Evolution of oxides and their microstructures at 800°C in a γ-γ’ stabilised Co-Ni-Al-Mo-Ta superalloy

• Published in: Corrosion science Vol. 155; pp. 46 - 54
• Main Authors: Das, Saurabh Mohan, Singh, Mahander Pratap, Chattopadhyay, Kamanio
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Ltd 15-07-2019; Elsevier BV
• Subjects: Aluminum oxide; Cobalt oxides; Grain boundaries; High temperature; Molybdenum; Multi oxide layers; Multilayers; Nickel base alloys; Oxidation; Solid solutions; STEM; Superalloys; γ-γ’ Cobalt based superalloys; Oxidation; γ-γ’ Cobalt based superalloys; Multi oxide layers; STEM
• ISSN: 0010-938X; 1879-0496
• DOI: 10.1016/j.corsci.2019.02.013

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•Detailed oxidation study of W-free low density Co base superalloy is presented.•Oxidation is primarily controlled by outward diffusion of Co and Ni.•S/TEM combined with EDS is applied to understand the oxide phase evolution.•A mechanism has been derived from these studies.
The paper reports mechanism of high temperature oxidation of the new γ’-strengthened Co-30Ni-10Al-5Mo-2Ta (at%) alloy. The plots of mass gain vs. time show a parabolic growth behaviour at a temperature range of 650–800 °C. The cross-sections of the oxidised samples reveal formation of a complex multilayer of oxides. The top layer comprises of Co3O4 followed by a solid solution of (Co1-x,Nix)O (x varying from 0-0.2). A middle layer evolves with time comprising hexagonal molybdate Co2Mo3O8, metastable TaO and amorphous Al2O3. The γ’ dissolve near the interface exposing the γ solid solution with growth of amorphous Al2O3 along the grain boundary.