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

852

4

Figure 2a shows the bright-field transmission electron microscope (TEM) images with corresponding selected area diffraction (SAD) pattern of the coarse-grained state of Ti-45Nb alloy. The microstructure is represented by matrix equiaxed subgrains of the β -phase and the dispersion-strengthened nanosized particles of ω -phase. Identification of the SAD patterns (Fig. 2 a, inset) shows the presence of high intensity reflexes from the main β -phase of the solid solution of titanium and niobium (bcc lattice). A distinctive feature of microdiffraction is the appearance of a group of additional low intensity reflexes , which are characteristic of the ω -phase (hexagonal primitive (hp) lattice). Microstructures of the coarse-grained Ti – 45 wt.% Nb alloy in the initial state and near the fracture surface after gigacycle test at  = 195 MPa and N = 1.7 · 10 9 cycles are shown in Fig. 2 b, c. The microstructure near the failure surface consisted of large equiaxed grains with an average size of 42 ± 2 µ m, as in the initial state with an average size of 44±16  m. In the coarse-grained state for the Ti-45 wt.% Nb alloy a net non-oriented dislocation substructure is observed (Fig. 2 b, indicated by arrows). A distinctive feature of the microstructure after fracture of the alloy in the coarse-grained state is a significant increase in the dislocation density and the formation of a developed dislocation substructure in the volume of β -phase matrix grains. Moreover, ellipsoidal ω -phase particles are present inside the β -phase grains (Fig. 2 c, the dark-field image).

a

110 

b

с

202 

112



220 

β

022 

dislocation substructure

002

ω

β

1.0  m

0.2  m

0.5  m

Fig. 2. (a) Initial state; (b, c) near failure surface after gigacycle test of coarse-grained Ti − 45 wt. % Nb alloy: (a, b) bright-field TEM images with SAD patterns; (c) dark-field TEM image.

The microstructure of the ultrafine-grained Ti – 45 wt.% Nb alloy is shown in Figure 3 а . SPD leads to grain refinement. The main structural elements are grains, subgrains and fragments. The TEM images reveal β -phase and α - phase subgrains, as well as ω - phase particles in the bulk of β -phase grains. When identifying the SAD pattern (Fig. 3 b), groups of reflexes from three phases were revealed: the high intensity reflexes from a solid solution of titanium and niobium  -phase, the low intensity reflexes from a non-equilibrium  -phase and the reflexes from the  -phase. The average size of the structural elements of the β -phase, α -phase, and ω -phase in the ultrafine-grained Ti-45Nb alloy was 0.28 ± 0.1 µm, 0.05± 0.02 µm and 10±2 nm, respectively. β

b 1 −  ( 101) 2 −  ( 101 ) 3 −  (101 ) 4 −  101) 5 −  ( 002 ) 6 −  ( 012 ) 7 −  ( 112 ) 8 −  ( 021) 9 −  ( 122) 10 −  ( 130 ) 11 −  ( 101 ) 12 −  (1 01)

a

d

c

b

β

a β + 

β

β

ω

3

β

7

9

5

α

β

1

2

dislocation substructure

4

11

8

6

10

12



α



α



0.2 µm

α 0.2 µ m

0.2 µ m

Fig. 3. (a, b) Initial failure surface and (c, d) near failure surface after gigacycle test of ultrafine-grained Ti – 45 wt. % Nb alloy: (a) bright field TEM image and (b) SAD pattern and phase identification; (c) bright-field TEM image with SAD pattern and (d) dark-field TEM image.