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. 5. Stress dependences of (a) the minimum creep rate m   , and (b) the time to fracture t f. . The minimum creep rate m   and the time to fracture t f , are plotted against the applied stress σ on a bilogarithmic scale for the GTD 111 superalloy as shown in Fig. 5. Fig. 5(a) shows that dependences m   (σ) are not linear and have a similar trend for all testing temperatures. The values of the stress exponent n of the minimum creep rate m   , were determined from the slopes of the best fit of the data at each applied stress and temperature (Table 2). The values of the stress exponent n under selected loading conditions show significant dependences on the applied stress and testing temperature, respectively. Fig. 5(a) and Table 2 show that determined values of n increase with increasing applied stress at given testing temperature and decrease with increasing testing temperature. It should be noted that these values of the apparent stress exponent n are consistent with the values n reported by Sajjadi et al. (2001) for GTD 111 superalloy over comparable ranges of stresses and temperatures and agree with the values reported by other researchers for a number of superalloys (Reed (2006), Kvapilova et al. (2021), Guguloth et al. (2021), Sourabh and Singh (2022)). However, these high observed values of the apparent stress exponents specific for precipitation strengthened materials can be decreased slightly by introducing threshold stress σ th (Gibeling and Nix (1980)) essential for mobile dislocations to overcome precipitates during creep.

Table 2. Experimentally determined values of the stress exponents n and m .

Testing temperature [°C]

Exponent n

Exponent m

800 900 950

12-18 8-11

10-19 7-10

6-7

5-6

The observed values of the stress exponent n of the creep rate suggest that all creep tests were carried out in the region of five-power-law (dislocation) creep (Kassner (2009)). It is generally accepted that in this region creep behaviour is controlled by a thermally activated climb of dislocations. Further, many authors reported that in power law creep the activation energy for creep Q C is equal to the activation enthalpy of lattice self-diffusion ∆ H L . The activation energy for creep Q C can be experimentally determined as a k- multiple of the slope of ∂ln m   versus 1/T plot as shown in Fig. 6. However, due to entire lack of the data points high estimated values of Q C should be considered as very informative quantitative data. The activation enthalpy for lattice self-diffusion in Ni is 287 kJ/mol (Hayes et al. (2017)) and the relation Q C ~ ∆ H L could be expected more likely in the case of solid solution strengthened alloys. Nevertheless, for precipitate strengthened GTD 111 superalloy higher values of Q C could be influenced by the γ´precipitation and a high dislocation density. Guguloth et a l. (2 021) on γ - γ´ nickel -base disk superalloy 720 reported the value Q C to be 598 kJ/mol. Higher values of the activation energy in this study could also be attributed to the precipitation of chromium-rich M 23 C 6 carbides which intensively occur at 800°C and higher temperatures. In conclusion, the high values of n and Q C can preferentially be attributed to the internal resistance of fine particles to movement of mobile dislocations during creep.