Role of high-pressure torsion temperature on microstructure, mechanical properties, and corrosion behavior of pure copper

High pressure torsion (HPT) serves as a method to refine the microstructure and manipulate different characteristics of metals and alloys [1]. In current research, the effect of HPT temperature on microstructure, mechanical properties and corrosion behavior of pure copper was investigated. Material and Methods Pure copper metal sheet with thickness 2 mm was cut and circular specimens with diameter of 10 mm were prepared. HPT was carried out at 3 GPa and turn number of 3. Temperature during HPT was controlled by a thermal element (Fig. 1). HPT tests were performed at three different temperatures namely room temperature (RT) (⁓ 25 °C), 100 and 200 °C. Fig. 1 a) Thermal element around the die. For microhardness testing load was 100 gf and dwell time was 13 sec. Electrochemical corrosion tests were conducted at room temperature (⁓25 °C). The electrolyte was a 3.5 wt.% NaCl solution. Results and discussion Microstructural images of the studied specimens (Fig. 2) show that grains for specimens deformed at room temperature (⁓ 500 nm) are smaller than those relating to specimens deformed at higher temperatures (⁓ 1-2 µm). Fig. 2 Microstructures of the studied specimens a) base metal, b) HPT-RT, c) HPT-100, d) HPT-200. Dynamic recrystallization is the main mechanism for grain refinement during HPT [2]. During HPT, some dislocations omit each other, and the others rearrange in a condition that form sub grain boundaries with low angle. As HPT proceeds, the misorientation angel between sub-grains increases and high angle grain boundaries develop and consequently, fine grains develop [3]. As temperature during HPT increases, dislocations evolution occurs more easily, and dynamic recrystallization occurs more extensively. But, temperature increase during HPT can result in grain growth. So, the effect of temperature increase during HPT on grain growth is higher than its effect on dynamic recrystallization. Hardness values decrease as temperature increases during HPT and additionally, the hardness increases as the distance from the center enhances (Fig. 3a). The base metal hardness was about 67.4 HV. Fig. 3 a) Microhardness measurements across radius for various studied specimens, b) Polarization and c) electrochemical impedance response curves of the studied specimens. Microhardness decrease by temperature increase can be related to role of grains growth. Grain boundaries impede the movement of dislocation and result in hardness increase [4]. As grains grow, the volume fraction of grain boundaries decreases and hardness decreases. Polarization curves are presented in Fig. 3b, c. The corrosion test results are presented in Table 1. It seems that corrosion rate decreases as HPT temperature increases. Low grain size and high amount of grain boundaries are normally numerated as parameters which increase the corrosion due to inhomogeneity enhancement [5]. Table 1 Corrosion current density and corrosion rate of the studied specimens. HPT-RT HPT-100 °C HPT-200 °C icorr 6.3E-6 5.62E-6 2.24E-6 Corrosion rate (mpy) 2.9 2.6 1.0 Conclusion It was concluded that increase in HPT temperature for pure copper alloy increased grain size and decreased the hardness, but it resulted in corrosion resistance enhancement