Mohsen K. Keshavarz

Strengthening Mechanisms and Electrochemical Behavior of Ultrafine-Grained Commercial Pure Copper Fabricated by Accumulative Roll Bonding

Abstract In this study, the four-cycle accumulative roll bonding (ARB) process at room temperature was successfully used for grain refining in commercial pure copper. Atomic force microscopy (AFM) images revealed that the average grain size reduced from about 26 µm in the unprocessed material to about 180 nm after four cycles of ARB. Also, transmission electron microscopy image indicated that the average grain size reached to 200 nm after four cycles. The yield strength of the ultrafine-grained pure copper after fourth cycle (360 MPa) was about 400 pct higher than that of the annealed unprocessed sample (70 MPa). The contribution of dislocations in strengthening of the pure copper decreased from ~30 to ~3 pct whit increasing the number of ARB cycles from 1 to 4. Scanning electron microscopy micrographs of fractured surfaces of the tensile test specimens revealed that ductile fracture of annealed sample with deep equiaxed dimples replaced by shear ductile rupture with shallow and small elongated dimples in ARB-processed samples. Moreover, electrochemical impedance spectroscopy and Mott–Schottky analysis showed that the electrochemical behavior improved by increasing the number of ARB cycle.

Electrochemical Behavior of Pure Copper in Phosphate Buffer Solutions: A Comparison Between Micro- and Nano-Grained Copper

In this work, electrochemical behavior of annealed (micro-) and nano-grained pure copper (fabricated by accumulative roll bonding process) in phosphate buffer solutions of various pH values ranging from 10.69 to 12.59 has been studied. Before any electrochemical measurements, evaluation of microstructure was obtained by optical microscope and transmission electron microscopy. To investigate the electrochemical behavior of the samples, the potentiodynamic polarization, Mott-Schottky analysis, and electrochemical impedance spectroscopy (EIS) were carried out. Potentiodynamic polarization plots and EIS measurements revealed that as a result of grain refinement, the passive behavior of the nano-grained sample was improved compared to that of annealed pure copper. Also, Mott-Schottky analysis indicated that the passive films behaved as p-type semiconductors and grain refinement did not change the semiconductor type of passive films.

Simultaneous Investigation of the Effect of Advanced Thermomechanical Treatment and Repetitive Cyclic Voltammetry on the Electrochemical Behavior of AISI 430 Ferritic Stainless Steel

In this study, it was revealed that the electrochemical behavior of AISI 430 ferritic stainless steel can be modified and improved to a large extent by the application of repetitive cyclic voltammetry in the anodic polarization branch of the alloy. The efficiency of this method was evaluated on the basis of the alloy grain size which is of great importance in corrosion studies. In fact, a coarse grain structure versus a fine grain structure was the subject of the used surface treatment method. Coarsening and refining of the grain size were conducted through a heat treatment and an advanced thermomechanical process. On the basis of cyclic voltammetry tests and also the electrochemical tests performed after that, it was shown that cyclic voltammetry had a significant improving effect on the passive behavior of both fine- and coarse-grained samples. Moreover, superior behavior of fine-grained sample in comparison with coarse-grained one was distinguished by its smaller cyclic voltammogram loops, more noble free potentials, larger capacitive arcs in the Nyquist plots, and less charge carrier densities within the passive film.