**3.1 Microstructure**

### **3.1.1 ARB processing**

Figure 3 shows the optical micrograph observed in the RD-ND section of the sample produced by 10 ARB cycles, indicating that a good bonding with no delamination has been achieved under the present ARB conditions. To investigate the microstructural evolution of

Nanostructure, Texture Evolution and Mechanical Properties of

LAGBs.

cycles

Aluminum Alloys Processed by Severe Plastic Deformation 433

(1.6<ε<4.8) the grain subdivision ratio decreases and strain induced transformation of low angle to high angle grain boundaries (HAGB) caused by the accumulation of dislocations in exiting subgrain boundaries is the controlling mechanism which results in a lamellar structure at the specimens processed by 4 and 6 ARB cycles (Figs. 4b and c). The fraction of HAGB presented in Figs. 4 and 5 confirm the increase of the HAGBs in expense of the

In comparison with the first sample, less pronounced unrefined regions of subgrains are also seen in these samples. The proposed mechanism of grain refinement at high levels of strain (ε>4.8) is the progressive fragmentation of the thin lamellar grains into more equiaxed grain structures which leads to development of a relatively homogeneous submicron

grain structure without the unrefined regions after the eighth cycle (Fig. 4d).

Fig. 4. Orientation scans obtained from the EBSD measurements of the AA1100 ARB processed by (a) 2, (b) 4, (c) 6 and (d) 8 cycles. Thin grey lines represent the misorientation

Fig. 5. Orientation scans of the AA3003 samples produced by (a) 2, (b) 4, (c) 6 and (d) 8 ARB

(*θ*) of 2° ≤*θ* <15° and bold black lines represent 15° ≤ *θ*

aluminium sheets processed by ARB, the boundary misorientation maps were extracted from the EBSD data. These mappings were recorded on a section near the centre of the RD– ND plane of the samples. Because of the severe deformation of the ARB processed samples, acquiring good Kikuchi-lines was too difficult. However, by means of electropolishing in Barker's reagent (5 mL HBF4 in 200 mL H2O) and preparing a very smooth surface and adjusting the parameters of SEM (e.g. gun high voltage, working distance and spot size) and the OIM Data Collection software (e.g. exposure time, binning and step size), it was significantly improved.

Fig. 3. Optical micrograph of longitudinal cross section of aluminum sheet (AA1100) after 10 ARB cycles

The orientation scans of the AA1100 and AA3003 aluminum sheets processed by various ARB cycles are depicted in Figs. 4 and 5. The boundary misorientation maps of these samples are also depicted in these figures. In these maps, the high angle grain boundaries (HAGB) with misorientations larger than 15° are drawn in bold black lines, while the low angle grain boundaries (LAGB) with misorientations between 2° and 15° are drawn in thin grey lines. In the case of AA1100 alloy, it is observed that after two cycles of ARB, the microstructure is covered with elongated grains surrounded by many low angle grain boundaries (Fig. 4a). The microstructures of the samples produced by four and six ARB cycles are generally similar to the sample produced by two ARB cycles. However, with the increasing number of cycles the grain sizes decrease gradually. After the sixth cycle, the structure consists of very fine elongated grains surrounded by high angle grain boundaries which are usually parallel to the rolling direction (Fig. 4c).

It is observed that the evolution of ultra-fine grains in the AA1100 alloy occurs by various mechanisms of grain refinement at different strains. At low strains (ε<1.6) grain subdivision is the dominant mechanism which leads to the formation of a fibrous microstructure of elongated grains and promotes the formation of subgrains surrounded by a high fraction of low angle grain boundaries (LAGB) (Fig. 4a) and some coarse unrefined bands which contain predominantly subgrains are seen in this specimen. At medium levels of strains

aluminium sheets processed by ARB, the boundary misorientation maps were extracted from the EBSD data. These mappings were recorded on a section near the centre of the RD– ND plane of the samples. Because of the severe deformation of the ARB processed samples, acquiring good Kikuchi-lines was too difficult. However, by means of electropolishing in Barker's reagent (5 mL HBF4 in 200 mL H2O) and preparing a very smooth surface and adjusting the parameters of SEM (e.g. gun high voltage, working distance and spot size) and the OIM Data Collection software (e.g. exposure time, binning and step size), it was

Fig. 3. Optical micrograph of longitudinal cross section of aluminum sheet (AA1100) after 10

The orientation scans of the AA1100 and AA3003 aluminum sheets processed by various ARB cycles are depicted in Figs. 4 and 5. The boundary misorientation maps of these samples are also depicted in these figures. In these maps, the high angle grain boundaries (HAGB) with misorientations larger than 15° are drawn in bold black lines, while the low angle grain boundaries (LAGB) with misorientations between 2° and 15° are drawn in thin grey lines. In the case of AA1100 alloy, it is observed that after two cycles of ARB, the microstructure is covered with elongated grains surrounded by many low angle grain boundaries (Fig. 4a). The microstructures of the samples produced by four and six ARB cycles are generally similar to the sample produced by two ARB cycles. However, with the increasing number of cycles the grain sizes decrease gradually. After the sixth cycle, the structure consists of very fine elongated grains surrounded by high angle grain boundaries

It is observed that the evolution of ultra-fine grains in the AA1100 alloy occurs by various mechanisms of grain refinement at different strains. At low strains (ε<1.6) grain subdivision is the dominant mechanism which leads to the formation of a fibrous microstructure of elongated grains and promotes the formation of subgrains surrounded by a high fraction of low angle grain boundaries (LAGB) (Fig. 4a) and some coarse unrefined bands which contain predominantly subgrains are seen in this specimen. At medium levels of strains

which are usually parallel to the rolling direction (Fig. 4c).

significantly improved.

ARB cycles

(1.6<ε<4.8) the grain subdivision ratio decreases and strain induced transformation of low angle to high angle grain boundaries (HAGB) caused by the accumulation of dislocations in exiting subgrain boundaries is the controlling mechanism which results in a lamellar structure at the specimens processed by 4 and 6 ARB cycles (Figs. 4b and c). The fraction of HAGB presented in Figs. 4 and 5 confirm the increase of the HAGBs in expense of the LAGBs.

In comparison with the first sample, less pronounced unrefined regions of subgrains are also seen in these samples. The proposed mechanism of grain refinement at high levels of strain (ε>4.8) is the progressive fragmentation of the thin lamellar grains into more equiaxed grain structures which leads to development of a relatively homogeneous submicron grain structure without the unrefined regions after the eighth cycle (Fig. 4d).

Fig. 4. Orientation scans obtained from the EBSD measurements of the AA1100 ARB processed by (a) 2, (b) 4, (c) 6 and (d) 8 cycles. Thin grey lines represent the misorientation (*θ*) of 2° ≤*θ* <15° and bold black lines represent 15° ≤ *θ*

Fig. 5. Orientation scans of the AA3003 samples produced by (a) 2, (b) 4, (c) 6 and (d) 8 ARB cycles

Nanostructure, Texture Evolution and Mechanical Properties of

and reaches a saturation value of about 72% after the sixth cycle.

Aluminum Alloys Processed by Severe Plastic Deformation 435

alloys, with ARB processing, the fraction of high angle grain boundaries increases and approaches to a near random misorientation distribution very much similar to the theoretical Mackenzie distribution. However, there are some differences between the misorientation profiles produced in the two alloys during the ARB process. It can be observed that in AA3003 alloy the fraction of HAGBs initially increases rapidly and reaches over 70% after the fourth cycle. During the next cycles, few further changes occur and the fraction of HAGBs after the eighth cycles reaches to 75%. In contrast, the fraction of HAGBs in AA1100 alloy increases more slowly with the increasing strain during the ARB process

Fig. 6. Variation of microstructural parameters of aluminium sheets during ARB process, (a)

A general definition for the materials with a submicrometer grain structure proposed by (Prangnell et al., 2001) includes two main criteria: the average spacing of HAGBs must be less than 1 μm, and the fraction of HAGBs must be greater than 70%. The authors' observations proved that the development of such submicrometer or ultrafine grains in a single phase (AA1100) and a particle containing alloy (AA3003) occurs at different levels of strains during the ARB process. It was shown that (Heason & Prangnell, 2002a) the

grain thickness and (b) grain length

Fig. 5 displays that in comparison with the AA1100 alloy, in the AA3003 alloy which contains a significant volume fraction of second phase particles, an ultra-fine grain structure develops at a much higher rate. It is reported that two types of precipitates evolve in the AA3003 alloy: orthorhombic (Fe,Mn)Al6 and cubic α-Al(Fe,Mn)Si. The total volume fraction of these precipitates is between 4 and 5% and the large majority of them are elongated and have an average length between 1-5 µm (Richert J. & Richert M., 1986). In a similar research on AA8097, it has been shown that these second phase particles cause the texture randomization and promotion of grain refinement (Heason and Prangnell, 2002b).

According to Fig. 5, the volume fraction of HAGBs after the second cycle in the AA3003 alloy is 67% compared to 57% in the AA1100 alloy. This rapid refinement of grains in the AA3003 alloy at relatively low strains results from extensive HAGBs generation in local deformation zones around the second-phase particles. During deformation, large lattice rotation around the second phase particles leads to the increase of local misorientation and production of new HAGBs. Dispersion of these particles also develops a random and more heterogeneous plastic flow behavior within the matrix and promotes the fragmentation of the lamellar structures seen in the AA1100 alloy (Pirgazi & Akbarzadeh, 2008c). This behavior as well as a gradual increase in the percentage of HAGBs and a reduction in the grain aspect ratio during the next cycles results in the formation of a more homogenous submicron grains structure in the AA3003 alloy (Fig. 5).

Although many low angle grain boundaries are also observed in the microstructure, they are usually perpendicular to the rolling direction. Figure 4d displays the microstructure developed in the AA1100 aluminium sheet processed by eight ARB cycles. Significant changes are observed at this level of strain and the grain morphology changes into more equi-axed structures. In comparison with the AA1100 alloy, the developed microstructures in AA3003 alloy during the ARB process are somewhat different. According to Fig. 5, it is clearly observed that in this alloy, the formation of ultrafine grains occurs at the earlier stages of the process. Figure 5a shows that even after the second cycle, the microstructure is completely covered with very fine grains whose morphology is more equi-axed than the same sample of AA1100 alloy (Fig. 4a).

With the number increasing of cycles, the grain size continuously decreases and after the eighth cycle the whole specimen shows a homogenous submicrometre grain size structure. Further analysis revealed that the microstructures of these samples differ in grain size and the fraction of HAGBs. Variations of microstructural parameters of the aluminium sheets during the ARB process are depicted in Fig. 6 with regard to the changes of the average thickness and length of grains. In the AA1100 alloy, the grain thickness (measured by the linear interception method on the EBSD maps) drastically decreases during the first four cycles. While with the increasing strain during the next cycles, the grain thickness reduces slightly and approached to a constant value of about 500 nm at large strains. Similar results are observed in the AA3003 alloy but the grain thickness in this case is smaller than the former case and reaches the minimum of 400 nm after the eighth cycle. Comparison of the grain length in the ARB processed samples revealed that there is a more dramatic difference between the evolved microstructures in the two alloys after a large number of ARB cycles. In the AA3003 alloy, the average length of grains decreases continuously while in the case of AA1100 alloy a drastic reduction is observed after the sixth cycle (Fig. 6b).

By analysing the EBSD data after each ARB cycle, quantitative measurements were made of the misorientation distributions and the results are summarised in Fig. 7. The Mackenzie distribution for boundaries in a random polycrystal is shown by the black bold line. In both

Fig. 5 displays that in comparison with the AA1100 alloy, in the AA3003 alloy which contains a significant volume fraction of second phase particles, an ultra-fine grain structure develops at a much higher rate. It is reported that two types of precipitates evolve in the AA3003 alloy: orthorhombic (Fe,Mn)Al6 and cubic α-Al(Fe,Mn)Si. The total volume fraction of these precipitates is between 4 and 5% and the large majority of them are elongated and have an average length between 1-5 µm (Richert J. & Richert M., 1986). In a similar research on AA8097, it has been shown that these second phase particles cause the texture

According to Fig. 5, the volume fraction of HAGBs after the second cycle in the AA3003 alloy is 67% compared to 57% in the AA1100 alloy. This rapid refinement of grains in the AA3003 alloy at relatively low strains results from extensive HAGBs generation in local deformation zones around the second-phase particles. During deformation, large lattice rotation around the second phase particles leads to the increase of local misorientation and production of new HAGBs. Dispersion of these particles also develops a random and more heterogeneous plastic flow behavior within the matrix and promotes the fragmentation of the lamellar structures seen in the AA1100 alloy (Pirgazi & Akbarzadeh, 2008c). This behavior as well as a gradual increase in the percentage of HAGBs and a reduction in the grain aspect ratio during the next cycles results in the formation of a more homogenous

Although many low angle grain boundaries are also observed in the microstructure, they are usually perpendicular to the rolling direction. Figure 4d displays the microstructure developed in the AA1100 aluminium sheet processed by eight ARB cycles. Significant changes are observed at this level of strain and the grain morphology changes into more equi-axed structures. In comparison with the AA1100 alloy, the developed microstructures in AA3003 alloy during the ARB process are somewhat different. According to Fig. 5, it is clearly observed that in this alloy, the formation of ultrafine grains occurs at the earlier stages of the process. Figure 5a shows that even after the second cycle, the microstructure is completely covered with very fine grains whose morphology is more equi-axed than the

With the number increasing of cycles, the grain size continuously decreases and after the eighth cycle the whole specimen shows a homogenous submicrometre grain size structure. Further analysis revealed that the microstructures of these samples differ in grain size and the fraction of HAGBs. Variations of microstructural parameters of the aluminium sheets during the ARB process are depicted in Fig. 6 with regard to the changes of the average thickness and length of grains. In the AA1100 alloy, the grain thickness (measured by the linear interception method on the EBSD maps) drastically decreases during the first four cycles. While with the increasing strain during the next cycles, the grain thickness reduces slightly and approached to a constant value of about 500 nm at large strains. Similar results are observed in the AA3003 alloy but the grain thickness in this case is smaller than the former case and reaches the minimum of 400 nm after the eighth cycle. Comparison of the grain length in the ARB processed samples revealed that there is a more dramatic difference between the evolved microstructures in the two alloys after a large number of ARB cycles. In the AA3003 alloy, the average length of grains decreases continuously while in the case of

By analysing the EBSD data after each ARB cycle, quantitative measurements were made of the misorientation distributions and the results are summarised in Fig. 7. The Mackenzie distribution for boundaries in a random polycrystal is shown by the black bold line. In both

AA1100 alloy a drastic reduction is observed after the sixth cycle (Fig. 6b).

randomization and promotion of grain refinement (Heason and Prangnell, 2002b).

submicron grains structure in the AA3003 alloy (Fig. 5).

same sample of AA1100 alloy (Fig. 4a).

alloys, with ARB processing, the fraction of high angle grain boundaries increases and approaches to a near random misorientation distribution very much similar to the theoretical Mackenzie distribution. However, there are some differences between the misorientation profiles produced in the two alloys during the ARB process. It can be observed that in AA3003 alloy the fraction of HAGBs initially increases rapidly and reaches over 70% after the fourth cycle. During the next cycles, few further changes occur and the fraction of HAGBs after the eighth cycles reaches to 75%. In contrast, the fraction of HAGBs in AA1100 alloy increases more slowly with the increasing strain during the ARB process and reaches a saturation value of about 72% after the sixth cycle.

Fig. 6. Variation of microstructural parameters of aluminium sheets during ARB process, (a) grain thickness and (b) grain length

A general definition for the materials with a submicrometer grain structure proposed by (Prangnell et al., 2001) includes two main criteria: the average spacing of HAGBs must be less than 1 μm, and the fraction of HAGBs must be greater than 70%. The authors' observations proved that the development of such submicrometer or ultrafine grains in a single phase (AA1100) and a particle containing alloy (AA3003) occurs at different levels of strains during the ARB process. It was shown that (Heason & Prangnell, 2002a) the

Nanostructure, Texture Evolution and Mechanical Properties of

sheets.

boundaries.

10 ARB cycles

**3.1.2 ASB processing** 

Aluminum Alloys Processed by Severe Plastic Deformation 437

subgrains are also seen in these samples. The proposed mechanism for grain refinement at high levels of strain (ε> 4.8) is the progressive fragmentation of thin lamellar grains into more equi-axed grain structures (Fig. 4d). The last mentioned mechanism enables the ARB process to develop a relatively homogeneous submicrometer grain structure in aluminium

Figure 5 displays that in comparison with the AA1100 alloy, in the AA3003 alloy which contains a significant volume fraction of second phase particles, an ultrafine grain structure develops at a much higher rate. It has been reported that two types of precipitates evolve in the AA3003 alloy: orthorhombic (Fe,Mn)Al6 and cubic α-Al(Fe,Mn)Si (Rios & Padilha, 2003). The total volume fraction of these precipitates is between 4 and 5% and the large majority of them are elongated and have an average length between 1 and 5 mm. According to Fig. 7, the volume fraction of HAGBs after the second cycle in the AA3003 alloy is 67% compared with 57% in the AA1100 alloy. At the same time, the transverse spacing of HAGBs in these specimens is 0.66 and 1.1 mm respectively (Fig. 6a). This rapid refinement of grains in the AA3003 alloy at relatively low strains results from extensive generation of HAGBs in local deformation zones around the second phase particles. During deformation, large lattice rotation around the second phase particles leads to the increase of the local misorientation and production of new HAGBs. Dispersion of these particles also develops a random and more heterogeneous plastic flow behaviour within the matrix and promotes fragmentation of the lamellar structures seen in the AA1100 alloy. This behaviour as well as the gradual increase in the percentage of HAGBs and reduction in the grain aspect ratio during the next cycles results in the formation of a more homogenous submicrometre grain structure in the AA3003 alloy (Fig. 6). TEM micrograph depicted in Fig. 8 confirms that the grain size of the sample processed by 10 ARB cycles reaches a nanoscale value. The microstructure mostly consists of grains with a size in the range of 200-300 nm which are surrounded by clear

Fig. 8. TEM micrograph showing ultrafine grains in rolling plane of the AA1100 sample after

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

evolution of ultrafine grains in the AA1100 alloy occurs by various mechanisms of grain refinement at different strains.

Fig. 7. Misorientation distributions in the samples of (a) AA100 and (b) AA3003 aluminium sheets processed by various ARB cycles

At low strains (ε< 1.6), grain subdivision is the dominant mechanism which leads to the formation of a fibrous microstructure of elongated grains. This mechanism also promotes the formation of subgrains surrounded by a high fraction of low angle grain boundaries (Fig. 4a) and some coarse unrefined bands which contain predominantly subgrains are seen in this specimen. At medium levels of strains (1.6 < ε < 4.8) the grain subdivision ratio decreases and the strain induced transformation of low angle to high angle grain boundaries caused by the accumulation of dislocations in exiting subgrain boundaries is the governing mechanism which results in a lamellar structure at the specimens processed by four and six ARB cycles (Figs. 4b and c).

The misorientation profiles plotted in Fig. 7 confirm the increase of HAGBs in expense of the LAGBs and the tendency of misorientation of the lamellar structure to a Mackenzie distribution. In comparison with the first sample, less pronounced unrefined regions of

evolution of ultrafine grains in the AA1100 alloy occurs by various mechanisms of grain

Fig. 7. Misorientation distributions in the samples of (a) AA100 and (b) AA3003 aluminium

At low strains (ε< 1.6), grain subdivision is the dominant mechanism which leads to the formation of a fibrous microstructure of elongated grains. This mechanism also promotes the formation of subgrains surrounded by a high fraction of low angle grain boundaries (Fig. 4a) and some coarse unrefined bands which contain predominantly subgrains are seen in this specimen. At medium levels of strains (1.6 < ε < 4.8) the grain subdivision ratio decreases and the strain induced transformation of low angle to high angle grain boundaries caused by the accumulation of dislocations in exiting subgrain boundaries is the governing mechanism which results in a lamellar structure at the specimens processed by four and six

The misorientation profiles plotted in Fig. 7 confirm the increase of HAGBs in expense of the LAGBs and the tendency of misorientation of the lamellar structure to a Mackenzie distribution. In comparison with the first sample, less pronounced unrefined regions of

refinement at different strains.

sheets processed by various ARB cycles

ARB cycles (Figs. 4b and c).

subgrains are also seen in these samples. The proposed mechanism for grain refinement at high levels of strain (ε> 4.8) is the progressive fragmentation of thin lamellar grains into more equi-axed grain structures (Fig. 4d). The last mentioned mechanism enables the ARB process to develop a relatively homogeneous submicrometer grain structure in aluminium sheets.

Figure 5 displays that in comparison with the AA1100 alloy, in the AA3003 alloy which contains a significant volume fraction of second phase particles, an ultrafine grain structure develops at a much higher rate. It has been reported that two types of precipitates evolve in the AA3003 alloy: orthorhombic (Fe,Mn)Al6 and cubic α-Al(Fe,Mn)Si (Rios & Padilha, 2003). The total volume fraction of these precipitates is between 4 and 5% and the large majority of them are elongated and have an average length between 1 and 5 mm. According to Fig. 7, the volume fraction of HAGBs after the second cycle in the AA3003 alloy is 67% compared with 57% in the AA1100 alloy. At the same time, the transverse spacing of HAGBs in these specimens is 0.66 and 1.1 mm respectively (Fig. 6a). This rapid refinement of grains in the AA3003 alloy at relatively low strains results from extensive generation of HAGBs in local deformation zones around the second phase particles. During deformation, large lattice rotation around the second phase particles leads to the increase of the local misorientation and production of new HAGBs. Dispersion of these particles also develops a random and more heterogeneous plastic flow behaviour within the matrix and promotes fragmentation of the lamellar structures seen in the AA1100 alloy. This behaviour as well as the gradual increase in the percentage of HAGBs and reduction in the grain aspect ratio during the next cycles results in the formation of a more homogenous submicrometre grain structure in the AA3003 alloy (Fig. 6). TEM micrograph depicted in Fig. 8 confirms that the grain size of the sample processed by 10 ARB cycles reaches a nanoscale value. The microstructure mostly consists of grains with a size in the range of 200-300 nm which are surrounded by clear boundaries.

Fig. 8. TEM micrograph showing ultrafine grains in rolling plane of the AA1100 sample after 10 ARB cycles
