PSI - Issue 80

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

274 6

which generally exhibit less elongation than their larger counterparts. Furthermore, fine, predominantly submicron MC-type carbides enriched in Ti, Ta, and Nb were observed along internal interfaces, particularly along cell walls. Notably, no topologically close-packed (TCP) or η phases, reported by (Kanagarajah et al., 2013), were detected. The creep tests were conducted on material that underwent additional heat treatment following the L-PBF process; therefore, the initial microstructure differed from that of the as-built, untreated state. Most notably, spherical γ′ precipitates formed with a bimodal size distribution: coarse precipitates were approximately 200 nm in diameter, while the fine ones measured around 20 nm. Additionally, almost no recrystallisation was detected after the heat treatment, only cell coarsening was evident. Fine, predominantly submicron MC carbides continued to decorate internal boundaries, especially along the cell walls Fig. 7.

(Ti, Ta, Nb)C

5 µ m

2 µ m

a)

b)

The state of the microstructure after creep exposure depended on the loading conditions – temperature and applied stress. At lower applied stresses, microcracks were observed along the grain boundaries, particularly near the fracture surface, while in other regions the matrix was able to accommodate the deformation through grain elongation. At higher stresses and greater strain rates (at 800°C, the limit stress is 400MPa), small cracks were observed even in regions at a greater distance from the fracture surface Fig.8. The grain shape after creep exposure is significantly elongated, and its uniaxial growth in the direction of loading is particularly evident at higher temperatures and lower stresses. While at lower temperatures (700°C, 800°C) the grain length in the loading direction is approximately 30– 50 µ m and the width around 20 µ m, at the highest temperature (900°C) and with prolonged exposure, the grain length increased to 200 µ m. Changes also occur in the size and shape of precipitates- the MC-type and gamma prime type. MC carbides are still located at the grain boundaries. At lower stresses and longer creep exposure, they tend to grow — up to 5 µ m at lower temperatures (700°C and 800°C), and up to 10 µ m at a temperature of 900°C. The growth of gamma prime precipitates is not as pronounced; their size increases from 200 nm to 300 nm, especially during prolonged creep exposure. Only at the highest temperature (900°C, 60 MPa) do they grow to a size of 0.5–1 µm and the smaller precipitates dissolved or coalesced to the less spheroidal, elongated shape. Fig. 7. Microstructure of heat treatment following INC 939 after heat- treatment. a) SEM micrographs with spherical g ´ precipitates, b) TEM micrographs of fine dispersion of spherical complex (Ti, Ta, Nb)C nanocarbides alongside dislocation wall.