3.1.1.4 Influence of depth of hardened zone

Relative gain of fatigue strength is being decreased with increasing of depth of hardened zone l under constant level of surface hardening K (Figures 18 and 19, Table 3). The higher level of K, the rather fatigue strength of alloy is being decreased with the rising of depth of hardened zone (Figure 19).

Solid solution hardening of surface layer of titanium alloys under conditions of thermodiffusion saturation by interstitial impurity (oxygen) can lead to the embitterment of hardened in this way layer and its brittle failure under conditions of the repeated loading. Because of this the investigations of fracture of samples were carried out after fatigue tests by rotating bending. Results of fractographical investigations of near-surface parts of fractures of titanium alloy VT1-0 samples with different levels of surface hardening after fatigue tests by rotating bending are presented in Figure 20.

#### Figure 20.

Fractograms of near-surface part of fractures of titanium alloy VT1-0 samples with different levels of surface hardening after fatigue tests by rotating bending: (a) K = 70%, l = 30 μm, σ<sup>1</sup> = 310 MPa; (b) K = 50%, l = 70 μm, σ<sup>1</sup> = 285 MPa; (c), (d), (e) K = 70%, l = 70 μm, σ<sup>1</sup> = 260 MPa.

words, fatigue strength has a maximum. Relative gain of fatigue strength Δσ<sup>1</sup> is the highest when K = 70%. Such character of changing σ<sup>1</sup> can be explained by the improvement of fatigue properties due to dissolution of oxygen in metal with formation of solid solution that is accompanied by the appearing of compressing stress. On the other hand, metal is embrittled due to solid solution hardening by oxygen dissolution. One or another factor dominates the defined conditions,

Fatigue strength of titanium alloy VT1-0, under rotating bending conditions, depending on depth of hardened

Fatigue strength of titanium alloy VT1-0, under rotating bending conditions, as a function of depth of hardened

Depth of hardened zone l, μm

Depth of hardened zone l, μm

1900 5 5 ≤225 0

1900 5 ≤5 ≤225 0

Fatigue strength σ1, MPa

Fatigue strength σ1, MPa Relative gain of fatigue strength Δσ1, %

Relative gain of fatigue strength Δσ1, %

Average relative gain of surface hardness К, %

R1 1800 2250 25 30–35 ≤245 8.9 R2 2700 50 30–35 ≤295 31.1 R3 3050 70 30–35 ≤310 37.7 R4 3420 90 30–35 ≤275 22.2

Fatigue strength of alloy VT1-0, under rotating bending conditions, depending on level of surface hardening K,

Average relative gain of surface hardness К, %

2. 1800 2700 50 30 ≤295 31.1 3. 2700 50 70 ≤285 28.4 4. 3050 70 30 ≤310 37.7 5. 3050 70 70 ≤260 15.5

zone, when level of surface hardening K is constant: (1) K = 50% (2) K = 70%.

Titanium Alloys - Novel Aspects of Their Manufacturing and Processing

Hardness of surface HS, MPa

when depth of hardened zone is l is constant.

Hardness of surface HS, MPa

zone l, when level of surface hardening K is constant.

Figure 19.

Table 22.

Table 23.

96
