PSI - Issue 23

Petr Král et al. / Procedia Structural Integrity 23 (2019) 287–292 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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3. Results and discussion

3.1. Microstructure investigations

The microstructure before creep testing is shown in Fig. 1. One can see that the microstructure contains coarse primary carbides M 23 C 6 which were located in the interdendritic regions and formed carbide skeleton. In the interior of the dendrites were observed fine secondary of M 23 C 6 type. Creep testing at 1173K led to the partial dissolution of coarse primary carbides and to dissolution and the coarsening of secondary M 23 C 6 carbides in the dendritic interiors (Fig. 2a). The secondary carbides were nearly dissolved during creep testing at 1373 K (Fig. 2b).

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Fig. 2. Microstructure of Ni-Cr-W based alloy after creep testing (a) at 1173 K; (b) at 1373 K

Fig. 3. shows the chemical composition and corresponding diffraction patterns of carbides found in the microstructure of Ni-Cr-W based alloy. Analysis of chemical composition revealed that carbides M 23 C 6 contain predominantly Cr and W. In the microstructure were also found carbides of M 6 C type. The chemical composition revealed that M 6 C carbides contain high amount of W and Nb in comparison with M 23 C 6 . From this reason they can be easily found in back-scatter electron (BSE) micrographs (Fig. 2a) due to different Z-contrast. They become brighter due to higher content of high atomic number elements such as W and Nb in comparison with surroundings. Previous studies, Djerdjare et al. (2008), Reddy et al. (2010) and Chomette et al. (2010), showed that M 6 C carbides are unstable and they can decompose into M 23 C 6 carbides in some alloys. But it was found that the addition of Si increases thermal stability of M 6 C carbides. However Si-rich M 6 C carbides reduce the grain boundary strength and lead to the deterioration of the tensile properties, Xu et al. (2015). Similar effect on thermal stability and grain boundary strength may be caused by W and Nb which were measured in M 6 C carbides in the present work. 3.2. Creep behavior and fracture The stress dependences of minimum creep rate determined at 1173 – 1373 K are shown in Fig. 4. The results demonstrate that the value of stress exponent n decreases with increasing testing temperature. Microstructure investigation suggests that decrease of experimentally determined values of stress exponent n is probably associated with the density of fine secondary M 23 C 6 carbides in the interior of dendrites. Creep behavior at 1173 K is more