**3. Results of researches and discussion**

According to results of mechanical tests, dependences are constructed *σ* = *f*(*ε*). The received dependences and their look do not contradict the settled ideas of behavior of metal polycrystals in the conditions of hot plastic deformation. So, in the course of process of plastic deformation of metal, tension smoothly increases and reaches a certain maximum (saturation) in which value is defined at the same time proceeding competing processes—hardenings and a weakening [9]. The growth rate of tension depends on temperature of heating and speed of deformation. At low temperatures and high speeds of deformation, flow stress continuously increases with deformation growth that is caused by the prevailing process of deformation hardening.

At the increased temperatures and low speeds of deformation, flow stress reaches a maximum and then goes down, reaching a certain constant value. In such type of charts, tension deformation is characteristic of the majority of the metals and alloys deformed at temperatures exceeding half the temperature of melting [10, 11].

Current tension size (σs) of the studied alloy depending on temperature and the speed of deformation is presented in **Table 2**.

Values of tension of a current at the set temperature and high-speed parameters of deformation were used for the subsequent calculation of effectiveness ratio of dissipation of energy η.

Change of coefficient η from temperature and high-speed parameters of deformation are presented in the form of the volume chart (**Figure 2**) and also in the form of the card of constant levels of effectiveness ratio of dissipation of energy (**Figure 3**).

Analyzing the results of change of effectiveness ratio of dissipation of energy presented on 3D plot and the map of constant levels of effectiveness ratio of dissipation depending on temperature and high-speed parameters of deformation, it is possible to note:

Temperature dependence *η* = *f(t, ε*;): It is characterized by a maximum at temperatures 900–940°С. And with increase in speed of deformation, maximum shift toward big speeds of deformation is observed.



**Table 2.**

*The flow stress of examined alloy at various temperatures and strain rate values for tensile strain of ε = 0.2.*

**71**

**Figure 3.**

**Figure 2.**

*Characteristics of the Dissipation of Energy at Hot Plastic Deformation of Near-Alpha Titanium…*

When heating alloy to temperature of 800°С, the first signs of recrystallization are observed, and further heating to temperatures of 920–940°С and endurance of

To process recrystallization, *α* → *β* phase transformation is followed. An increase

of the *β*-phase contents in the alloy when heated is represented in **Table 3**.

*Processing map for titanium alloy. Constant levels of coefficient efficiency of dissipation at ε = 0.2.*

15 min. Process of recrystallization proceeds completely.

*DOI: http://dx.doi.org/10.5772/intechopen.88845*

*3D plot of effectiveness ratio dissipations of energy at ε = 0.2.*

*Characteristics of the Dissipation of Energy at Hot Plastic Deformation of Near-Alpha Titanium… DOI: http://dx.doi.org/10.5772/intechopen.88845*

**Figure 2.** *3D plot of effectiveness ratio dissipations of energy at ε = 0.2.*

**Figure 3.** *Processing map for titanium alloy. Constant levels of coefficient efficiency of dissipation at ε = 0.2.*

When heating alloy to temperature of 800°С, the first signs of recrystallization are observed, and further heating to temperatures of 920–940°С and endurance of 15 min. Process of recrystallization proceeds completely.

To process recrystallization, *α* → *β* phase transformation is followed. An increase of the *β*-phase contents in the alloy when heated is represented in **Table 3**.

*Titanium Alloys - Novel Aspects of Their Manufacturing and Processing*

melting [10, 11].

(**Figure 3**).

possible to note:

equilibrium state.

dissipation of energy η.

speed of deformation is presented in **Table 2**.

toward big speeds of deformation is observed.

with increase in extent of deformation from 0.1 to 0.3.

*Т***, °C σs, MPa**

 **s<sup>−</sup><sup>1</sup> 10<sup>−</sup><sup>3</sup>**

**10<sup>−</sup><sup>4</sup>**

course of process of plastic deformation of metal, tension smoothly increases and reaches a certain maximum (saturation) in which value is defined at the same time proceeding competing processes—hardenings and a weakening [9]. The growth rate of tension depends on temperature of heating and speed of deformation. At low temperatures and high speeds of deformation, flow stress continuously increases with deformation growth that is caused by the prevailing process of deformation hardening. At the increased temperatures and low speeds of deformation, flow stress reaches a maximum and then goes down, reaching a certain constant value. In such type of charts, tension deformation is characteristic of the majority of the metals and alloys deformed at temperatures exceeding half the temperature of

Current tension size (σs) of the studied alloy depending on temperature and the

Values of tension of a current at the set temperature and high-speed parameters of deformation were used for the subsequent calculation of effectiveness ratio of

Change of coefficient η from temperature and high-speed parameters of deformation are presented in the form of the volume chart (**Figure 2**) and also in the form of the card of constant levels of effectiveness ratio of dissipation of energy

Analyzing the results of change of effectiveness ratio of dissipation of energy presented on 3D plot and the map of constant levels of effectiveness ratio of dissipation depending on temperature and high-speed parameters of deformation, it is

Temperature dependence *η* = *f(t, ε*;): It is characterized by a maximum at temperatures 900–940°С. And with increase in speed of deformation, maximum shift

• The studied alloy is characterized by high efficiency of dissipation of energy in the studied range of temperature and high-speed parameters of process of hot deformation. Efficiency of energy of dissipation significantly does not change

• Mechanical properties of alloy at hot plastic deformation substantially depend on initial structure and temperature and high-speed parameters of deformation.

• In an initial state the studied alloy has the coarse-grained (not recrystallized) structure and the increased maintenance of a β-phase in comparison with an

 60 100 160 240 44 75 130 195 24 53 94 149 14 28 60 95 8.0 16 30 60 5.8 10 18 32

*The flow stress of examined alloy at various temperatures and strain rate values for tensile strain of ε = 0.2.*

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

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

 **s<sup>−</sup><sup>1</sup>**

**70**

**Table 2.**


**Table 3.**

*Change of volume fraction of a β-phase when heating alloy.*

Increasing the heating temperature of the alloy leads to an increase in the phase change rate due to an increase in self-diffusion. The largest speed of phase transformation is observed at a temperature of heating of 900–950°C. The quantity of *α*- and *β*-phases decreases with temperature increase and increase in hold time. The quantity of a phase decreases with temperature increase and increase in hold time. At alloy heating temperatures (1040°С), the phase transformation which is followed by sharp integration of grains of a β-phase completely comes to the end (**Table 3**).

It follows from the provided data that the microstructure and phase composition of alloys undergo significant changes at a temperature of heating to temperatures more than 950°С owing to full completion of process of recrystallization and phase *α*→*β* transformations.

Results of researches on the change of structure of alloy in the course of deformation at various temperatures and extent of deformation 0.4 are presented in **Table 4**.

Grain size change in phase *α*→*β* transformation is characteristic for structure at hot deformation of alloy. If the size of grains *α*-phases decreases, then the size of grains *β*-phases on the contrary increases.

As appears from the data provided in **Table 4** in the course of deformation of alloy, there is an increase in quantity of *β*-phases. The considerable difference in the number of phases of composition of alloy is observed at deformation speeds 10<sup>−</sup><sup>3</sup> s<sup>−</sup><sup>1</sup> . Further increase in speed of deformation practically does not lead to significant change of quantity of *β*-phases in comparison with an initial condition of alloy. Increased rate of deformation results in intensive phase transformation in alloy at small degrees of deformation.

So, at extent of deformation *ε* = 0.4, quantity of *β*-phases reaches 12–20%, and at extent of deformation from 0.4 to 1.0, it reaches only 3–4%.

Change of phase composition in the course of deformation is often connected with intensity of diffusive processes. The authors of [11] note that heating to the temperature of deformation of the titanium alloy does not lead to the achievement of phase equilibrium. The reason for this phenomenon is the relatively low diffusion mobility of *β*-stabilizing elements. For example, the *β*-phase content is 49% at a strain temperature of 950°C and 30 minutes.


**73**

**Figure 4.**

*(b) δ = 200%. ×1000.*

*Characteristics of the Dissipation of Energy at Hot Plastic Deformation of Near-Alpha Titanium…*

The phase *α*↔*β* transformation is accompanied by a volumetric effect. It is known that various authors estimate this value to be about 0.15% [12]. Transformation of *α*↔*β* is accompanied by a volumetric effect and *α*→*β*

transformation-negative volumetric effect. Therefore with an external pressure, there is a temperature change of polymorphic transformation. The speed of phase transformations generally depends on the difference of free energy of an initial and final state and also the size of change of volume upon this transition. As the size of free energy and volume depend on pressure, it is possible to expect that the speed of

In that case when phase transformations are carried out in the diffusive way, the kinetics of phase transformations is defined by change of speed of the course of

The driving force of phase *β*→*α* transformation in titanium alloys is shift of phase equilibrium temperature under action of external tensions. The rate of phase change is determined by the diffusion mobility of the *β* stabilizing elements' atoms. The interesting fact established when studying changes of a microstructure of alloys at hot deformation is transformation of initial lamellar structure in granular, which is most brightly shown at a temperature of deformation of 920°C and strain rate of

metallography method Кф = l*α*/d*α*; where l*α* is the length of the plates and d*α* is the width of the plates *α*-phases. The results of the calculations showed that intensive change of grain shape occurs up to deformation of 100%; at higher deformation, Kf

Equiaxial grains of structure 200÷ 300 microns in size are observed in the field

The nature of dissipative processes described above finally defines indicators of plasticity of alloy. Maximum stability of plastic deformation of alloy is observed at compliance of temperature-speed deformation parameters and maximum

Practically the same quantity of *β*-phases is observed at 2.5 min. Endurance with extent of deformation ε = 0.5. This circumstance allows to make the assumption that not only the increase in diffusive mobility of atoms is caused by deformation but also temperature change of phase balance is the reason of phase transformation

*DOI: http://dx.doi.org/10.5772/intechopen.88845*

phase transformations will also depend on pressure.

Grain shape coefficient was determined by quantitative

*An alloy microstructure after deformation at 920°*С *and speeds of deformation 1,1*.*10<sup>−</sup><sup>3</sup>*

 *s<sup>−</sup><sup>1</sup>*

*. (a) δ = 55%,* 

at action of external tension.

diffusive processes with a pressure.

(**Figure 4**).

stabilizes at values of ≈ 1.2–1.5.

of temperatures of *β*-phases (t ≥ 1040°С).

1.1∙10<sup>−</sup><sup>3</sup>

s<sup>−</sup><sup>1</sup>

**Table 4.**

*The size of grain and phase composition of alloy before deformation at various temperatures.*

Practically the same quantity of *β*-phases is observed at 2.5 min. Endurance with extent of deformation ε = 0.5. This circumstance allows to make the assumption that not only the increase in diffusive mobility of atoms is caused by deformation but also temperature change of phase balance is the reason of phase transformation at action of external tension.

The phase *α*↔*β* transformation is accompanied by a volumetric effect. It is known that various authors estimate this value to be about 0.15% [12]. Transformation of *α*↔*β* is accompanied by a volumetric effect and *α*→*β* transformation-negative volumetric effect. Therefore with an external pressure, there is a temperature change of polymorphic transformation. The speed of phase transformations generally depends on the difference of free energy of an initial and final state and also the size of change of volume upon this transition. As the size of free energy and volume depend on pressure, it is possible to expect that the speed of phase transformations will also depend on pressure.

In that case when phase transformations are carried out in the diffusive way, the kinetics of phase transformations is defined by change of speed of the course of diffusive processes with a pressure.

The driving force of phase *β*→*α* transformation in titanium alloys is shift of phase equilibrium temperature under action of external tensions. The rate of phase change is determined by the diffusion mobility of the *β* stabilizing elements' atoms. The interesting fact established when studying changes of a microstructure of alloys at hot deformation is transformation of initial lamellar structure in granular, which is most brightly shown at a temperature of deformation of 920°C and strain rate of 1.1∙10<sup>−</sup><sup>3</sup> s<sup>−</sup><sup>1</sup> (**Figure 4**).

Grain shape coefficient was determined by quantitative

metallography method Кф = l*α*/d*α*; where l*α* is the length of the plates and d*α* is the width of the plates *α*-phases. The results of the calculations showed that intensive change of grain shape occurs up to deformation of 100%; at higher deformation, Kf stabilizes at values of ≈ 1.2–1.5.

Equiaxial grains of structure 200÷ 300 microns in size are observed in the field of temperatures of *β*-phases (t ≥ 1040°С).

The nature of dissipative processes described above finally defines indicators of plasticity of alloy. Maximum stability of plastic deformation of alloy is observed at compliance of temperature-speed deformation parameters and maximum

#### **Figure 4.** *An alloy microstructure after deformation at 920°*С *and speeds of deformation 1,1*.*10<sup>−</sup><sup>3</sup> s<sup>−</sup><sup>1</sup> . (a) δ = 55%, (b) δ = 200%. ×1000.*

*Titanium Alloys - Novel Aspects of Their Manufacturing and Processing*

*Change of volume fraction of a β-phase when heating alloy.*

*α*→*β* transformations.

grains *β*-phases on the contrary increases.

alloy at small degrees of deformation.

strain temperature of 950°C and 30 minutes.

*Note: I, longitudinal section of a sample; II, the cross-section of a sample.*

extent of deformation from 0.4 to 1.0, it reaches only 3–4%.

**Table 4**.

**Table 3.**

10<sup>−</sup><sup>3</sup> s<sup>−</sup><sup>1</sup>

Increasing the heating temperature of the alloy leads to an increase in the phase change rate due to an increase in self-diffusion. The largest speed of phase transformation is observed at a temperature of heating of 900–950°C. The quantity of *α*- and *β*-phases decreases with temperature increase and increase in hold time. The quantity of a phase decreases with temperature increase and increase in hold time. At alloy heating temperatures (1040°С), the phase transformation which is followed by sharp integration of grains of a β-phase completely comes to the end (**Table 3**). It follows from the provided data that the microstructure and phase composition of alloys undergo significant changes at a temperature of heating to temperatures more than 950°С owing to full completion of process of recrystallization and phase

*Т*, °С 750 800 850 900 950 1000 1040 β-phase, % 15 20 28 40 65 82 100

Results of researches on the change of structure of alloy in the course of deformation at various temperatures and extent of deformation 0.4 are presented in

Grain size change in phase *α*→*β* transformation is characteristic for structure at hot deformation of alloy. If the size of grains *α*-phases decreases, then the size of

As appears from the data provided in **Table 4** in the course of deformation of alloy, there is an increase in quantity of *β*-phases. The considerable difference in the number of phases of composition of alloy is observed at deformation speeds

. Further increase in speed of deformation practically does not lead to significant change of quantity of *β*-phases in comparison with an initial condition of alloy. Increased rate of deformation results in intensive phase transformation in

So, at extent of deformation *ε* = 0.4, quantity of *β*-phases reaches 12–20%, and at

Change of phase composition in the course of deformation is often connected with intensity of diffusive processes. The authors of [11] note that heating to the temperature of deformation of the titanium alloy does not lead to the achievement of phase equilibrium. The reason for this phenomenon is the relatively low diffusion mobility of *β*-stabilizing elements. For example, the *β*-phase content is 49% at a

*Т***, °C Grain size, μm Phase composition, %**

840 12.7 4.7 6.8 2.4 6.6 5.5 3.6 2.5 70 30 54 46 900 11.0 4.9 6.6 3.0 7.9 7.7 4.3 4.2 60 40 42 58 960 10.4 6.7 6.8 5.1 9.9 12.5 6.7 6.9 35 65 22 78

**I II I II**

*The size of grain and phase composition of alloy before deformation at various temperatures.*

*ε* **= 0** *ε* **= 0.4** *ε* **= 0** *ε* **= 0.4**

*α β α β α β α β α β α β*

**72**

**Table 4.**
