**Dayan Nugmanov 1,\*, Andrey Mazilkin 1,2, Horst Hahn <sup>1</sup> and Yulia Ivanisenko <sup>1</sup>**


**\*** Correspondence: dayan.nugmanov@kit.edu; Tel.: +49-(0)721-608-28902

Received: 9 September 2019; Accepted: 25 September 2019; Published: 7 October 2019

**Abstract:** The microstructure and mechanical properties of rod-shaped samples (measuring 11.8 mm in diameter and 35 mm in length) of commercially pure (CP) copper were characterized after they were processed by high pressure torsion extrusion (HPTE). During HPTE, CP copper was subjected to extremely high strains, ranging from 5.2 at central area of the sample to 22.4 at its edge. This high but varying strain across the sample section resulted in HPTE copper displaying a gradient structure, consisting of fine grains in the central area and of ultrafine grains both in the middle-radius area and at the sample edge. A detailed analysis of the tensile characteristics showed that the strength of HPTE copper with its gradient structure is similar to that of copper after severe plastic deformation (SPD) techniques, typically displaying a homogeneous structure. Detailed analysis of the contributions of various strengthening mechanisms to the overall strength of HPTE coper revealed the following: The main contribution comes from Hall–Petch strengthening due to the presence of high and low angle grain boundaries in gradient structure, which act as effective obstacles to dislocation motion. Therefore, both types of boundaries should be taken into account in the Hall–Petch equation. This study on CP copper demonstrated the potential of using the HPTE method for producing high-strength metallic materials in bulk form for industrial use.

**Keywords:** high pressure torsion extrusion; severe plastic deformation; gradient structure; microstructure; hardness distribution; tensile properties; copper
