Investigation of the effect of dwell period in load controlled fatigue tests of inconel 718 superalloy

Abstract

The use of nickel alloys has become widespread in many industries during this last century, nickel with its alloys providing relatively better mechanical properties than conventional ferrous alloys for various industrial applications. Especially with its decent high-temperature oxidation and mechanical resistance makes this alloy the material of choice for engine applications. An example of a mostly used alloy in turbine engine hot sections is Inconel 718 is a niobium-modified and nickel-based superalloy that will be examined in this work. Inconel 718 superalloy is a face-centered cubic latticed nickel-chromium alloy that can relatively retain its good mechanical strength up to 649°C. These superior mechanical properties are provided with γ″-Ni3Nb precipitate phases present in the structure of the alloy, above mentioned 649°C temperature level secondary δ phase formation causes rapid strength loss of the material. In industrial applications materials are exposed to various hazardous factors and these can be exemplified as; temperature, residual stresses, corrosion, etc. Depending on the application type these hazardous factors generate different types of deformations can be summed as; creep, fatigue, overload failure, etc. Unlike commonly known deformation types, the exampled engine application of this alloy includes different factors. In this study, the mechanical property of Inconel 718 is examined in terms of fatigue behavior. The main concern of this work is to determine the material's fatigue properties under pure bending and different conditions. Test conditions will include room temperature, 450°C, and dwell times to simulate material's real usage in the engine application. Additional to fatigue testing hardness measurements and nanoindentation testing were performed at ambient temperature. Fractographic and microstructural examinations of the tested specimens are performed with an optical microscope and SEM (scanning electron microscope) to understand the material's reaction for different deformation factors. To understand the material's mechanical behavior first room temperature and high-temperature load controlled fatigue tests without dwell application have been performed. Parameters are selected to examine mainly the dwell effect on the material by maximizing the dwell cycle. It is observed that Inconel718 alloy's dwell sensitivity is depending on temperature and applied stress, change in mechanical properties is also observed with hardness testing. Fractographic examinations have shown that different features become visible under different test conditions. In this work in addition to mechanical testing statistical analysis is also studied for observed data sets.