PSI - Issue 80

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

270

2

high-temperature applications, primarily in cast form -for example, in turbine blade production. Its typical operating temperature reaches up to 850 °C. The alloy’s microstructure consists of a dendritic matrix of the γ solid solution, strengthened by primarily spherical γ′ precipitates, which make up around 30–40% of the volume after heat treatment. Additional phases include a eutectic γ–γ′ structure, coarse MC-type carbides (where M is mainly Ti, Ta, or Nb), and finer M 23 C 6 -type carbides (with M mostly represented by chromium). A critical factor that may affect the performance of cast IN 939 components in operation is microstructural evolution, such as changes in γ′ particle size and morphology or degradation of carbides at grain boundaries -which can reduce ductility and creep strength. Unlike other types of nickel-based superalloys, however, no precipitation of the degrading σ or η phases has been observed. (Jahangiri et al., 2014) Compared to the frequently used IN 738 alloy, IN 939 offers significantly better corrosion resistance (Gibbons & Stickler, 1982) Despite these advantages, the casting and especially the post-casting heat treatment of IN 939 are time-consuming and often lead to both chemical and structural heterogeneity. In addition, conventional manufacturing results in high material losses, and the finishing processes - such as machining and surface treatment - are labor-intensive. Nowadays, additive manufacturing (AM), commonly referred to as 3D printing, has emerged as a promising method for producing high-temperature components. Although this method was on the verge of usability for IN 939 until recently, it has been shown that by optimizing printing parameters, it is possible to produce defect-free material (Marchese et al., 2020; Mišković et al., 1992). Among AM techniques, laser powder bed fusion (L-PBF) has shown strong potential for manufacturing energy components with complex geometries and intricate internal features that would be difficult or even impossible to achieve using conventional processes (Kulkarni, 2018; Raza et al., 2024). However, the microstructure of L-PBF-processed IN 939 differs markedly from that of the conventionally cast alloy due to high thermal gradients and rapid solidification rates inherent in the process (Jahangiri et al., 2014). A distinctive characteristic of printed IN 939 is its high dislocation density (~10¹⁵ m/m³), typical for heavily deformed metals, which can significantly influence mechanical behaviour. The rapid cooling during L-PBF prevents the formation of strengthening phases. As a result, the alloy retains a non-equilibrium microstructure, and prolonged exposure to elevated temperatures may lead to uncontrolled γ′ precipitation, drastically reducing ductility (Banoth et al., 2020). This necessitates appropriate post-processing steps such as heat treatment and precipitation hardening (Kvapilova et al., 2021; Šulák et al., 2023). To fully leverage the potential of L-PBF-manufactured IN 939 in high-temperature applications, knowledge of its mechanical loading behaviour is essential. The fatigue resistance of cast IN 939 is generally inferior to that of its additively manufactured counterpart, mainly due to the presence of casting defects and a higher amount of carbides in the cast state, both of which lead to premature failure, as well as grain size differences (Šulák et al., 2023). So far, the creep behaviour of printed IN 939 has only been investigated in one study, which focused on the influence of heat treatment and related grain recrystallization (Banoth et al., 2020). The present work aims to analyze the creep behaviour of IN 939 produced by L-PBF under various loading conditions, including temperature and applied stress. 2. Experimental procedures The composition of the commercial EOS powder and the resulting alloy samples is shown in Table 1. The alloy was printed vertically (Fig. 1), followed by solution annealing at 1175°C for 45 minutes and two-step aging at 1000°C/6h and 800°C/4h. Table 1: Chemical composition of IN939 (wt.%) C Cr Co W Ta Nb Al Ti NI powder 0.15 22.5 19.0 2.0 1.4 1.0 2.05 3.7 Bal. L-PBF 0.15 22.5 18.95 2.0 1.37 1.0 2.04 3.68 Bal.