2. Regularities of solid solution hardening of surface layers of titanium alloys (α, near-α, and α + β) depending on conditions of thermodiffusion saturation in rarefied gaseous medium containing oxygen

Accordingly, with the results obtained previously, the required level of solid solution hardening of surface layers of titanium alloys by interstitial impurities depends on the phase-structural state of metal and relative depth of hardened (gassaturated) zone. In this connection the problem of purposeful control of intensity of physicochemical processes in the system "titanium (titanium alloy)/gas medium" for the formation of required phase-structural state of surface layers aimed to ensure the corresponding operating performance of materials (fatigue strength, endurance, etc.) arises. In turn, this envisages the study of kinetic regularities of interaction of titanium alloys in rarefied gas medium.

## 2.1 Kinetic parameters of interaction of VT1-0, VT5, OT4-1, and VT16 alloys with rarefied gas medium

As a result of interaction of titanium alloys with rarefied gas medium containing oxygen, the processes of sublimation and phase formation may take place [1]. The processes of gas saturation and phase formation will be predominated at selected temperatures (923…1023 K) and pressures (6.6 <sup>10</sup><sup>3</sup> …6.6 <sup>10</sup><sup>2</sup> Pa), according to the analysis of changing of free energy of formation of solid solution of oxygen in titanium and titanium monoxide (Figure 1).

At the individual case, for alloys alloyed by manganese, molybdenum, and vanadium, the sublimation is possible due to the formation of oxide compounds with high pressure of saturated vapor [2]. Thus the gas saturation and phase formation increase the specimen mass of investigated titanium alloy, while sublimation decreases.

Surface Treatment of Titanium Alloys in Oxygen-Containing Gaseous Medium DOI: http://dx.doi.org/10.5772/intechopen.82545

Figure 1.

operating characteristics of metal. Therefore the aim of investigations on the second stage of the project was to determine (a) regularities of solid solution hardening of surface layers of titanium alloys (α, near-α, α + β) depending on the conditions of thermodiffusion saturation in rarified gas medium containing oxygen and (b) general regularities of influence of methods and regimes of surface deformation on phase, structural, and substructural state of various titanium alloys. It is expected that obtained results will allow to forecast the influence of the regime of CTT on the phase-structural state of surface layers of metal and level of hardening and determine the parameters of thermal treatments for achieving the regulated level of

Titanium Alloys - Novel Aspects of Their Manufacturing and Processing

Increasing of fatigue strength and durability of titanium alloy workpieces remains an actual modern problem. It is known that fatigue properties of titanium alloys can be increased sufficiently by means of optimization of phase-structural state of surface layer. The aim of the investigations of paper was to determine the correlations between parameters of surface-hardened layers (surface hardness, depth of hardened zone, microstructure) and fatigue properties of titanium alloys VT1-0, VT5, ОТ4-1, VT16, and VT22 under various methods of surface hardening: thermodiffusion saturation in gas medium containing oxygen (CTT). Determination of such correlations allow to define the parameters of surface hardening of titanium alloys necessary for increasing of fatigue properties and approximate these

2. Regularities of solid solution hardening of surface layers of titanium

thermodiffusion saturation in rarefied gaseous medium containing

Accordingly, with the results obtained previously, the required level of solid solution hardening of surface layers of titanium alloys by interstitial impurities depends on the phase-structural state of metal and relative depth of hardened (gassaturated) zone. In this connection the problem of purposeful control of intensity of physicochemical processes in the system "titanium (titanium alloy)/gas medium" for the formation of required phase-structural state of surface layers aimed to ensure the corresponding operating performance of materials (fatigue strength, endurance, etc.) arises. In turn, this envisages the study of kinetic regularities of

2.1 Kinetic parameters of interaction of VT1-0, VT5, OT4-1, and VT16 alloys

As a result of interaction of titanium alloys with rarefied gas medium containing oxygen, the processes of sublimation and phase formation may take place [1]. The processes of gas saturation and phase formation will be predominated at selected

to the analysis of changing of free energy of formation of solid solution of oxygen in

At the individual case, for alloys alloyed by manganese, molybdenum, and vanadium, the sublimation is possible due to the formation of oxide compounds with high pressure of saturated vapor [2]. Thus the gas saturation and phase formation increase the specimen mass of investigated titanium alloy, while sublima-

…6.6 <sup>10</sup><sup>2</sup> Pa), according

alloys (α, near-α, and α + β) depending on conditions of

methods of surface hardening to the practical application.

interaction of titanium alloys in rarefied gas medium.

temperatures (923…1023 K) and pressures (6.6 <sup>10</sup><sup>3</sup>

titanium and titanium monoxide (Figure 1).

with rarefied gas medium

hardening.

oxygen

tion decreases.

78

Changes of free energy of formation of oxygen solid solutions in alpha-titanium and titanium monoxide as a function of (a) oxygen pressure and (b) temperature [1].

The influence of temperature-time and gas-dynamical parameters (T = 650, 700, 750°С, <sup>τ</sup> = 1, 3, 5 h, <sup>P</sup> = 6.6 � <sup>10</sup>�<sup>3</sup> , 1.33 � <sup>10</sup>�<sup>2</sup> , 6.6 � <sup>10</sup>�<sup>2</sup> Pa) of thermodiffusion saturation in controlled gas medium on the regularities of interaction of titanium alloys VT1-0, VT5, OT4-1, and VT16 is investigated by means of gravimetric analysis. The kinetic parameters of interaction of investigated titanium alloys determined by means of gravimetric analysis are shown in Tables 1–4.

According to the obtained results for α-titanium alloys (VT1-0, VT5), the process of gas saturation is intensified with the increasing of interaction temperature and pressure of gas medium (partial pressure of oxygen). The α-alloy VT1-0 (technical titanium) has the largest rate of interaction with rarefied gas medium containing oxygen under the all conditions. Alloying of titanium by 5% Al-alloy VT5-slows down slightly the rate of mass gain under the same conditions of interaction.

The mass loss caused by intensification of sublimation of alloying element Mn is possible for near-α-alloy OT4-1 (2% β-phase) under definite conditions of interaction with rarefied gas medium. The ratio of parameters T and P exists for this alloy when the rates of the gas saturation and sublimation processes become comparable. For the predomination of gas saturation processes, it is necessary to increase the partial pressure of oxygen or decrease the temperature of interaction.

Rate of gas saturation in rarefied gas medium decreased substantially with the increasing of quantity of β-phase in alloys (VT16 ! VT22). This is caused by the decreasing of maximal solubility of oxygen in β-phase of titanium (6 at.%) in comparison with α-phase (33 at.%). With the increasing of interaction temperature, this difference appeared most appreciably. Therefore, it was concluded that alloys with large quantity of β-phase are the ones less sensitive to the changing of the conditions of interaction with rarefied gas medium containing oxygen.


Table 1.

The specific mass gain of specimens of titanium alloy VT1-0 as a result of interaction with rarefied gas medium containing oxygen.


kP

DOI: http://dx.doi.org/10.5772/intechopen.82545

interval with a possibility of 0.98.

zone depth

Table 5.

81

presented in Table 5.

ð Þ¼ <sup>T</sup> <sup>B</sup> � exp ð Þ� �Eекс=RT <sup>C</sup>, g m�<sup>2</sup> <sup>h</sup>�<sup>1</sup> (2)

<sup>k</sup><sup>T</sup>ð Þ¼ <sup>P</sup> <sup>½</sup><sup>H</sup> <sup>þ</sup> <sup>J</sup> � ln ð Þ <sup>P</sup> � � <sup>K</sup>, g m<sup>2</sup> <sup>h</sup>�<sup>1</sup> (4)

, g=m<sup>2</sup> (3)

where B is the constant depending on temperature, Eекс is the total energy of process activation, and C is the confidence interval with a possibility of 0.98. Constants for Eqs. (1) and (2) and experimental activation energy of process are

All kinetic regularities follow linear dependence satisfactorily under isothermal conditions and various pressures accordingly with the data of thermogravimetry:

where k<sup>T</sup> is the coefficient of linear rate under constant temperature (700°C) and F is the confidence interval with a possibility of 0.98. Dependence of coefficient of linear rate under isothermal conditions on the residual pressure of medium is

where H and J are the constants depending on pressure and K is the confidence

Formation of interstitial solid solution in the metal during diffusion saturation of titanium alloys by gases in rarefied gas medium (mainly by oxygen) is bound up with strong distortion of crystallographic lattice (Figure 2) and as a result of this with essential increasing of hardness of metal. Therefore, the parameters of gassaturated layers were determined by means of two methods: using microhardness

Using formula (1) Using formula (2)

ВТ1-0 650 3.23 1.67 4 � <sup>10</sup><sup>7</sup> 7447.6 4.66

ВТ5 650 2.44 0.59 1 � <sup>10</sup><sup>8</sup> 8140.6 3.91

ВТ16 650 2.6 0.1 29.21 1350.8 1.07

ОТ4-1 650 3.2 0.2 16585.2 4148.6 3.34

,gm�<sup>2</sup> h�<sup>1</sup> A,gm�<sup>2</sup> B,gm�<sup>2</sup> h�<sup>1</sup> Eекс, J mol�<sup>1</sup> C,gm�<sup>2</sup> h�<sup>1</sup>

2.2 Effect of temperature and time on surface metal hardness and hardening

<sup>Δ</sup>M=<sup>S</sup> <sup>¼</sup> <sup>k</sup><sup>Т</sup> � <sup>τ</sup> � <sup>F</sup> � <sup>10</sup>�<sup>2</sup>

Surface Treatment of Titanium Alloys in Oxygen-Containing Gaseous Medium

approximated satisfactorily by logarithmic dependence:

Coefficients for Eqs. (3) and (4) are presented in Table 6.

Alloy T, °C At Т = 700°С

kP

700 7.46 3.53 750 15.8 4.21

700 6.02 2.96 750 13.49 4.10

700 2.86 0.87 750 3.3 1.10

700 5.0 1.41 750 7.38 2.87

Kinetic parameters of gas saturation of titanium alloys under isothermal conditions.

Table 2.

The specific mass gain of specimens of titanium alloy VT5 as a result of interaction with rarefied gas medium containing oxygen.


#### Table 3.

The specific mass change of specimens of titanium alloy OT4-1 as a result of interaction with rarefied gas medium containing oxygen.


#### Table 4.

The specific mass gain of specimens of titanium alloy VT16 as a result of interaction with rarefied gas medium containing oxygen.

Let us calculate the kinetic parameters as the function of temperature accordingly with the data of mass changing of specimens of titanium alloys in rarefied gas medium containing oxygen.

All kinetic dependences in the 5-h interval at the residual pressure <sup>P</sup> = 1.33 � <sup>10</sup>�<sup>2</sup> Pa follow linear dependence (1) satisfactorily. This indicates that surface reactions at the "metal-gas" interface are the controlling stage of the processes [3]:

$$
\Delta \mathbf{M} / \mathbf{S} = \left( \mathbf{k}^{\mathrm{P}} \cdot \mathbf{r} \pm \mathbf{A} \right) \cdot \mathbf{10}^{-2}, \left[ \mathbf{g} / \mathrm{m}^2 \right] \tag{1}
$$

where k<sup>p</sup> is the coefficient of linear rate under constant pressure and A is the confidence interval with a possibility of 0.98.

The coefficient of linear rate under isobaric conditions of thermally activated process depends on the absolute temperature T accordingly with Arrhenius equations [1]:

Surface Treatment of Titanium Alloys in Oxygen-Containing Gaseous Medium DOI: http://dx.doi.org/10.5772/intechopen.82545

$$\mathbf{k}^{\mathrm{p}}(T) = \mathbf{B} \cdot \exp\left(-\mathbf{E}\_{\mathrm{exc}}/\mathbf{R}T\right) \pm \mathbf{C}\_{\mathrm{s}} \left[\mathbf{g} \ \mathrm{m}^{-2} \ \mathrm{h}^{-1}\right] \tag{2}$$

where B is the constant depending on temperature, Eекс is the total energy of process activation, and C is the confidence interval with a possibility of 0.98. Constants for Eqs. (1) and (2) and experimental activation energy of process are presented in Table 5.

All kinetic regularities follow linear dependence satisfactorily under isothermal conditions and various pressures accordingly with the data of thermogravimetry:

$$
\Delta \mathbf{M} / \mathbf{S} = \left( \mathbf{k}^T \cdot \boldsymbol{\pi} \pm \mathbf{F} \right) \cdot \mathbf{10}^{-2}, \left[ \mathbf{g} / \mathbf{m}^2 \right] \tag{3}
$$

where k<sup>T</sup> is the coefficient of linear rate under constant temperature (700°C) and F is the confidence interval with a possibility of 0.98. Dependence of coefficient of linear rate under isothermal conditions on the residual pressure of medium is approximated satisfactorily by logarithmic dependence:

$$\boldsymbol{k}^T(\mathbf{P}) = [\mathbf{H} + \mathbf{J} \cdot \ln \left(\mathbf{P}\right)] \pm \mathbf{K}, \left[\mathbf{g} \text{ m}^2 \,\text{h}^{-1}\right] \tag{4}$$

where H and J are the constants depending on pressure and K is the confidence interval with a possibility of 0.98.

Coefficients for Eqs. (3) and (4) are presented in Table 6.

### 2.2 Effect of temperature and time on surface metal hardness and hardening zone depth

Formation of interstitial solid solution in the metal during diffusion saturation of titanium alloys by gases in rarefied gas medium (mainly by oxygen) is bound up with strong distortion of crystallographic lattice (Figure 2) and as a result of this with essential increasing of hardness of metal. Therefore, the parameters of gassaturated layers were determined by means of two methods: using microhardness


Table 5.

Kinetic parameters of gas saturation of titanium alloys under isothermal conditions.

Let us calculate the kinetic parameters as the function of temperature accordingly with the data of mass changing of specimens of titanium alloys in rarefied gas

The specific mass gain of specimens of titanium alloy VT16 as a result of interaction with rarefied gas medium

650 2.00 6.00 9.66 2.50 7.80 13.00 3.50 10.60 17.80 700 2.32 7.00 11.60 2.80 8.80 14.20 4.10 12.60 22.50 750 2.80 8.50 13.29 3.20 9.75 16.60 5.30 16.70 27.40

) at residual pressure of gas medium

) at residual pressure of gas medium

) at residual pressure of gas medium

<sup>P</sup> = 6.6 � <sup>10</sup>�<sup>3</sup> Pa <sup>P</sup> = 1.33 � <sup>10</sup>�<sup>2</sup> Pa <sup>P</sup> = 6.6 � <sup>10</sup>�<sup>2</sup> Pa 1h 3h 5h 1h 3h 5h 1h 3h 5

<sup>P</sup> = 6.6 � <sup>10</sup>�<sup>3</sup> Pa <sup>P</sup> = 1.33 � <sup>10</sup>�<sup>2</sup> Pa <sup>P</sup> = 6.6 � <sup>10</sup>�<sup>2</sup> Pa 1h 3h 5h 1h 3h 5h 1h 3h 5h

650 2.24 6.53 10.65 2.57 7.44 12.10 3.31 9.48 15.31 700 5.62 16.2 26.29 6.43 18.42 29.77 8.24 23.31 37.36 750 12.83 36.64 59.07 14.64 41.49 66.60 18.68 52.14 82.86

The specific mass gain of specimens of titanium alloy VT5 as a result of interaction with rarefied gas medium

650 �0.10 �0.31 �0.52 3.18 9.55 16.0 7.25 22.00 36.75 700 �0.16 �0.49 �0.82 4.98 15.0 25.0 11.50 34.75 57.75 750 �0.24 �0.73 �1.23 7.47 22.0 37.0 17.25 52.00 86.50

The specific mass change of specimens of titanium alloy OT4-1 as a result of interaction with rarefied gas

<sup>P</sup> = 6.6 � <sup>10</sup>�<sup>3</sup> Pa <sup>P</sup> = 1.33 � <sup>10</sup>�<sup>2</sup> Pa <sup>P</sup> = 6.6 � <sup>10</sup>�<sup>2</sup> Pa 1h 3h 5h 1h 3h 5h 1h 3h 5h

<sup>P</sup> = 1.33 � <sup>10</sup>�<sup>2</sup> Pa follow linear dependence (1) satisfactorily. This indicates that surface reactions at the "metal-gas" interface are the controlling stage of the

where k<sup>p</sup> is the coefficient of linear rate under constant pressure and A is the

The coefficient of linear rate under isobaric conditions of thermally activated

, g=m<sup>2</sup> (1)

All kinetic dependences in the 5-h interval at the residual pressure

<sup>Δ</sup>M=<sup>S</sup> <sup>¼</sup> <sup>k</sup><sup>P</sup> � <sup>τ</sup> � <sup>A</sup> � <sup>10</sup>�<sup>2</sup>

process depends on the absolute temperature T accordingly with Arrhenius

medium containing oxygen.

T, °C ΔM/S (μg/cm2

T, °C ΔM/S (μg/cm2

Titanium Alloys - Novel Aspects of Their Manufacturing and Processing

T, °C ΔM/S (μg/cm<sup>2</sup>

confidence interval with a possibility of 0.98.

processes [3]:

Table 4.

Table 3.

medium containing oxygen.

Table 2.

containing oxygen.

containing oxygen.

equations [1]:

80


#### Table 6.

Kinetic parameters of gas saturation of titanium alloys under isothermal conditions.

#### Figure 2.

Changing of ratio of parameters of crystallographic lattice (c/a) of titanium alloy surface layer VT1-0 as a function of CTT regime: (1) in initial state, (2) after CTT: <sup>Т</sup> = 650°С, <sup>P</sup> = 6.6 <sup>10</sup><sup>3</sup> Pa, <sup>τ</sup> = 5 h, (3) after CTT: <sup>Т</sup> = 750°С, <sup>P</sup> = 6.6 <sup>10</sup><sup>3</sup> Pa, <sup>τ</sup> = 5 h.

that was measured on both surface and cross section of metallographic sample made of the gas-saturated specimen and changing parameters of crystallographic lattice. The last one method is more complex and laborious; therefore, it was used only for determination of parameters of maximally hardened layer. Depth of gas-saturated layer recognizes as a distance from the surface where increasing of hardness caused by dissolution of oxygen is equal to the measurement error.

The experimentally obtained results of influence of parameters of thermodiffusion saturation on the gain of surface hardness K of investigated titanium alloys are presented in Tables 7–14 (K = ((H<sup>μ</sup> <sup>s</sup> <sup>H</sup><sup>μ</sup> c )/H<sup>μ</sup> с ) 100%, where H<sup>μ</sup> <sup>s</sup> is the surface hardness of metal and H<sup>μ</sup> <sup>c</sup> is the bulk hardness) and depth of gassaturated zone l determined by durometry.

The results concerning the determination of parameters of crystallographic lattice of specimens of investigated alloys after different regimes of thermodiffusion saturation are presented in Tables 15–18.

The regularities of thermodiffusion saturation intrinsic for all investigated alloys were revealed basing on the analysis of obtained results, namely, parameters of gas-

Gain of surface hardness of titanium alloy VT16 as a result of interaction with rarefied gas medium containing

T, °С K (%) at residual pressure of gas medium

T, °С K (%) at residual pressure of gas medium

Surface Treatment of Titanium Alloys in Oxygen-Containing Gaseous Medium

DOI: http://dx.doi.org/10.5772/intechopen.82545

T, °С K (%) at residual pressure of gas medium

T, °C K (%) at residual pressure of gas medium

<sup>P</sup> = 6.6 <sup>10</sup><sup>3</sup> Pa <sup>P</sup> = 1.33 <sup>10</sup><sup>2</sup> Pa <sup>P</sup> = 6.6 <sup>10</sup><sup>2</sup> Pa 1h 3h 5h 1h 3h 5h 1h 3h 5h

<sup>P</sup> = 6.6 <sup>10</sup><sup>3</sup> Pa <sup>P</sup> = 1.33 <sup>10</sup><sup>2</sup> Pa <sup>P</sup> = 6.6 <sup>10</sup><sup>2</sup> Pa 1h 3h 5h 1h 3h 5h 1h 3h 5h

650 41 66 81 56 85 103 80 121 143 700 37 61 75 49 78 96 74 113 134 750 22 37 47 30 48 60 45 72 88

Gain of surface hardness of titanium alloy VT1-0 as a result of interaction with rarefied gas medium

<sup>P</sup> = 6.6 <sup>10</sup><sup>3</sup> Pa <sup>P</sup> = 1.33 <sup>10</sup><sup>2</sup> Pa <sup>P</sup> = 6.6 <sup>10</sup><sup>2</sup> Pa 1h 3h 5h 1h 3h 5h 1h 3h 5h

<sup>P</sup> = 6.6 <sup>10</sup><sup>3</sup> Pa <sup>P</sup> = 1.33 <sup>10</sup><sup>2</sup> Pa <sup>P</sup> = 6.6 <sup>10</sup><sup>2</sup> Pa 1h 3h 5h 1h 3h 5h 1h 3h 5h

650 30 47 58 37 58 70 52 78 92 700 28 45 56 35 56 67 50 75 89 750 6.5 11 15 10 18 22 20 32 40

Gain of surface hardness of titanium alloy VT5 as a result of interaction with rarefied gas medium containing

650 38 66 86 33 58 75 11 18 24 700 17 30 38.9 28 50 64 26 45 59 750 9 17 22 8 15 19 13 23 29

Gain of surface hardness of titanium alloy OT4-1 as a result of interaction with rarefied gas medium

650 25 47 60.2 24 41 54 22 38 48.8 700 5 9 11.8 3 5 6.7 1 2 2.5 750 0.45 0.77 1 0.4 0.8 0.76 0.4 0.8 1

, and l increase with the increasing of saturation time

saturated layer H<sup>μ</sup>

Table 8.

Table 7.

containing oxygen.

oxygen.

Table 9.

Table 10.

oxygen.

83

containing oxygen.

s , ΔH<sup>μ</sup> s Surface Treatment of Titanium Alloys in Oxygen-Containing Gaseous Medium DOI: http://dx.doi.org/10.5772/intechopen.82545


#### Table 7.

Gain of surface hardness of titanium alloy VT1-0 as a result of interaction with rarefied gas medium containing oxygen.


#### Table 8.

Gain of surface hardness of titanium alloy VT5 as a result of interaction with rarefied gas medium containing oxygen.


#### Table 9.

that was measured on both surface and cross section of metallographic sample made of the gas-saturated specimen and changing parameters of crystallographic lattice. The last one method is more complex and laborious; therefore, it was used only for determination of parameters of maximally hardened layer. Depth of gas-saturated layer recognizes as a distance from the surface where increasing of hardness caused

Changing of ratio of parameters of crystallographic lattice (c/a) of titanium alloy surface layer VT1-0 as a function of CTT regime: (1) in initial state, (2) after CTT: <sup>Т</sup> = 650°С, <sup>P</sup> = 6.6 <sup>10</sup><sup>3</sup> Pa, <sup>τ</sup> = 5 h, (3) after

thermodiffusion saturation on the gain of surface hardness K of investigated tita-

The results concerning the determination of parameters of crystallographic lattice of specimens of investigated alloys after different regimes of thermodiffusion

<sup>s</sup> <sup>H</sup><sup>μ</sup> c )/H<sup>μ</sup> с

<sup>c</sup> is the bulk hardness) and depth of gas-

) 100%, where

by dissolution of oxygen is equal to the measurement error.

Alloy <sup>P</sup> <sup>10</sup><sup>2</sup> Pa At <sup>Т</sup> = 700°<sup>С</sup>

Titanium Alloys - Novel Aspects of Their Manufacturing and Processing

1.33 9.57 1.53 6.6 15.2 2.24

1.33 6.02 0.90 6.6 7.57 1.68

1.33 2.86 0.74 6.6 4.41 1.05

1.33 5.00 0.41 6.6 11.56 0.77

Kinetic parameters of gas saturation of titanium alloys under isothermal conditions.

Using formula (3) Using formula (4)

ВТ1-0 0.66 7.46 1.25 24.357 3.3804 1.05

ВТ5 0.66 5.3 0.64 10.243 0.98 0.77

ВТ16 0.66 2.32 0.24 6.8862 0.9116 0.37

ОТ4-1 0.66 0.16 0.063 25.189 4.8973 2.11

kT,gm<sup>2</sup> h<sup>1</sup> F,gm<sup>2</sup> H,gm<sup>2</sup> h<sup>1</sup> J,gm<sup>2</sup> h<sup>1</sup> K,gm<sup>2</sup> h<sup>1</sup>

nium alloys are presented in Tables 7–14 (K = ((H<sup>μ</sup>

<sup>s</sup> is the surface hardness of metal and H<sup>μ</sup>

saturated zone l determined by durometry.

CTT: <sup>Т</sup> = 750°С, <sup>P</sup> = 6.6 <sup>10</sup><sup>3</sup> Pa, <sup>τ</sup> = 5 h.

saturation are presented in Tables 15–18.

H<sup>μ</sup>

82

Table 6.

Figure 2.

The experimentally obtained results of influence of parameters of

Gain of surface hardness of titanium alloy OT4-1 as a result of interaction with rarefied gas medium containing oxygen.


#### Table 10.

Gain of surface hardness of titanium alloy VT16 as a result of interaction with rarefied gas medium containing oxygen.

The regularities of thermodiffusion saturation intrinsic for all investigated alloys were revealed basing on the analysis of obtained results, namely, parameters of gassaturated layer H<sup>μ</sup> s , ΔH<sup>μ</sup> s , and l increase with the increasing of saturation time


#### Table 11.

Dimension of gas-saturated layer on titanium alloy VT1-0 as a result of interaction with rarefied gas medium containing oxygen.


#### Table 12.

Dimension of gas-saturated layer on titanium alloy VT5 as a result of interaction with rarefied gas medium containing oxygen.


#### Table 13.

Dimension of gas-saturated layer on titanium alloy OT4-1 as a result of interaction with rarefied gas medium containing oxygen.


#### Table 14.

Dimension of gas-saturated layer on titanium alloy VT16 as a result of interaction with rarefied gas medium containing oxygen.

The first regularity is connected with the increasing of concentration of interstitial impurity in surface layer of metal with time and its penetration on the greater depth. The second regularity can be explained by acceleration of withdrawal of interstitial impurities from the surface due to the increasing of its diffusivity in αand β-titanium with the increasing of temperature (Figure 6). Under such conditions, the flow of oxygen from the medium to the metal surface becomes smaller in comparison with withdrawal flow owing to diffusion from the surface into the

Changing of parameter of crystallographic lattice of alloy VT16 as a result of interaction with rarefied gas

Operating mode of CTT a c c/a Initial state 2.9481 4.6842 1.5889 750°С, 5.3 <sup>10</sup><sup>4</sup> Pa, 5 h 2.9484 4.6860 1.5893 750°С, 1.3 <sup>10</sup><sup>2</sup> Pa, 5 h 2.9496 4.6979 1.5927 750°С, 6.6 <sup>10</sup><sup>2</sup> Pa, 5 h 2.948 4.7248 1.6027

Surface Treatment of Titanium Alloys in Oxygen-Containing Gaseous Medium

DOI: http://dx.doi.org/10.5772/intechopen.82545

Changing of parameter of crystallographic lattice of alloy VT1-0 as a result of interaction with rarefied gas

Operating mode of CTT a c c/a Initial state 2.9286 4.6746 1.5962 750°С, 6,.6 <sup>10</sup><sup>3</sup> Pa, 5 h 2.9281 4.6753 1.5967 750°С, 1.3 <sup>10</sup><sup>2</sup> Pa, 5 h 2.9263 4.6729 1.5968 750°С, 6.6 <sup>10</sup><sup>2</sup> Pa, 5 h 2.9329 4.7050 1.6042

Changing of parameter of crystallographic lattice of alloy VT5 as a result of interaction with rarefied gas

Changing of parameter of crystallographic lattice of alloy OT4-1 as a result of interaction with rarefied gas

Operating mode of CTT a c c/a Beta Initial state 2.9287 4.6674 1.5936 3.2265 750°С, 6.6 <sup>10</sup><sup>3</sup> Pa, 5 h 2.9298 4.6707 1.5942 3.2266 750°С, 1.3 <sup>10</sup><sup>2</sup> Pa, 5 h 2.9284 4.6687 1.5942 3.2273 750°С, 6.6 <sup>10</sup><sup>2</sup> Pa, 5 h 2.9301 4.6706 1.5940 3.2280

Operating mode of CTT a c c/a Initial state 2.9427 4.6823 1.5911 750°С, 6.6 <sup>10</sup><sup>3</sup> Pa, 5 h 2.9419 4.6856 1.5927 750°С, 1.3 <sup>10</sup><sup>2</sup> Pa, 5 h 2.9426 4.6879 1.5931 750°С, 6.6 <sup>10</sup><sup>2</sup> Pa, 5 h 2.9426 4.6911 1.5942

metal depth.

85

Table 15.

Table 16.

Table 17.

Table 18.

medium containing oxygen.

medium containing oxygen.

medium containing oxygen.

medium containing oxygen.

under the same pressure of gas medium and temperature (Figures 3 and 4); the depth of gas-saturated zone l is increased, and value of relative gain of surface hardness ΔH<sup>μ</sup> <sup>s</sup> is decreased with the increasing of saturated temperature in the range 650–750°C (Figure 5).

Surface Treatment of Titanium Alloys in Oxygen-Containing Gaseous Medium DOI: http://dx.doi.org/10.5772/intechopen.82545


Table 15.

Changing of parameter of crystallographic lattice of alloy VT1-0 as a result of interaction with rarefied gas medium containing oxygen.


#### Table 16.

Changing of parameter of crystallographic lattice of alloy VT5 as a result of interaction with rarefied gas medium containing oxygen.


#### Table 17.

Changing of parameter of crystallographic lattice of alloy OT4-1 as a result of interaction with rarefied gas medium containing oxygen.


#### Table 18.

Changing of parameter of crystallographic lattice of alloy VT16 as a result of interaction with rarefied gas medium containing oxygen.

The first regularity is connected with the increasing of concentration of interstitial impurity in surface layer of metal with time and its penetration on the greater depth. The second regularity can be explained by acceleration of withdrawal of interstitial impurities from the surface due to the increasing of its diffusivity in αand β-titanium with the increasing of temperature (Figure 6). Under such conditions, the flow of oxygen from the medium to the metal surface becomes smaller in comparison with withdrawal flow owing to diffusion from the surface into the metal depth.

under the same pressure of gas medium and temperature (Figures 3 and 4); the depth of gas-saturated zone l is increased, and value of relative gain of surface

Dimension of gas-saturated layer on titanium alloy VT16 as a result of interaction with rarefied gas medium

650 19 48 71 22 53 76 26 60 85 700 3 10 33 5 40 70 27 73 109 750 2 5 40 1 55 100 37 108 163

T, °C l (μm) at residual pressure of gas medium

Titanium Alloys - Novel Aspects of Their Manufacturing and Processing

T, °C l (μm) at residual pressure of gas medium

T, °C l (μm) at residual pressure of gas medium

T, °C l (μm) at residual pressure of gas medium

<sup>P</sup> = 6.6 <sup>10</sup><sup>3</sup> Pa <sup>P</sup> = 1.33 <sup>10</sup><sup>2</sup> Pa <sup>P</sup> = 6.6<sup>10</sup><sup>2</sup> Pa 1h 3h 5h 1h 3h 5h 1h 3h 5h

650 6 13 18 7 14 20 8 16 22 700 10 22 30 11 24 33 13 27 36 750 19 38 53 21 41 57 24 46 62

Dimension of gas-saturated layer on titanium alloy VT1-0 as a result of interaction with rarefied gas medium

650 5 11 16 6 12 18 7 14 20 700 7 17 25 9 20 28 11 22 31 750 8 22 32 12 28 40 17 35 49

Dimension of gas-saturated layer on titanium alloy VT5 as a result of interaction with rarefied gas medium

650 13 22 29 26 45 58 36 62 80 700 20 35 45 33 57 73 45 77 100 750 30 52 67 40 70 90 54 93 120

Dimension of gas-saturated layer on titanium alloy OT4-1 as a result of interaction with rarefied gas medium

<sup>P</sup> = 6.6 <sup>10</sup><sup>3</sup> Pa <sup>P</sup> = 1.33 <sup>10</sup><sup>2</sup> Pa <sup>P</sup> = 6.6 <sup>10</sup><sup>2</sup> Pa 1h 3h 5h 1h 3h 5h 1h 3h 5h

<sup>P</sup> = 6.6 <sup>10</sup><sup>3</sup> Pa <sup>P</sup> = 1.33 <sup>10</sup><sup>2</sup> Pa <sup>P</sup> = 6.6 <sup>10</sup><sup>2</sup> Pa 1h 3h 5h 1h 3h 5h 1h 3h 5h

<sup>P</sup> = 6.6 <sup>10</sup><sup>3</sup> Pa <sup>P</sup> = 1.33 <sup>10</sup><sup>2</sup> Pa <sup>P</sup> = 6.6 <sup>10</sup><sup>2</sup> Pa 1h 3h 5h 1h 3h 5h 1h 3h 5h

<sup>s</sup> is decreased with the increasing of saturated temperature in the

hardness ΔH<sup>μ</sup>

84

containing oxygen.

Table 14.

Table 11.

Table 12.

Table 13.

containing oxygen.

containing oxygen.

containing oxygen.

range 650–750°C (Figure 5).

Figure 3.

Changing of the relative gain of surface hardness of titanium alloy VT1-0 under CTT (Т = 750°С, <sup>P</sup> = 1.3 <sup>10</sup><sup>3</sup> Pa) depending on exposure time.

The phase composition of titanium alloys influences essentially on the quantitative index of saturation at not influencing on the qualitative appearance of determined regularities. The maximal dissolution of oxygen in α-phase of titanium composes 33 at.%, while in β-phase, only 6 at.%. Therefore in the titanium alloys with a large content of α-phase (VT1-0, VT5, and near-α-alloy OT4-1) during thermodiffusion saturation in gas medium, the gas-saturated layers with high

Gas-saturated layer with high gradient of hardness is formed on the (α + β)-alloy

The values of coefficient K representing the relative gain of surface hardness are higher sufficiently for α-alloys VT1-0 and VT5 in comparison with near-α-alloy OT4-1 and (α + β)-alloy VT16 under the same conditions of thermodiffusion saturation. On the other hand, the oxygen diffusion coefficient in β-titanium is high

Gas saturation and sublimation influence during thermodiffusion saturation not only the hardness but also the change of the state of surface and phase-structural state of near-surface layer of metal. Thus, because of sublimation and surface diffusion, the grain boundaries showed up; on some grains the characteristic step-

Distribution of microhardness in the surface layer of alloy (a) VT5 and (b) OT4-1 after CTT (Т = 750°С,

/с at 800°С [3, 4]). Therefore, with the increasing in the alloys

/с;

of order of magnitude in comparison with <sup>α</sup>-phase (Dβ-Ti = 3 <sup>10</sup><sup>10</sup> cm2

treatment under condition <sup>Т</sup> = 750°С, <sup>P</sup> = 6.6 <sup>10</sup><sup>2</sup> Pa, <sup>τ</sup> = 5 h).

of volumetric content of β-phase, the depth of gas-saturated zone l is increased. Thus on the surface of alloy VT16, the gas-saturated layers with a depth 2–2.5 times bigger than α-alloy VT1-0 are formed (160 and 60 μm correspondingly after

gradient of hardness are formed (Figure 7).

like microrelief is developed (Figure 9).

<sup>D</sup>α-Ti = 2 <sup>10</sup><sup>9</sup> <sup>с</sup>m<sup>2</sup>

Figure 7.

87

<sup>P</sup> = 6.6 <sup>10</sup><sup>2</sup> Pa, <sup>τ</sup> = 5 h).

Figure 6.

VT16 at the low temperatures of saturation (Figure 8).

Temperature dependence of oxygen diffusion coefficient in titanium [3].

Surface Treatment of Titanium Alloys in Oxygen-Containing Gaseous Medium

DOI: http://dx.doi.org/10.5772/intechopen.82545

#### Figure 4.

Distribution of microhardness through the section of the specimens of alloy VT1-0 after CTT (<sup>P</sup> = 1.3 <sup>10</sup><sup>3</sup> Pa, <sup>τ</sup> = 5 h) at temperatures (1) 650°С, (2) 700°С, (3) 750°С.

#### Figure 5.

Changing of the gain of surface hardness depending on interaction temperature with gas medium: (a) <sup>P</sup> = 6.6 <sup>10</sup><sup>3</sup> Pa, (b) <sup>P</sup> = 1.3 <sup>10</sup><sup>2</sup> Pa, (c) <sup>P</sup> = 6.6 <sup>10</sup><sup>2</sup> Pa.

Surface Treatment of Titanium Alloys in Oxygen-Containing Gaseous Medium DOI: http://dx.doi.org/10.5772/intechopen.82545

#### Figure 6.

Figure 3.

Figure 4.

Figure 5.

86

<sup>P</sup> = 1.3 <sup>10</sup><sup>3</sup> Pa) depending on exposure time.

Changing of the relative gain of surface hardness of titanium alloy VT1-0 under CTT (Т = 750°С,

Titanium Alloys - Novel Aspects of Their Manufacturing and Processing

Distribution of microhardness through the section of the specimens of alloy VT1-0 after CTT (<sup>P</sup> = 1.3 <sup>10</sup><sup>3</sup> Pa, <sup>τ</sup> = 5 h) at temperatures (1) 650°С, (2) 700°С, (3) 750°С.

Changing of the gain of surface hardness depending on interaction temperature with gas medium:

(a) <sup>P</sup> = 6.6 <sup>10</sup><sup>3</sup> Pa, (b) <sup>P</sup> = 1.3 <sup>10</sup><sup>2</sup> Pa, (c) <sup>P</sup> = 6.6 <sup>10</sup><sup>2</sup> Pa.

Temperature dependence of oxygen diffusion coefficient in titanium [3].

The phase composition of titanium alloys influences essentially on the quantitative index of saturation at not influencing on the qualitative appearance of determined regularities. The maximal dissolution of oxygen in α-phase of titanium composes 33 at.%, while in β-phase, only 6 at.%. Therefore in the titanium alloys with a large content of α-phase (VT1-0, VT5, and near-α-alloy OT4-1) during thermodiffusion saturation in gas medium, the gas-saturated layers with high gradient of hardness are formed (Figure 7).

Gas-saturated layer with high gradient of hardness is formed on the (α + β)-alloy VT16 at the low temperatures of saturation (Figure 8).

The values of coefficient K representing the relative gain of surface hardness are higher sufficiently for α-alloys VT1-0 and VT5 in comparison with near-α-alloy OT4-1 and (α + β)-alloy VT16 under the same conditions of thermodiffusion saturation. On the other hand, the oxygen diffusion coefficient in β-titanium is high of order of magnitude in comparison with <sup>α</sup>-phase (Dβ-Ti = 3 <sup>10</sup><sup>10</sup> cm2 /с; <sup>D</sup>α-Ti = 2 <sup>10</sup><sup>9</sup> <sup>с</sup>m<sup>2</sup> /с at 800°С [3, 4]). Therefore, with the increasing in the alloys of volumetric content of β-phase, the depth of gas-saturated zone l is increased. Thus on the surface of alloy VT16, the gas-saturated layers with a depth 2–2.5 times bigger than α-alloy VT1-0 are formed (160 and 60 μm correspondingly after treatment under condition <sup>Т</sup> = 750°С, <sup>P</sup> = 6.6 <sup>10</sup><sup>2</sup> Pa, <sup>τ</sup> = 5 h).

Gas saturation and sublimation influence during thermodiffusion saturation not only the hardness but also the change of the state of surface and phase-structural state of near-surface layer of metal. Thus, because of sublimation and surface diffusion, the grain boundaries showed up; on some grains the characteristic steplike microrelief is developed (Figure 9).

#### Figure 7.

Distribution of microhardness in the surface layer of alloy (a) VT5 and (b) OT4-1 after CTT (Т = 750°С, <sup>P</sup> = 6.6 <sup>10</sup><sup>2</sup> Pa, <sup>τ</sup> = 5 h).

#### Figure 8.

Distribution of microhardness through the section of specimens of alloy VT16 after CTT (<sup>P</sup> = 1.3 <sup>10</sup><sup>2</sup> Pa, τ = 5 h) at temperatures (a) 650°С and (b) 750°С.

of metal and changing of its chemical composition on the alloy OT4-1, but on the

Distribution of alloying elements in the surface layer of alloys (a) OT4-1, (b) VT5, and (c) VT16 after CTT

2.3 Effect of oxygen partial pressure on the surface hardness and hardening

Apart from the temperature-time parameters, the gas-dynamical parameters of gas medium (partial pressure of chemically active components and dynamics—I of leaking) influence the level of surface hardening. This influence should be taken into account during prediction of the consequences of thermodiffusion saturation of

Let us consider in detail the influence of the changing of pressure of gas medium

hardness and depth of hardened zone of most of titanium alloys are increased with the increasing of pressure under constant temperature accordingly with the obtained results (Tables 13 and 14 and Figure 12). This is most appreciably for αtitanium alloys VT1-0 and VT5. The derivation from the regularity mentioned above is observed for near-α-alloy OT4-1 and (α + β)-alloy VT16 alloyed by elements with high volatility of oxides under saturation at 650°C (OT4-1, VT16) and 700°C (VT16)—decreasing of gain of surface hardness. In our opinion, this is caused by activation of sublimation process of Mn and V at relatively low intensity of gas saturation process and small solubility of oxygen in β-phase of titanium. At higher temperatures, the increasing of pressure of gas medium leads to the changing

of tendencies in gain of surface hardness: the change in the inclination of

temperature dependences is observed that can be connected with the increasing of oxygen flow from the medium, which becomes commensurable with the flow of

Thermodiffusion saturation was performed under dynamic conditions of rarefied gas medium. That is to say, the residual pressure of medium is determined by dynamic equilibrium of gas flows pumped out and leaking into the reaction camera from the outside. The rate of leaking should be restricted because the increase of the

, 6.6 <sup>10</sup><sup>3</sup> Pa, <sup>I</sup> = 5 <sup>10</sup><sup>5</sup> Pa s<sup>1</sup>

). The surface

other alloys, such changes are not observed (Figure 11).

Surface Treatment of Titanium Alloys in Oxygen-Containing Gaseous Medium

DOI: http://dx.doi.org/10.5772/intechopen.82545

titanium alloys in the rarefied gas medium.

, 1.33 <sup>10</sup><sup>2</sup>

withdrawal due to diffusion (Figure 12c).

zone depth

(<sup>Т</sup> = 750°С, <sup>P</sup> = 6.6 <sup>10</sup><sup>2</sup> Pa, <sup>τ</sup> = 5 h).

Figure 11.

(<sup>P</sup> = 6.6 <sup>10</sup><sup>2</sup>

89

#### Figure 9.

Microstructure of top surface of alloy VT1-0 after CTT (<sup>Т</sup> = 700°С, <sup>P</sup> = 6.6 <sup>10</sup><sup>3</sup> Pa, <sup>τ</sup> = 5 h). Larger indentation (loading 0.49 N) is obtained before thermal treatment, the smaller one after thermal treatment.

Gas saturation stabilizes the α-phase of titanium in the surface layer of metal and increases its hardness. The diffusion layer consists of α-phase rich layer and transition layer. The α-phase rich layer differs in structure from the base metal by increased content of α-phase that is easily revealed with metallography (Figure 10a). This layer is represented often by only one α-phase. Transition layer is not visibly different from the base metal (Figure 10b), but for this layer the larger hardness is inherent.

Saturation of surface layer by oxygen and in some cases the sublimation of alloying elements lead to the redistribution of alloying elements in the surface layer

#### Figure 10.

Microstructure of surface layer of alloy OT4-1 (a) and VT1-0 (b) after CTT (<sup>Т</sup> = 750°С, <sup>P</sup> = 6.6 <sup>10</sup><sup>3</sup> Pa, τ = 5 h).

Surface Treatment of Titanium Alloys in Oxygen-Containing Gaseous Medium DOI: http://dx.doi.org/10.5772/intechopen.82545

#### Figure 11.

Gas saturation stabilizes the α-phase of titanium in the surface layer of metal and increases its hardness. The diffusion layer consists of α-phase rich layer and transition layer. The α-phase rich layer differs in structure from the base metal by increased content of α-phase that is easily revealed with metallography

Microstructure of top surface of alloy VT1-0 after CTT (<sup>Т</sup> = 700°С, <sup>P</sup> = 6.6 <sup>10</sup><sup>3</sup> Pa, <sup>τ</sup> = 5 h). Larger indentation (loading 0.49 N) is obtained before thermal treatment, the smaller one after thermal treatment.

Distribution of microhardness through the section of specimens of alloy VT16 after CTT (<sup>P</sup> = 1.3 <sup>10</sup><sup>2</sup> Pa,

Titanium Alloys - Novel Aspects of Their Manufacturing and Processing

(Figure 10a). This layer is represented often by only one α-phase. Transition layer is not visibly different from the base metal (Figure 10b), but for this layer the

Saturation of surface layer by oxygen and in some cases the sublimation of alloying elements lead to the redistribution of alloying elements in the surface layer

Microstructure of surface layer of alloy OT4-1 (a) and VT1-0 (b) after CTT (<sup>Т</sup> = 750°С, <sup>P</sup> = 6.6 <sup>10</sup><sup>3</sup> Pa,

larger hardness is inherent.

Figure 9.

Figure 10.

τ = 5 h).

88

Figure 8.

τ = 5 h) at temperatures (a) 650°С and (b) 750°С.

Distribution of alloying elements in the surface layer of alloys (a) OT4-1, (b) VT5, and (c) VT16 after CTT (<sup>Т</sup> = 750°С, <sup>P</sup> = 6.6 <sup>10</sup><sup>2</sup> Pa, <sup>τ</sup> = 5 h).

of metal and changing of its chemical composition on the alloy OT4-1, but on the other alloys, such changes are not observed (Figure 11).

### 2.3 Effect of oxygen partial pressure on the surface hardness and hardening zone depth

Apart from the temperature-time parameters, the gas-dynamical parameters of gas medium (partial pressure of chemically active components and dynamics—I of leaking) influence the level of surface hardening. This influence should be taken into account during prediction of the consequences of thermodiffusion saturation of titanium alloys in the rarefied gas medium.

Let us consider in detail the influence of the changing of pressure of gas medium (<sup>P</sup> = 6.6 <sup>10</sup><sup>2</sup> , 1.33 <sup>10</sup><sup>2</sup> , 6.6 <sup>10</sup><sup>3</sup> Pa, <sup>I</sup> = 5 <sup>10</sup><sup>5</sup> Pa s<sup>1</sup> ). The surface hardness and depth of hardened zone of most of titanium alloys are increased with the increasing of pressure under constant temperature accordingly with the obtained results (Tables 13 and 14 and Figure 12). This is most appreciably for αtitanium alloys VT1-0 and VT5. The derivation from the regularity mentioned above is observed for near-α-alloy OT4-1 and (α + β)-alloy VT16 alloyed by elements with high volatility of oxides under saturation at 650°C (OT4-1, VT16) and 700°C (VT16)—decreasing of gain of surface hardness. In our opinion, this is caused by activation of sublimation process of Mn and V at relatively low intensity of gas saturation process and small solubility of oxygen in β-phase of titanium. At higher temperatures, the increasing of pressure of gas medium leads to the changing of tendencies in gain of surface hardness: the change in the inclination of temperature dependences is observed that can be connected with the increasing of oxygen flow from the medium, which becomes commensurable with the flow of withdrawal due to diffusion (Figure 12c).

Thermodiffusion saturation was performed under dynamic conditions of rarefied gas medium. That is to say, the residual pressure of medium is determined by dynamic equilibrium of gas flows pumped out and leaking into the reaction camera from the outside. The rate of leaking should be restricted because the increase of the

ð6Þ

ð7Þ

Mass changing of specimen under saturation by oxygen, dimension of diffusion zone up to 150 μm, and specimen thickness 3 mm per unit area is approximated by

In the framework of physico-mathematical model of interaction of solid with gaseous medium under conditions of third type at the metal/gas interface, the coefficients of phase-boundary reaction (α) for alpha-titanium alloys VT1-0 and

The proposed approach for describing of the gas saturation processes with the use of coefficient of phase-boundary reaction α allows to calculate the mass change and concentration distribution of diffusant in the surface layer of metal under various temperature-time regimes of thermodiffusion saturation of α-titanium alloys, to determine the characteristics of gas-saturated layer (hardness distribution, depth of hardened zone) by using known correlation between hardness of surface layer and concentration of oxygen. The calculated nomograms for determination of permissible parameters of CTT of alloys VT1-0 and VT5 under condition of regulated level of surface hardening K = 25% which is presented

The obtained analytical data are in a good accordance with the experimental results that allow using this approach for evaluation and prediction of parameters of

and (α + β)-titanium alloys is necessary since at the selected temperatures the change in the ratio of phase components in alloys and sublimation of alloying

<sup>650</sup> 5.017 � <sup>10</sup>�<sup>9</sup> 5.792 � <sup>10</sup>�<sup>9</sup> 7.565 � <sup>10</sup>�<sup>9</sup> <sup>700</sup> 1.206 � <sup>10</sup>�<sup>8</sup> 1.393 � <sup>10</sup>�<sup>8</sup> 1.819 � <sup>10</sup>�<sup>8</sup> <sup>750</sup> 2.662 � <sup>10</sup>�<sup>8</sup> 3.073 � <sup>10</sup>�<sup>8</sup> 4.014 � <sup>10</sup>�<sup>8</sup>

<sup>650</sup> 1.76 � <sup>10</sup>�<sup>9</sup> 2.04 � <sup>10</sup>�<sup>9</sup> 2.66 � <sup>10</sup>�<sup>9</sup> <sup>700</sup> 4.48 � <sup>10</sup>�<sup>9</sup> 5.18 � <sup>10</sup>�<sup>9</sup> 6.76 � <sup>10</sup>�<sup>9</sup> <sup>750</sup> 1.04 � <sup>10</sup>�<sup>8</sup> 1.20 � <sup>10</sup>�<sup>8</sup> 1.56 � <sup>10</sup>�<sup>8</sup>

T, °C α (cm/s) at residual pressure of gas medium

T, °C α (cm/s) at residual pressure of gas medium

The additional investigation for determination of model dependences for near-α

<sup>P</sup> = 6.6 � <sup>10</sup>�<sup>3</sup> Pa <sup>P</sup> = 1.33 � <sup>10</sup>�<sup>2</sup> Pa <sup>P</sup> = 6.6 � <sup>10</sup>�<sup>2</sup> Pa

<sup>P</sup> = 6.6 � <sup>10</sup>�<sup>3</sup> Pa <sup>P</sup> = 1.33 � <sup>10</sup>�<sup>2</sup> Pa <sup>P</sup> = 6.6 � <sup>10</sup>�<sup>2</sup> Pa

thermodiffusion hardening of surface layer of α-titanium alloys.

elements during gas saturation are possible.

Coefficients of phase-boundary reaction (α) for titanium alloy VT1-0.

Coefficients of phase-boundary reaction (α) for titanium alloy VT5.

VT5 (see Tables 19 and 20) are determined using the experimental data

Surface Treatment of Titanium Alloys in Oxygen-Containing Gaseous Medium

DOI: http://dx.doi.org/10.5772/intechopen.82545

expression [5]:

(Tables 1 and 2).

in Figure 13.

Table 19.

Table 20.

91

Figure 12.

The relative gain of surface hardness of titanium alloys for 5 h depending on the oxygen pressure in rarefied gas medium at temperatures (a) 650°С, (b) 700°С, and (c) 750°С.

flow of the leaking gases influences the kinetics of interaction similarly to the increasing of pressure [5]. It should be noticed that all mentioned results are obtained under the conditions of low enough specific rate of leaking into the reaction chamber of vacuum equipment—<sup>I</sup> = 5 � <sup>10</sup>�<sup>5</sup> Pa s�<sup>1</sup> . The increasing of leaking into the vacuum system intensifies sufficiently the oxidation and gas saturation of titanium alloys VT1-0, VT5, OT4-1, and VT16; however the regularities of this factor were not studied enough.

### 2.4 Relationship between the treatment parameters (pressure, temperature, duration) and level of interstitial solid

To predict the parameters of surface hardening of titanium alloys as a function of thermodiffusion saturation by interstitial impurities, the improved physicomathematical model of gas saturation of titanium alloys in rarefied gas medium is proposed for using [6].

Intensity of thermodiffusion processes is determined by phase-boundary reaction and concentration distribution of diffusant and diffusion coefficient. Phaseboundary reaction consists of a number of processes occurring in vacuum: adsorption of molecules and atoms of gases of residual atmosphere, their dissolution in the metal, oxidation, etc.; rate of this reaction is changed depending on the degree of medium discharging. Actually, the boundary concentration of oxygen on the surface of the metal is not being steadied at once but is being increased gradually with the rate depending on the degree of gas medium discharging. Based on the thermodynamic analysis (see first milestone), the titanium oxides are in the equilibrium with pressure of oxygen at all degrees of discharging that provides the boundary solubility of oxygen in α-titanium, С<sup>0</sup> = 33 аt.% О2, that causes formation of stoichiometric oxides [3, 4]. Thus the calculation of diffusion saturation of metal by oxygen must be performed basing on the next boundary condition [6]:

$$-D\frac{d\mathcal{L}}{d\mathcal{L}} = \alpha \cdot |C\_{\cdot \cdot} - C(\vee, t)| \quad , \quad \mathcal{N} = 0 \tag{5}$$

where α is the coefficient of rate of phase-boundary reaction, C<sup>0</sup> is the equilibrium concentration of oxygen in metal, C(0, t) is the actual concentration, and t is the time. These boundary conditions describe the mass change for period of time before the formation of oxide with certain thickness, which can already influence substantially the rate of the processes.

The function C(x, t) is the known solution of the diffusion task:

Surface Treatment of Titanium Alloys in Oxygen-Containing Gaseous Medium DOI: http://dx.doi.org/10.5772/intechopen.82545

$$\mathcal{L}(\mathbf{x}, t) = \mathcal{L}\_{\mathbf{y}} \cdot \left[ \mathbf{e} \operatorname{\mathbf{r}} \mathbf{f} \mathbf{z} \left( \mathbf{x} \big[ \operatorname{\mathbf{U}} \big[ \operatorname{\mathbf{U}} \big] \big] - \mathbf{e} \operatorname{\mathbf{u}} \big[ \operatorname{\mathbf{L}} \mathbf{v} - \operatorname{\mathbf{h}}^{\mathsf{T}} \operatorname{\mathbf{D}} \mathbf{t} \big] \cdot \operatorname{\mathbf{e}} \mathbf{v} \left[ \operatorname{\mathbf{L}} \big[ \operatorname{\mathbf{L}} \big] \big] \mathbf{v} + \operatorname{\mathbf{h}} \big[ \operatorname{\mathbf{D}} \big] \big] \mathbf{v} \right], \text{ where } \operatorname{\mathbf{h}} = \boldsymbol{\alpha} \cdot \boldsymbol{\mathsf{D}} \qquad (\boldsymbol{\Theta}) \text{ is the } \mathbf{z} \text{ matrix} \right]$$

Mass changing of specimen under saturation by oxygen, dimension of diffusion zone up to 150 μm, and specimen thickness 3 mm per unit area is approximated by expression [5]:

$$M(t) = (C\_0 \wedge \hbar) |\exp(\hbar \bar{\epsilon} \cdot Dt) \text{erfc}(\hbar \sqrt{\mathcal{D}t}) - 1 \quad \exists \hbar \sqrt{\mathcal{D}t} / \pi \vert . \tag{7}$$

In the framework of physico-mathematical model of interaction of solid with gaseous medium under conditions of third type at the metal/gas interface, the coefficients of phase-boundary reaction (α) for alpha-titanium alloys VT1-0 and VT5 (see Tables 19 and 20) are determined using the experimental data (Tables 1 and 2).

The proposed approach for describing of the gas saturation processes with the use of coefficient of phase-boundary reaction α allows to calculate the mass change and concentration distribution of diffusant in the surface layer of metal under various temperature-time regimes of thermodiffusion saturation of α-titanium alloys, to determine the characteristics of gas-saturated layer (hardness distribution, depth of hardened zone) by using known correlation between hardness of surface layer and concentration of oxygen. The calculated nomograms for determination of permissible parameters of CTT of alloys VT1-0 and VT5 under condition of regulated level of surface hardening K = 25% which is presented in Figure 13.

The obtained analytical data are in a good accordance with the experimental results that allow using this approach for evaluation and prediction of parameters of thermodiffusion hardening of surface layer of α-titanium alloys.

The additional investigation for determination of model dependences for near-α and (α + β)-titanium alloys is necessary since at the selected temperatures the change in the ratio of phase components in alloys and sublimation of alloying elements during gas saturation are possible.


Table 19.

flow of the leaking gases influences the kinetics of interaction similarly to the increasing of pressure [5]. It should be noticed that all mentioned results are obtained under the conditions of low enough specific rate of leaking into the reac-

into the vacuum system intensifies sufficiently the oxidation and gas saturation of titanium alloys VT1-0, VT5, OT4-1, and VT16; however the regularities of this

The relative gain of surface hardness of titanium alloys for 5 h depending on the oxygen pressure in rarefied gas

2.4 Relationship between the treatment parameters (pressure, temperature,

To predict the parameters of surface hardening of titanium alloys as a function of thermodiffusion saturation by interstitial impurities, the improved physicomathematical model of gas saturation of titanium alloys in rarefied gas medium is

Intensity of thermodiffusion processes is determined by phase-boundary reaction and concentration distribution of diffusant and diffusion coefficient. Phaseboundary reaction consists of a number of processes occurring in vacuum: adsorption of molecules and atoms of gases of residual atmosphere, their dissolution in the metal, oxidation, etc.; rate of this reaction is changed depending on the degree of medium discharging. Actually, the boundary concentration of oxygen on the surface of the metal is not being steadied at once but is being increased gradually with the rate depending on the degree of gas medium discharging. Based on the thermodynamic analysis (see first milestone), the titanium oxides are in the equilibrium with pressure of oxygen at all degrees of discharging that provides the boundary solubility of oxygen in α-titanium, С<sup>0</sup> = 33 аt.% О2, that causes formation of stoichiometric oxides [3, 4]. Thus the calculation of diffusion saturation of metal by

where α is the coefficient of rate of phase-boundary reaction, C<sup>0</sup> is the equilibrium concentration of oxygen in metal, C(0, t) is the actual concentration, and t is the time. These boundary conditions describe the mass change for period of time before the formation of oxide with certain thickness, which can already influence

oxygen must be performed basing on the next boundary condition [6]:

The function C(x, t) is the known solution of the diffusion task:

. The increasing of leaking

ð5Þ

tion chamber of vacuum equipment—<sup>I</sup> = 5 � <sup>10</sup>�<sup>5</sup> Pa s�<sup>1</sup>

Titanium Alloys - Novel Aspects of Their Manufacturing and Processing

medium at temperatures (a) 650°С, (b) 700°С, and (c) 750°С.

duration) and level of interstitial solid

factor were not studied enough.

Figure 12.

proposed for using [6].

substantially the rate of the processes.

90

Coefficients of phase-boundary reaction (α) for titanium alloy VT1-0.


Table 20.

Coefficients of phase-boundary reaction (α) for titanium alloy VT5.

Figure 13.

Nomograms for determination of parameters of CTT of titanium alloys (a) VT1-0 and (b) VT5 (the curves correspond to the level of surface hardening K = 25%).
