On the High-Temperature Oxidation Behavior of a Ta-Containing Quaternary Co-Base Model Alloy System with γ/γ′-Microstructure - Influence of γ′-Volume Fraction, Surface State, and Heating Condition on Alumina Growth

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.

Protective Alumina Scale Growth at 900 °C for a Ni- and Cr-Free Co-Base Model Alloy with γ/γ’-Microstructure: Synergistic Effects by Combining Shot-Peening and Halogenation

“…In contrast, all alloys in polished surface state were prone to exceptionally high oxidation kinetics caused by massive internal oxidation and external growth of Co oxides. In accordance with other authors, it was concluded that shot-peening-and by that plastic deformation-induces additional lattice defects that can act as short circuit paths for diffusion and thereby increase the effective solute diffusivity [18][19][20][21].…”

“…When the exposure procedure abstained from heating in argon atmosphere, for shot-peened samples clearly less continuous and protective alumina scales with higher amounts of transient oxides developed [18]. In contrast, for polished samples, no significant effect of the heating atmosphere on oxidation kinetics was found [18].…”

“…In a recent study, we demonstrated a strategy that allowed to tremendously increase the oxidation resistance of four Co-base model alloys with γ/γ'microstructure [18]. Based on a combination of shot-peening (prior to oxidation) and a temporary exposure to oxygen deficient atmosphere (impure argon during heating), protective alumina scale growth could be achieved.…”

“…When the exposure procedure abstained from heating in argon atmosphere, for shot-peened samples clearly less continuous and protective alumina scales with higher amounts of transient oxides developed [18]. In contrast, for polished samples, no significant effect of the heating atmosphere on oxidation kinetics was found [18]. Therefore, it could be concluded that the shot-peening treatment did increase the tendency of the alloys to form protective alumina scales, whereas alone, it was not sufficient to initiate the latter.…”

“…Therefore, it could be concluded that the shot-peening treatment did increase the tendency of the alloys to form protective alumina scales, whereas alone, it was not sufficient to initiate the latter. For more details on the interaction of surface deformation and atmosphere, the reader is referred to [18].…”