Nanodipoles of partial disclinations as quasi-ductile strain carriers responsible for nanocrystalline structure formation in metals and alloys under severe plastic deformation

Abstract

Two-level nanostructural states were detected by transmission electron microscopy in V- and Mo-Re-based alloys and Ni under plastic deformation by high-pressure torsion at a true logarithmic strain e ≈3–6. The two-level nano structural states represent nanocrystals of size 50–100 nm with nanobands of width less than 10 nm and dipole and multipole misorientations. In nickel, the nanobands are bounded by nanodipoles of partial disclinations or dislocations of noncrystallographic shear with effective Burgers vectors several times smaller than Burgers vectors of lattice dislocations. In the vicinity of the nanodipoles, structural states with extraordinarily high (hundreds of degrees per micrometer) elastic crystal lattice curvature and local internal stress gradients up to ≈(10–20) E μm −1 are found. The mechanism proposed for the formation of the nanostructures is quasi-ductile motion of nanodipoles of partial disclinations (or dislocations of noncrystallographic shear) controlled by flows of nonequilibrium point defects in high local gradient fields of normal stress tensor components. In V-based alloy at e > 6, the nanobands are grouped in bundles of width 1–2 μm forming vortex deformation mesobands with dipole misorientations and high density of equiaxial nanocrystals of size several nanometers. It is supposed that these mesobands result from collective effects in the disclination substructure and attendant collective motion of nanodipoles of partial disclinations in inhomo-geneous couple stress fields.