Issue 78

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

Figure 1: Dimensions of the coupons for the fatigue tests.

Fractography examinations of all the tested coupons were carried out using an “EVO 40” SEM (Carl Zeiss). Based on the measured values of fatigue striation spacing for different crack lengths of each fractured specimen, the crack propagation duration N p was determined according to the following equation [21]:

k



a

 

i

N

p

i 



i

1

where Δ a i = a i – a i − 1 , a i and a i − 1 were the crack lengths at which the fatigue striation spacing δ i and δ i − 1 were measured, respectively; k was the number of the fatigue striation spacing measurements. Stress intensity factors (SIFs) corresponding to the transition to the final fracture stage for the coupons were determined using the formula [22]:

  F E k c t W      , , a a c S a

  





K

,

I, max

where F S was a correction function for a surface semi-elliptical crack; E ( k ) was a complete elliptic integral of the second kind; W and t were the half-width and thickness of a coupon, respectively; a and c were the semi-axes of the semi-elliptical crack front during the transition to the final fracture stage, estimated during the fractographic analysis; φ was the angle defining the crack front point for calculating the SIF (Fig. 2).

Figure 2: A sketch of the semi-elliptical crack front in the cross-section of a flat coupon for determining the SIF value corresponding to the transition to the final fracture stage. (Notations are given in the text.).

R ESULTS AND DISCUSSION ig. 3 shows SEM micrographs of the treated samples as well as their X-ray diffraction patterns. For both heat treatments, hierarchically organized microstructures were observed with rounded particles of the primary  -phase at grain/subgrain boundaries and nanosized precipitates of the secondary  -phase inside subgrains of the prior β phase (Fig. 3a, b). After the RSR and aging at T =450 °C (heat treatment 1), the particle sizes of the primary  -phase were F

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