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