Issue 78

A. P. Soldatenkov et alii, Fracture and Structural Integrity, 78 (2026) 248-259; DOI: 10.3221/IGF-ESIS.78.15

0.4–0.7 μ m, enlarging up to 0.5–1.0 μ m at T =520 °C (heat treatment 2). In the latter case, lamellar precipitates of the  /  - phase also increased from 20 up to 40 nm during the decomposition of the metastable β -phase formed by the RSR. According to the results of the X-ray diffraction analysis, the volume fraction of the  -phase enhanced from 58 up to 71 % with increasing the aging temperature from 450 to 520  C, while its internal stresses decreased almost threefold at virtually unchanged those of the β -phase (Tab. 2). These data were in good agreement with the thickening of lamellar precipitates of the  /  -phase revealed by SEM.

a

b

primary  -phase

secondary  -phase

1 µm

1 µm

c

d



 phase 58%

 phase 71%







I, a.u.

I, a.u.





























30 40 50 60 70 80 90

30 40 50 60 70 80 90

2  , deg

2  , deg

Figure 3: The SEM micrographs (a, b) and the X-ray diffraction patterns (c, d) of the samples after heat treatment 1 (a, c) and 2 (b, d).

Treatment

α / β phase ratio, %

Size of coherent scattering regions, nm

Lattice parameter, Å

Microdistortions

α

c α /a α

a β



 ε 2 

 ε

2 

α

β

RSR + aging 450 °C, 5 h RSR + aging 520 °C, 5 h

58/42

14

13

2.9226 4.6593 2.9240 4.6674

3.2050

1.5  10 -3

1.9  10 –3

71/29

45

34

3.2111

6.6  10 -4

1.5  10 -3

Table 2. The microstructure parameters of the treated samples.

The results of the tensile tests of the specimens cut out from the treated samples are presented in Fig. 4 and Tab. 3. The registered stress-strain diagrams show (Fig. 4a) that enhancing the aging temperature from 450  C (heat treatment 1) up to 520  C (heat treatment 2) slightly reduced the ultimate tensile strength from 1695 down to 1570 MPa, respectively, with a two-fold increase in ductility (from 3 up to 6 % according to Tab. 3). At the same time, the slope angle at the linear (elastic strain) stage of the stress-strain curves (Fig. 4a), determined by Young  s modulus, was enhanced after heat treatment 2. This

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