Combining Experiments and Atom Probe Tomography‐Informed Simulations on γ′ Precipitation Strengthening in the Polycrystalline Ni‐Base Superalloy A718Plus

Abstract

The strength of superalloys is strongly influenced by γ′ precipitates, whose size and volume fraction which can be adjusted by heat treatments. According to classical precipitation strengthening models, an increasing precipitate diameter should lead to a transition from weak to strong coupling of the dislocation pairs that form superdislocations in the γ′ phase. We show that long‐term annealing of the Ni‐base superalloy A718Plus at 670 and 680 °C increases the alloy's strength without significantly changing the grain size and η fraction. To understand the effect of the slight increase in γ′ size, detailed atom probe tomography (APT) was performed. Here, different field evaporation rates of the phases strongly affect the determination of the volume fraction when using the usual isosurface construction. This can be mitigated by considering the number density of atoms inside and outside the γ′ precipitates. Using an approximation of the precipitate shapes and arrangements from the APT data in atomistic simulations revealed that precipitate shearing by both, weakly and strongly coupled dislocations can occur in the same sample due to the wide distribution of precipitate sizes. These results highlight the need for advanced strengthening models that take into account the γ′ size distribution.

Atomic-scale modeling of superalloys

“…Kohler et al 145 used a similar methodology but focused on even smaller precipitates. The more relevant case of studying the interaction of super dislocations with spherical precipitates was recently simulated by Hocker et al 146 and Kirchmayer et al 147 . The former group used a similar setup as in 144,145 using regular arrays of spherical precipitates.…”

“…dislocations is larger or smaller than the precipitate diameter (corresponding to weak or strong coupling between the superpartial dislocations 148 ), different partial dislocations govern the critical resolved shear stress (CRSS) to pass the precipitate. Kirchmayer et al, however, used realistic precipitate morphologies and arrangement obtained from atom probe tomography (APT), and were thus able to show that for a relatively wide distribution of precipitate sizes weak and strong pair coupling can be at play simultaneously 147 . Spherical precipitates were also studied by Takahashi et al 149 and Kondo et al 150 , however, in their simulations the precipitates consisted of the γ phase which were cut by superdislocations from the surrounding Ni 3 Al γ phase.…”

Near-Surface and Bulk Dissolution Behavior of γ′ Precipitates in Nickel-Based VDM® Alloy 780 Studied with In-Situ Lab-Source and Synchrotron X-ray Diffraction

“…In recent decades, many efforts have been made to develop new polycrystalline Ni-based alloys with operating temperatures above 650 • C, while retaining the good mechanical and processing characteristics of alloy 718 [20][21][22]. One very promising candidate is the new VDM ® Alloy 780 [23,24].…”

Implementation of Nanoscale Secondary‐Ion Mass Spectrometry Analyses: Application to Ni‐Based Superalloys

“…A strong knowledge of the sample or a correlative approach with data obtained by TEM can be used to correct these artefacts. [4][5][6][7] Recent work reports the comparison or correlation between APT and secondary-ion mass spectrometry (SIMS) techniques demonstrating the complementarity of the two chemical analysis methods. [8,9] The SIMS method is commonly used to detect trace elements and light elements due to its ultimate sensitivity.…”