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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a) b) Fig.10. Fracture surfaces of crept specimen (a) 800°C, 500MPa, creep fracture elongation 3,7%, time to fracture 2 hours; (b) 900°C, 60MPa, creep fracture elongation 27.1%, time to fracture 1143 hours.

4. Discussion This study investigated the creep behaviour of the IN939 nickel-based superalloy fabricated by laser powder bed fusion (L-PBF) under various combinations of applied stress and temperature. The conventional steady-state stage of the creep curve was found to be suppressed to a distinct inflection point, from which the minimum creep rate was determined. Stress-dependent analysis of the minimum creep rate and time to fracture revealed that the stress exponents n and m vary with applied stress. The comparable values of these exponents suggest that the deformation and fracture mechanisms are governed by the same controlling process—most likely diffusion-controlled dislocation climb. Furthermore, the analysis indicates that at lower stress levels, a shift in the dominant creep mechanism may occur, potentially involving grain boundary sliding or diffusional creep. These transitions underline the importance of accounting for variable creep behaviour across different loading regimes in high-temperature applications. The microstructure of L-PBF-manufactured IN939 superalloy undergoes significant changes during creep exposure, strongly influenced by temperature and applied stress. At lower stresses, creep deformation was accommodated primarily through grain elongation, while microcracks were observed along grain boundaries near fracture regions. In contrast, at higher stresses and strain rates, microcracks extended even into areas distant from the fracture surface, indicating more widespread damage. Grain morphology was notably altered by creep exposure, with grains becoming highly elongated in the loading direction, especially at high temperatures and low stresses. Grain lengths increased from approximately 30–50 μm at 700–800°C up to 200 μm at 900°C, reflecting significant grain growth under prolonged thermal exposure. Precipitate evolution was also evident. MC-type carbides remained located at grain boundaries and exhibited notable coarsening, growing to 5 μm at lower temperatures and up to 10 μm at 900°C. Gamma prime (γ′) precipitates showed moderate growth from 200 nm to 300 nm, but at 900°C and low stress levels, they coarsened substantially to 0.5–1 μm, with smaller particles dissolving or merging. These changes suggest a thermally driven degradation of the strengthening phase, particularly under long-term, high-temperature exposure. A transition in fracture mode was observed as a function of temperature and exposure duration. In summary, the observed microstructural evolution underscores the necessity of carefully balancing creep resistance and microstructural stability in L-PBF IN939 components, especially for long-duration service at elevated temperatures.