**4. Some basics of plastic deformation mechanism**

We know that the plastic deformation permanently changes the dimension and shape of metal, whereas in terms of microstructural changes only the number density of dislocations increases, whereas crystal structures including lattice parameters of metals typically remain unchanged. Slip and twinning processes, which are shown in a simple model presentation in **Figure 4**, are responsible for this macroscopic change of shape and dimensions. Slip implicates sliding of abutting blocks of a crystal along definite crystallographic planes, called slip planes. A slip occurs when shear stress applied to the material exceeds a critical value. During slip, each atom usually moves the same integral number of atomic distances along the slip plane producing a step, without change of the crystal orientation (**Figure 4b**). Grain boundaries represent obstacles for the slip movement as the slip direction, according to **Figure 4a**, will be usually changed across the boundary. This implies

**7**

**5.1 Work hardening**

*Plastic Deformation Behavior in Steels during Metal Forming Processes: A Review*

that the strength of polycrystalline materials will be higher than that of a single

however different from the untwinned region (**Figure 4a**).

their terminology are treated in detail in the following.

**5. Terminology and summary of TMP related mechanisms**

In twinning, each atom moves by only a fraction of an interatomic distance relative to its neighboring atoms (see **Figure 4c**). The twinned portion of the crystal is a mirror image of the parent crystal. The orientation of the twinned region is

*Schematic representation of slip and twinning mechanisms in metals during plastic deformation (a) original position of atoms within a crystal lattice, (b) atoms movement by slip, (c) atoms movement by twinning [43].*

Metallurgical incidents during the TMP may act statically or dynamically on the material. This depends upon the rate of load and temperature conditions and strongly affects grain refinement. Microstructural evolution during TMP largely depends on the ability of dislocation movement during plastic deformation, which has consequently also a considerable impact on the mechanical properties of materials. The terminology of several mechanisms related to TMP is introduced in the following. These can be understood with the help of the flow stress–strain diagram (**Figure 5**) interpretation [44]. The flow stresses σc, σp, and σs mean the critical, peak, and steady state conditions, respectively. The combined effect of work hardening (WH) and softening mechanisms on flow curves are categorized into distinct regions: I) hardening, II) critical, III) softening and IV) steady-state. WH and dynamic recovery (DRV) occur in the first region where WH dominates and flow stress rises steeply. The second region is the critical zone where DRV and WH both are decreased and new dynamic recrystallization (DRX) initiates. Subsequently, DRX is clearly observed in the third region associated with softening. The fourth region is a steady-state where only DRX occurs. Key mechanisms and

Work hardening (WH) is also called strain hardening or cold hardening. It is the process of making a metal stronger and harder below its recrystallization

*DOI: http://dx.doi.org/10.5772/intechopen.97607*

crystal of the same material.

**Figure 4.**

*Plastic Deformation Behavior in Steels during Metal Forming Processes: A Review DOI: http://dx.doi.org/10.5772/intechopen.97607*

**Figure 4.**

*Material Flow Analysis*

Some important torsion test setups are listed for shear testing with a wide range of strain rates within the framework of SPD. All of the listed setups are supportive for controlled and taylored TMP in order to achieve an optimized balance of processing

The morphology of materials can be defined through shape, size, and structure that plays an important role in both mechanical and corrosion resistance properties. It is well known that all materials are composed of atoms that are arranged in short/long-range order with regular/irregular patterns, those solids are familiar as crystalline and non-crystalline, respectively. The crystalline metals with different crystal structures, such as body-centered cubic, face-centered cubic, or hexagonally closed packed, are prorated into the single crystal and polycrystalline categories. Conversely, most polycrystalline metals are composed of a collection of many small single crystals named grains and are similar to pomegranate fruit, which is made up of many small seeds (see **Figure 3a**). The grains are separated from each other by grain boundaries while preserving the integrity of the metal. Similarly **Figure 3b** shows one grain (shown by yellow dotted line) that has a subgroup of several laths,

costs, time, and materials properties for various industrial applications.

**3. A basic understanding of microstructure**

and every lath having several crystal atoms.

**4. Some basics of plastic deformation mechanism**

We know that the plastic deformation permanently changes the dimension and shape of metal, whereas in terms of microstructural changes only the number density of dislocations increases, whereas crystal structures including lattice parameters of metals typically remain unchanged. Slip and twinning processes, which are shown in a simple model presentation in **Figure 4**, are responsible for this macroscopic change of shape and dimensions. Slip implicates sliding of abutting blocks of a crystal along definite crystallographic planes, called slip planes. A slip occurs when shear stress applied to the material exceeds a critical value. During slip, each atom usually moves the same integral number of atomic distances along the slip plane producing a step, without change of the crystal orientation (**Figure 4b**). Grain boundaries represent obstacles for the slip movement as the slip direction, according to **Figure 4a**, will be usually changed across the boundary. This implies

*The photographs of (a) pomegranate fruit, which compound of grains and separated by grain boundary like* 

*metal structure, (b) high strength steel structure consists of bainitic ferrite and martensite [27].*

**6**

**Figure 3.**

*Schematic representation of slip and twinning mechanisms in metals during plastic deformation (a) original position of atoms within a crystal lattice, (b) atoms movement by slip, (c) atoms movement by twinning [43].*

that the strength of polycrystalline materials will be higher than that of a single crystal of the same material.

In twinning, each atom moves by only a fraction of an interatomic distance relative to its neighboring atoms (see **Figure 4c**). The twinned portion of the crystal is a mirror image of the parent crystal. The orientation of the twinned region is however different from the untwinned region (**Figure 4a**).

### **5. Terminology and summary of TMP related mechanisms**

Metallurgical incidents during the TMP may act statically or dynamically on the material. This depends upon the rate of load and temperature conditions and strongly affects grain refinement. Microstructural evolution during TMP largely depends on the ability of dislocation movement during plastic deformation, which has consequently also a considerable impact on the mechanical properties of materials. The terminology of several mechanisms related to TMP is introduced in the following. These can be understood with the help of the flow stress–strain diagram (**Figure 5**) interpretation [44]. The flow stresses σc, σp, and σs mean the critical, peak, and steady state conditions, respectively. The combined effect of work hardening (WH) and softening mechanisms on flow curves are categorized into distinct regions: I) hardening, II) critical, III) softening and IV) steady-state. WH and dynamic recovery (DRV) occur in the first region where WH dominates and flow stress rises steeply. The second region is the critical zone where DRV and WH both are decreased and new dynamic recrystallization (DRX) initiates. Subsequently, DRX is clearly observed in the third region associated with softening. The fourth region is a steady-state where only DRX occurs. Key mechanisms and their terminology are treated in detail in the following.

#### **5.1 Work hardening**

Work hardening (WH) is also called strain hardening or cold hardening. It is the process of making a metal stronger and harder below its recrystallization

**Figure 5.** *Schematic flow stress–strain diagram [44].*

temperature by increasing dislocation density via plastic deformation. Dislocations will be pinned by each other. Also, as a consequence this highly "faulted" microstructure will prevent the propagation of cracks. With increasing the temperature, the chance of rearrangement of matter and also dislocations is higher which contributes to lower strength at increased ductility.

#### **5.2 Recovery**

Recovery is a softening process that refers to the relieve of part of the internal energy stored within the microstructure, taking place before recrystallization in a deformed material. It normally occurs above the recrystallization temperature where the movement of atoms, i.e. the atomic mobilities and derived diffusion is considerably facilitated. Diffusion increases rapidly with rising temperatures and tends to recover strained regions to the "original" unstrained structure (**Figure 6a**). The extent of recovery depends, among other parameters, on the stacking fault energy (SFE), the type and amount of solute atoms of the material, particularly in the context of dislocation dissociations, which determine the rate of dislocation climb and cross slip. In low SFE metals, recovery as well as cross slip and climb of dislocation is difficult, while the climb is rapid and significant recovery may occur in metals and alloys with a high SFE [46].

Two types of recovery are known, static and dynamic recovery. Static recovery (SRV) occurs at high strain rates where jerky microstructural response of dislocation dynamics prevails. Technologically, this is the case for instance during friction stir welding (FSW) and other torsion processing. Dynamic recovery (DRV) occurs at slower strain rates where thermal activation of the metastable positions within the dislocation structure leads to steady-state during metal processing e.g. hotrolling, extrusion, and forging processes, It is commonly accepted that both DRV and SRV reduce the stresses through changes in dislocation structure due to subgrain growth, dislocation annihilation, and dislocation rearrangement into lowerenergy configurations (such as planar dislocation boundaries). Overall, ductility is improved by recovery, while the strength of materials is reduced [47].

#### **5.3 Recrystallization**

The recrystallization associates with the nucleation of new strain-free grains and their subsequent growth in deformed microstructure when internal energy reaches a critical value (**Figure 6b** and **c**). When the recrystallization process arises during

**9**

Where,

**Figure 6.**

*(c) area of full recrystallization [45].*

**Table 2.**

*Plastic Deformation Behavior in Steels during Metal Forming Processes: A Review*

deformation processes, it is called dynamic recrystallization (DRX). In contrast, when it takes place after deformation or during post-processing like the annealing process, it is known as static recrystallization (SRX) [48]. When DRX is not completed within deformation, this is termed meta dynamic or post dynamic DRX (mDRX) [48]. Moreover, two types of DRX can be distinguished. In discontinuous DRX (dDRX) strain-free grains nucleate and grow rapidly, thus consuming the surrounding strain hardened matrix, while continuous DRX (cDRX) involves the generation of new grain boundaries by the continuous misorientation of nearby subgrains. The combined effect of cDRX and dDRX phenomena takes place during higher strain conditions which are possible during torsion, other severe plastic deformation processes [29]. Since the rate of annihilation due to dynamic recovery is not sufficient to complete with the strain hardening rate in low SFE materials, the dislocation density increases continuously in this case. Contrary, high SFE materials act in favor for higher mobility of dislocation, and consequently dynamic recovery becomes involved as an operating mechanism [49]. The details of materials and the

**Type of process Mechanism Materials** 

*The optical microstructure of deformed stainless steel samples: (a) recovered grains (b) partial recrystallization* 

Hot deformation (T > 0.5Tm) dDRX Category L

Cold/warm deformation (T < 0.5Tm) cDRX All Categories Hot torsion (T > 0.5Tm), other SPD processes DRV + dDRX+cDRX All Categories

**Low and medium-range SFE materials** (*Category L & M*): Copper, Gold, Lead, γ-iron, Ni and their alloys.

**High SFE materials** (*Category H*): Aluminum, Magnesium, α-iron, and their alloys.

*The details of materials and acting mechanisms during the hot deformation process [34, 49, 50].*

**type**

& M

cDRX and DRV Category H

The range of dDRX and cDRX can be understood through the schematic diagram between processing temperature and strain rate (see **Figure 7a**). dDRX phenomena increase above the melting temperature (Tm) when the strain rate decreases while the cDRX phenomena decrease with decreases in processing temperature and increases in strain rate. cDRX occurs in all SFE materials [53] when the temperature falls below 0.5 Tm, however, the dDRX takes place only in low and medium-range SFE materials above 0.5 Tm wherein dynamic recovery is slow after accessing a critical strain value, as can be seen in **Figure 7a** [34, 54, 55]. The grain nucleation and growth during dDRX is the same as for primary DRX which occurs during heating in cold-worked materials. Localized nucleation and growth at local grain boundary

type of possible phenomena are illustrated in **Table 2**.

*DOI: http://dx.doi.org/10.5772/intechopen.97607*

*Plastic Deformation Behavior in Steels during Metal Forming Processes: A Review DOI: http://dx.doi.org/10.5772/intechopen.97607*

#### **Figure 6.**

*Material Flow Analysis*

**5.2 Recovery**

**Figure 5.**

temperature by increasing dislocation density via plastic deformation. Dislocations will be pinned by each other. Also, as a consequence this highly "faulted" microstructure will prevent the propagation of cracks. With increasing the temperature, the chance of rearrangement of matter and also dislocations is higher which con-

Recovery is a softening process that refers to the relieve of part of the internal energy stored within the microstructure, taking place before recrystallization in a deformed material. It normally occurs above the recrystallization temperature where the movement of atoms, i.e. the atomic mobilities and derived diffusion is considerably facilitated. Diffusion increases rapidly with rising temperatures and tends to recover strained regions to the "original" unstrained structure (**Figure 6a**). The extent of recovery depends, among other parameters, on the stacking fault energy (SFE), the type and amount of solute atoms of the material, particularly in the context of dislocation dissociations, which determine the rate of dislocation climb and cross slip. In low SFE metals, recovery as well as cross slip and climb of dislocation is difficult, while the climb is rapid

and significant recovery may occur in metals and alloys with a high SFE [46].

improved by recovery, while the strength of materials is reduced [47].

Two types of recovery are known, static and dynamic recovery. Static recovery (SRV) occurs at high strain rates where jerky microstructural response of dislocation dynamics prevails. Technologically, this is the case for instance during friction stir welding (FSW) and other torsion processing. Dynamic recovery (DRV) occurs at slower strain rates where thermal activation of the metastable positions within the dislocation structure leads to steady-state during metal processing e.g. hotrolling, extrusion, and forging processes, It is commonly accepted that both DRV and SRV reduce the stresses through changes in dislocation structure due to subgrain growth, dislocation annihilation, and dislocation rearrangement into lowerenergy configurations (such as planar dislocation boundaries). Overall, ductility is

The recrystallization associates with the nucleation of new strain-free grains and their subsequent growth in deformed microstructure when internal energy reaches a critical value (**Figure 6b** and **c**). When the recrystallization process arises during

tributes to lower strength at increased ductility.

*Schematic flow stress–strain diagram [44].*

**8**

**5.3 Recrystallization**

*The optical microstructure of deformed stainless steel samples: (a) recovered grains (b) partial recrystallization (c) area of full recrystallization [45].*


Where,

**Low and medium-range SFE materials** (*Category L & M*): Copper, Gold, Lead, γ-iron, Ni and their alloys. **High SFE materials** (*Category H*): Aluminum, Magnesium, α-iron, and their alloys.

#### **Table 2.**

*The details of materials and acting mechanisms during the hot deformation process [34, 49, 50].*

deformation processes, it is called dynamic recrystallization (DRX). In contrast, when it takes place after deformation or during post-processing like the annealing process, it is known as static recrystallization (SRX) [48]. When DRX is not completed within deformation, this is termed meta dynamic or post dynamic DRX (mDRX) [48]. Moreover, two types of DRX can be distinguished. In discontinuous DRX (dDRX) strain-free grains nucleate and grow rapidly, thus consuming the surrounding strain hardened matrix, while continuous DRX (cDRX) involves the generation of new grain boundaries by the continuous misorientation of nearby subgrains. The combined effect of cDRX and dDRX phenomena takes place during higher strain conditions which are possible during torsion, other severe plastic deformation processes [29]. Since the rate of annihilation due to dynamic recovery is not sufficient to complete with the strain hardening rate in low SFE materials, the dislocation density increases continuously in this case. Contrary, high SFE materials act in favor for higher mobility of dislocation, and consequently dynamic recovery becomes involved as an operating mechanism [49]. The details of materials and the type of possible phenomena are illustrated in **Table 2**.

The range of dDRX and cDRX can be understood through the schematic diagram between processing temperature and strain rate (see **Figure 7a**). dDRX phenomena increase above the melting temperature (Tm) when the strain rate decreases while the cDRX phenomena decrease with decreases in processing temperature and increases in strain rate. cDRX occurs in all SFE materials [53] when the temperature falls below 0.5 Tm, however, the dDRX takes place only in low and medium-range SFE materials above 0.5 Tm wherein dynamic recovery is slow after accessing a critical strain value, as can be seen in **Figure 7a** [34, 54, 55]. The grain nucleation and growth during dDRX is the same as for primary DRX which occurs during heating in cold-worked materials. Localized nucleation and growth at local grain boundary

**Figure 7.**

*(a) Schematic correlation between cDRX and dDRX [51], (b) dDRX nuclei in austenitic stainless steel at 800°C with strain rate 0.001 s−1 [52].*

bulging can be seen in **Figure 7b**. It is obvious that the dDRX nuclei contain a much lower dislocation density than the deformed region and these nuclei are distinct from highly disturbed substructures with twin boundaries and low angle dislocation sub boundaries.

One additional terminology has recently been denoted as post-DRX which occurs during the annealing process in deformed materials [55].
