PSI - Issue 80

Marie Kvapilová et al. / Procedia Structural Integrity 80 (2026) 269–278 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 3. Time dependence of strain (a) and strain rate (b) for the INC939 AT alloy at a temperature of 700 °C and an applied stress of 300 MPa.

The time to fracture of the specimen during creep exposure can be expressed by an analogous relationship. , (3) where B is a material constant and Q F is the activation energy for fracture. The stress exponent m for the time to fracture can thus be determined as follows . (4) The stress dependencies of the minimum creep rate and the time to fracture, from which the exponents n and m can be determined, are shown in Fig. 4 and 5. It can be seen that the values of the stress exponents are stress-dependent, which may imply a change in the active creep deformation or fracture mechanism governing the creep behaviour. The exponents n and m were determined from the slopes of the linear fits of the curves at each stress and temperature. The stress exponent n lies within the range of 5-17 at 700 °C, 5-13 at 800 °C, and 3-8 at 900 °C, indicating that the creep tests at all temperatures were performed in the power-law creep regime. Similarly, the exponent m varies between 5 6 at 700 °C, 5-13 at 800 °C, and 3-7 at 900 °C. The nearly identical values of the stress exponents n and m suggest that the governing mechanism for both deformation and fracture behaviour is the same. Comparing the determined values of n and m with those in the literature, it can be assumed that the controlling mechanism in the present creep tests is diffusion-controlled dislocation climb. However, at lower stress levels, where the stress exponents decrease to lower values, deformation processes related to grain boundary behaviour may occur (such as grain boundary sliding, grain boundary cavitations, or diffusional creep). (Čadek, 1988; Kassner, 2004, 2009; Weertman, 1955). The dependence of the fracture elongation of the tested samples on the applied stress and temperature is shown in Fig. 6. While at lower temperatures the fracture strain ranges from 2 to 8 % (occasionally up to 10%), at 900°C the material becomes more ductile and reaches fracture elongation in the range of 12 to 28%. ! ! " # $ $ % & ' = ( !" # B $ & ' & !"# ) ! " # $ ! ! " # $ $ % & ' ' ( = ) !" !"