Precipitation kinetics of γ′ and γ′′ particles in Inconel 718 and its influence on mechanical properties

Abstract

The as-received state of Inconel 718 showcases a γ austenitic structure containing δ intergranular precipitates and a blend of (Nb,Ti)C type carbides. Furthermore, examination via transmission electron microscopy identified γ′′ precipitates (elongated disks) and γ′ precipitates (spherical in shape), with γ′′ being the prevailing phase. After quenching at 990 °C for 30 min, the structure comprises an fcc γ matrix hosting randomly dispersed (Nb,Ti)C carbides. Additionally, the TEM examination of the quenched state showed an absence of the δ phase and γ′′ and γ′ precipitates. The precipitation of γ′′ and γ′ phases was studied in Inconel 718 alloy using specimens which were subjected to isothermal aging for various durations at temperatures between 600 and 750 °C and were previously quenched in water at 990 °C for 30 min. The tensile and the micro-hardness mechanical properties were studied for aging at 680 and 750 °C at 4, 50 and 100 h aging duration. The grain growth kinetic of γ′′ and γ′ precipitates has been analyzed following the power–law relationship Dn= Kt and the rate constant K = K0exp−QRT. The optimum grain growth exponent n of γ′′ and γ′ phases has been determined as equal to 2.3. The activation energy values for γ′′ and γ′ coarsening have been determined as equal to 209 kJ mol−1 and 139 kJ mol−1, respectively. Based on the experimental results, a mathematical model was established to describe the grain growth behavior of the studied Inconel 718 alloy for different tempering temperatures and holding times. The predicted grain size growth is in good agreement with the measured one. For aging at 680 °C, the optimum aging time which corresponds to the highest value of yield strength (1164 MPa) and Vickers micro-hardness (500) due to γ′′ strengthening phase, was determined as 50 h. Nonetheless, when subjected to aging durations surpassing 50 h at 750 °C, the reduction in both yield strength and micro-hardness can be attributed primarily to the coarsening of γ′′ precipitates, followed by a decrease in the volume fraction of the γ′′ phase. This reduction is a result of the concurrent increase in the volume fraction of the δ phase. For γ′′ particle size lower or equal to 35 nm, strengthening is governed by the shearing mechanism. Beyond 35 nm γ′′ particle size, the γ′′ precipitates are crossed preferably by bypassing mechanism. A relationship between the 0.2% yield strength and the Vickers micro-hardness was determined.