**2. An examination of reports of microstructures produced in Mg and magnesium alloys**

To make a competent design, it is essential to analyze different grain structures of magnesium and its alloys, which processed through the ECAP [17]. An initial documented reports organized, and general review presented in **Table 1** for pure magnesium and magnesium alloys, which processed through ECAP.

The outcome in the given **Table 1** indicates the utilization of an ECAP die set up with the 90 internal channel angle, operating with and without back pressure, and the expected outcomes notified in the column number six of **Table 1**. From the table, A and B represent the heterogeneous and homogeneous grain structure,


**Table 1.**

*Comparison of experiments conducted on pure magnesium and a range of magnesium alloy.*

respectively. BP indicates the back pressure, and U is for the channel angle within the die. Then ECAP routes defined by route A indicate that ECAP processed without any specimen rotation in-between the two passes, BA indicates the specimen rotation of 90° in the alternative direction in-between the passes and C denotes the rotation of 180° in-between the passes.

From **Table 1**, we can observe that materials represented in the first column, the grain sizes of the material before and after ECAP provided in the following columns. Then followed by that intermediate stage in the ECAP process is given in the fourth column and the fifth column provided with the additional information regarding the total number of passes. Finally, the references provided in the last column. Additionally, the structure of the grain after processing with the ECAP have given in the notation of Bi-m and Trim to indicate the grain distribution, whether it is tri-modal or Bimodal correspondingly.

From the interference of **Table 1** observation of the distribution of the heterogeneous grain size has done after many passes, particularly while the grain size is large at the initial stage. Certainly, grains were heterogeneous at the initial stage in which the size of the grain ranges from the minimum 45.5 μm for magnesium alloy ZK60 to a higher grain size range of 640 μm magnesium alloy AZ31. From the observation made from the existing investigation, with the minimum number of ECAP passes, the homogeneous grain arrays can obtain with the average grain size of ~40 μm. The high-temperature ECAP processed help to form the homogenization: for instance, the grain size with bi-modal configuration with homogeneity attained with six ECAP passes at the temperature of 423 K in the magnesium AZ31 alloy. In Magnesium-Zn alloy bi-modal distribution is obtained in grain size in which initially the fine and coarse grains have found.

From **Table 1**, there were two significant limitations found; one is the results illustrated that by considering the process limitations, the initial structure might exhibit the homogenous distribution, tri-modal or bi-modal grain size distribution. And another observed one is when the grain size is larger, or it formed in coarse then the bi-modal distribution in the grain size is preferred. These were the two important consideration which made based on the grain structure.

## **3. A model for grain refinement in Mg and magnesium alloys processed by ECAP**

From **Figure 1**, which shows the patterned design model which created based on the grain structure formation of ECAP processed of magnesium alloys. The design raised based on the assumption of grain size and distribution on the initial stage and mechanism which proposed earlier regarding the grain structure.

The various grain structure at the initial stage, which is before the ECAP is shown in **Figure 1a, d, g**, and **j**. In which dc, the critical grain size integrates with the concept which effectively reduced the size, which initiates the nucleation with proper homogenization during the processing of the materials. The structure of the grain and its refinement process is given below.

Particularly, the initial structure of the coarse grain at the initial stage with the size, d and the grain size, d, is larger than dc (d >> dc), which is the critical grain size which shown in **Figure 1a**, **d**, and **g**. From **Figure 1j**, d < dc, which means the initial grain structure is granular than the critical size. In certain, the grains which attained homogenous nucleation in which entire grain structure is non-basal slips. Furthermore a complete has been yet to be considered to improve the standard of the processing parameters, and it is important to expect that it will rely on kind of alloy and temperature for processing of ECAP and present investigations show that back pressure also a dependent factor for the grain structure.

**35**

**Figure 1.**

*the pass [8].*

From the second column of **Figure 1**, which illustrates the grain structure formed after one pass. Followed by that third column indicated the grain structure formed after more than one number of passes. Where the structure of the initial size of the grain is comparably larger than critical diameter dc, as shown in the first column of **Figure 1**, the grain structure develops in the initial pass, such as the bimodal or multimodal distribution of the grain as shown in the second column of **Figure 1**. Subsequently, the region occupied by the extended cores of the grains at the initial stage and by the grains which newly formed after refinement have significantly relied on the initial size of the grains which observed during the analysis of

*Patterned design on the refinement of grain of the ECAP processed Mg alloys. (a), (d), (g), (j) are the initial grain structure whose grain size can be termed as d>>dc, d>dc, d>dc and dc respectively. (b), (e) and (h) are the intermediate structure at the shear zone during the ECAP. (c), (f), (i) and (k) are the final structure after* 

*Severely Plastic Deformed Magnesium Based Alloys DOI: http://dx.doi.org/10.5772/intechopen.88778*

*Severely Plastic Deformed Magnesium Based Alloys DOI: http://dx.doi.org/10.5772/intechopen.88778*

*Magnesium - The Wonder Element for Engineering/Biomedical Applications*

rotation of 180° in-between the passes.

is tri-modal or Bimodal correspondingly.

initially the fine and coarse grains have found.

grain and its refinement process is given below.

back pressure also a dependent factor for the grain structure.

**processed by ECAP**

respectively. BP indicates the back pressure, and U is for the channel angle within the die. Then ECAP routes defined by route A indicate that ECAP processed without any specimen rotation in-between the two passes, BA indicates the specimen rotation of 90° in the alternative direction in-between the passes and C denotes the

From **Table 1**, we can observe that materials represented in the first column, the grain sizes of the material before and after ECAP provided in the following columns. Then followed by that intermediate stage in the ECAP process is given in the fourth column and the fifth column provided with the additional information regarding the total number of passes. Finally, the references provided in the last column. Additionally, the structure of the grain after processing with the ECAP have given in the notation of Bi-m and Trim to indicate the grain distribution, whether it

From the interference of **Table 1** observation of the distribution of the heterogeneous grain size has done after many passes, particularly while the grain size is large at the initial stage. Certainly, grains were heterogeneous at the initial stage in which the size of the grain ranges from the minimum 45.5 μm for magnesium alloy ZK60 to a higher grain size range of 640 μm magnesium alloy AZ31. From the observation made from the existing investigation, with the minimum number of ECAP passes, the homogeneous grain arrays can obtain with the average grain size of ~40 μm. The high-temperature ECAP processed help to form the homogenization: for instance, the grain size with bi-modal configuration with homogeneity attained with six ECAP passes at the temperature of 423 K in the magnesium AZ31 alloy. In Magnesium-Zn alloy bi-modal distribution is obtained in grain size in which

From **Table 1**, there were two significant limitations found; one is the results illustrated that by considering the process limitations, the initial structure might exhibit the homogenous distribution, tri-modal or bi-modal grain size distribution. And another observed one is when the grain size is larger, or it formed in coarse then the bi-modal distribution in the grain size is preferred. These were the two

From **Figure 1**, which shows the patterned design model which created based on the grain structure formation of ECAP processed of magnesium alloys. The design raised based on the assumption of grain size and distribution on the initial stage

The various grain structure at the initial stage, which is before the ECAP is shown in **Figure 1a, d, g**, and **j**. In which dc, the critical grain size integrates with the concept which effectively reduced the size, which initiates the nucleation with proper homogenization during the processing of the materials. The structure of the

Particularly, the initial structure of the coarse grain at the initial stage with the size, d and the grain size, d, is larger than dc (d >> dc), which is the critical grain size which shown in **Figure 1a**, **d**, and **g**. From **Figure 1j**, d < dc, which means the initial grain structure is granular than the critical size. In certain, the grains which attained homogenous nucleation in which entire grain structure is non-basal slips. Furthermore a complete has been yet to be considered to improve the standard of the processing parameters, and it is important to expect that it will rely on kind of alloy and temperature for processing of ECAP and present investigations show that

important consideration which made based on the grain structure.

**3. A model for grain refinement in Mg and magnesium alloys** 

and mechanism which proposed earlier regarding the grain structure.

**34**

**Figure 1.**

*Patterned design on the refinement of grain of the ECAP processed Mg alloys. (a), (d), (g), (j) are the initial grain structure whose grain size can be termed as d>>dc, d>dc, d>dc and dc respectively. (b), (e) and (h) are the intermediate structure at the shear zone during the ECAP. (c), (f), (i) and (k) are the final structure after the pass [8].*

From the second column of **Figure 1**, which illustrates the grain structure formed after one pass. Followed by that third column indicated the grain structure formed after more than one number of passes. Where the structure of the initial size of the grain is comparably larger than critical diameter dc, as shown in the first column of **Figure 1**, the grain structure develops in the initial pass, such as the bimodal or multimodal distribution of the grain as shown in the second column of **Figure 1**. Subsequently, the region occupied by the extended cores of the grains at the initial stage and by the grains which newly formed after refinement have significantly relied on the initial size of the grains which observed during the analysis of

**Figure 1**. For a better explanation, differentiation among the cores of the initial grains which are usually larger which exist even after one pass of ECAP, the core regions mentioned above illustrated in the second column and the third column of **Figure 1**, in particular recently refined grains which indicated in the dotted region.

In **Figure 1**, the first row illustrates the condition of the grain structure at initial stage which is particularly coarse, because of that even after the one pass of ECAP the initial grains last and hold an extended region which shown in **Figure 1b** and the same grains may get unrefined until multiple passes which are shown in **Figure 1c**. From **Figure 1b**, it can be observed that twinning takes place across the larger grains, so it leads to refinement of grain accompanying with the twinning features. The condition where the critical size of the grain is very smaller than the initial grain size which made bi-modal or multi-modal grain distribution possible which continues even after the multiple passes of ECAP which shown in **Figure 1c**.

In defining the features of the newly developed grains, it is important to explain whether the grain size distribution is multimodal or homogeneous. The dc grain size variations observed from the first two rows in **Figure 1** were the result of variation in the processing parameters particularly the temperature through the initial structure is similar which illustrated in **Figure 1a** and **d**. The fractional volume of the newly developed grains after one pass is small when compared to the initial structure because it is too coarse which observed from the first row of **Figure 1** and this will be the cause for the formation of bimodal and multimodal distribution among the grain size after the multi passes of ECAP. Subsequently, the same initial structure subjected to the one pass of ECAP with different processing conditions such as lower strain rate and elevated temperature which resulted in the formation of new grains and occupy the extended region which illustrated in **Figure 1e** and also after multiple passes of ECAP the sample exhibits uniform grain structure as illustrated in **Figure 1f**.

After a single pass of ECAP, the grain size distribution becomes bimodal or multimodal by forming reasonably fine structures grains. But the existing grains or grain which not affected by the pass occupied lesser fraction area that the newly formed ones which illustrated in **Figure 1g**. **Figure 1i** shows, by multiple ECAP, passes, the grains get refined and resulted in the homogeneous distribution.

By having the initial grain size as smaller than the critical grain diameter as shown in **Figure 1j**, the homogeneous array distribution achieved through the single pass of ECAP as illustrated in **Figure 1i** and subsequently after many passes homogeneity remains the same.

From the mode, it concluded that bi- and multi-modal grain size distributions which formed through the ECAP were transitional and the distribution of the grains gets altered as ECAP passes the increase.

Spitale et al. processed ECAP in minimal temperature, which is about 250°C with the channel angle of 90° and with 45°of the radius of curvature. The plunger speed for the process is 0.1 mm/s. They observed the evolution of grain structure at different location of the deformation zone. **Figure 2** shows the appearance of the structure entering the deformation zone [37].

The initial structure of the grain exists with the witness of twinning action. Grain boundaries observed with the serration like features. Grains with fine size within the range of 20 μm have witnessed within the region of grain boundaries and twins, which is developed from the initial grain structure, as shown in **Figure 3**. Apart from the certain limits from the grain boundaries and twins, fine grains were not witnessed with then fine grains.

From **Figure 4**, it witnessed that a large area occupied by the fine grains. The fine grains average size is around ~15 μm, which observed from the deformation zone. The fine grain distribution followed the necklace pattern which exists around the area of unrefined grain (>100 μm).

**37**

**Figure 2.**

**Figure 3.**

*Severely Plastic Deformed Magnesium Based Alloys DOI: http://dx.doi.org/10.5772/intechopen.88778*

*The grain structure in the region of deformation [37].*

*Structure of the grain inside the region of deformation [37].*

**4. Improved processing routes for ECAP with magnesium alloys**

failure of the coarse grain structure in the magnesium and its alloys.

To rectify the issues in the ECAP process of the magnesium and its alloys, some measures have been improvised and designed such as the processing route along with temperature, die angle and the back pressure range and its usage. To bring down the tenor for shear localization the grain cores along with the boundary extents have to reduce. Thus a primary step of extrusion is made to refine the microstructure which can change the initial grain structure as illustrated as **Figure 1** an into refined grain structure like **Figure 1g** or **j**. As shown in the second row of **Figure 1a**, the grain refinement sequence of the newly formed grains can alter through the rise in the processing temperature. This alteration in the ECAP process

The processing routes influenced the final structure of the grain, failure of billet, and shear localization effect through the grain structure size distribution and refinement mechanism of the magnesium and its alloys. The formation of the shear bands is due to the concentration of the thin layer which belongs to the newly formed grains along with the existing grain boundaries. This shear concentration occurs in a layer due to the nearby regions shear, and it develops damage pile; thus, the failure in the billet takes place. The rise in the initial grain coarse volume leads to the rise in the shear amount, which is in the shear band. The possibilities of the localization of the shear are higher, which is shown in the first row of **Figure 1** successively the growth shown in below rows. This decision is in Correspond with the

*Severely Plastic Deformed Magnesium Based Alloys DOI: http://dx.doi.org/10.5772/intechopen.88778*

#### **Figure 2.**

*Magnesium - The Wonder Element for Engineering/Biomedical Applications*

**Figure 1**. For a better explanation, differentiation among the cores of the initial grains which are usually larger which exist even after one pass of ECAP, the core regions mentioned above illustrated in the second column and the third column of **Figure 1**, in particular recently refined grains which indicated in the dotted region. In **Figure 1**, the first row illustrates the condition of the grain structure at initial stage which is particularly coarse, because of that even after the one pass of ECAP the initial grains last and hold an extended region which shown in **Figure 1b** and the same grains may get unrefined until multiple passes which are shown in **Figure 1c**. From **Figure 1b**, it can be observed that twinning takes place across the larger grains, so it leads to refinement of grain accompanying with the twinning features. The condition where the critical size of the grain is very smaller than the initial grain size which made bi-modal or multi-modal grain distribution possible which continues even after the multiple passes of ECAP which shown in **Figure 1c**.

In defining the features of the newly developed grains, it is important to explain whether the grain size distribution is multimodal or homogeneous. The dc grain size variations observed from the first two rows in **Figure 1** were the result of variation in the processing parameters particularly the temperature through the initial structure is similar which illustrated in **Figure 1a** and **d**. The fractional volume of the newly developed grains after one pass is small when compared to the initial structure because it is too coarse which observed from the first row of **Figure 1** and this will be the cause for the formation of bimodal and multimodal distribution among the grain size after the multi passes of ECAP. Subsequently, the same initial structure subjected to the one pass of ECAP with different processing conditions such as lower strain rate and elevated temperature which resulted in the formation of new grains and occupy the extended region which illustrated in **Figure 1e** and also after multiple passes of ECAP the sample exhibits uniform grain structure as illustrated in **Figure 1f**. After a single pass of ECAP, the grain size distribution becomes bimodal or multimodal by forming reasonably fine structures grains. But the existing grains or grain which not affected by the pass occupied lesser fraction area that the newly formed ones which illustrated in **Figure 1g**. **Figure 1i** shows, by multiple ECAP, passes, the grains get refined and resulted in the homogeneous distribution. By having the initial grain size as smaller than the critical grain diameter as shown in **Figure 1j**, the homogeneous array distribution achieved through the single pass of ECAP as illustrated in **Figure 1i** and subsequently after many passes homo-

From the mode, it concluded that bi- and multi-modal grain size distributions which formed through the ECAP were transitional and the distribution of the grains

Spitale et al. processed ECAP in minimal temperature, which is about 250°C with the channel angle of 90° and with 45°of the radius of curvature. The plunger speed for the process is 0.1 mm/s. They observed the evolution of grain structure at different location of the deformation zone. **Figure 2** shows the appearance of the

The initial structure of the grain exists with the witness of twinning action. Grain boundaries observed with the serration like features. Grains with fine size within the range of 20 μm have witnessed within the region of grain boundaries and twins, which is developed from the initial grain structure, as shown in **Figure 3**. Apart from the certain limits from the grain boundaries and twins, fine grains were

From **Figure 4**, it witnessed that a large area occupied by the fine grains. The fine grains average size is around ~15 μm, which observed from the deformation zone. The fine grain distribution followed the necklace pattern which exists around

**36**

geneity remains the same.

gets altered as ECAP passes the increase.

structure entering the deformation zone [37].

not witnessed with then fine grains.

the area of unrefined grain (>100 μm).

*The grain structure in the region of deformation [37].*

**Figure 3.** *Structure of the grain inside the region of deformation [37].*
