PSI - Issue 80

Vaclav Sklenicka et al. / Procedia Structural Integrity 80 (2026) 493–500 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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transmission electron microscopy. It was found that precipitation of the intermetallic phases, mostly hexagonal (Fe, Cr) 2 Zr, occurs relatively homogeneously at grain boundaries but more intensively in the grain interiors. The mean size of precipitates was estimated to be ~ 100 nm, and the density was ~ 5.6 ± 0.3 x 10 18 m -3 . No significant changes in size and density of precipitation were observed in the crept specimens loaded under different conditions. TEM analyses revealed an intensive presence of a hexagonal screw dislocation network on the basal planes (Fig.5(a)). The density of dislocations was 1.75±0.25 x 10 13 m -2 . A more detailed study on dislocation substructure was reported by Kombaiah and Murty (2015). No creep intergranular cavitation and/or microcracks were observed along the gauge length. The final ductile transgranular fracture proceeds by local intensive necking of the cross-section of the crept specimens (Fig. 5(b) and (c)).

Fig. 5. Dislocation substructure and creep fracture: (a) a bright field TEM image depicting a dislocation network, (b) SEM fractography of ductile transgranular creep fracture, and (c) a detail of fracture (creep at 350°C, 280 MPa, ε f = 0.43).

4. Discussion and Conclusions Fig. 2 (a) shows that the stress dependences of the minimum creep rate are not generally given by straight lines, which implies that a simple power function with a constant value of the stress exponent n cannot describe it. A dramatic increase in the value of the stress exponent n at higher stresses suggests that creep tests were carried out in the transition between the power-law (low and medium applied stresses) and power-law breakdown creep regimes. While Eq. (1) is frequently used for the power-law region, the power-law breakdown can be described by a single relationship ̇ m = C (sinhDσ) n , as reported by Murty et al.(2013). The values of the stress exponent of the creep rate, n , and the stress exponent of the time to fracture, m , were found near each other, indicating the close relationship between creep deformation and fracture processes. Further, the Monkman-Grant empirical relationship strongly supports the idea that creep deformation and fracture in Zry-4 are interconnected under the loading conditions used. However, despite an important understanding of creep deformation mechanisms of the power-law regime in zirconium alloys, creep mechanisms in power-law breakdown remain poorly understood. It is, therefore, surprising because the power-law breakdown can preclude disaster situations, e.g. LOCA. Therefore, our future experiments will be carried out within the power-law breakdown regime at various loading conditions.