PSI - Issue 52

Marie Kvapilova et al. / Procedia Structural Integrity 52 (2024) 89–98 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 6. Temperature dependence of the minimum creep rate m  

Fig. 7. Monkman-Grant relationship.

The stress dependence of the time to fracture t f is shown in Fig. 5(b). In a similar way as in Fig. 5(a) the double logarithmic plots are non-linear from which the slopes and, therefore, the values of the stress exponents m (Eq. (5)) of the time to fracture were determined (Table 2). The estimated values of the stress exponents n and m are near to each other, indicating that both the creep deformation and fracture could be controlled by the same operating mechanism(s). The interrelation between the minimum creep rate m   and the time to fracture t f can be described by the empirical Monkman-Grant relationship (Monkman and Grant (1956)) (6) where the exponent k is frequently found to range from 0.8 to 1.0 in the literature and C MG is constant. A double logarithmic plot of the Monkman-Grant relationship applied to all experimental GTD 111 superalloy data can be fitted with very good confidence by a single line in Fig.7. The fact, that the experimental results obey the Monkman- Grant relationship strongly supports the idea about a close link between creep deformation and damage processes. Nevertheless, the Monkman-Grant relationship itself does not give an account of the development of creep damage and fracture modes. Accordingly, the fractographic analyses will be discussed in greater detail in what follows. 3.4. Creep damage development and fracture After creep exposures, longitudinal polished and etched metallographic cross-sections parallel to the tensile axis of the fractured specimens and their fracture surfaces were examined by scanning microscope. The low-resolution SEM images in Fig. 8, taken from the cross-sections, show the fracture profiles at different testing temperature. The inspection of Fig. 8 clearly indicates that the fracture is a brittle mostly interdendritic and/or intergranular modes resulting in low values of the fracture strain ε f ~ 4÷5% (Figs. 3 and 4). The fractured specimens at all temperatures exhibit small surface cracks along the gauge length, which could be responsible for the formation and propagation of the main crack (Sklenicka (1997)). f MG C / k m ( ) =   t ,

Fig. 8. SEM images of the cross-sections of the fracture after creep at: (a) 800°C/400MPa, (b) 900°C/200MPa, and (c) 950°C/125MPa, respectively. The stress axis is horizontal.