Issue 78

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

To determine the causes of the prolonged fatigue performance with increasing the aging temperature from 450 up to 520  C, regularities of the initiation and propagation of cracks were analyzed for heat treatment 2 with the help of SEM. The specific features of fracture surfaces formed in specimens with the heat treatment 1 by testing at load frequencies of 5 and 2  10 4 Hz were investigated previously [15]. Fig. 6 shows the facture surfaces of the specimens cut out from the sample with the heat treatment 2, according to which fatigue cracks initiated with the formation of surface origin on the extended side of the cross-section. The fatigue crack propagation zone is of semi-elliptical shape. The crack fronts were elongated in the direction of crack propagation from the origin towards the opposite surface (Fig. 6). For the previously investigated specimens cut out from the sample with heat treatment 1, crack origin is located at corner of cross-section or close to it [15]. This difference in the origin locations indicated that increasing the aging temperature from 450 up to 520 °C decreased the sensitivity of the Ti-5Al-5V-5Mo-1Cr-1Fe alloy to such stress concentrators.

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Figure 7: SEM-micrographs of the fatigue crack initiation zones on the specimens with heat treatment 2 with the fibrous (a, b) and fan shaped (c) relief, formed in the tests at 850 (a), 900 (b) and 1000 (c) MPa. The main directions of the crack propagation are indicated by arrows. For the heat treatment 2, the initial stage of the fatigue crack propagation was characterized by both smoothed fibrous and fan-shaped reliefs of fracture surface at all the applied stresses (Fig. 7). As the fatigue cracks propagated, local oriented shear “quasi”-dimples formed at the smoothed areas (Fig. 8). The “quasi”-dimples indicated are not determine the general morphology of smoothed fan-shaped relief of fracture surface in the zone 2. It is suggested that such relief features characterize the effect of residual compressive stresses during the crack propagation on the descending load half-cycle.

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Figure 8: SEM-micrographs of smoothed fan-shaped reliefs with oriented shear “quasi”-dimples on the fracture surface in specimens with heat treatment 2, revealed in the fatigue tests at 900 (a) and 1000 MPa (b). The main directions of the crack propagation are indicated by arrows. Further propagation of the fatigue cracks was accompanied with the formation of striations possessing curved arcuate shapes due to their short fronts (Fig. 9). The fatigue striation spacing increased from 100 up to 350 nm, after which an abrupt transition to unstable crack propagation occurred. As a result, dimpled reliefs (typical for failure under static loading) were observed with vortex-like zones, reflecting texture of the Ti-5Al-5V-5Mo-1Cr-1Fe alloy created by the RSR (Fig. 10). Thus, the relief formation sequences were the same for both the heat treatments 2 (discussed above) and 1 (investigated in [15]). However, the semi-elliptical crack fronts at the transition to the final fracture stages were much longer in the specimen

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