The role of interfaces in evolution of structure and thermal stability of Cu-Nb composite processed by high-pressure torsion

In the present work the structure and thermal stability of Cu-18Nb multi-core nanocomposites fabricated by repeated cold-drawing of in situ melted mixture of Cu and Nb in the Bochvar Institute of Inorganic Materials has been studied by SEM, TEM, X-ray analysis and microhardness measurements. Then the composite was subjected to high-pressure torsion (HPT), and evolution of its structure under this severe plastic deformation and further annealing was studied as well.

In the cold-drawn state Nb ribbon-like filaments the thickness of about 70 nm are located in Cu-matrix with sharp axial texture _Nb║_Cu║drawing axis. According to the X-ray analysis, the Nb lattice is strongly distorted, namely, the interplane distances (110)Nb are 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 400ºС, actively develops at 600ºС and finishes 800ºС 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 about 20-30 nm are formed, which gives rise to a dramatic increase of microhardness.

The thermal stability of Cu-Nb nanocomposites after cold drawing and HPT is appreciably higher than that of Nb and Cu nanostructured by severe plastic deformation.

This work was performed within the State Assignment of FASO of Russian Federation (theme “Crystal” no. 01201463333) and supported in part by the Ural Branch of the Russian Academy of Sciences (project no. 15-17-2-11).