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

Transition of dislocation structures in ultrafine-grained copper processed by simple shear extrusion (SSE) and its effects on dissolution weremanifested 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.


Introduction
Grain refinement to grain sizes below 1 μm by severe plastic deformation (SPD) is now well known for improving strength of bulk metallic materials for structural application [1].Simple shear extrusion (SSE) technique is one of the SPD methods wherein deformation proceeds by pressing the material through a die with a specifically created direct extrusion path [2].SSE as well as other SPD techniques represented by equal-channel angular pressing (ECAP) and accumulative roll bonding (ARB) produces ultrafine grain (UFG) materials with residual dislocation inside grains, which may cause unique physical and mechanical properties [3].Dislocation density increases to the order of 10 15 m −2 with increasing number of passes in SPD, forming finally the so-called deformation-induced grain boundaries with some dislocations remained inside grains.However, we reported in the previous study that the softening occurred with further passes after UFG formation in pure copper processed by SSE, and this softening was considered as a result of a decrease in dislocation density, which was revealed by scanning transmission electron microscope (STEM).is decrease in dislocation density after UFG formation may be caused by the dynamic recovery [3][4][5].As a classic approach, a dislocation etchant has been used to locate discrete dislocations and its density in low plastic strain [6].However, it was reported that the dissolution rate in a modified solution becomes sensitive to dislocation density in very high range and grain boundaries state with residual dislocations after SPD, and the dissolution rate was changed by the flush annealing in spite that grain size is not changed [4,7].is study was carried out in order to evaluate dislocation density in very high strain range using a modified Livingston etchant which is very sensitive to dislocations and to examine the dissolution behavior of UFG copper.

Simple Shear Extrusion (SSE).
A schematic representation of the SSE channel is shown in Figure 1 [3].rough the deformation channel, the shear strain is gradually applied to the material while its cross-sectional area remains constant [2].e direction of the shear is reversed at the middle of the channel where the specimen distorts with a maximum distortion angle of α max .Another parameter of SSE processing is the inclination angle (β), which exerts a profound e ect on the deformation zone, strain rate, and the load of the process [2,3,8,9].e SSE can be conducted with a lower cost of production and easier installation than many other SPD methods owing to the straight movement of the billets through the channel.
A bisection die with the maximum distortion angle (α max ) and the maximum inclination angle (β max ) of 45 °and 22.2 °, respectively, was designed and constructed, which applies an equivalent strain of 1.155 per pass, as shown in Figure 1 [3].
e deformation channel's cross section is a square with the side of 10 mm. e copper billets of commercial purity with a dimension of 10 mm × 10 mm × 50 mm were machined, annealed for 2 h at 923 K in argon atmosphere and then furnace cooled to room temperature as an initial material.A screw press with a ram speed of 0.2 mm/s was used for SSE processing, namely, route C where the sample is rotated in the same view by 180 °between each pass was used for repeating SSE. e initial materials were subjected to two, four, eight, and twelve passes of SSE (see [3] for further details of SSE).

Microstructural Characterization.
e microstructure of samples after two, four, eight and twelve passes of SSE was observed using a transmission electron microscope (JEM-2100F, JEOL) with an acceleration voltage of 200 kV.All the TEM specimens were prepared from extrusion direction (ED) section of the samples.e surface of the specimens was mechanically grounded to the thickness of 100 μm using a SiC abrasive paper.en, the TEM specimens were thinned by a twin-jet polisher Tenupol 5 (Struers Co., Ltd.) at an applied voltage of 10 V in a mixture of 250 ml ethanol, 500 ml phosphoric acid, 50 ml propanol, and 5 g urea at 273 K until perforation.After that, the specimens were nally polished by an ion beam using the Gatan 691 precision ion polishing system.

X-Ray Di raction.
e measurement of X-ray di raction (XRD) on the SSE-processed sample was conducted by the SmartLab, Rigaku.e integral breadth was determined after appropriate tting of the scattered XRD pattern.e broadening in XRD data line consists of contributions due to coherent domain size, D, and microstrain, ε [10].e following equation was used to separate the contributions from each other for calculating the dislocation density ρ [10]: where β is the value of the integral breadth (in radians), θ is Bragg's di raction angle, and λ is the wavelength of the X-ray beam ( Å).
Several XRD peaks of high intensity (2θ 30-130 °) were taken, and the plot of (β cosθ/λ) 2 versus ( sinθ/λ) 2 was constructed [10].e plots D and ε were calculated using intercept 1/D 2 and slope 16ε 2 .en, the dislocation density can be calculated from D and ε, with η integer number, as [10] follows: Finally, the dislocation density (ρ) can be estimated by X-ray line broadening and obtained from dislocation  Advances in Materials Science and Engineering densities that are related to D • (ρ D ) and ε • (ρ ε ) as follows [10]:

Immersion Testing.
e immersion tests were performed using the modi ed Livingston etchant (HCl: 30 ml; CH 3 COOH: 10 ml; H 2 O: 410 ml; pH 0.41), which is known to attack dislocations and grain boundaries exclusively, leaving the other area intact [7].e surface except the ED plane was coated with a nail lacquer to avoid a contact with the etchant solution.e immersion tests were conducted at stable room temperature condition for 1, 2, 4, 8, and 16 h.e morphology after 16 h immersion tests was observed by an optical microscope.

Results and Discussion
3.1.Microstructural Observation.Figure 2 shows the TEM images of the ED plane in the deformed specimens of two, four, eight, and twelve passes.After two passes, the microstructure showed a mixture of nondeformed and deformed grains with some dislocations.Up to eight SSE passes, the dislocation density increased and cell structures were observed.With increasing number of passes, the microstructure became more uniform and some grains with an average grain size of 1 µm formed.When the strain was increased through twelve SSE passes, grain size is overall constant with an average of 0.9 µm.Most interestingly, the dislocation density visible inside grains appeared to be lower than that after eight passes.
e decrease in dislocation density in very high strain range after several passes in SPD was reported by Dalla Torre et al. [11], who ascribed it to the dynamic recovery during SPD.
Figure 3(a) shows the {111} peaks obtained from XRD analysis of the specimens.A signi cant line broadening can be observed with increasing SSE passes, which indicates a high density of dislocations and resultant longrange elastic stresses [12,13].
e dislocation density calculated from ( 4) is shown in Figure 3(b) with other data estimated by direct observation by TEM [3].e dislocation density increases gradually for two to eight passes and decreases after eight passes until twelve passes.So, the maximum dislocation density was achieved around eight passes.ough this trend is similar to the results obtained by STEM observation [3], there is a large discrepancy between two results.One possible reason for the discrepancy is that the direct observation by STEM counted only visible dislocations inside grains, whereas line broadening of XRD re ects the strain eld by grain boundaries as well as discrete dislocations inside grains.Particularly, grain boundaries at nonequilibrium state are regarded as having extrinsic grain boundaries dislocations and may exert a long-range stress eld [14].
erefore, decrease in dislocation density after eight passes is associated with decease in extrinsic grain boundary dislocations, in other words, structural change from nonequilibrium to equilibrium state.

Dissolution Behavior.
Figure 4 shows the weight loss of the as-annealed and SSE-processed specimens in the immersion tests in the modi ed Livingston etchant.e weight loss increased with increasing number of SSE passes until eight passes and decreased after twelve SSE passes. is trend was obtained for all immersion times.is result suggests that the dissolution of copper was promoted by SSE process until eight passes, but suppressed after higher passes until twelve passes.Advances in Materials Science and Engineering e surface morphologies after immersion for 16 h in the Livingston etchant are shown in Figure 5.As shown in Figure 5(a), the grain boundaries groove in the asannealed (CG) copper can be clearly observed (indicated by the red arrow), indicating that the grain boundaries were attacked predominantly while the grain interior is immune in the Livingston etchant.
is is because the grain boundaries have higher energy than the grains and, therefore, work as an anode, while the grain interior works as a cathode.With increasing SSE passes and higher dislocation density, the grain boundary grooves are not visible, and the surface became smoother, as shown in Figures 5(b)-5(e).When dislocation density becomes higher than 10 12 m −2 , interspace between dislocations is lower than 1 μm in average, with stress eld by dislocations being overlapped.us, dislocation pits are not isolated rending dissolution from selective to more uniform manner.4 Advances in Materials Science and Engineering e most important result in this study is that weight loss by dissolution reflects the line broadening by XRD, namely, weight loss decreases with higher strain after UFG formation.
e decrease in the dislocation density after severe plastic deformation was also reported along with softening caused by the dislocation annihilation [15][16][17][18][19][20][21]; for example, after four passes of equal-channel angular pressing (ECAP) of pure copper, a decrease in the dislocation density was reported by Della Torre et al. [11].According to their result, it is suggested that the decrease of stress field is also related to the operation of recovery mechanism, causing an increase in the dislocationfree grains [11].
e high extrinsic dislocation density at nonequilibrium grain boundaries may affect the dissolution rate because of lower activation energy for dissolution [4].e dislocation annihilation inside grains as well as transition from nonequilibrium to equilibrium state therefore may cause the decreasing dissolution behavior after UFG formation in pure copper in the modified Livingston etchant.

Conclusions
e effect of SSE passes on the dissolution was investigated by the modified Livingston etchant which is very sensitive to dislocations.Consequently, the following major conclusions can be drawn: (1) e internal strain field evaluated by X-ray line broadening measurements increased gradually until eight SSE passes and decreased until twelve SSE passes.Advances in Materials Science and Engineering dislocation density inside grains and causes structural change of grain boundaries from nonequilibrium to equilibrium state.

Figure 1 :
Figure 1: Schematic presentation of the SSE channel.