PSI - Issue 47

Yurii Sharkeev et al. / Procedia Structural Integrity 47 (2023) 849–854 Yurii Sharkeev et al. / Structural Integrity Procedia 00 (2023) 000 – 000

851

3

obtain an ultrafine-grained state in the Ti – 45 wt.% Nb alloy. The final multi-pass rolling at the second stage forms an ultrafine-grained state, homogeneous over the bulk volume of the billet, with the average size of the structure elements equal to 0. 28±0 . 10 µm. To compare the mechanical properties and microstructure formed in the alloy under severe plastic deformation, the coarse-grained state in the alloy was formed during recrystallization annealing in vacuum at 800 °C for one hour . The average grain size in the coarse-grained alloy state was 44 ±16 μm. Table 1 demonstrates the mechanical properties of Ti – 45 wt.% Nb alloy in the coarse-grained and ultrafine-grained states. The yield strength and ultimate strength are 610 MPa and 920 MPa for ultrafine-grained specimens, respectively, which exceeds the yield strength and ultimate strength for coarse-grained specimens by 1.5 times. At the same time, the ultimate plasticity for the coarse-grained state is 15.0%, and for the ultrafine-grained state – 5.5%. However, it should be noted that the ultimate plasticity value for the UFG state satisfies the requirements for implant materials. Finally, the elastic modulus value did not change during severe plastic deformation.

Table 1. Mechanical properties of the Ti – 45 wt.% Nb alloy.

Elastic modulus E (GPa)

Plasticity  (%)

Microhardness H  (MPa)

Ultimate strength  в , (MPa)

Yield strength  0.2 (MPa)

State

Coarse-grained state

420

660

15.0

1740

50-60

Ultrafine-grained state

610

920

5.5

3000

58

Analysis of the results from Table 1 demonstrates that the implementation of the severe plastic deformation technique by a complex method, including multi-step pressing and multi-pass rolling in grooved rolls, followed by pre-recrystallization annealing, allowed to obtain ultrafine-grained state in the alloy, which provided a significant increase in mechanical properties while maintaining the elastic modulus and a moderate decrease in ultimate plasticity. Fig. 1 shows the results of cyclic tests of the Ti – 45 wt.% Nb alloy samples in gigacycle mode for coarse-grained (curve 1) and ultrafine-grained (curve 2) alloy states. The samples in the coarse-grained state failed after 10 7 cycles at the loading amplitude of 230 MPa. At the same time, samples in the ultrafine-grained state did not fail after 10 7 cycles at of 300 MPa. The fatigue limit based on 10 6 cycles of Ti-45Nb alloy specimens in the ultrafine-grained state was 380 MPa, which exceeds the fatigue limit of 280 MPa in the coarse-grained state by 1.4 times. The fatigue limit of ultrafine-grained specimens based on 10 9 cycles was 295 MPa, but in the coarse-grained state it was 195 MPa, which is 1.5 times higher. Thus, the ultrafine-grained structure in the alloy provides a significant increase in the fatigue limit when compared to the coarse-grained state.

200 240 280 320 360 400

2

 , MPa

1

10 6

10 7

10 8

10 9

10 10

N, Number of cycles

Fig. 1. Dependence of the number of cycles to failure on the stress amplitude for Ti – 45 wt.% Nb samples: 1 – coarse-grained samples 2 – ultrafine- grained samples.