in situ melted mixture of Cu and Nb and subjected to high-pressure torsion (HPT) have been studied by SEM, TEM, X-ray analysis and microhardness measurements. In the cold-drawn state ribbon-like Nb filaments the thickness of 30-70 nm are located in Cu-matrix with sharp texture <110>_(Nb) || <111>_(Cu) || drawing axis. The Nb lattice is distorted, the interplanar spacing (110)_(Nb) being extended along the drawing axis and compressed perpendicular to it, which testifies a semi-coherent character of Cu/Nb interfaces. At annealing these distorsions gradually vanish, and coagulation of Nb ribbons starts at 400°C, actively develops at 600°C and finishes at 800°C with the formation of sausage-like filaments with round transverse sections, which is accompanied with about two-fold decreasing of microhardness. Under the HPT the composite structure is considerably refined, and almost equiaxed grains the sizes of 20-30 nm are formed, which gives rise to a dramatic increase of microhardness. The thermal stability of Cu-Nb composite after cold drawing and HPT is appreciably higher than that of pure Nb and Cu nanostructured by severe plastic deformation.