**3. Coarse-grained superplasticity**

In 1982, Wadsworth et al. [21] fabricated a commercial C103 niobium alloy (Ni + 10 wt.% Hf + 1 wt.% Ti) by duplex hipping process for high-performance missile and space application systems. The microstructure of the fabricated metal had an average grain size of 75 μm. It exhibited a rupture at 125% elongation during a creep test at 1650°C. The deformation mechanism in this work was predicted as solute drag of dislocations caused by the diffusion of hafnium in niobium, which is of type class I solid solutions. Morgan and Hammond [22] used a method of pre-strain at high strain rate, region III of superplastic regime, followed by cyclic strain to generate subgrain boundaries inside the coarse-grained beta-Ti alloys. The material exhibited maximum elongation of 101% at 830°C. The superplastic deformation was due to diffusional creep mechanism. The vacancy generation and absorption were accommodated by the subgrain boundaries during the diffusional creep. In coarse-grained (62 μm) Ti40 alloy, the grains initially elongated and are then refined to lesser than 10 μm due to CDRA at 840°C and at a strain rate of 1 × 10<sup>−</sup><sup>3</sup> s<sup>−</sup><sup>1</sup> . It exhibited 436% elongation attributed lattice diffusion. Lin and Sun [23] reported 287.5% elongation in TiAl with 95 μm grain size at 1100°C and at a strain rate of 4 × 10<sup>−</sup><sup>5</sup> s<sup>−</sup><sup>1</sup> due to CDRX grain refinement followed by dislocation glide and climb accommodated GBS.

Woo et al. [24] observed superplasticity (300% elongation) in 150 μm grainsized Al-Mg alloy due to diffusion of Mg in Al, solute-drag creep. Reduction in strain-rate sensitivity during solute-drag creep was observed in Al-Mg alloy after the addition of Mn or Zr. It is due to the change in the failure mechanism from necking-controlled-failure to cavitation-controlled-failure [25]. Further, Si addition on coarse-grained Al-Mg alloys also exhibited 350% elongation by solute-drag creep. Thus, Si addition, which did not affect the superplasticity but the high Si content of 0.2 wt%, formed higher volume fraction of Mg2Si particles, which generates more cavity nucleation. These cavities reduce the post-forming tensile properties of the material [26]. On the addition Cu with Al-Mg alloy, the superplastic elongation (320%) was more than that of Al-Mg alloy (260% at 450°C and at a strain rate of 10<sup>−</sup><sup>2</sup> s<sup>−</sup><sup>1</sup> ) [27]. Though the solute drag was explained as responsible deformation mechanism, the effect of Cu addition was not reported. When solute-drag creep was the responsible deformation mechanism, the material exhibited superplasticity at high homologous temperature as well as at high strain rate. Similarly, addition of Zn in Al-Mg alloy also improved the superplastic elongation due to solute drag coupled with CDRX before fracture [28]. Unlike GBS, solute-drag creep mechanism was independent of grain size. Hong [29] reported that if solid solution strength increases due to the increase in strain rate, during solute drag, it would increase the strain-rate sensitivity and superplastic deformation. Meanwhile, the microstructure would contain homogeneously distributed dislocations rather than dislocation clusters.

**101**

**Figure 2.**

*tested strain rate of 5 × 10<sup>−</sup><sup>4</sup>*

 *s<sup>−</sup><sup>1</sup> [30].*

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

Malek [32] observed superplastic elongation of 150–200% in the coarse-

coarse-grained (70 μm) in this work was refined to fine grain size (20 μm) with straining due to CDRX. Similar results of CDRX was observed during superplastic deformation in the Fe3Al-Ti alloy with 100 μm large grains when tested at 850°C at

tion of up to 180% in Fe-27Al alloy with an initial grain size of 700–800 μm. The grain size reduced to 100–200 μm with strain due to continuous grain boundary migration, also called CDRX. CDRX is accountable only for grain refinement, but these works did not report about the superplastic deformation mechanisms. Mohri et al. [35] observed CDRX during superplastic deformation of rolled AZ91 Mg alloy with an initial grain size of 39.5 μm. The grain size reduced to 9.1 μm at a true strain 0.6 due to CDRX. The CDRX in Mg alloy was not because of the effect of SFE but due to the easily activated non-basal slip systems at elevated temperature. After grain refinement, GBS was observed to contribute in 604% superplastic elongation at 300°C and at a strain rate of 1.5 × 10<sup>−</sup><sup>3</sup>

On the similar mechanism of grain refinement followed by GBS, elongation of 512% was obtained by Lin et al. [36] in 155 μm grain-sized Mg-Li alloy at 250°C

coarse grains were refined to 25 μm at a true strain of 0.024. In this work, grain refinement was reported due to dislocation slip and climb, while lattice diffusion controls the superplastic deformation, as the test was performed at higher

The coarse-grained materials exhibit superplastic elongation from 100% to maximum of 600% by AZ91 Mg alloy. The superplastic deformation mechanisms reported in coarse-grained materials are two types. First is the solute-drag creep in solid solution alloys like Al-Mg alloy with elongated grains as shown in **Figure 2**. The second one is the dynamic recovery and/or dynamic recrystallization followed

*Light optical micrographs of changes in the grain structure at a true strain of (a) zero, (b) 0.6, and (c) 1.5 at a* 

s<sup>−</sup><sup>1</sup>

by grain boundary sliding as shown in **Figure 3**.

in 300 μm large grain Az31 alloy at 500°C and at a strain rate of 1 × 10<sup>−</sup><sup>3</sup>

[33]. Chu et al. [34] also observed superplastic elonga-

. Wu and Liu [37] observed 320% elongation

 s<sup>−</sup><sup>1</sup> . The

> s<sup>−</sup><sup>1</sup> .

 s<sup>−</sup><sup>1</sup> . The

grained Zn-1.1 wt.% Al alloy at 227°C and at a strain rate of 4.2 × 10<sup>−</sup><sup>4</sup>

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

s<sup>−</sup><sup>1</sup>

a strain rate of 1 × 10<sup>−</sup><sup>3</sup>

and at a strain rate of 5 × 10<sup>−</sup><sup>4</sup>

temperature.

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

*Aluminium Alloys and Composites*

called GDRX [20].

1 × 10<sup>−</sup><sup>3</sup>

s<sup>−</sup><sup>1</sup>

strain rate of 4 × 10<sup>−</sup><sup>5</sup>

s<sup>−</sup><sup>1</sup>

glide and climb accommodated GBS.

**3. Coarse-grained superplasticity**

of new grains and grain growth at the expense of regions full of dislocations are called DDRX [18] which is mostly observed during hot deformation of materials with low to medium SFE. In materials with high SFE, the subgrains or cell structure with low-angle grain boundaries (LAGBs) formed during deformation is gradually evolved into high-angle grain boundaries (HAGBs) due to efficient dynamic recovery, which is known as CDRX [19]. In material like aluminum, the grains elongate with local serrations at high temperatures and large strain. On further increase in strain, these serrations pinch off and form high-angle grain boundaries which are

In 1982, Wadsworth et al. [21] fabricated a commercial C103 niobium alloy (Ni + 10 wt.% Hf + 1 wt.% Ti) by duplex hipping process for high-performance missile and space application systems. The microstructure of the fabricated metal had an average grain size of 75 μm. It exhibited a rupture at 125% elongation during a creep test at 1650°C. The deformation mechanism in this work was predicted as solute drag of dislocations caused by the diffusion of hafnium in niobium, which is of type class I solid solutions. Morgan and Hammond [22] used a method of pre-strain at high strain rate, region III of superplastic regime, followed by cyclic strain to generate subgrain boundaries inside the coarse-grained beta-Ti alloys. The material exhibited maximum elongation of 101% at 830°C. The superplastic deformation was due to diffusional creep mechanism. The vacancy generation and absorption were accommodated by the subgrain boundaries during the diffusional creep. In coarse-grained (62 μm) Ti40 alloy, the grains initially elongated and are then refined to lesser than 10 μm due to CDRA at 840°C and at a strain rate of

. It exhibited 436% elongation attributed lattice diffusion. Lin and Sun

due to CDRX grain refinement followed by dislocation

[23] reported 287.5% elongation in TiAl with 95 μm grain size at 1100°C and at a

Woo et al. [24] observed superplasticity (300% elongation) in 150 μm grainsized Al-Mg alloy due to diffusion of Mg in Al, solute-drag creep. Reduction in strain-rate sensitivity during solute-drag creep was observed in Al-Mg alloy after the addition of Mn or Zr. It is due to the change in the failure mechanism from necking-controlled-failure to cavitation-controlled-failure [25]. Further, Si addition on coarse-grained Al-Mg alloys also exhibited 350% elongation by solute-drag creep. Thus, Si addition, which did not affect the superplasticity but the high Si content of 0.2 wt%, formed higher volume fraction of Mg2Si particles, which generates more cavity nucleation. These cavities reduce the post-forming tensile properties of the material [26]. On the addition Cu with Al-Mg alloy, the superplastic elongation (320%) was more than that of Al-Mg alloy (260% at 450°C and at a strain rate of

) [27]. Though the solute drag was explained as responsible deformation mechanism, the effect of Cu addition was not reported. When solute-drag creep was the responsible deformation mechanism, the material exhibited superplasticity at high homologous temperature as well as at high strain rate. Similarly, addition of Zn in Al-Mg alloy also improved the superplastic elongation due to solute drag coupled with CDRX before fracture [28]. Unlike GBS, solute-drag creep mechanism was independent of grain size. Hong [29] reported that if solid solution strength increases due to the increase in strain rate, during solute drag, it would increase the strain-rate sensitivity and superplastic deformation. Meanwhile, the microstructure would contain homogeneously distributed dislocations rather than dislocation

**100**

clusters.

10<sup>−</sup><sup>2</sup> s<sup>−</sup><sup>1</sup>

Malek [32] observed superplastic elongation of 150–200% in the coarsegrained Zn-1.1 wt.% Al alloy at 227°C and at a strain rate of 4.2 × 10<sup>−</sup><sup>4</sup> s<sup>−</sup><sup>1</sup> . The coarse-grained (70 μm) in this work was refined to fine grain size (20 μm) with straining due to CDRX. Similar results of CDRX was observed during superplastic deformation in the Fe3Al-Ti alloy with 100 μm large grains when tested at 850°C at a strain rate of 1 × 10<sup>−</sup><sup>3</sup> s<sup>−</sup><sup>1</sup> [33]. Chu et al. [34] also observed superplastic elongation of up to 180% in Fe-27Al alloy with an initial grain size of 700–800 μm. The grain size reduced to 100–200 μm with strain due to continuous grain boundary migration, also called CDRX. CDRX is accountable only for grain refinement, but these works did not report about the superplastic deformation mechanisms. Mohri et al. [35] observed CDRX during superplastic deformation of rolled AZ91 Mg alloy with an initial grain size of 39.5 μm. The grain size reduced to 9.1 μm at a true strain 0.6 due to CDRX. The CDRX in Mg alloy was not because of the effect of SFE but due to the easily activated non-basal slip systems at elevated temperature. After grain refinement, GBS was observed to contribute in 604% superplastic elongation at 300°C and at a strain rate of 1.5 × 10<sup>−</sup><sup>3</sup> s<sup>−</sup><sup>1</sup> . On the similar mechanism of grain refinement followed by GBS, elongation of 512% was obtained by Lin et al. [36] in 155 μm grain-sized Mg-Li alloy at 250°C and at a strain rate of 5 × 10<sup>−</sup><sup>4</sup> s<sup>−</sup><sup>1</sup> . Wu and Liu [37] observed 320% elongation in 300 μm large grain Az31 alloy at 500°C and at a strain rate of 1 × 10<sup>−</sup><sup>3</sup> s<sup>−</sup><sup>1</sup> . The coarse grains were refined to 25 μm at a true strain of 0.024. In this work, grain refinement was reported due to dislocation slip and climb, while lattice diffusion controls the superplastic deformation, as the test was performed at higher temperature.

The coarse-grained materials exhibit superplastic elongation from 100% to maximum of 600% by AZ91 Mg alloy. The superplastic deformation mechanisms reported in coarse-grained materials are two types. First is the solute-drag creep in solid solution alloys like Al-Mg alloy with elongated grains as shown in **Figure 2**. The second one is the dynamic recovery and/or dynamic recrystallization followed by grain boundary sliding as shown in **Figure 3**.

#### **Figure 2.**

*Light optical micrographs of changes in the grain structure at a true strain of (a) zero, (b) 0.6, and (c) 1.5 at a tested strain rate of 5 × 10<sup>−</sup><sup>4</sup> s<sup>−</sup><sup>1</sup> [30].*

#### **Figure 3.**

*Light optical micrographs of Al 5052 alloy deformed at 425°C and at a strain rate of 1 × 10<sup>−</sup><sup>2</sup> s<sup>−</sup><sup>1</sup> to (a) 10%, (b) 100%, and (c) 130% [31].*

## **4. Fine-grained superplasticity**

Fine-grained materials are generally preferred for superplastic deformation. Most of the materials deform by GBS; fine grains offer more fraction of grain boundaries when compared to coarse grains in a given area. As grain boundary slides, the moving dislocations need to be accommodated during the deformation process. Otherwise, dislocations would pile up at ledges or triple junction and generates stress concentration, eventually leads to cavitation and failure. Further, the slower accommodation mechanism if any or slower deformation mechanism which controls the superplastic deformation is termed as rate controlling mechanism. Hence, the deformation mechanism, accommodation mechanism, and ratecontrolling mechanism are important in the fine-grained material. Zhou et al. [38] observed low-temperature superplasticity in 6 μm grain-sized Ti-6Al-4 V alloy at 700°C due to GBS accommodated by slip. However, the accommodation mechanism becomes grain boundary diffusion on increasing the temperature to 850°C.

In a work by Rao and Mukerjee [39] in fine-grained Al 7475 alloy, 840% elongation was attained during the tensile test at 457°C and at a strain rate of 1 × 10<sup>−</sup><sup>4</sup> s<sup>−</sup><sup>1</sup> . Grain elongation was observed with high dislocation density that leads to strain hardening in the material. Dislocation slip and climb through grains to the grain boundary was reported to cause grain elongation, together with grain boundary sliding. The measured activation energy, approximately 165 kJ/mol, was equal to self-diffusion of Al (142 kJ/mol). It describes that diffusion creep is the deformation mechanism rate controlled by dislocation climb. Mikhaylovskaya et al. [40] compared the superplastic deformation behavior in fine-grained Al 7475 alloy at primary stage and steady stage deformation. It was found that grain elongation occurred at primary stage due to diffusional creep, while at steady stage, diffusion creep becomes the accommodation process for GBS without any further change in the shape and size of the grain. Possibility of dislocation creep as an accommodation mechanism along with dislocation creep is also suggested in this work. This detailed study on the comparison between primary and steady stage deformation by Mikhaylovskaya et al. [40] elaborated and concurred with the work by Rao and Mukerjee [39].

In an interesting work by Fukuyo et al. [41], the accommodation mechanism for GBS in a fine-grained (6 μm) 10 wt.% Al-added ultrahigh carbon steel (UHCS-10Al) was studied. It was alluded that if stress exponent is equal to 2, then the accommodation mechanism was dislocation climb due to iron self-diffusion where if stress exponent reduced below 2, then dislocation glide due to solute drag is the accommodation mechanism. These values were confirmed by matching the measured Q values 191 kJ/mol and 292 kJ/mol with the theoretical Q values for selfdiffusion (270 kJ/mol) and Al diffusion in Fe (155 kJ/mol), respectively.

**103**

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

In a fine-grained (2 μm) Mg-9Li alloy, the grain boundary diffusion accommodated GBS was reported as measured Q value of 67 kJ/mol was approximately equal to that of GBD (70 kJ/mol) [42]. This material exhibited superplastic elongation of 450% at 100°C. In a work by Kin and Chung [43], the deformation mechanism of 5 μm grain-sized AZ61 Mg alloy was reported to vary with the strain rate. At low strain rate, grain boundary diffusion-controlled GBS was deformation mechanism, whereas at high strain rates, pipe diffusion-controlled slip was the deformation mechanism. Maximum elongation of 560% was obtained at 400°C and at a low

tion of highest Schmidt factor has easier mobility which facilitates GBS accommodation. Anisotropy in superplastic elongation was observed in the fine-grained AZ61 alloy at 300–400°C by Wang and Huang [44]. However, the influence of Schmidt factors was limited to initial stages of deformation and below 400°C. At large strain, GBS and GBM would greatly reduce the texture effect, and above 400°C the difference in CRSS between different slip systems becomes negligible. GBS mechanism was compared in Al and Mg alloys by Wang and Huang [45]. The contribution of GBS was approximately 60% in Mg, while in the case of Al alloys, its contribution is around 20–30% at a true strain of below 0.5. It was because warmextruded Mg alloy contains 88% HAGBs, while warm-worked Al alloy had 45–65% HAGBs. HAGBs favor GBS; hence, in Mg alloy almost every single grain contributes for GBS, whereas in Al alloys, the presence of LAGBs and CSL boundaries did not favor GBS. Hence, during initial stages of deformation, the group of grains with a common HAGB would undergo GBS, known as cooperative GBS. This initial strain promotes DRV/DRX to convert LAGBs into HAGBs and increase the fraction HAGBs at higher strain values. Further, in slip-accommodated GBS, slip process takes place by subsequent glide and climb. The distance of climb is an important factor in determining the strain rate. If the climb distance is of grain size, it would be absorbed by the grain boundary, and it would contribute for the elementary process of GBS. If the climb distance is of only core size of the grain, it would climb through the lattice and act as rate controlling process [46]. The superplasticity of AA 5083 alloy with and without Cr of grain sizes 7 and 10 μm was studied by Mikhaylovskaya et al. [47]. The elongation in alloy with Cr is found to be marginally higher than the alloy without Cr attributed to it difference in the grain size. It was found that contribution of GBS negligible in both these alloys, instead diffusion creep, was observed to be contributed for 50% of the strain rate. In particle-containing Mg alloys, which exhibit superplastic deformation at low temperature or room temperature, the GBS is accompanied by grain boundary migration. The cooperative effect of grain boundary sliding and migration creates stress concentration around the grains. The larger the GBS distance, the larger is the stress concentration, and depending on the grain orientation, migration aids in either stress relaxation or stress concentration [48]. This effect nucleates cracking of the particles inside the grain due to disclination; a rotational line defect arises out of rotational misfit between the lattices [49].

. As slip accommodates the GBS, slip systems in the direc-

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

s<sup>−</sup><sup>1</sup>

**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

strain rate of 2 × 10<sup>−</sup><sup>4</sup>

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

*Aluminium Alloys and Composites*

**4. Fine-grained superplasticity**

*(b) 100%, and (c) 130% [31].*

**Figure 3.**

Fine-grained materials are generally preferred for superplastic deformation. Most of the materials deform by GBS; fine grains offer more fraction of grain boundaries when compared to coarse grains in a given area. As grain boundary slides, the moving dislocations need to be accommodated during the deformation process. Otherwise, dislocations would pile up at ledges or triple junction and generates stress concentration, eventually leads to cavitation and failure. Further, the slower accommodation mechanism if any or slower deformation mechanism which controls the superplastic deformation is termed as rate controlling mechanism. Hence, the deformation mechanism, accommodation mechanism, and ratecontrolling mechanism are important in the fine-grained material. Zhou et al. [38] observed low-temperature superplasticity in 6 μm grain-sized Ti-6Al-4 V alloy at 700°C due to GBS accommodated by slip. However, the accommodation mechanism

*Light optical micrographs of Al 5052 alloy deformed at 425°C and at a strain rate of 1 × 10<sup>−</sup><sup>2</sup>*

becomes grain boundary diffusion on increasing the temperature to 850°C.

Grain elongation was observed with high dislocation density that leads to strain hardening in the material. Dislocation slip and climb through grains to the grain boundary was reported to cause grain elongation, together with grain boundary sliding. The measured activation energy, approximately 165 kJ/mol, was equal to self-diffusion of Al (142 kJ/mol). It describes that diffusion creep is the deformation mechanism rate controlled by dislocation climb. Mikhaylovskaya et al. [40] compared the superplastic deformation behavior in fine-grained Al 7475 alloy at primary stage and steady stage deformation. It was found that grain elongation occurred at primary stage due to diffusional creep, while at steady stage, diffusion creep becomes the accommodation process for GBS without any further change in the shape and size of the grain. Possibility of dislocation creep as an accommodation mechanism along with dislocation creep is also suggested in this work. This detailed study on the comparison between primary and steady stage deformation by Mikhaylovskaya et al. [40] elaborated and concurred with the work by Rao and

In an interesting work by Fukuyo et al. [41], the accommodation mechanism for GBS in a fine-grained (6 μm) 10 wt.% Al-added ultrahigh carbon steel (UHCS-10Al) was studied. It was alluded that if stress exponent is equal to 2, then the accommodation mechanism was dislocation climb due to iron self-diffusion where if stress exponent reduced below 2, then dislocation glide due to solute drag is the accommodation mechanism. These values were confirmed by matching the measured Q values 191 kJ/mol and 292 kJ/mol with the theoretical Q values for self-

diffusion (270 kJ/mol) and Al diffusion in Fe (155 kJ/mol), respectively.

In a work by Rao and Mukerjee [39] in fine-grained Al 7475 alloy, 840% elongation was attained during the tensile test at 457°C and at a strain rate of 1 × 10<sup>−</sup><sup>4</sup>

 s<sup>−</sup><sup>1</sup> .

 *s<sup>−</sup><sup>1</sup>*

 *to (a) 10%,* 

**102**

Mukerjee [39].

In a fine-grained (2 μm) Mg-9Li alloy, the grain boundary diffusion accommodated GBS was reported as measured Q value of 67 kJ/mol was approximately equal to that of GBD (70 kJ/mol) [42]. This material exhibited superplastic elongation of 450% at 100°C. In a work by Kin and Chung [43], the deformation mechanism of 5 μm grain-sized AZ61 Mg alloy was reported to vary with the strain rate. At low strain rate, grain boundary diffusion-controlled GBS was deformation mechanism, whereas at high strain rates, pipe diffusion-controlled slip was the deformation mechanism. Maximum elongation of 560% was obtained at 400°C and at a low strain rate of 2 × 10<sup>−</sup><sup>4</sup> s<sup>−</sup><sup>1</sup> . As slip accommodates the GBS, slip systems in the direction of highest Schmidt factor has easier mobility which facilitates GBS accommodation. Anisotropy in superplastic elongation was observed in the fine-grained AZ61 alloy at 300–400°C by Wang and Huang [44]. However, the influence of Schmidt factors was limited to initial stages of deformation and below 400°C. At large strain, GBS and GBM would greatly reduce the texture effect, and above 400°C the difference in CRSS between different slip systems becomes negligible. GBS mechanism was compared in Al and Mg alloys by Wang and Huang [45]. The contribution of GBS was approximately 60% in Mg, while in the case of Al alloys, its contribution is around 20–30% at a true strain of below 0.5. It was because warmextruded Mg alloy contains 88% HAGBs, while warm-worked Al alloy had 45–65% HAGBs. HAGBs favor GBS; hence, in Mg alloy almost every single grain contributes for GBS, whereas in Al alloys, the presence of LAGBs and CSL boundaries did not favor GBS. Hence, during initial stages of deformation, the group of grains with a common HAGB would undergo GBS, known as cooperative GBS. This initial strain promotes DRV/DRX to convert LAGBs into HAGBs and increase the fraction HAGBs at higher strain values. Further, in slip-accommodated GBS, slip process takes place by subsequent glide and climb. The distance of climb is an important factor in determining the strain rate. If the climb distance is of grain size, it would be absorbed by the grain boundary, and it would contribute for the elementary process of GBS. If the climb distance is of only core size of the grain, it would climb through the lattice and act as rate controlling process [46]. The superplasticity of AA 5083 alloy with and without Cr of grain sizes 7 and 10 μm was studied by Mikhaylovskaya et al. [47]. The elongation in alloy with Cr is found to be marginally higher than the alloy without Cr attributed to it difference in the grain size. It was found that contribution of GBS negligible in both these alloys, instead diffusion creep, was observed to be contributed for 50% of the strain rate. In particle-containing Mg alloys, which exhibit superplastic deformation at low temperature or room temperature, the GBS is accompanied by grain boundary migration. The cooperative effect of grain boundary sliding and migration creates stress concentration around the grains. The larger the GBS distance, the larger is the stress concentration, and depending on the grain orientation, migration aids in either stress relaxation or stress concentration [48]. This effect nucleates cracking of the particles inside the grain due to disclination; a rotational line defect arises out of rotational misfit between the lattices [49].
