Evolution of Y2O3 dispersoids during laser powder bed fusion of oxide dispersion strengthened Ni-Cr-Al-Ti γ/γ’ superalloy

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

doi:10.1016/j.addma.2021.102224

Abstract

The successful synthesis of oxide-dispersion-strengthened (ODS) alloys via laser powder bed fusion ( L -PBF) requires a better understanding of the interaction of the oxide dispersoids with the metallic melt pool. Here, a γ/γ’-strengthened Ni-8Cr-5.5Al-1Ti (wt%) model alloy is studied, as a simplified version of the commercial CM247LC alloy, by melting pre-alloyed powders in which 0.5–1 wt% Y 2 O 3 nanoparticles were added via mechanical alloying. The Y 2 O 3 nanoparticles follow three distinct paths. First, the strong affinity between Y 2 O 3 and Al leads to the formation of Y 4 Al 2 O 9 slag which floats on the melt pool; if in excess, the slag leads to vertically aligned mm-size cavities, preventing complete consolidation of the alloy. Second, a high number density of oxide nanodispersoids is distributed within the alloy’s grain inducing a strong (100) texture and noticeably reduces grain size compared to the unmodified base alloy. Third, despite the high stability of Y 2 O 3 , the extreme temperatures achieved in the melt pool decompose some of the Y 2 O 3 precipitates leading to the formation of Ni- and Y-rich particles (16 nm in radius) and Y segregation to the alloy’s grain boundaries. The local composition on cracked grain boundaries is consistent with Ni 17 Y 2 having an embrittling and liquation effect. Based on these results, the critical role of Al in reacting with oxide nanodispersoids during L -PBF manufacturing is discussed, and various types of potentially more successful dispersoids are suggested. Powders of a Ni-8Cr-5.5Al-1Ti (wt%) model superalloy containing 0.5–1 wt% Y 2 O 3 nanodispersoids are melted via laser powder bed fusion. The Y 2 O 3 addition leads to various microstructural changes. The columnar grain structure is refined and a strong (100) texture is observed. Some Y 2 O 3 undergoes partial dissolution/reaction in the melt pool, despite being one of the most stable dispersoids conventionally used in powder metallurgy processed ODS alloys. The resulting free Y leads to significant grain boundary segregation with an embrittling effect, and the formation of Ni-Y-rich nanodispersoids throughout the alloy. Y 2 O 3 nanodisperoids that are not reduced are either incorporated directly into the matrix or scavenge S and form Y-O-S dispersoids. A fraction of the added oxide is not incorporated but reacts with Al to form Y 4 Al 2 O 9 slag, a reaction which is influenced by the energy density employed during the LPBF processing.

Highlights

The field of additive manufacturing (AM) of metallic alloys has rapidly developed in recent years as it represents an emerging process ing route for geometrically complex parts, from novel and established alloys [1,2,3]
The strong affinity between Y2O3 and Al leads to the formation of Y4Al2O9 slag which floats on the melt pool; if in excess, the slag leads to vertically aligned mm-size cavities, preventing complete consolidation of the alloy
While the part consolidation remains acceptable with 0.5 wt% Y2O3 dispersoids (~99.5% density of the Ni-Cr-Al-Ti alloy), a higher con centration of 1 wt% Y2O3 leads to the formation of very wide slag lenses (>100 μm), limiting the amount of dispersoids that can be added

Summary

Introduction

The field of additive manufacturing (AM) of metallic alloys has rapidly developed in recent years as it represents an emerging process ing route for geometrically complex parts, from novel and established alloys [1,2,3]. In addition to the capability to create parts with complex geometries, AM allows the use metal matrix composites [4,5] which are not processable by other methods. One such class of metallic material is oxide-dispersion-strengthened (ODS) alloys, which typically contain 1–3 vol% nanometric oxide nanoparticles to strengthen the metal matrix at ambient and elevated temperatures [6]. The high strength of these alloys makes it difficult to machine complex shapes, motivating research in AM as an alternative manufacturing method. Processing of ODS alloys through the liquid state brings novel capabilities, such as supersaturating metallic matrixes or entrapping small dispersoids, and poses challenges to avoid undesired reactions, melting or decom position of the oxide dispersoids due to the high local energy input

Results

Discussion

Conclusion