In-Situ Determination of Precipitation Kinetics During Heat Treatment of Superalloy 718

Abstract

Two in-situ test techniques were used to obtain insight into the isothermal precipitation kinetics of c¢¢ and c¢ in superalloy 718. The first method consisted of measurements of Young's modulus using a dynamic-resonance method (DRM), and the other comprised determination of the evolution of the lattice parameter of the c-matrix phase via neutron diffraction. For both techniques, solution-treated-and-water-quenched samples were heated to a nominal test temperature of 923 K, 953 K, 1013 K, or 1053 K and held for a time of~5 to 50 hours, at the end of which a near-steady condition in terms of an apparently-constant modulus/lattice parameter had been achieved. The observations from each test technique suggested sequential periods of rapid initial increase in volume fraction followed by a gradually-decreasing rate of change. The data were converted to transformed fraction as a function of time and interpreted in terms of phenomenological (JMAK) and mechanistic (nucleation-and-growth) models. From the former approach, Avrami exponents which decreased from approximately unity at the lower two temperatures to~0.5 at the highest temperature, were deduced. The transformed fraction-versus-time behaviors were well replicated using fast-acting numerical simulations based on the mechanistic model. These simulations highlighted the competition between nucleation and growth in determining overall transformation kinetics.

The Application of Differential Scanning Calorimetry to Investigate Precipitation Behavior in Nickel-Base Superalloys Under Continuous Cooling and Heating Conditions

“…Key Input Parameters for 718 Simulations in Refs. [23] and [38] Parameter Value III) and the present measurements.…”

“…A number of the key input parameters for the present cooling/heating simulations were identical to those used in previous analyses of precipitation under isothermal conditions (Table II). [23,38] In this prior work, special attention was paid to the sensitivity of kinetic predictions to precipitate-matrix (c¢¢-c and c¢-c) interface energies/elastic (misfit) strain energies, which play a key role in nucleation, and effective diffusivities, which control subsequent precipitate growth. In the present work, two key thermodynamic coefficients, which affect nucleation through their influence on the evolution of supersaturation and the bulk free energy of phase transformation during cooling or heating, were investigated.…”

“…For this reason, other (non-destructive) techniques such as those based on electrical resistivity, differential thermal analysis (DTA), ''single-sensor'' DTA, ultrasonic/modulus measurements, and neutron/X-ray diffraction have been applied to obtain qualitative and sometimes quantitative results. [19][20][21][22][23] Experimental challenges in the determination of precipitation behavior in nickel-base superalloys can also be mitigated using modeling-and-simulation techniques, provided the assumed physical basis underlying microstructure evolution (e.g., nucleation, growth, and coarsening) is sound and the material properties required for numerical calculations are known accurately. Modeling techniques fall into two main categories, mean field and phase field.…”

“…The work reported here was part of ongoing efforts [16,18,23,29,38] to develop novel test methods and fast-acting simulation tools to quantify precipitation behavior in nickel-base superalloys. In the present research, the focus was on behavior under continuous cooling and heating conditions for both wrought and PM superalloys.…”

Post-Weld Heat Treatment of Additively Manufactured Inconel 718 Welded to Forged Ni-Based Superalloy AD730 by Linear Friction Welding

“…Therefore, interactions between these fields can block the dislocation movements and increase the strength of the material. [7,40,64] Therefore, the precipitation of mainly c¢¢ and c¢ after PWHT and grain refinement [17] and solid-solution strengthening [12,16,17,65,66] could enhance the hardness of the IN718 alloy. Although grain refinement is a mechanism used to increase the hardness in as-linear friction welded conditions (W1 and W2), the grain sizes did not change after PWHT.…”