Atom Probe Microscopy of Strengthening Effects in Alloy 718

Abstract

Polycrystalline Ni-based superalloys for aerospace and power generation applications are often precipitation hardened to achieve strengthening at elevated temperatures. Here, atom probe microscopy has become an essential tool to study the complex morphology of nanoscale precipitates. This study focuses on Alloy 718, which is hardened by semi-coherent, ordered γ′ (Ni3(Al, Ti)) and γ″ (Ni3(Nb)) particles. According to previous research, these particles often occur as duplets or triplets with a stacking sequence dependent on prior processing. This creates various interfaces with a strong impact on the mechanical properties, highlighting the importance of quantitative studies which are challenging with electron microscopy. We present atom probe data reconstruction and analysis approaches particularly suited for precipitation hardened superalloys. While voltage atom probe allows for an accurate reconstruction, the acquired data volume is often limited. Laser-assisted atom probe provides statistically significant data, but the loss of crystallographic information requires correlation with voltage-mode datasets. We further describe an advanced iso-surface method where initially arbitrarily chosen concentration thresholds of Al + Ti for γ′ and Nb for γ″ particles are optimized. Recognizing the importance of the precipitate stacking order, the different types of precipitate interfaces are quantified, and these methods may be applicable to other engineering alloys.

On the early stages of precipitation during direct ageing of Alloy 718

“…Voltage-mode APM datasets were calibrated using atom probe crystallography routines [31], and laser-assisted mode datasets were calibrated based on the precipitate morphology obtained from voltage-mode [32]. Full-size datasets approximated the tip volume as truncated cone to determine the precipitate volume fractions, radii, number density, morphology and chemical composition.…”

“…Full-size datasets approximated the tip volume as truncated cone to determine the precipitate volume fractions, radii, number density, morphology and chemical composition. The datasets were cropped to identical volumes for improved visualisation, determination of the bulk chemical composition, and the interfacemethod [32]. Figures 7 and 8 are projections of datasets which were cropped to cylinders of 30x30x50 nm 3 in voltage-mode, or 50x50x300 nm 3 in laser-assisted mode.…”

“…Therefore, all of the APM data shown in this study are either 30 nm (voltage-mode) or 50 nm (laser-assisted) thick. Details to the enhanced iso-surface and interface methods are provided elsewhere [32].…”

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

“…[14,15] For example, volume fractions reported in the literature for heat-treated 718 have varied from approximately 4 to 15 pct for c¢¢ and from 4 to 18 pct for and c¢. [16][17][18][19][20] Challenges in characterizing the very fine microstructure typically formed in solution-treated-and-aged 718 via transmission or high-resolution-scanning electron microscopy and atom-probe tomography have spurred the development of other approaches to understand/quantify phase-transformation kinetics. These include non-destructive evaluation (in-situ) methods such as electrical resistivity, various ultrasonic methods, neutron diffraction (e.g., to determine the temporal evolution of lattice parameters under isothermal conditions [21] ), and differential thermal analysis (e.g., to establish the onset of precipitation or dissolution of c¢¢, c¢, and d during heating and cooling [22,23] ).…”

On the Microstructures and Anisotropic Mechanical Behaviours of Additively Manufactured IN718

“…The thermal stability of γ ′′ limits the main applications of IN718 under 650 • C [11,12]. In the aged conventional IN718, the morphologies of γ ′ and γ ′′ can be in the form of γ ′ or γ ′′ monoliths, γ ′ /γ ′′ duplets, sandwich-like γ ′′ /γ ′ /γ ′′ or γ ′ /γ ′′ /γ ′ [13]. By increasing the (Al + Ti)/Nb atomic ratio beyond 0.9 (which is 0.69 for conventional IN718), a compact morphology with cube-shaped γ ′ particles coated with a γ ′′ shell over their six faces, has been proved to associate with better thermal stability than the conventional IN718 [14].…”