PSI - Issue 58

Juraj Belan et al. / Procedia Structural Integrity 58 (2024) 109–114

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Juraj Belan et al. / Structural Integrity Procedia 00 (2023) 000–000

1. Introduction IN 718 alloy represents about half of their world tonnage and is considered as a refractory superalloy since it can be used permanently above 600 °C. Young's modulus is almost twice that of Ti6Al4V and similar to that of unalloyed hardenable carbon steel. The alloy associates good creep and rupture strength with high resistance to fatigue. It possesses long-time strength and toughness at higher temperatures along with confinement of corrosion resistance up to high temperatures (Silva et al. 2017). IN 718 accounts for up to 50% of the weight of aircraft turbojet engines, being the main component for discs, blades and casing of the high-pressure section of the compressor and discs as well as some blades of the turbine section. It also finds several applications in rocket engines and cryogenic environments due to its good toughness at low temperatures (preserving parts from a brittle fracture). Nomenclature BCT Body Centred Tetragonal DIC Differential Interference Contrast DF Dark Field observing SEM Scanning Electron Microscopy EDS Energy Dispersive Spectroscopy VIM Vacuum Induction Melting R fatigue cycle asymmetry parameter RT Room Temperature IN 718 contains significant amounts of Fe, lowering its price per kilogram while endowing it with precipitation hardening effect. Fe low mobility in the matrix confers the main strengthening phase (γ  -phase) a sluggish precipitation kinetics that reduces susceptibility to post-weld cracking. Indeed, IN 718 was designed to overcome the low weldability of this class of materials, generally susceptible to cracks (microstructural segregation of alloying elements in the heat-affected zone of welds). Specific alloying elements give IN 718 a strong resistance to corrosion up to 1000 °C (Sulzer et al. 2020). For instance, Ni is useful in combating chloride-ion stress-corrosion cracking and protects from corrosion in many inorganic and organic oxidizing compounds, in a wide range of acidity and alkalinity. Cr imparts an ability to withstand attacks from oxidizing media and S compounds while Mo is known to improve resistance to pitting corrosion. Moreover, like other Ni-Fe-based superalloys, IN 718 can be hardened. It is desirable for structural components operating at high temperatures. Two strengthening modes are combined: solid solution hardening (atoms of Fe, Cr, Mo and Nb can substitute for Ni within the metallic matrix) and hardening by precipitation of ordered intermetallic phases, γ  and γ  . Ti and Al form by precipitation of the intermetallic phase γ  , Ni 3 (Ti, Al), metastable and hardenable again by the solid solution of Nb and Ti (at room temperature) and of W or Mo (at high temperature). At a temperature close to 650 °C, Nb combines with Ni to form by precipitation the γ  phase (Ni 3 Nb) (Maj et al. 2017, Sundararaman et al. 1988), which has very high mechanical properties at very low and moderately high temperatures. Although γ  and γ  are present in the aged condition, the amount of γ  is much lower and γ  is recognized as the primary strengthening phase. The γ  precipitates are disk-shaped, with a thickness of 5-9 nm and an average diameter of roughly 60 nm. The next structural components are primary carbides (created by elements such as Cr and Ti) and secondary carbides (created by elements such as Cr, Co, Mo and W). However, except of these structural components, undesirable phases can also appear (Sundararaman et al. 1997, Belan 2016). HFHC High Frequency and High Cycle fatigue loading LFHC Low Frequency and High Cycle fatigue loading N f Number of cycles to failure  c Fatigue limits stress PSB  s Persistent Slip Bands  pa Plastic deformation amplitude  a Fatigue stress amplitude