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

The as-received state of Inconel 718 showcases a gamma austenitic structure containing delta intergranular precipitates and a blend of (Nb,Ti)C type carbides. Furthermore, examination via transmission electron microscopy identified gamma '' precipitates (elongated disks) and gamma ' precipitates (spherical in shape), with gamma '' being the prevailing phase. After quenching at 990 degrees C for 30 min, the structure comprises an fcc gamma matrix hosting randomly dispersed (Nb,Ti)C carbides. Additionally, the TEM examination of the quenched state showed an absence of the delta phase and gamma '' and gamma ' precipitates. The precipitation of gamma '' and gamma ' phases was studied in Inconel 718 alloy using specimens which were subjected to isothermal aging for various durations at temperatures between 600 and 750 degrees C and were previously quenched in water at 990 degrees C for 30 min. The tensile and the micro-hardness mechanical properties were studied for aging at 680 and 750 degrees C at 4, 50 and 100 h aging duration. The grain growth kinetic of gamma '' and gamma ' precipitates has been analyzed following the power-law relationship D (n) = Kt and the rate constant K = K-0 exp-(Q/RT). The optimum grain growth exponent n of gamma '' and gamma ' phases has been determined as equal to 2.3. The activation energy values for gamma '' and gamma ' 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 degrees C, the optimum aging time which corresponds to the highest value of yield strength (1164 MPa) and Vickers micro-hardness (500) due to gamma '' strengthening phase, was determined as 50 h. Nonetheless, when subjected to aging durations surpassing 50 h at 750 degrees C, the reduction in both yield strength and micro-hardness can be attributed primarily to the coarsening of gamma '' precipitates, followed by a decrease in the volume fraction of the gamma '' phase. This reduction is a result of the concurrent increase in the volume fraction of the delta phase. For gamma '' particle size lower or equal to 35 nm, strengthening is governed by the shearing mechanism. Beyond 35 nm gamma '' particle size, the gamma '' precipitates are crossed preferably by bypassing mechanism. A relationship between the 0.2% yield strength and the Vickers micro-hardness was determined.