**3.1.2 ASB processing**

Optical micrographs of the longitudinal sections of ASB samples are shown in Fig. 9. Previous study on the bond quality of spin-bonding process showed that the threshold thickness reduction for Al/Al bonding by conditions of this work is about 24% (Mohebbi & Akbarzadeh, 2010a). Therefore, it is expected that a good bond is obtained at thickness

Nanostructure, Texture Evolution and Mechanical Properties of

4-cycle

and 4 cycles of ASB

Aluminum Alloys Processed by Severe Plastic Deformation 439

Fig. 10. TEM micrographs and SAD patterns of AA1050 specimens processed by one, two

reduction of 50%. While no interface was observed on the sections after polishing, the bond interfaces are distinguished by etching (Fig. 9). Since subsequent ASB cycles improve the previous bonds, the last bond interface of each specimen is clearer than the previous ones. It is obvious in this figure that the external layer undertakes more thickness reduction than the internal one at each cycle, so that the ratio of external layer thickness to internal one is between 0.40 and 0.45. It should be noted that during the process more elongation were observed in the length of external tube than the internal one, confirming the mentioned thickness strain distribution.


Table 3. Results of the EBSD analysis

TEM micrographs and the corresponding selected area diffraction (SAD) patterns of the specimens ASBed by one, two and four cycles are shown in Fig. 10. The specimen after one cycle shows a grain size of about 1.5 μm. Large number of subgrain boundaries can be observed in this specimen. The specimen after two cycles illustrates ultra-fine grains with the size of approximately 600 nm. The grain size of about 150 nm is observed in the 4-cycle specimen. The SAD pattern illustrates a more complex shape comparing to the first and second cycles of ASB.

Fig. 9. Optical micrographs obtained from the longitudinal section of the AA1050 specimens after various cycles of ASB

reduction of 50%. While no interface was observed on the sections after polishing, the bond interfaces are distinguished by etching (Fig. 9). Since subsequent ASB cycles improve the previous bonds, the last bond interface of each specimen is clearer than the previous ones. It is obvious in this figure that the external layer undertakes more thickness reduction than the internal one at each cycle, so that the ratio of external layer thickness to internal one is between 0.40 and 0.45. It should be noted that during the process more elongation were observed in the length of external tube than the internal one, confirming the mentioned

Number of ASB cycles 1 2 3 4 Fraction of the HAGBs 0.54 0.76 0.83 0.84 Grain thickness (nm) 409 306 176 186 Grain length (nm) 885 717 465 419

TEM micrographs and the corresponding selected area diffraction (SAD) patterns of the specimens ASBed by one, two and four cycles are shown in Fig. 10. The specimen after one cycle shows a grain size of about 1.5 μm. Large number of subgrain boundaries can be observed in this specimen. The specimen after two cycles illustrates ultra-fine grains with the size of approximately 600 nm. The grain size of about 150 nm is observed in the 4-cycle specimen. The SAD pattern illustrates a more complex shape comparing to the first and

Fig. 9. Optical micrographs obtained from the longitudinal section of the AA1050 specimens

thickness strain distribution.

Table 3. Results of the EBSD analysis

second cycles of ASB.

after various cycles of ASB

Fig. 10. TEM micrographs and SAD patterns of AA1050 specimens processed by one, two and 4 cycles of ASB

Nanostructure, Texture Evolution and Mechanical Properties of

to a random distribution during the next ASB cycles.

cycles obtained from the EBSD analysis

both subgrain and grain boundaries.

Aluminum Alloys Processed by Severe Plastic Deformation 441

grain refinement and increment of HAGBs fraction by increasing the ASB cycles. Quantitative analysis of the EBSD patterns is presented in Table 3 and Figs. 12 and 13. Regarding the elongated grains, grain size is described by the grain thickness and grain length in radial and longitudinal directions, respectively. This is considered and applied for

Fig. 12 demonstrates that both grain thicknesses and lengths decrease by increasing the ASB cycles, so that the grain thickness and length are, respectively, 186 and 419 nm after the fourth cycle. In addition to grain refinement, it can be seen in Figs. 11 and 12 that the HAGBs fraction is increased from 54% after the first cycle to 84% after the fourth cycle of ASB. The grain boundary misorientation increments through the ASB cycles are clearer in depiction of misorientation distributions, Fig. 13. The Mackenzie distribution for a random polycrystalline material is shown by black line. It is observed that while the misorientation distribution after the first cycle is considerably far from the random, it becomes much closer

Fig. 12. Evolution of grain thickness and length and fraction of the HAGBs during the ASB

An analytical model was recently developed (Mohebbi & Akbarzadeh, 2010a) to study the strain history of this process. In this model, the radial strain increment at each revolution is calculated. Total strain at each time is then determined by summation of these increments. The strain rate is calculated by dividing these strain increments to the deformation time of each one (Mohebbi & Akbarzadeh, 2010b). It was demonstrated that the material undergoes a small deformation with high strain rate at any exposure to roller, while at the time between the steps, the strain rate downfalls to zero. For thermally activated deformation

Fig. 11 illustrates the low angle and high angle grain boundary maps recorded on the radial (r)-longitudinal (z) plane of the ASBed specimens. In this figure, which is obtained from the EBSD analysis, low angle grain boundaries (LAGBs) are depicted by red lines where the misorientation between two points of a step is between 2° and 15° and high angle grain boundaries (HAGBs) with misorientations larger than 15° are drawn in the black lines. Comparing to the very locally analysis of TEM micrographs, this figure clearly shows the

Fig. 11. High angle (black) and low angle (red) grain boundary maps of AA1050 specimens processed by various cycles of ASB

Fig. 11 illustrates the low angle and high angle grain boundary maps recorded on the radial (r)-longitudinal (z) plane of the ASBed specimens. In this figure, which is obtained from the EBSD analysis, low angle grain boundaries (LAGBs) are depicted by red lines where the misorientation between two points of a step is between 2° and 15° and high angle grain boundaries (HAGBs) with misorientations larger than 15° are drawn in the black lines. Comparing to the very locally analysis of TEM micrographs, this figure clearly shows the

Fig. 11. High angle (black) and low angle (red) grain boundary maps of AA1050 specimens

processed by various cycles of ASB

grain refinement and increment of HAGBs fraction by increasing the ASB cycles. Quantitative analysis of the EBSD patterns is presented in Table 3 and Figs. 12 and 13. Regarding the elongated grains, grain size is described by the grain thickness and grain length in radial and longitudinal directions, respectively. This is considered and applied for both subgrain and grain boundaries.

Fig. 12 demonstrates that both grain thicknesses and lengths decrease by increasing the ASB cycles, so that the grain thickness and length are, respectively, 186 and 419 nm after the fourth cycle. In addition to grain refinement, it can be seen in Figs. 11 and 12 that the HAGBs fraction is increased from 54% after the first cycle to 84% after the fourth cycle of ASB. The grain boundary misorientation increments through the ASB cycles are clearer in depiction of misorientation distributions, Fig. 13. The Mackenzie distribution for a random polycrystalline material is shown by black line. It is observed that while the misorientation distribution after the first cycle is considerably far from the random, it becomes much closer to a random distribution during the next ASB cycles.

Fig. 12. Evolution of grain thickness and length and fraction of the HAGBs during the ASB cycles obtained from the EBSD analysis

An analytical model was recently developed (Mohebbi & Akbarzadeh, 2010a) to study the strain history of this process. In this model, the radial strain increment at each revolution is calculated. Total strain at each time is then determined by summation of these increments. The strain rate is calculated by dividing these strain increments to the deformation time of each one (Mohebbi & Akbarzadeh, 2010b). It was demonstrated that the material undergoes a small deformation with high strain rate at any exposure to roller, while at the time between the steps, the strain rate downfalls to zero. For thermally activated deformation

Nanostructure, Texture Evolution and Mechanical Properties of

one (Mackenzie distribution).

**3.2 Microtexture** 

Aluminum Alloys Processed by Severe Plastic Deformation 443

by applying strain in increments small enough to determine the negligible temperature rises during the incremental cooling periods (Sevillano et al., 1981). Therefore, it can be said that increasing the strain rate in common SPD processes causes a high temperature rise due to continuous induction of a large strain, which prevents significant promotion of Z. In case of ASB, however, it is possible to increase the strain rate without temperature rise. Deformation with high strain rate and low temperature rise is a very interesting characteristic of ASB process for evaluation of the pure effects of strain rate on the microstructural evolutions and resulted mechanical properties without temperature rise. This research group's work has shown (Mohebbi & Akbarzadeh, 2010b) that due to large amount of redundant strain and high Zener–Hollomon parameter, as the characteristics of ASB process, the grain refinement and the rate of microstructural evolution are expected to be higher in comparison to other SPD processes. This grain refinement occurred by common mechanism of grain refinement in SPDs of pure aluminum. Increase of dislocation density and its accumulation in cell structure by dynamic recovery leads to formation of subgrains at early stages. This is followed by increase of the misorientations by strain induced transition of low angle to high angle grain boundaries at next stages (Pirgazi et al., 2008b). This mechanism is confirmed by the TEM micrographs and SAD patterns as well as the EBSD analysis. As can be seen in Figs. 11–13, a large fraction of LAGBs is formed after the first cycle. After the fourth cycle, while the grains are more refined, the fraction of HAGBs is considerably increased and the boundary misorientation distribution is close to the random

Figure 14 shows φ2 = 45, 65 and 90° sections of the initial orientation distribution functions (ODFs) of both aluminium sheets before the ARB process (fully annealed sheets). It is seen that the initial texture of these samples is mainly composed of a dominant rotated cube {001}<100> which is characteristic of the recrystallisation state. The ODFs corresponding to the samples of Figs. 3 and 4 are illustrated in Figs. 15 and 16 respectively. Only the φ2 = 45 and 90° sections of ODF are represented because these sections contain the most important texture components for ARB processed aluminium sheets. It is seen that the main texture components in the mid layers of both materials are Copper {1 1 2}<1 1 1> located at φ1, Φ, φ<sup>2</sup> ≡ 45, 65 and 90° and Dillamore {4 4 11}<11 11 8> at φ1, Φ, φ2 ≡ 90, 27 and 45° sections and there is no evidence of shear components. The ODFs of these ARB processed samples were generally similar and the overall texture intensity and the concentration of the contour lines increase continuously with increasing strain. However, it should be noted that in comparison with the AA3003 alloy, a more severe texture was observed in AA1100 alloy during the ARB process and the strength of the overall textures developed at the eighth

The presence of second phase particles can also change the microtextural evolution in the ARB processed aluminium sheets. The comparison of the ODFs presented in Figs. 15 and 16 indicates that the developed texture in the AA1100 alloy is much stronger than the particle containing alloy. The evolution of a strong texture during the ARB process is attributed to the cyclic nature of this process. Heason and Prangnell (2002a) investigated the texture evolution during the ARB process of the AA1100 aluminium sheets by using a model based on the FC-Taylor approach. According to their model, the surface shear texture which deforms in plane strain compression (in the mid layer) rotates towards the copper orientation during the next cycle. In contrast, this model predicts that all the rolling

cycle was 18.4 and 28.2 times the random levels, respectively.

and restoration processes, which occur even at room temperature by large strains (Estrin et al., 1998), the microstructural evolution and flow stress are dependent on the deformation temperature, strain and strain rate. The strain rate ( ε ) and deformation temperature (T) are included into the Zener–Hollomon Parameter (Z), which is defined as:

$$\mathbf{Z} = \dot{\mathbf{z}} \cdot \exp\left(\mathbf{Q}/\mathbf{RT}\right) \tag{1}$$

where Q is the activation energy for operative process and R is the gas constant (Zener & Hollomon, 1944).

Fig. 13. Misorientation distribution of the specimens processed by various cycles of ASB comparing to the Mackenzie distribution

Generally, by increasing this parameter, subgrain size at dynamic recovery and steady state grain size at dynamic recrystallization are decreased and the flow stress is increased (Humphreys & Hatherly, 2004). Tsuji et al. (2003b) have studied the effect of strain rate on microstructural changes in deformation of the ultra-fine grained (UFG) aluminum produced by ARB. They had expected that higher speed of deformation can result in finer grain subdivision, because larger amount of dislocations would operate at higher strain rates. However, their results showed that grain size rather increases with increasing the strain rate, due to heat of deformation. Therefore, they concluded that higher strain rate and simultaneous cooling are favorable for producing finer grain size in SPD. However, as deformations with strain rates higher than 10 s−1 are adiabatic processes (Ryazanov et al., 2003), simultaneous cooling is not effective on deformation temperature and therefore, conclusion of Tsuji et al. (2003b) cannot be correct.

Controlled cooling of the specimen in tensile test maintains the normal temperatures of the tests conducted at 10−2 to 10−1 s−1. For higher strain rates, the only way is to perform the test

by applying strain in increments small enough to determine the negligible temperature rises during the incremental cooling periods (Sevillano et al., 1981). Therefore, it can be said that increasing the strain rate in common SPD processes causes a high temperature rise due to continuous induction of a large strain, which prevents significant promotion of Z. In case of ASB, however, it is possible to increase the strain rate without temperature rise. Deformation with high strain rate and low temperature rise is a very interesting characteristic of ASB process for evaluation of the pure effects of strain rate on the microstructural evolutions and resulted mechanical properties without temperature rise. This research group's work has shown (Mohebbi & Akbarzadeh, 2010b) that due to large amount of redundant strain and high Zener–Hollomon parameter, as the characteristics of ASB process, the grain refinement and the rate of microstructural evolution are expected to be higher in comparison to other SPD processes. This grain refinement occurred by common mechanism of grain refinement in SPDs of pure aluminum. Increase of dislocation density and its accumulation in cell structure by dynamic recovery leads to formation of subgrains at early stages. This is followed by increase of the misorientations by strain induced transition of low angle to high angle grain boundaries at next stages (Pirgazi et al., 2008b). This mechanism is confirmed by the TEM micrographs and SAD patterns as well as the EBSD analysis. As can be seen in Figs. 11–13, a large fraction of LAGBs is formed after the first cycle. After the fourth cycle, while the grains are more refined, the fraction of HAGBs is considerably increased and the boundary misorientation distribution is close to the random one (Mackenzie distribution).
