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. 9. The formation and propagation of extensive surface crack: (a) creep damage development, (b) decohesion between primary bulk carbides and the matrix, and (c) EDS spectrum of primary bulk carbides (wt.%:49.6Ta,30.6Ti,11.1W).

Fig. 10. Breakage of intergranular chromium-rich M 23 C 6 carbides: (a) destruction of carbides, (b) breakage of large grain boundary carbides, and (c) EDS spectrum of M 23 C 6 carbides (wt.%:74.4Cr).

Fig. 9 shows in more detail the extensive surface crack and its further propagation closely connected with decohesion between the primary Ta-, Ti- and W-rich bulk carbides/matrix. The decohesion at the interface between carbides and matrix results from a loss of coherency of the primary carbides. Lee et al. (2012) reported that during thermal exposure of GTD 111 superalloy the dominant carbide reaction is the formation of M 23 C 6 according to the following reaction: MC + γ → M 23 C 6 + γ´. During the decomposition process of MC, Cr-rich M 23 C 6 were formed along the grain boundaries and coarsened with exposure time. In this study it was observed that intergranular M 23 C 6 carbides are connected with the formation of creep damage and creep fracture - Fig. 10. This leads to conclusion that there are two different mechanisms of the formation of creep damage: (i) interface decohesion between the refractory elements rich carbides/matrix, and (ii) breakage of the grain boundary M 23 C 6 carbides. “Classical” mode of intergranular cavitational fracture associated with the wedge -type microcracks or r-type cavities (Cadek (1988), Beere (1981), Riedel (1987), Svoboda et al. (1990)) were not observed.

Fig. 11. SEM images of the fracture surface of specimen crept at 800°C/400 MPa: (a) the low-resolution image, and (b) detail of transition zone between the quasi-cleavage and interdendritic modes of creep fracture.