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

Anthony De Luca,Christoph Kenel,Joanna Pado,Shreyas S Joglekar,David C Dunand,Christian Leinenbach

doi:10.1016/j.jmrt.2021.09.076

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.

Highlights

Oxide dispersion strengthened (ODS) alloys have been extensively studied for their superior strength, corrosion- and creep resistance at temperatures beyond those where precipitation strengthening is effective [1e3]
No ternary Y4Al2O9 nanodispersoids were observed by Transmission electron microscopy (TEM), only large slag particle which formed during solidification
To investigate the high-temperature stability of an additively manufactured Y2O3-modified nickel-base superalloy, as well as the microstructural changes induced by the presence of dispersoids, a high-g0 NieCreAleTi alloy, with and without 0.5% Y2O3 additions, was studied after various postprocessing treatments

Summary

Introduction

Oxide dispersion strengthened (ODS) alloys have been extensively studied for their superior strength, corrosion- and creep resistance at temperatures beyond those where precipitation strengthening is effective [1e3]. The development of mechanical alloying sparked the development of a range of ODS alloys starting in the 1970s including Ni-, Fe-, Co-, W-, Cu- and Al-based alloys [1]. Ni alloys included thoriated Nickel and NieCr-based ODS with improved oxidation resistance [2,5e7]. While superior at high homologous temperatures, these alloys suffered from low strength at intermediate temperatures, being outperformed by conventionally cast Ni-base superalloys. This was addressed by the development of ODS Ni-base superalloys combining matrix solution-strengthening, g0 precipitation, and dispersoid strengthening, culminating in the development of commercial MA6000 (SpecialMetals, Nie15Cre4.5Ale2.5Tie4We2Tae2.5Fee1.1Y2O3), PM3030

Results

Discussion

Conclusion