Formation of a Gradient Structure in a Material by Twist Extrusion

Abstract

Hexagonal samples with an initial coarse-grained structure (CG) in the near-axis zone and a gradient ultrafine-grained (UFG) structure at the periphery are formed by twist extrusion (TE) of commercial-purity copper through a hexagonal twist die with a small twist-line slope. This hybrid structure of the samples provides conditions for a large uniform deformation before necking (due to the CG core) and a high yield strength (due to the gradient UFG periphery). The hybrid CG–UFG structure can be formed due to the threshold nature of metal grain refinement during cyclic deformation. A simple relation is derived for estimating the CG core diameter of the sample. The structure and the texture of the samples are investigated by X‑ray diffraction and optical microscopy.