Date

2023-1-26

Author

Yalçın, Merve Yeşim

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The efficiency of the systems generally increases with increasing service temperatures. For instance, high operation temperatures in the energy industry reduce operation costs, fuel consumption, and environmental pollution. Similarly, high service temperatures result in a longer range, higher carrying capacity, and performance in aerospace applications. However, service temperatures of currently used Nickel-based superalloys especially in aerospace and defense applications are limited below 700 °C. The formation of nano-sized oxide particles in the materials has been shown to improve the thermal stability and mechanical properties of the materials. In this study, three grades of oxide dispersion strengthened (ODS) Inconel 718 alloys were produced with the selective laser melting (SLM) method, and their microstructural and mechanical properties were investigated. The compositions of the alloys were determined as IN718-Y (IN718 – Y2O3), IN718-YF (IN718 – Y2O3 – FeO), and IN718-YFH (IN718 – Y2O3 – FeO – Hf) using thermodynamic CALPHAD (CALculation of PHAse Diagrams) calculations. Before the production of the samples, various power (P) and velocity (V) parameters were used to find the optimum SLM processing parameters for each alloy. As-built (AB) alloys produced with the optimized parameters had > 99.7% densification, but the existence of detrimental phases such as Laves, which deteriorate mechanical properties, was detected in the microstructures of all the grades. Since the part undergoes fast heating and cooling cycles with thermal gradients up to 106 K/s during SLM production, detrimental phases start to form in the microstructure. In order to remove the detrimental phases which are resulted from rapid cooling during processing and promote the further precipitation of the nano-oxide particles, heat treatment was applied at 1050°C for 1 hour to all the grades. In heat-treated (HT) conditions, according to scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analyses, increases in oxide amounts, and some remaining Laves phase particles in IN718-Y, IN718-YF, and IN718-YFH grades were detected. Transmission electron microscopy (TEM) analyses revealed a significant increase in nano-oxide number density and in-situ tensile tests conducted at room temperature (RT) and 700°C showed an increase in tensile strengths for all the grades in HT condition.

Subject Keywords

Additive Manufacturing, Inconel 718, Nano-Oxide, Oxide Dispersion Strengthening, Selective Laser Melting

URI

https://hdl.handle.net/11511/102041

Collections

Graduate School of Natural and Applied Sciences, Thesis