Papers by Keyword: Alloy 718

Abstract: Understanding damage mechanisms across scales is crucial to ensure the structural integrity of nickel-based superalloy components under demanding conditions. This study highlights key aspects of a multi-scale experimental approach for analyzing oxygen-induced cracking in Alloy 718. Microcantilever bending tests on specific grain boundaries were combined with corrosion tests and detailed analyses using high-resolution scanning electron microscopy, electron backscatter diffraction, and energy-dispersive X-ray spectroscopy. The results suggest that susceptibility to oxidative attack is significantly impacted by the type of grain boundary, emphasising the importance of local crystallography in oxygen diffusion and elemental redistribution. By bridging local microstructural features with global mechanical response, the presented multi-scale approach allows the parameterization of physically based material models and identifies grain boundary engineering as a promising strategy for improving damage tolerance.

Abstract: In order to get the insights about microstructural changes that occurs under the thermo-mechanical processing conditions, the physics based modelling approach is very useful. Therefore, the flow curves of alloy 718 are theoretical simulated using a dislocation density dependent constitutive model for different conditions. Presented model considers the microstructural ingredients that are immobile dislocation density, effective grain size and dislocation cell size as the variables to address the creep. The simulated flow curves show a good agreement with the experimental flow curves. The magnitude of immobile dislocation density and dislocation cell size in between 3.87× 10¹⁴ - 3.87× 10^14­ m^-2 and 8.29-8.45 μm, respectively, at the completion of the simulation. Furthermore, this approach also provides the possibility to quantify and depict the variation in each strengthening contributions.

Abstract: The evolution of microstructural features such as local grain size and local grain size distribution are essential in view of the final physical and mechanical properties of the nickel base alloy 718 for aircraft parts forged in a multi-step production route. Due to increasing standards and the need of the prediction of fracture mechanical properties, a multi-class grain size model for a more detailed microstructure prediction is necessary. Therefore, a multi-class model considers the real initial non-uniform grain size distribution and structure of the pre-material at the beginning of the forging process, which affects the evolution of grain sizes during thermo-mechanical treatment and leads to different results than commonly used uniform grain structures. The initial distribution is defined by grain classes according the ASTM standard. It is shown that the presence of different classes and distributions of grains are as import as the applied strain, strain rate and temperature on dynamic, meta-dynamic and static recrystallization. Additionally, dissolution processes of delta phase and grain growth kinetics are included in the model to properly indicate the recrystallized fractions and represent the resulting multi-class microstructure. A series of simulations with different initial distributions is discussed and compared with examined forged samples in terms of the resulting microstructure for typical forging parameters. Based on these results the microstructure model can be used in combination with collected process data to predict the resulting properties and for the design of new aircraft parts.

Abstract: It is well known that the introduction of sustained tensile loads during high-temperature fatigue (dwell-fatigue) significantly increases the crack propagation rates in many superalloys. One such superalloy is the Ni-Fe based Alloy 718, which is a high-strength corrosion resistant alloy used in gas turbines and jet engines. As the problem is typically more pronounced in fine-grained materials, the main body of existing literature is devoted to the characterization of sheets or forgings of Alloy 718. However, as welded components are being used in increasingly demanding applications, there is a need to understand the behavior. The present study is focused on the interaction of the propagating crack with the complex microstructure in Alloy 718 weld metal during cyclic and dwell-fatigue loading at 550 °C and 650 °C.

Abstract: With the application upgrade of alloy 718 in the marine oil exploitation, higher corrosion resistance demands has been presented for structure design and reasonable material application of drilling and production equipments; thus there is an urgent need to understand influences of grain size on corrosion behavior of such alloy in sea water environment. In this experiment, different solution treatments are applied to nickel-based alloy 718. As a result, specimens with grain size varied from ASTM No. 9 to No. 3.5 grades. The influences of grain size on the electrochemical behaviors in simulated sea water (3.5% sodium chloride solution) are investigated using electrochemical impedance spectroscopy and polarization curves. Alloy 718 shows good corrosion resistance in this electrolyte solution, however, grain size still has an effect on its electrochemical corrosion behaviors to a certain extent. With the time going, testing results show that coarse-grained alloys with larger grain size have higher impedance values and smaller corrosion current densities, indicating a better corrosion resistance than fine-grained specimens. However, when the grain size increases further, the corrosivity resistant nature of alloy 718 would decline. Testing results show that specimen with size of ASTM No. 5 has the best corrosion resistance.

Abstract: The effects of different solid-solution temperature and holding time on the alloy structure and mechanical properties of alloy 718 were investigated. The results show that the grain size grows up as the solid-solution temperature elevates and the holding time prolongs. The grain size increases slowly because undissolved delta phase restrains the growth of grains when solid-solution temperature is below 1000°C, whereas the grain size increases quickly when solid-solution temperature is beyond 1050°C. At room temperature, the strength and hardness of alloy 718 decrease with solution treatment temperature increasing, however, the ductility and toughness of alloy 718 both increase, and the brittle fracture of sample turns into ductile fracture.

Abstract: It is known that the stiffened cylinder structure supports external pressure loads inaerospace and marine application subjected to hydrostatic pressure. For relatively low temperature,aluminum or composite cylinder can be appropriate, but at higher temperature, titanium or steel alloymust be considered. Nickel based alloys show excellent corrosion resistance and elevated temperaturemechanical strength so that these alloys are now successfully utilized for aerospace and engineapplication. This paper provides innovative

Abstract: It is known that the stiffened cylinder structure supports external pressure loads in aerospace and marine application subjected to hydrostatic pressure. For relatively low temperature, aluminum or composite cylinder can be appropriate, but at higher temperature, titanium or steel alloy must be considered. Nickel based alloys show excellent corrosion resistance and elevated temperature mechanical strength so that these alloys are now successfully utilized for aerospace and engine application. This paper provides innovative manufacturing process of producing stiffened cylinder for elevated temperature application.

Abstract: A dislocation density based plasticity model is applied to two variants of steels. One is an austenitic (fcc) stainless steel with ordered precipitates and the other is a Ti-Nb microalloyed (bcc) steel. Precipitate distributions are measured and this information is combined with appropriate precipitate hardening models. The flow stress model is also calibrated for an nickel-based superalloy where it is combined with a model for precipitate growth.

Abstract: It is well known that alloy 718 shows excellent mechanical strength and corrosion resistance and this alloy is now successfully utilized for aerospace and oil industry application. In order to design and manufacture components to satisfy the design requirements of the applied system, it is necessary to consider formability and weldability of the material. This paper provides experimental results of lap shear strength of welded specimens of alloy 718 with EBW(Electron Beam Welding) and LBW(Laser Beam Welding) and Brazing. The purpose is to determine the optimum welding method for overlap welding of alloy 718 for blow forming.