Unraveling the Complexity of Nano-Dispersoids in the Oxide Dispersion Strengthened Alloy 617

Abstract

Nanocrystalline oxides are mainly responsible for Ni-base oxide dispersion strengthened (ODS) superalloys excellent thermo-mechanical properties. To establish the microstructural correlations between the metallic matrix and various oxide dispersoids, we report here the atomic-scale structure and chemistry of the complex nano-oxide dispersoids. Ultrahigh-resolution Cs-aberration-corrected scanning transmission electron microscopy (STEM) based techniques have been used to resolve nano-dispersoids in the Alloy 617 ODS. These nano-oxides, interestingly, possess a variety of high-angle annular dark-field (HAADF) contrasts, that is, bright, dark, and bi-phases. Both the light and heavy atoms have been found to be present in Y–Al–O complex-oxide nanostructures in varying quantities and forming a characteristic interface with the metallic matrix. In overcoming the limitation of conventional STEM-HAADF imaging, the integrated differential phase-contrast imaging technique was employed to investigate the oxygen atoms along with other elements in the dispersoids and its interface with the matrix. The most intriguing aspect of the study is the discovery of a few atoms thick Al2O3 interlayer (shell) around a monoclinic Y–Al–O core in the Ni-matrix. On the other hand, when the dispersoid is a hexagonal type Y–Al–O complex, the interface energy is already low, maintaining a semi-coherent interface and it was devoid of a shell.

Production of Oxide Dispersion Strengthened Inconel 718 Alloys Using Conventional Powder Metallurgy and Additive Manufacturing Methods

“…While the particle in Figure 7a has an incoherent interface around its periphery, the particle in Figure 7b exhibits a crystalline structure with semi-coherency with the matrix on the left side and a certain level of amorphization on the right side. A similar phenomenon which is called 'bi-phase' formation has been reported by Sinha et al in ODS IN617 alloys [43]. This has been attributed to the formation mechanisms of the Y-Al-O particles due to the reaction between Y2O3 and Al2O3 [44].…”

“…Figures 5e and f show an HRTEM micrograph of ~7 nm size nano-particle and corresponding FFT diffractogram, respectively. It has been found that the measured interplanar distances and angles in Table 3 are close to the parameters of hexagonal Y-Al-O (PDF# 00-054-0621) whose lattice parameters are, a = b = 0.368 nm, c = 1.052 nm, and γ = 120° [43]. However, the stoichiometry of the observed particle seems to be slightly different which is inferred from the values which are slightly off.…”