Lattice Curvature and Mesoscopic Strain-Induced Defects as the Basis of Plastic Deformation in Ultrafine-Grained Metals

Abstract

Here, in the context of space, time, and energy, we analyze the nanoscale mesosubstructure of ultrafinegrained nickel and copper after equal channel angular pressing and subsequent rolling and its changes after lowtemperature annealing. The analysis, including scanning tunnel microscopy and positron lifetime spectroscopy, shows that the basis for plastic deformation in such materials is provided by their lattice curvature and associated nanoscale mesoscopic strain-induced defects. Under equal channel angular pressing and rolling, for example, these structural elements increase the role of nonequilibrium point defects, plastic distortion, and low-angle subboundaries. We also analyze the energy of internal interfaces (grain boundaries) estimated from dihedral angles of etch grooves of different scales and their relative energy from cumulative energy distribution functions. In ultrafinegrained nickel, the integral energy distribution function is Gaussian both after equal channel angular and rolling and after further low-temperature annealing, and this is because of the presence of low-angle subboundaries. In ultrafine-grained copper, the integral energy distribution function is Gaussian after equal channel angular pressing and rolling, and after low-temperature annealing it assumes a power form because of the absence of lattice curvature and low-angle subboundaries. Both metals reveal vacancy clusters due to their lattice curvature and to dissolved low-angle subboundaries. In ultrafine-grained copper at T> 180°C, dynamic recrystallization occurs as nonequilibrium low-angle subboundaries inside nanograins are dissolved. It is the lattice curvature that controls the formation and evolution of mesoscopic substructures on different scales under low-temperature annealing.