Transition of Dislocation Structures in Severe Plastic Deformation and Its Effect on Dissolution in Dislocation Etchant

Abstract

Transition of dislocation structures in ultrafine-grained copper processed by simple shear extrusion (SSE) and its effects on dissolution were manifested by simple immersion tests using a modified Livingston dislocation etchant, which attacks dislocations and grain boundaries selectively. e SSE process increased the internal strain evaluated by X-ray line broadening analysis until eight passes but decreased it with further extrusion until twelve passes. e weight loss in the immersion tests reflected the variation in the internal strain: namely, it increased until eight passes and then decreased with further extrusion to twelve passes. Taking our previous report on microstructural observation into account, it is suggested that variation in the internal strain is caused by both the variation in dislocation density and structural change of grain boundaries from equilibrium to nonequilibrium states or vice versa. Decreased dislocation density and structural change back to equilibrium state of grain boundaries in very high strain range by possibly dynamic recovery as pointed out by Dalla Torre were validated by X-ray and dissolution in the modified Livingston etchant in addition to the direct observation by TEM reported in our former report.

Corrosion Behavior of Severely Deformed Pure and Single-Phase Materials

“…Nevertheless, several studies present the corrosion trends over a wide strain range from conventional up to SPD values. 34,36,39,58,95,107114) Some of these studies report an anomalous change in corrosion behavior 34,36,95,110,113) and suggest the possible role of UFG structures on corrosion kinetics, as described later. 36) 3.…”

“…34) Similarly, the corrosion rate increases with increasing stored energy in Livingston's dislocation etchant, but it starts to decrease after 8 SSE passes, presumably upon the possible formation of a UFG structure. 110) In both cases, the change in corrosion behavior does not conform to the change in stored energy, suggesting that the transition to UFG may affect corrosion kinetics. Inversely, the corrosion rate of UFG copper increased by post-SPD annealing which results in little grain growth, possibly due to the recovery of UFG structures of as-ECAPed copper.…”

“…129) Fig. 4 Variation of total stored energy, 104) resistance, passive film resistance Rp, 34) and weight loss in immersion test in Livingston's dislocation etchant 110) of pure copper with increasing strain.…”

“…Figure 4 shows the variations in the polarization resistance, corrosion rate, and total stored energy upon plastic deformation of pure copper as a function of equivalent strain by SPD; the three data sets are from different sources. 34,104,110) The polarization resistance, R p , of pure copper deformed by ARB was obtained by electrochemical impedance spectroscopy (EIS) in a 3.5% NaCl solution 34) and the corrosion rate of pure copper deformed by SSE was obtained by immersion tests in Livingston etchant. 110) The total internal energy, stored in lattice defects mainly as dislocations, grain boundaries and excessive vacancies by ECAP, is also plotted.…”

“…34,104,110) The polarization resistance, R p , of pure copper deformed by ARB was obtained by electrochemical impedance spectroscopy (EIS) in a 3.5% NaCl solution 34) and the corrosion rate of pure copper deformed by SSE was obtained by immersion tests in Livingston etchant. 110) The total internal energy, stored in lattice defects mainly as dislocations, grain boundaries and excessive vacancies by ECAP, is also plotted. 104) It is important to note that R p , increases rapidly after 4 ARB passes despite the almost constant total stored energy.…”