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. 2. Cross- sectional SEM detail images showing primary γ´ precipitates of the GTD 111 superalloy after creep exposures at different test ing temperatures: (a) 800°C, (b) 900°C, and (c) 950°C.

3.2. Creep curves Standard creep curve is the graphical illustration of the time dependence of creep strain. The typical shapes of the standard creep curves obtained by testing the GTD 111 superalloy are presented in Figs. 3(a) and 4(a). As can be seen from these figures, standard creep curves consist of different creep regions. The instantaneous strain, ε 0 ≈ 0.005, which is a result of sudden loading, consists of elastic, anelastic and non-recoverable plastic components. After an instantaneou s strain ε 0 , a short primary (transient) stage occurs in which a creep rate decreases with time – Figs. 3(b) and 4(b). This can be explained by easier slip of mobile dislocations in the matrix and an increase of the density of dislocations causing strain hardening to decrease the strain rate. In the following short secondary and/or steady- state stage a creep rate remains nearly constant (Fig. 3(b)) or reduces more likely to an inflection point (Fig. 4(b)). In fact, steady-state region is the result of the equilibrium between the processes of strain hardening and recovery. The creep rate in this region is designated as the steady state or secondary creep rate, s   , or as the minimum creep rate, m   . In the last creep stage called the tertiary creep stage the creep rate increases with time and terminates by the fracture of the specimen (Figs. 3 and 4). It should be noted that the standard creep curves shown in Figs. 3(a) and 4(a) do not clearly indicate the individual creep regions. Nevertheless, these standard creep curves can be easily replotted in the modified form of the strain rate   vs time t as shown in Figs. 3(b) and 4(b).

Fig. 3. Different creep curves for the GTD 111 superalloy at 800°C and 400MPa: (a) standard creep curve, (b) modified creep curve.