Thermal stability and influence of Y2O3 dispersoids on the heat treatment response of an additively manufactured ODS Ni–Cr–Al–Ti γ/γ′ superalloy

Abstract

The post-processing treatment response of a Y2O3 oxide-dispersion-strengthened (ODS) and γ/γ′-strengthened Ni–8Cr–5.5Al–1Ti (wt.%) model alloy, produced by laser powder bed fusion (L-PBF) is studied. The solutionizing treatment at 1260 °C induces significant recrystallization of the initial elongated grain microstructure, characteristic of L-PBF processing. Grain-boundary pinning by dispersoids affects the recrystallization efficiency, leaving behind pockets of fine 1–10 μm grains in-between 200–400 μm large grains with complex shapes. The Y2O3/Y2O2S dispersoids exhibit excellent coarsening resistance during solutionizing, and are stable during subsequent aging heat treatments at 850 °C. The slow cooling from solutionizing promotes the formation of secondary and tertiary γ′ precipitates, with the ODS particles located within the γ channels. Within the timescale of the isothermal aging, up to ∼1000 h, the coarsening rate of the γ′ precipitates is slightly faster in the ODS material. This allows to independently optimize the γ′ volume fraction and size for balanced properties. Additionally, the capability of hot isostatic pressing (HIP) to close and heal cracks is investigated under various consolidation parameters. This post-process treatment allows to significantly widen the L-PBF processing window of the alloy.

Keywords

• Additive manufacturing
• Laser powder bed fusion
• Nickel superalloy
• Oxide dispersion strengthening
• Precipitation strengthening