Hardness of matrix HO, MPa

1. 1800 (initial state)

Cup-shaped fracture mode is fixed on the fractograms of near-surface layer of samples that testify in ductile failure (Figure 20a–c). That is, the total embrittlement of surface layer does not take place even if the level of fatigue strength is maximal (Figure 7a). Brittle mode of fracture of thin (1…2 μm) near-surface layer is observed only on the specimen with K = 70%, l = 70 μm (Figure 20d and e). The time till origin of fatigue crack under alternate stress is being decreased providing that such layer exists on the samples and in turn the fatigue strength is being decreased. The decreasing of fatigue strength of alloy VT1-0 hardened by CTT with the increasing of depth of hardened zone l at a high level of surface hardening K is connected with this fact.

Thus, it can be concluded that for each of surface hardening level K of titanium alloy VT1-0 under conditions of thermodiffusion saturation in controlled gas medium, an optimal depth of hardened (gas saturated) zone l ensuring the highest level of fatigue characteristics exists. And vice versa for each depth of hardened zone, the optimal level of surface hardening exists. The aim of the next work stage is to search the optimal ratio of parameters K and l.

#### 3.1.2 α-Alloy VT5

The parameters of regimes of thermodiffusion saturation of titanium alloy VT5 in the controlled gas medium containing oxygen and parameters of surfacehardened layer and cross section hardness distribution are presented in Table 24.

#### 3.1.2.1 Influence of CTT on the fatigue properties of α-alloy VT5

The results of fatigue tests of samples of alloy VT5 after regulated surface hardening by thermodiffusion saturation in the gas medium are presented in Figures 21–23 and Table 25.

It should be noticed that in the presented case, the character of dependence of fatigue strength σ<sup>1</sup> on the level of surface hardening K has a maximum level which depends on the depth of hardened zone l (Figure 22). The relative gain of fatigue strength Δσ<sup>1</sup> of alloy VT5 reaches 20% under conditions K = 19%, l = 45–50 μm.

The fatigue strength is being decreased with the increasing of depth of gassaturated layer under constant K (Figure 23).

It should be a supposition that under the analyzing of dependences presented in Figures 22 and 23, the maximal gain of fatigue strength Δσ<sup>1</sup> for alloy VT5 can be reached by the creation of gas-saturated layer of parameters K ≈ 45–55%, l ≈ 40– 50 μm. Such parameters of gas-saturated layer can be determined as optimal parameters of the hardening of alloy VT5. It is the determination of optimal parameters of hardening to provide the highest gain of fatigue strength that is the aim of the next project step.

> The sensitivity of titanium alloys to the presence of gas-saturated layers is increased with the increasing of β-phase value. As a result the increasing of fatigue characteristics is attained under smaller values of relative gain K %. The value of fatigue strength gain is being decreased. Results of VT16 alloy tests can be a confir-

Parameters of VT5 titanium alloy surface-hardened layers and thermodiffusion saturation's regimes.

Surface Treatment of Titanium Alloys in Oxygen-Containing Gaseous Medium

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

Thus it can be concluded that for each level of surface hardening K of investi-

thermodiffusion saturation in controlled gas medium, the optimal depth l of hardened (gas-saturated) zone exists which provides the highest level of fatigue

gated titanium alloys VT1-0, VT5, OT4-1, and VT16 under conditions of

mation (Table 27).

Table 24.

99

#### 3.1.3 Near-α-alloy ОТ4-1 and (α + β)-alloy VT16

The regularities of the analogue presented above (see Tables 26 and 27) are observed for the near α-alloy OT4-1 and (α + β)-alloy VT16. The sufficient effect of solid solution hardening is observed for alloy OT4-1: relative fatigue strength gain Δσ<sup>1</sup> reaches 38% under relative gain of surface hardness K = 35%, l = 45–50 μm. It can be concluded that for titanium alloys with low or middle level of strength (VT1-0 and OT4-1 alloys), the positive effect of thermodiffusion saturation of metal surface layers by interstitial impurities is the highest: Δσ<sup>1</sup> = 35–40%.

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

Table 24. Parameters of VT5 titanium alloy surface-hardened layers and thermodiffusion saturation's regimes.

The sensitivity of titanium alloys to the presence of gas-saturated layers is increased with the increasing of β-phase value. As a result the increasing of fatigue characteristics is attained under smaller values of relative gain K %. The value of fatigue strength gain is being decreased. Results of VT16 alloy tests can be a confirmation (Table 27).

Thus it can be concluded that for each level of surface hardening K of investigated titanium alloys VT1-0, VT5, OT4-1, and VT16 under conditions of thermodiffusion saturation in controlled gas medium, the optimal depth l of hardened (gas-saturated) zone exists which provides the highest level of fatigue

Cup-shaped fracture mode is fixed on the fractograms of near-surface layer of samples that testify in ductile failure (Figure 20a–c). That is, the total embrittlement of surface layer does not take place even if the level of fatigue strength is maximal (Figure 7a). Brittle mode of fracture of thin (1…2 μm) near-surface layer is observed only on the specimen with K = 70%, l = 70 μm (Figure 20d and e). The time till origin of fatigue crack under alternate stress is being decreased providing that such layer exists on the samples and in turn the fatigue strength is being decreased. The decreasing of fatigue strength of alloy VT1-0 hardened by CTT with the increasing of depth of hardened zone l at a high level of surface hardening K is

Titanium Alloys - Novel Aspects of Their Manufacturing and Processing

Thus, it can be concluded that for each of surface hardening level K of titanium

The parameters of regimes of thermodiffusion saturation of titanium alloy VT5

in the controlled gas medium containing oxygen and parameters of surfacehardened layer and cross section hardness distribution are presented in Table 24.

The results of fatigue tests of samples of alloy VT5 after regulated surface hardening by thermodiffusion saturation in the gas medium are presented in

It should be noticed that in the presented case, the character of dependence of fatigue strength σ<sup>1</sup> on the level of surface hardening K has a maximum level which depends on the depth of hardened zone l (Figure 22). The relative gain of fatigue strength Δσ<sup>1</sup> of alloy VT5 reaches 20% under conditions K = 19%, l = 45–50 μm. The fatigue strength is being decreased with the increasing of depth of gas-

It should be a supposition that under the analyzing of dependences presented in Figures 22 and 23, the maximal gain of fatigue strength Δσ<sup>1</sup> for alloy VT5 can be reached by the creation of gas-saturated layer of parameters K ≈ 45–55%, l ≈ 40– 50 μm. Such parameters of gas-saturated layer can be determined as optimal parameters of the hardening of alloy VT5. It is the determination of optimal parameters of hardening to provide the highest gain of fatigue strength that is the aim of

The regularities of the analogue presented above (see Tables 26 and 27) are observed for the near α-alloy OT4-1 and (α + β)-alloy VT16. The sufficient effect of solid solution hardening is observed for alloy OT4-1: relative fatigue strength gain Δσ<sup>1</sup> reaches 38% under relative gain of surface hardness K = 35%, l = 45–50 μm. It can be concluded that for titanium alloys with low or middle level of strength (VT1-0 and OT4-1 alloys), the positive effect of thermodiffusion saturation of metal surface layers by interstitial impurities is the highest: Δσ<sup>1</sup> = 35–40%.

3.1.2.1 Influence of CTT on the fatigue properties of α-alloy VT5

alloy VT1-0 under conditions of thermodiffusion saturation in controlled gas medium, an optimal depth of hardened (gas saturated) zone l ensuring the highest level of fatigue characteristics exists. And vice versa for each depth of hardened zone, the optimal level of surface hardening exists. The aim of the next work stage is

connected with this fact.

3.1.2 α-Alloy VT5

Figures 21–23 and Table 25.

the next project step.

98

to search the optimal ratio of parameters K and l.

saturated layer under constant K (Figure 23).

3.1.3 Near-α-alloy ОТ4-1 and (α + β)-alloy VT16

#### Figure 21.

Fatigue curves of titanium alloy VT5, under rotating bending conditions, depending on level of surface hardening K, when depth of hardened (gas saturated) zone l is constant (a) l = 30–35 μm and (b) l = 60– 65 μm: (1) initial state K = 5%; l = 5–10 μm; (2) K = 32%; (3) K = 82%; (4) K = 34%; (5) K = 60%.

#### Figure 22.

Fatigue strength of titanium alloy VT5, under rotating bending conditions, as a function of level of surface hardening when depth of hardened zone (gas saturated) is constant: (1) l = 30–35 μm (2) l = 60–65 μm.

4. Conclusions
