**3. Transmission electron microscopy in the study of Al/BN Nanocomposite**

The fabrication of bulk Al-BN nanocomposites have strongly been considered due to several advantages such as their light weight, superior mechanical properties even at low contents of BN as a reinforcement, and good thermal stability at elevated temperatures. These advantages are definitely related to the special characteristic of BN. It is well recognized that BN decomposed and dissolved in the Al matrix during high energy ball milling. Then, the possible AlN and AlB2 as the in-situ phases are generated during a heating state in the Al matrix [10–17]. Due to presence of in-situ particles in matrix, the microstructural observation and mechanical behavior of the in-situ Al/BN nanocomposite should be investigated by high resolution transmission electron microscopy.

#### **3.1 Phase evaluation**

In the recent study, Al-1,2 and 4 wt. % BN bulk nanocomposite was fabricated by planetary ball milling of the composite powders and a post-process of hot extrusion at 580°C. As can be seen in **Figure 10**, the rod-shape phase was detected in TEM observation. Although AlN and AlB2 as the in-situ phases were expected to create in microstructure, the accurate investigation using STEM has shown different results. As can be seen in **Figure 11**, the nature of the rod-shape phase was recognized by high-resolution STEM micrographs. The plane of atoms is clearly visible in the micrograph. The inter-planar spacing as the finger effect can lead to phase characterization. This parameter for the rod-shaped phases is measured to be about 0.55 nm, which is corresponding to the (006) planes of unwanted in-situ Al4C3 phase, respectively.

**Figure 12** shows the bright-field TEM image of Al4C3 phase with the corresponding result of the EDS line scan the matrix and in-situ phase in the Al-2 wt. % BN nanocomposite. As can be, the high amount of carbon in analysis confirms the presence of Al4C3. In addition, the decreasing blue line and increasing the gray line in scanning through the in-situ phase corresponded to the contents of Al and carbon elements, respectively which were proved the formation of Al4C3.

The EDS analysis as a valuable technique in TEM is used to characterize Al4C3 insitu phases in the microstructure of the Al-2 wt. % BN nanocomposite (**Figure 13**). Although the EDS technique does not accurately determine the composition, the results show the increasing the carbon content in the in-situ phase against the matrix in composition. Stearic acid as the process control agent (PCA) was recognized as the source of carbon in the nanocomposite. In fact, the PCA dissolved in the matrix during the planetary ball milling and Al4C3 was formed after hot extrusion as following reaction [18]:

$$\mathsf{4Al} + \mathsf{BC} = \mathsf{Al}\_4\mathsf{C}\_3 \tag{2}$$

The SAED pattern of nanocomposite was shown in **Figure 14**. The continuous ring shape in the SAED pattern confirms that the nano crystallite structure was developed in the nanocomposite. In addition, the Al4C3 (012) and Al4C3 (0015) rings were observed in this pattern. Although the characterization of phases with the light elements such as nitrogen is troublous, the presence of AlN phase was shown in **Figure 14**. The diffraction reflections of AlN (100) in the SAED pattern indicate its in-situ formation through the extrusion process by the following equation:

$$\text{Al} \downarrow \text{N} = \text{AlN} \tag{3}$$

**17**

not found.

**Figure 11.**

**Figure 10.**

*(b) higher magnification.*

of phases is impossible using XRD.

*Transmission Electron Microscopy of Nanomaterials DOI: http://dx.doi.org/10.5772/intechopen.92212*

AlN in the matrix. Furthermore, the green line in the result of the EDS line scanning shows the higher nitrogen content of the spherical shape phase. According to the composition table in point (1), the high fraction of Nitrogen and all the Boron content remained as a solid solution in the matrix. These elements appeared as a solid solution during high energy ball milling in the Al matrix. Therefore, AlN phase was created with the reaction of Al and N in the hot extrusion process. On the other hand, AlB2 which can be formed from the reaction of Al and B was

*TEM micrograph of Al-1 wt. % BN representing (a) the uniform distribution of a rod-shape in-situ phase and* 

*High resolution STEM image of the in-situ Al4C3 phase with orientation of (006).*

**Figure 16** shows the fraction of Al2O3 phase in the microstructure. As can be seen, the high content of oxygen compared to the matrix is clearly observed as a yellow line in the result of line scanning and the EDS analysis. Therefore, due to the limitation of XRD to detect the in-situ phase which has less than 5 vol. % in the matrix, the phase evaluation of this nanocomposite with the nano- and low content

AlN phase as spherical shape and nanometric size was found in microstructure observation using STEM and the EDS analysis (**Figure 15**). As can be seen, increasing the nitrogen content compared to the matrix confirms the creation of *Transmission Electron Microscopy of Nanomaterials DOI: http://dx.doi.org/10.5772/intechopen.92212*

#### **Figure 10.**

*Electron Crystallography*

**3.1 Phase evaluation**

Al4C3 phase, respectively.

sion as following reaction [18]:

even at low contents of BN as a reinforcement, and good thermal stability at elevated temperatures. These advantages are definitely related to the special characteristic of BN. It is well recognized that BN decomposed and dissolved in the Al matrix during high energy ball milling. Then, the possible AlN and AlB2 as the in-situ phases are generated during a heating state in the Al matrix [10–17]. Due to presence of in-situ particles in matrix, the microstructural observation and mechanical behavior of the in-situ Al/BN nanocomposite should be investigated by

In the recent study, Al-1,2 and 4 wt. % BN bulk nanocomposite was fabricated

by planetary ball milling of the composite powders and a post-process of hot extrusion at 580°C. As can be seen in **Figure 10**, the rod-shape phase was detected in TEM observation. Although AlN and AlB2 as the in-situ phases were expected to create in microstructure, the accurate investigation using STEM has shown different results. As can be seen in **Figure 11**, the nature of the rod-shape phase was recognized by high-resolution STEM micrographs. The plane of atoms is clearly visible in the micrograph. The inter-planar spacing as the finger effect can lead to phase characterization. This parameter for the rod-shaped phases is measured to be about 0.55 nm, which is corresponding to the (006) planes of unwanted in-situ

**Figure 12** shows the bright-field TEM image of Al4C3 phase with the corresponding result of the EDS line scan the matrix and in-situ phase in the Al-2 wt. % BN nanocomposite. As can be, the high amount of carbon in analysis confirms the presence of Al4C3. In addition, the decreasing blue line and increasing the gray line in scanning through the in-situ phase corresponded to the contents of Al and

The EDS analysis as a valuable technique in TEM is used to characterize Al4C3 insitu phases in the microstructure of the Al-2 wt. % BN nanocomposite (**Figure 13**). Although the EDS technique does not accurately determine the composition, the results show the increasing the carbon content in the in-situ phase against the matrix in composition. Stearic acid as the process control agent (PCA) was recognized as the source of carbon in the nanocomposite. In fact, the PCA dissolved in the matrix during the planetary ball milling and Al4C3 was formed after hot extru-

The SAED pattern of nanocomposite was shown in **Figure 14**. The continuous ring shape in the SAED pattern confirms that the nano crystallite structure was developed in the nanocomposite. In addition, the Al4C3 (012) and Al4C3 (0015) rings were observed in this pattern. Although the characterization of phases with the light elements such as nitrogen is troublous, the presence of AlN phase was shown in **Figure 14**. The diffraction reflections of AlN (100) in the SAED pattern indicate its in-situ formation through the extrusion process by the following

AlN phase as spherical shape and nanometric size was found in microstructure observation using STEM and the EDS analysis (**Figure 15**). As can be seen, increasing the nitrogen content compared to the matrix confirms the creation of

4Al + 3C = Al4 C3 (2)

Al + N = AlN (3)

carbon elements, respectively which were proved the formation of Al4C3.

high resolution transmission electron microscopy.

**16**

equation:

*TEM micrograph of Al-1 wt. % BN representing (a) the uniform distribution of a rod-shape in-situ phase and (b) higher magnification.*

#### **Figure 11.**

*High resolution STEM image of the in-situ Al4C3 phase with orientation of (006).*

AlN in the matrix. Furthermore, the green line in the result of the EDS line scanning shows the higher nitrogen content of the spherical shape phase. According to the composition table in point (1), the high fraction of Nitrogen and all the Boron content remained as a solid solution in the matrix. These elements appeared as a solid solution during high energy ball milling in the Al matrix. Therefore, AlN phase was created with the reaction of Al and N in the hot extrusion process. On the other hand, AlB2 which can be formed from the reaction of Al and B was not found.

**Figure 16** shows the fraction of Al2O3 phase in the microstructure. As can be seen, the high content of oxygen compared to the matrix is clearly observed as a yellow line in the result of line scanning and the EDS analysis. Therefore, due to the limitation of XRD to detect the in-situ phase which has less than 5 vol. % in the matrix, the phase evaluation of this nanocomposite with the nano- and low content of phases is impossible using XRD.

#### **Figure 12.**

*Bright field TEM image of Al4C3 phase with the corresponding result of the EDS line scan.*

#### **Figure 13.**

*Bright field TEM images and the corresponding results of EDS analysis of the matrix and in-situ Al4C3 phases in Al-4 wt. % BN nanocomposite.*

#### **Figure 14.**

*(a) SAD pattern of Al-4 wt. % BN nanocomposite (b) higher magnification of SAD pattern of Al phase (blue color), the in-situ Al4C3 (yellow color) and AlN (red color) phases.*

**19**

**Figure 16.**

**Figure 15.**

*In-situ AlN phases in Al-4 wt. % BN nanocomposite (a) high resolution STEM image, (b) the EDS line* 

*scanning analysis, and (c) composition of the Al matrix and AlN phases.*

*The EDS line scanning analysis of Al2O3 phases in Al-1 wt. % BN nanocomposite.*

*Transmission Electron Microscopy of Nanomaterials DOI: http://dx.doi.org/10.5772/intechopen.92212*

*Transmission Electron Microscopy of Nanomaterials DOI: http://dx.doi.org/10.5772/intechopen.92212*

#### **Figure 15.**

*Electron Crystallography*

**18**

**Figure 14.**

**Figure 13.**

**Figure 12.**

*in Al-4 wt. % BN nanocomposite.*

*Bright field TEM images and the corresponding results of EDS analysis of the matrix and in-situ Al4C3 phases* 

*Bright field TEM image of Al4C3 phase with the corresponding result of the EDS line scan.*

*(a) SAD pattern of Al-4 wt. % BN nanocomposite (b) higher magnification of SAD pattern of Al phase (blue* 

*color), the in-situ Al4C3 (yellow color) and AlN (red color) phases.*

*In-situ AlN phases in Al-4 wt. % BN nanocomposite (a) high resolution STEM image, (b) the EDS line scanning analysis, and (c) composition of the Al matrix and AlN phases.*

**Figure 16.** *The EDS line scanning analysis of Al2O3 phases in Al-1 wt. % BN nanocomposite.*

#### **3.2 Dislocation**

Dislocation recognized as a linear defect is the justifier deformation mechanism. Density dislocation is a key factor is Calculated or estimated with a different method. Absolutely, the measuring corresponding to the observation of dislocations has more accurate than estimation with equations. In the mentioned nanocomposite, the TEM observation in **Figure 17** shows the cell wall dislocation and pile-up dislocation behind the grain boundary. As can be seen, the grain boundary in grain 1 acts as an obstacle against the dislocation movement. It led to pile up dislocation and stored strain in grain 1, while relaxation happened in the other adjacent grains with low dislocation density.

According to the following equation and TEM observations, the density dislocation (ρ) is calculated as a function of average separation between dislocations (¯ *<sup>l</sup>*) [19].

$$I = \mathbf{1}/\rho^{\mathbf{n}} \mathbf{0}.\mathbf{5} \tag{4}$$

Based on TEM observations and mentioned above equation, the dislocation density in the cell walls, accumulated behind the grain boundaries and in the grain interior are measured as 1.8 × 1016 m−<sup>2</sup> , 7 × 1014 m−<sup>2</sup> , and 1.5 × 1014 m−<sup>2</sup> , respectively.

**Figure 18** shows the incidence of extended dislocation was intersected on two slip planes. In this phenomenon, the new partial dislocation was created by intersecting of two leading partial dislocation, which is known as a stair-rod dislocation. In this dislocation, three dislocations were connected by wedge shape stacking fault and were immobile which is named Lomer-Cottrell lock. This is a barrier that pins the dislocation and led to work hardening in FCC materials [20].

Orowan dislocation loops in **Figure 19** indicate that dispersed phases such as Aluminum oxide particles in this nanocomposite act as the obstacle in dislocation movement.

#### **3.3 Microstructural observations**

**Figure 20** displays the microstructure of nanocomposite after the hot extrusion process. As can be seen, the low angle boundary with the thick wall with an average thickness of 20 nm is surrounded by high angle boundaries. The cell

#### **Figure 17.**

*TEM observation of dislocation in Al-4 wt. % BN nanocomposite (a) dislocation in cell wall and pile up dislocations behind the cell wall (b) accumulated dislocation near a grain boundary.*

**21**

high angle boundaries [20].

**Figure 18.**

**Figure 19.**

*Image of in Al-4 wt. % BN nanocomposite.*

*Transmission Electron Microscopy of Nanomaterials DOI: http://dx.doi.org/10.5772/intechopen.92212*

structure happens in hot deformation of high SFE materials with converting threedimensional dislocation tangles to two-dimensional dislocations with the more than 10° crystallographic misorientation. Then, the dislocation substructure is obtained from cell structure when the misorientation of cells exceeds to 2°. The dislocation substructures are converted to high angle sub boundaries which are surrounded by

*Orowan dislocation loop (a) pin obstacles (b) higher magnification of Orowan dislocation loop.*

**Figure 21** shows the microstructure of Al-1 wt. % BN nanocomposite from the extrusion direction and cross-section of the extruded rod. As can be seen, the matrix consists of recrystallized nano/ultrafine grains with high angle boundaries and free dislocation density. It means the in-situ nanocomposite has thermal stabil-

Based on TEM observations and the high true strain value in hot extrusion process (2.3 mm/mm) which is higher than required critical strain for happening dynamic recrystallization in Al matrix (0.5 mm/mm), the recrystallization is the dominant mechanism in hot extrusion and high fraction of boundaries are formed as the high angle in microstructure [21]. On the other hand, the low angle boundaries and the grains with the high dislocation density were observed in microstructure

ity against the abnormal grain at high extrusion temperature.

*Transmission Electron Microscopy of Nanomaterials DOI: http://dx.doi.org/10.5772/intechopen.92212*

*Electron Crystallography*

adjacent grains with low dislocation density.

*<sup>l</sup>*) [19].

interior are measured as 1.8 × 1016 m−<sup>2</sup>

**3.3 Microstructural observations**

Dislocation recognized as a linear defect is the justifier deformation mechanism. Density dislocation is a key factor is Calculated or estimated with a different method. Absolutely, the measuring corresponding to the observation of dislocations has more accurate than estimation with equations. In the mentioned nanocomposite, the TEM observation in **Figure 17** shows the cell wall dislocation and pile-up dislocation behind the grain boundary. As can be seen, the grain boundary in grain 1 acts as an obstacle against the dislocation movement. It led to pile up dislocation and stored strain in grain 1, while relaxation happened in the other

According to the following equation and TEM observations, the density dislocation (ρ) is calculated as a function of average separation between

Based on TEM observations and mentioned above equation, the dislocation density in the cell walls, accumulated behind the grain boundaries and in the grain

**Figure 18** shows the incidence of extended dislocation was intersected on two slip planes. In this phenomenon, the new partial dislocation was created by intersecting of two leading partial dislocation, which is known as a stair-rod dislocation. In this dislocation, three dislocations were connected by wedge shape stacking fault and were immobile which is named Lomer-Cottrell lock. This is a barrier that pins

Orowan dislocation loops in **Figure 19** indicate that dispersed phases such as Aluminum oxide particles in this nanocomposite act as the obstacle in dislocation

**Figure 20** displays the microstructure of nanocomposite after the hot extrusion process. As can be seen, the low angle boundary with the thick wall with an average thickness of 20 nm is surrounded by high angle boundaries. The cell

*TEM observation of dislocation in Al-4 wt. % BN nanocomposite (a) dislocation in cell wall and pile up* 

*dislocations behind the cell wall (b) accumulated dislocation near a grain boundary.*

the dislocation and led to work hardening in FCC materials [20].

, 7 × 1014 m−<sup>2</sup>

¯*l* = 1/ρ^0.5 (4)

, and 1.5 × 1014 m−<sup>2</sup>

, respectively.

**3.2 Dislocation**

dislocations (¯

movement.

**20**

**Figure 17.**

**Figure 18.** *Image of in Al-4 wt. % BN nanocomposite.*

**Figure 19.**

*Orowan dislocation loop (a) pin obstacles (b) higher magnification of Orowan dislocation loop.*

structure happens in hot deformation of high SFE materials with converting threedimensional dislocation tangles to two-dimensional dislocations with the more than 10° crystallographic misorientation. Then, the dislocation substructure is obtained from cell structure when the misorientation of cells exceeds to 2°. The dislocation substructures are converted to high angle sub boundaries which are surrounded by high angle boundaries [20].

**Figure 21** shows the microstructure of Al-1 wt. % BN nanocomposite from the extrusion direction and cross-section of the extruded rod. As can be seen, the matrix consists of recrystallized nano/ultrafine grains with high angle boundaries and free dislocation density. It means the in-situ nanocomposite has thermal stability against the abnormal grain at high extrusion temperature.

Based on TEM observations and the high true strain value in hot extrusion process (2.3 mm/mm) which is higher than required critical strain for happening dynamic recrystallization in Al matrix (0.5 mm/mm), the recrystallization is the dominant mechanism in hot extrusion and high fraction of boundaries are formed as the high angle in microstructure [21]. On the other hand, the low angle boundaries and the grains with the high dislocation density were observed in microstructure

#### **Figure 20.**

*Image of low angle grain boundary surrounded by high angle grain boundaries (Higher magnification views from Figure 19(b)).*

#### **Figure 21.**

*Microstructure of Al-1 wt. % BN nanocomposite from (a) cross section of extruded rod (b) extrusion direction. Arrows show the scalloped boundaries.*

(**Figure 22**). As can be seen, it seems that the rotation and coalescence is the main mechanism to decrease the low angle boundary. Due to high extrusion temperature, high stacking fault energy of Al matrix and high straining, the dynamic recovery occurred in the initial stage of deformation and the dislocations rearrangement themselves as low angle boundaries and subgrains is formed.

The geometrically necessary dislocation in metal matrix composite is high and led to increase in the kinetics of recrystallization [22–24]. However, the identification of dynamic recrystallization mechanisms in materials is difficult. The continuous dynamic recrystallization (CDRX) and the geometrical dynamic recrystallization (GDRX) can also occur simultaneously. During deformation of high stacking fault energy materials in elevated temperature, dynamic recovery prevents the accumulation of dislocation and the occurrence of discontinuous dynamic recrystallization (DDRX) will sustain. In CDRX, the subgrains are developed and

**23**

**4. Conclusions**

**Figure 22.**

**Figure 23.**

*Transmission Electron Microscopy of Nanomaterials DOI: http://dx.doi.org/10.5772/intechopen.92212*

then with increasing the misorientation of subgrain boundaries, the high angle grain boundaries will appear. On the other hand, in pure, solute drag and particlecontaining alloy systems with high stacking fault energy which are deformed to large strain at high temperature, the GDRX will take place. In GDRX, the serrations are developed on the high angle boundaries and the scalloped boundaries are formed. In large straining, the grain elongation and thinning occurred. Then, the impingement of serrated boundaries led to equiaxed grains formation [22–26]. **Figure 23(a)** shows the ultrafine grains were recrystallized by CDRX. As can be seen in **Figures 21** and **23(b)**, the serrated high angle boundaries were observed. According to TEM results, the CDRX and GDRX are recognized as the dominant

*Bright field TEM images of recrystallized grains in Al-4 wt. % BN (a) CDRX grains (b) GDRX grains.*

*(a) Microstructure of Al-4 wt. % BN nanocomposite from and (b) higher magnification.*

In summary, TEM/STEM with an energy-dispersive X-ray spectrometry corresponding to selected area diffraction patterns (SADP) is a powerful and accurate instrument to analysis and results from interpretation of nanomaterial lattice

mechanisms in high angle grain boundary formation.

*Transmission Electron Microscopy of Nanomaterials DOI: http://dx.doi.org/10.5772/intechopen.92212*

#### **Figure 22.**

*Electron Crystallography*

**22**

**Figure 21.**

**Figure 20.**

*from Figure 19(b)).*

*Arrows show the scalloped boundaries.*

(**Figure 22**). As can be seen, it seems that the rotation and coalescence is the main mechanism to decrease the low angle boundary. Due to high extrusion temperature, high stacking fault energy of Al matrix and high straining, the dynamic recovery occurred in the initial stage of deformation and the dislocations rearrangement

*Microstructure of Al-1 wt. % BN nanocomposite from (a) cross section of extruded rod (b) extrusion direction.* 

*Image of low angle grain boundary surrounded by high angle grain boundaries (Higher magnification views* 

The geometrically necessary dislocation in metal matrix composite is high and led to increase in the kinetics of recrystallization [22–24]. However, the identification of dynamic recrystallization mechanisms in materials is difficult. The continuous dynamic recrystallization (CDRX) and the geometrical dynamic recrystallization (GDRX) can also occur simultaneously. During deformation of high stacking fault energy materials in elevated temperature, dynamic recovery prevents the accumulation of dislocation and the occurrence of discontinuous dynamic recrystallization (DDRX) will sustain. In CDRX, the subgrains are developed and

themselves as low angle boundaries and subgrains is formed.

*(a) Microstructure of Al-4 wt. % BN nanocomposite from and (b) higher magnification.*

then with increasing the misorientation of subgrain boundaries, the high angle grain boundaries will appear. On the other hand, in pure, solute drag and particlecontaining alloy systems with high stacking fault energy which are deformed to large strain at high temperature, the GDRX will take place. In GDRX, the serrations are developed on the high angle boundaries and the scalloped boundaries are formed. In large straining, the grain elongation and thinning occurred. Then, the impingement of serrated boundaries led to equiaxed grains formation [22–26]. **Figure 23(a)** shows the ultrafine grains were recrystallized by CDRX. As can be seen in **Figures 21** and **23(b)**, the serrated high angle boundaries were observed. According to TEM results, the CDRX and GDRX are recognized as the dominant mechanisms in high angle grain boundary formation.
