Abstract
The improvement of the high-temperature oxidation resistance remains an ambitious goal for the design of new γ/γ′-strengthened Co-base superalloys, since their oxidation resistance beyond 800 °C still ranks behind their Ni-base counterparts. To better understand the origin of the poor oxidation resistance at higher temperatures, this study focuses on early stages of oxidation of four quaternary (Co-Al-W-Ta system) Co-base model alloys with a two-phase γ/γ′-microstructure and varying γ′-volume fraction at 800 °C, 850 °C and 900 °C. Based on time-resolved isothermal gravimetric analysis (TGA) in synthetic air and detailed electron microscopic analysis, the role of the γ-channel width (or γ′-volume fraction), the surface preparation prior to exposure (polishing versus shot-peening), and the heating conditions (synthetic air versus argon) on protective alumina growth is elucidated. Firstly, for alloys of increased γ′-volume fractions slower oxidation kinetics prevailed. Secondly, the two-phase microstructure was found to decisively affect the propagation of the internal oxidation front at the early stages of oxidation. Thirdly, shot-peening prior to exposure together with a lack of oxygen availability during heating was identified to foster protective alumina growth, accompanied by TCP-phase formation in the substrate. The critical role of a high Al availability in the alloy for a rapid growth of protective alumina and the relating challenges in alloy development regarding, for example, phase stability in this relatively novel Co-base alloy class are discussed in detail.
Highlights
In 2006, Sato et al [1] discovered an ordered Co3(Al,W) phase (γ′) with L12 crystal structure in the ternary Co-Al-W-system
Since the opportunities for further optimisation of the well-established Ni-base superalloys are considered to be largely exhausted and an appropriate hardening mechanism for Co-alloys is available, the fact of a 40 K higher melting point of pure Co compared to pure Ni generated major expectations toward the high-temperature properties of these materials [2, 5, 6]
Each scale is composed of an outer Co-rich oxide layer, possessing a significant amount of porosity that is especially pronounced after exposure at 850 °C and 900 °C
Summary
In 2006, Sato et al [1] discovered an ordered Co3(Al,W) phase (γ′) with L12 crystal structure in the ternary Co-Al-W-system. Regarding the content of elements forming protective scales (Cr or Al), a main obstacle in Co-based systems with γ/γ′-microstructure is the narrow phase field in which the hardening-phase is stable, and no TCP-phase formation is present This issue is elucidated by Omori et al for an isothermal section of the ternary Co-Al-W phase diagram at 900 °C [12]. Weiser et al demonstrated for a ternary Co-base alloy with γ/γ′-microstructure the exclusive formation of discrete alumina particles in the γ-channels at the internal oxidation front (IOF) These oxide particles were identified to grow due to the preferential Al delivery from the surrounding γ-phase during the first hours of exposure to air. The susceptibility of the investigated alloys to accelerated alumina formation due to plastic deformation in the subsurface region (shot-peening (SP) prior to exposure) and due to heating conditions (influence of oxygen partial pressure) is assessed
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
