Effect of microstructural constituents on mechanical properties and fracture toughness of Inconel 718 with anomalous deformation behavior at 650 °C

Abstract

In the current work, Inconel 718 superalloy was subjected to five different heat treatment schedules to develop five different microstructures for studying the effect of γ’, γ” and δ phases on mechanical properties of IN-718 at room temperature and 650 °C as the working temperature of IN-718 is near 650 °C. Microstructural characterization was performed with Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). Vickers hardness tests, tensile tests and fracture toughness (JIC) tests were performed for different heat-treated samples at room temperature. Further tensile tests were carried out at 650 °C for all heat-treated specimens and one high-temperature JIC test at 650 °C was performed for Standard heat-treated compact tension specimen. It had been demonstrated that γ′ and γ″ phases caused a substantial increase in hardness of the material while δ phase was attributed to low hardness of this superalloy. Also, it was found that γ″ phase provided maximum strength to IN-718 superalloy followed by γ′ at room temperature but on contrary, γ′ was found to be primarily responsible for higher strength than γ″ at 650 °C. The δ phase furnished poor strength at all temperatures. However, the formation of Nb + Ti carbides and oxides at 650 °C caused premature failure of the tensile specimens with lower elongation. In case of fracture toughness test, the maximum value of critical J-integral (JIC) and tearing modulus (T) were demonstrated by γ′ phase. A decrease in the JIC value of the material was observed at higher temperature (650 °C). Both critical J-integral (JIC) and tearing modulus (T) were found to be critical parameters for crack growth characterization of this superalloy.