**5. Ultrafine-grained superplasticity**

Xu et al. [50] stated that superplastic deformation is one of the innovative applications of UFG alloys. Reducing the grain size aids in increasing the rate of superplastic deformation where GBS is the operative deformation mechanism. Kim et al. [51] proposed lattice diffusion-controlled GBS as a deformation mechanism for high-strain-rate superplasticity in 0.3–0.4-μm-sized grain Al alloy. Superplastic elongation of 9000% was achieved in an ultrafine-grained Ti-6Al-4 V alloy at 850°C [52]. However, Cu-Zn-Zr alloy exhibited superplasticity at a temperature

above 400°C and at strain rates below 1 × 10<sup>−</sup><sup>3</sup> s<sup>−</sup><sup>1</sup> . It was because UFG was obtained by ECAP method which contains elongated grains. At temperature higher than the recrystallization temperature of the alloy, it exhibited superplasticity. Mere 0.18% addition of Zr helps in increasing the recrystallization temperature from 100 to 500°C that prevents grain growth. Zn inhibits dislocation creep and assists GBS in the process [53]. Ma et al. [54] achieved superplasticity in UFG Al alloy at a strain rate of 1 s<sup>−</sup><sup>1</sup> and at a temperature of 425°C due to lattice diffusion-controlled GBS.

Kawasaki and Langdon [56–58] developed deformation mechanism map for UFG polycrystalline materials depending on the strain rate and temperature range. Most of the materials, namely, Zn-22% Al and Pb-62% Sn alloys, fall in the range of region II of the superplasticity regime. Lee and Horita [59] observed low-temperature superplasticity in Al7075 alloy at 200–250°C due to UFG generated by highpressure torsion process. In Al 5024 alloy, UFG grains of 0.7 μm was produced by ECAP and tested for superplasticity in the temperature range of 175–450°C and at a strain rate ranging from 10<sup>−</sup><sup>3</sup> s<sup>−</sup><sup>1</sup> to 10<sup>−</sup><sup>1</sup> s<sup>−</sup><sup>1</sup> . The material exhibited superplasticity in all the tested conditions due to grain boundary diffusion-controlled GBS. At 175°C the maximum elongation of 365% was obtained at a strain rate of 1.4 × 10<sup>−</sup><sup>4</sup> s<sup>−</sup><sup>1</sup> , and at 450°C the maximum elongation of 3300% was obtained at a strain rate of 5.6 × 10<sup>−</sup><sup>1</sup> s<sup>−</sup><sup>1</sup> . At low temperature, strain hardening was observed at the primary deformation stage due to the accumulation of lattice dislocations, whereas at high temperature, strain-induced grain elongation causes the strain hardening which accounted for the high-strain-rate superplasticity [55]. Bobruk et al. [60] demonstrated that UFG grain promoted low-temperature and high-strain-rate superplasticity in Al 7050 alloy. Edalati et al. [61] observed low-temperature superplasticity in Mg-Li alloy with 0.25 μm. The deformation mechanism was grain boundary sliding controlled by fast grain boundary diffusion due to Li segregation along the grain boundaries. The low-temperature superplasticity in this case was not attributed to UFG alone, but Li segregation in the boundary favors the grain boundary diffusion and GBS. Similar observation of maximum elongation of approximately 850% was found in UFG Ti-6Al-4 V (~0.4 μm) alloy due to high volume fraction of beta phase in a continuous network fashion acted as soft lubricant layer and increases the elongation [62]. Various deformation mechanisms that contribute for superplastic deformation in UFG regime were mapped with the optimum strain rate and temperature in **Figure 4**. It is evident that there

**105**

higher temperatures.

**Figure 5.**

*alloys [55].*

**6. Nano-grained superplasticity**

*Effect of Grain Size on Superplastic Deformation of Metallic Materials*

is a relationship between temperature and strain rate. In **Figure 5**, an empirical relationship was presented with an illustration that HSRSP would take place at

*Variation of optimum strain rate for maximum superplasticity (a) and optimum strain rate for the highest value of strain-rate sensitivity (b) with temperatures for ECAP Al-Mg-Sc-Zr and other UFG aluminum* 

The kinetics of nano-grained superplasticity is different from the higher scale sized grains. Nano-grains show higher flow stress for superplastic deformation. It makes difficulty in activating the slip systems for the accommodation process [63]. The hypothesis for difficult slip accommodation is displayed in **Figure 6**. In the absence of slip as accommodation mechanism for GBS during nano-grained superplasticity, one of the possible accommodation mechanisms in nano-grains could be mobile triple junctions [64]. In nano-grained materials, partial dislocation slips can be activated at a lower stress level when compared to the perfect

*DOI: http://dx.doi.org/10.5772/intechopen.86017*

**Figure 4.** *Superplastic mechanism map of FSP Al alloys [54].*

*Effect of Grain Size on Superplastic Deformation of Metallic Materials DOI: http://dx.doi.org/10.5772/intechopen.86017*

#### **Figure 5.**

*Aluminium Alloys and Composites*

strain rate ranging from 10<sup>−</sup><sup>3</sup>

s<sup>−</sup><sup>1</sup>

rate of 1 s<sup>−</sup><sup>1</sup>

5.6 × 10<sup>−</sup><sup>1</sup>

above 400°C and at strain rates below 1 × 10<sup>−</sup><sup>3</sup>

s<sup>−</sup><sup>1</sup>

and at a temperature of 425°C due to lattice diffusion-controlled GBS.

. At low temperature, strain hardening was observed at the primary

by ECAP method which contains elongated grains. At temperature higher than the recrystallization temperature of the alloy, it exhibited superplasticity. Mere 0.18% addition of Zr helps in increasing the recrystallization temperature from 100 to 500°C that prevents grain growth. Zn inhibits dislocation creep and assists GBS in the process [53]. Ma et al. [54] achieved superplasticity in UFG Al alloy at a strain

Kawasaki and Langdon [56–58] developed deformation mechanism map for UFG polycrystalline materials depending on the strain rate and temperature range. Most of the materials, namely, Zn-22% Al and Pb-62% Sn alloys, fall in the range of region II of the superplasticity regime. Lee and Horita [59] observed low-temperature superplasticity in Al7075 alloy at 200–250°C due to UFG generated by highpressure torsion process. In Al 5024 alloy, UFG grains of 0.7 μm was produced by ECAP and tested for superplasticity in the temperature range of 175–450°C and at a

s<sup>−</sup><sup>1</sup>

all the tested conditions due to grain boundary diffusion-controlled GBS. At 175°C the maximum elongation of 365% was obtained at a strain rate of 1.4 × 10<sup>−</sup><sup>4</sup>

and at 450°C the maximum elongation of 3300% was obtained at a strain rate of

deformation stage due to the accumulation of lattice dislocations, whereas at high temperature, strain-induced grain elongation causes the strain hardening which accounted for the high-strain-rate superplasticity [55]. Bobruk et al. [60] demonstrated that UFG grain promoted low-temperature and high-strain-rate superplasticity in Al 7050 alloy. Edalati et al. [61] observed low-temperature superplasticity in Mg-Li alloy with 0.25 μm. The deformation mechanism was grain boundary sliding controlled by fast grain boundary diffusion due to Li segregation along the grain boundaries. The low-temperature superplasticity in this case was not attributed to UFG alone, but Li segregation in the boundary favors the grain boundary diffusion and GBS. Similar observation of maximum elongation of approximately 850% was found in UFG Ti-6Al-4 V (~0.4 μm) alloy due to high volume fraction of beta phase in a continuous network fashion acted as soft lubricant layer and increases the elongation [62]. Various deformation mechanisms that contribute for superplastic deformation in UFG regime were mapped with the optimum strain rate and temperature in **Figure 4**. It is evident that there

s<sup>−</sup><sup>1</sup>

to 10<sup>−</sup><sup>1</sup>

. It was because UFG was obtained

. The material exhibited superplasticity in

 s<sup>−</sup><sup>1</sup> ,

**104**

**Figure 4.**

*Superplastic mechanism map of FSP Al alloys [54].*

*Variation of optimum strain rate for maximum superplasticity (a) and optimum strain rate for the highest value of strain-rate sensitivity (b) with temperatures for ECAP Al-Mg-Sc-Zr and other UFG aluminum alloys [55].*

is a relationship between temperature and strain rate. In **Figure 5**, an empirical relationship was presented with an illustration that HSRSP would take place at higher temperatures.

#### **6. Nano-grained superplasticity**

The kinetics of nano-grained superplasticity is different from the higher scale sized grains. Nano-grains show higher flow stress for superplastic deformation. It makes difficulty in activating the slip systems for the accommodation process [63]. The hypothesis for difficult slip accommodation is displayed in **Figure 6**. In the absence of slip as accommodation mechanism for GBS during nano-grained superplasticity, one of the possible accommodation mechanisms in nano-grains could be mobile triple junctions [64]. In nano-grained materials, partial dislocation slips can be activated at a lower stress level when compared to the perfect

**Figure 6.** *A comparison of theoretical flow stress and empirical flow stress for slip accommodation in Ti-6Al-3.2Mo [63].*

dislocation slips. As GBS occurs, dislocation accumulates at the triple junction which increases the system strain energy. The energy can be relaxed as Shockley partial dislocations emitted in to the neighboring grain. Thus, the emission of partial dislocations acts as an accommodation mechanism in nano-grained materials [65]. Similarly, dislocation transfer along a triple junction during grain boundary migration would increase the triple junction angle to accommodate the GBS [66]. Atomic shuffling accommodated GBS also suggested in nanomaterials [67]. The mechanism of atomic shuffling in tilt grain boundary during a shear is illustrated in **Figure 7**. This is called stress-induced grain boundary diffusion. In a defect-free single crystal, Au nano-wire superplastic deformation was found due to coherent twin propagation [68]. The test was carried in situ with SEM and HRTEM at room temperature, and the maximum elongation is only 50%, but the deformation mechanism reported in his work was interesting and specific to

#### **Figure 7.**

*Deformation mechanisms of a tilt GB under shear. (a) Early stage of GB sliding initiated by localized shuffling events (dashed circles) of atoms at the interface. Arrows correspond to the atomic displacement between two loading steps. (b) Subsequent stage of GB sliding where sites of atomic shuffling in the GB nucleate partial dislocations traveling into the grain and leaving a stacking fault (SF) [65].*

**107**

*Effect of Grain Size on Superplastic Deformation of Metallic Materials*

nano-grain superplasticity. The nano-twins observed to originate at the beginning of plastic deformation contain partial dislocations. Generally, the 1.54 GPa stress required to nucleate a partial in Au nano-wire, but due to nano-twins, the partials nucleated at the yield stress value of approximately 200 MPa. This nano-twin propagates by the glide of these partial dislocations. Ovid'ko and Skiba [69] figured out that the dislocation pile-up at the triple junction due to GBS caused nano-twin formation. The sequential step of GBS, dislocation pile-up, and twin

*Illustration of nano-twin formation during superplastic deformation. (a) Nanometer sized grains under tensile load, (b) GBS created dislocation pile up at triple junction O, (c) climb of dislocations on grain boundary AC', and (d) Nano-twin formation due to mobile partial dislocations that move into the adjacent grain interior, as* 

The piled-up dislocation splits in to two types: immobile GB dislocation and mobile partial dislocation. This partial dislocations then move cooperatively on

helps in improving the post-forming properties of the material [70]. Alizadeh et al. [71] reported grain boundary diffusion-controlled GBS in a nano-grained Mg-Gd-Y-Zr alloy when tested in the temperature range of 300–500°C. Similar to Li segregation at grain boundaries in UFG Mg-Li alloy, Edalati et al. [72] observed superplastic elongation at 300°C in nano-grained Al-Zn alloy due to the Zn

Mansoor and Ghosh [73] studied the effect of multi-pass FSP on extruded ZK60 Mg plate. They found improvement in room-temperature mechanical properties of processed alloy, which was attributed to layered microstructure with grain size of 2–5 μm and 100 μm. Similarly, Witkin et al. [74] and Oskooie et al. [75] worked on Al alloys through cryo-milling and high-energy planetary ball-milling methods, respectively, to fabricate mixed-coarse and fine-grained structures and obtain optimum strength and ductility. Wang et al. [76] achieved enhanced strength and ductility in Cu after thermomechanical treatment with a mixed grain size distribution of micrometer-sized grains embedded inside a matrix of nanocrystalline and ultrafine (<300 nm) grains. The matrix grains impart high strength, whereas the coarse grains facilitate strain hardening mechanisms to give high tensile

The evolution of nano-grains in to UFGs was observed in Al and Ti alloys which

every slip plane in a nanoscale region to generate nano-twin.

**7. Superplasticity in materials with mixed grain sizes**

*DOI: http://dx.doi.org/10.5772/intechopen.86017*

formation are presented in **Figure 8**.

*grain boundary dislocations become sessile dislocations [69].*

**Figure 8.**

segregation-enhanced GBS.

*Effect of Grain Size on Superplastic Deformation of Metallic Materials DOI: http://dx.doi.org/10.5772/intechopen.86017*

#### **Figure 8.**

*Aluminium Alloys and Composites*

**Figure 6.**

dislocation slips. As GBS occurs, dislocation accumulates at the triple junction which increases the system strain energy. The energy can be relaxed as Shockley partial dislocations emitted in to the neighboring grain. Thus, the emission of partial dislocations acts as an accommodation mechanism in nano-grained materials [65]. Similarly, dislocation transfer along a triple junction during grain boundary migration would increase the triple junction angle to accommodate the GBS [66]. Atomic shuffling accommodated GBS also suggested in nanomaterials [67]. The mechanism of atomic shuffling in tilt grain boundary during a shear is illustrated in **Figure 7**. This is called stress-induced grain boundary diffusion. In a defect-free single crystal, Au nano-wire superplastic deformation was found due to coherent twin propagation [68]. The test was carried in situ with SEM and HRTEM at room temperature, and the maximum elongation is only 50%, but the deformation mechanism reported in his work was interesting and specific to

*A comparison of theoretical flow stress and empirical flow stress for slip accommodation in Ti-6Al-3.2Mo [63].*

*Deformation mechanisms of a tilt GB under shear. (a) Early stage of GB sliding initiated by localized shuffling events (dashed circles) of atoms at the interface. Arrows correspond to the atomic displacement between two loading steps. (b) Subsequent stage of GB sliding where sites of atomic shuffling in the GB nucleate partial* 

*dislocations traveling into the grain and leaving a stacking fault (SF) [65].*

**106**

**Figure 7.**

*Illustration of nano-twin formation during superplastic deformation. (a) Nanometer sized grains under tensile load, (b) GBS created dislocation pile up at triple junction O, (c) climb of dislocations on grain boundary AC', and (d) Nano-twin formation due to mobile partial dislocations that move into the adjacent grain interior, as grain boundary dislocations become sessile dislocations [69].*

nano-grain superplasticity. The nano-twins observed to originate at the beginning of plastic deformation contain partial dislocations. Generally, the 1.54 GPa stress required to nucleate a partial in Au nano-wire, but due to nano-twins, the partials nucleated at the yield stress value of approximately 200 MPa. This nano-twin propagates by the glide of these partial dislocations. Ovid'ko and Skiba [69] figured out that the dislocation pile-up at the triple junction due to GBS caused nano-twin formation. The sequential step of GBS, dislocation pile-up, and twin formation are presented in **Figure 8**.

The piled-up dislocation splits in to two types: immobile GB dislocation and mobile partial dislocation. This partial dislocations then move cooperatively on every slip plane in a nanoscale region to generate nano-twin.

The evolution of nano-grains in to UFGs was observed in Al and Ti alloys which helps in improving the post-forming properties of the material [70]. Alizadeh et al. [71] reported grain boundary diffusion-controlled GBS in a nano-grained Mg-Gd-Y-Zr alloy when tested in the temperature range of 300–500°C. Similar to Li segregation at grain boundaries in UFG Mg-Li alloy, Edalati et al. [72] observed superplastic elongation at 300°C in nano-grained Al-Zn alloy due to the Zn segregation-enhanced GBS.

### **7. Superplasticity in materials with mixed grain sizes**

Mansoor and Ghosh [73] studied the effect of multi-pass FSP on extruded ZK60 Mg plate. They found improvement in room-temperature mechanical properties of processed alloy, which was attributed to layered microstructure with grain size of 2–5 μm and 100 μm. Similarly, Witkin et al. [74] and Oskooie et al. [75] worked on Al alloys through cryo-milling and high-energy planetary ball-milling methods, respectively, to fabricate mixed-coarse and fine-grained structures and obtain optimum strength and ductility. Wang et al. [76] achieved enhanced strength and ductility in Cu after thermomechanical treatment with a mixed grain size distribution of micrometer-sized grains embedded inside a matrix of nanocrystalline and ultrafine (<300 nm) grains. The matrix grains impart high strength, whereas the coarse grains facilitate strain hardening mechanisms to give high tensile ductility. Moreover, modifying the surfaces was reported to improve fracture and fatigue properties. Cáceres and Selling [77] mentioned that the area fraction of defects on the fracture surface influences the tensile properties rather than the bulk defect content. Begum et al. [78] found that that the fatigue cracks were originated from the surfaces with large grains. Perron et al. [79] performed molecular dynamic simulation on polycrystalline Al samples and found a linear relationship between grain size and surface roughness to an extent of applied strain, while Liu et al. [80] reported that surface modification by shot peening decreases surface roughness due to grain refinement and becomes the reason for improvement in mechanical properties in Mg alloy.

At high temperature, the effect of fine grains is substantial; even roomtemperature superplasticity was observed after HPT in pure Mg [81] and in Mg-Li alloy [61]. However, few studies showed superplasticity in coarse-grained Mg alloys also. Watanabe et al. [82] achieved superplastic elongation of 196% in AZ31 Mg alloy with an average grain size of 130 μm. Wu and Liu [37] reported superplasticity of 320% in hot-rolled AZ31 Mg alloy with a mean grain size of 300 μm. Grain size was found to reduce to 25 μm during deformation due to dislocation slip and climb. Hence, the idea of mixed grain size microstructure which showed promising improvement in room-temperature mechanical properties was explored at high temperature also. Fan et al. [83] observed more superplastic elongation in AA8090 Al-Li alloy which inherently contains three distinct microstructural layers through the thickness when compared to superplasticity of surface layers (equiaxed grains) and middle layers (elongated grains) alone as shown in the **Figure 9**. Pancholi and Kashyap [84] obtained better bulge profile during superplastic bulge forming of AA8090 Al-Li alloy.

#### **Figure 9.**

*Three-dimensional view of the microstructure after annealing at 540°C for 5 min: (a) Li-depleted layer, (b) layer with recrystallized microstructure, and (c) middle layer of un-recrystallized grains [84].*

**109**

**Figure 10.**

*and (g–i) CL [87].*

*Effect of Grain Size on Superplastic Deformation of Metallic Materials*

microstructure evolved into bimodal microstructure.

Later, Pradeep and Pancholi [85] deliberately fabricated layered microstructure containing coarse and fine grains in Al 5082 alloy using FSP and found that the inhomogeneous microstructures possess superior superplastic formability than the homogeneously grain-sized materials. It can be clearly observed in **Figure 10** that the composite layer (CL) contains the mixture of thermomechanically affected layer (TL) and nugget layer (NL) contains lesser fraction for cavities and maximum elongation as evident from its necking. Álvarez-Leal et al. [86] observed superplasticity in extruded ZK30 Mg alloy. During deformation the complex as received

In general, inhomogeneity in initial material would lead to inhomogeneous deformation and strain localization which eventually lead to failure [89]. In classical superplastic literature, uniform fine-grained microstructure was considered favorable for superplasticity. However in Al alloys, inhomogeneous microstructure has shown to exhibit higher ductility than homogeneous fine-grained microstructure [74–76, 87]. Pancholi et al. [90] demonstrated that at least 50% fine grain microstructure is required in the Al 5086 alloy for it to exhibit superior superplasticity when compared to the homogeneous fine-grained material. It was argued that microstructural refinement in the remaining 50% of coarse-grained microstructure, which leads to fully homogenous microstructure in the material, was the reason for higher superplasticity [84, 87]. However in the work by Raja and Pancholi [88], layered microstructure material exhibited elongation lower than fully fine-grained microstructure. The coarse grain with as-cast material is deformed by twins as shown in **Figure 11(a)**. The nature twins were found out to be extension-type from the misorientation distribution in **Figure 11(b)**. Lack of superplasticity in half thickness fine-grained material (HFG) and surface modified by fine-grained material (SFG) can be attributed to insignificant microstructural refinement in the coarse-grained as-cast microstructure during deformation, as shown in **Figures 11(c)** and **9(d)**. It is well established that Al alloys exhibit continuous dynamic recrystallization (CDRX) [91, 92], whereas AZ91 Mg alloys exhibit discontinuous dynamic recrystallization [93]. It appears that CDRX of coarse grains in the layer microstructure of Al alloys was the primary reason for higher

*Cavitation along the gauge length (in the thickness direction) at different points (tip end, mid-section, and* 

 *s<sup>−</sup><sup>1</sup>*

*. (a–c) NL, (d–f) TL,* 

*shoulder end) of the tensile samples deformed at 500°C with a strain rate of 1 × 10<sup>−</sup><sup>3</sup>*

*DOI: http://dx.doi.org/10.5772/intechopen.86017*

#### *Effect of Grain Size on Superplastic Deformation of Metallic Materials DOI: http://dx.doi.org/10.5772/intechopen.86017*

*Aluminium Alloys and Composites*

properties in Mg alloy.

AA8090 Al-Li alloy.

ductility. Moreover, modifying the surfaces was reported to improve fracture and fatigue properties. Cáceres and Selling [77] mentioned that the area fraction of defects on the fracture surface influences the tensile properties rather than the bulk defect content. Begum et al. [78] found that that the fatigue cracks were originated from the surfaces with large grains. Perron et al. [79] performed molecular dynamic simulation on polycrystalline Al samples and found a linear relationship between grain size and surface roughness to an extent of applied strain, while Liu et al. [80] reported that surface modification by shot peening decreases surface roughness due to grain refinement and becomes the reason for improvement in mechanical

At high temperature, the effect of fine grains is substantial; even roomtemperature superplasticity was observed after HPT in pure Mg [81] and in Mg-Li alloy [61]. However, few studies showed superplasticity in coarse-grained Mg alloys also. Watanabe et al. [82] achieved superplastic elongation of 196% in AZ31 Mg alloy with an average grain size of 130 μm. Wu and Liu [37] reported superplasticity of 320% in hot-rolled AZ31 Mg alloy with a mean grain size of 300 μm. Grain size was found to reduce to 25 μm during deformation due to dislocation slip and climb. Hence, the idea of mixed grain size microstructure which showed promising improvement in room-temperature mechanical properties was explored at high temperature also. Fan et al. [83] observed more superplastic elongation in AA8090 Al-Li alloy which inherently contains three distinct microstructural layers through the thickness when compared to superplasticity of surface layers (equiaxed grains) and middle layers (elongated grains) alone as shown in the **Figure 9**. Pancholi and Kashyap [84] obtained better bulge profile during superplastic bulge forming of

*Three-dimensional view of the microstructure after annealing at 540°C for 5 min: (a) Li-depleted layer, (b) layer with recrystallized microstructure, and (c) middle layer of un-recrystallized grains [84].*

**108**

**Figure 9.**

Later, Pradeep and Pancholi [85] deliberately fabricated layered microstructure containing coarse and fine grains in Al 5082 alloy using FSP and found that the inhomogeneous microstructures possess superior superplastic formability than the homogeneously grain-sized materials. It can be clearly observed in **Figure 10** that the composite layer (CL) contains the mixture of thermomechanically affected layer (TL) and nugget layer (NL) contains lesser fraction for cavities and maximum elongation as evident from its necking. Álvarez-Leal et al. [86] observed superplasticity in extruded ZK30 Mg alloy. During deformation the complex as received microstructure evolved into bimodal microstructure.

In general, inhomogeneity in initial material would lead to inhomogeneous deformation and strain localization which eventually lead to failure [89]. In classical superplastic literature, uniform fine-grained microstructure was considered favorable for superplasticity. However in Al alloys, inhomogeneous microstructure has shown to exhibit higher ductility than homogeneous fine-grained microstructure [74–76, 87]. Pancholi et al. [90] demonstrated that at least 50% fine grain microstructure is required in the Al 5086 alloy for it to exhibit superior superplasticity when compared to the homogeneous fine-grained material. It was argued that microstructural refinement in the remaining 50% of coarse-grained microstructure, which leads to fully homogenous microstructure in the material, was the reason for higher superplasticity [84, 87]. However in the work by Raja and Pancholi [88], layered microstructure material exhibited elongation lower than fully fine-grained microstructure. The coarse grain with as-cast material is deformed by twins as shown in **Figure 11(a)**. The nature twins were found out to be extension-type from the misorientation distribution in **Figure 11(b)**. Lack of superplasticity in half thickness fine-grained material (HFG) and surface modified by fine-grained material (SFG) can be attributed to insignificant microstructural refinement in the coarse-grained as-cast microstructure during deformation, as shown in **Figures 11(c)** and **9(d)**. It is well established that Al alloys exhibit continuous dynamic recrystallization (CDRX) [91, 92], whereas AZ91 Mg alloys exhibit discontinuous dynamic recrystallization [93]. It appears that CDRX of coarse grains in the layer microstructure of Al alloys was the primary reason for higher

#### **Figure 10.**

*Cavitation along the gauge length (in the thickness direction) at different points (tip end, mid-section, and shoulder end) of the tensile samples deformed at 500°C with a strain rate of 1 × 10<sup>−</sup><sup>3</sup> s<sup>−</sup><sup>1</sup> . (a–c) NL, (d–f) TL, and (g–i) CL [87].*

#### **Figure 11.**

*IPF maps of deformed samples with tensile axis parallel to process direction (parallel to X-axis shown above) at a strain rate of 5 × 10–4 s<sup>−</sup><sup>1</sup> ; (a) AC material with highlighted (yellow) extension twins indicated by arrows in its IPF map and (b) grain boundary misorientation distribution of as-cast material showing predominant tensile twin population around 86.3°, (c) HFG, (d) SFG, and (e) FFG [88].*

superplastic elongation in Al alloys which was not present in the AZ91 Mg alloy. However, elongation of more than 600% and strain-rate sensitivity of more than 0.5 in FFG material confirmed that friction stir processed AZ91 behave superplastically at temperature of 350°C and strain rate of 5 × 10<sup>−</sup><sup>4</sup> s<sup>−</sup><sup>1</sup> due to dynamic grain growth can be observed from the **Figure 11(e)**.
