Transport mechanisms during the high-temperature oxidation of ternary γ/γ′ Co-base model alloys

Abstract

Over a decade ago, γ′-strengthened Co-base alloys were introduced as potential replacement for conventional Ni-base Superalloys. Insufficient resistance against high-temperature oxidation restricts the number of possible applications. The present study contributes to the understanding of elementary mechanisms such as material transport during extensive oxide scale formation on γ/γ′ Co-base alloys to explain their inferior oxidation behaviour. A clear dependency of the scale growth kinetics on W content and oxidation temperature is demonstrated by thermogravimetry and subsequent analysis of cross-sections. By means of electron backscattered diffraction (EBSD), the evolution of microstructures in the outer oxide layers were examined depending on the oxidation temperature. Sequential exposure of samples in 16O2- and 18O2-containing atmospheres proved counter-current material transport. The combination of focused ion beam (FIB) and secondary ion mass spectroscopy (SIMS) visualised the formation of new oxide phases mainly on the outer and inner interface of the oxide scale. An elaborate review of available transport paths for oxygen is given during the discussion of results. All experimental findings were combined to a coherent explanation of the inferior oxidation resistance of this relatively new class of high-temperature materials at temperatures above 800 °C.

High-temperature oxidation behaviour of AlxFeCrCoNi and AlTiVCr compositionally complex alloys

“…As such, the ability of high-temperature alloys to form an adherent, slow-growing and protective surface oxide (scale), nominally consisting of chromia (Cr 2 O 3 ) and alumina (Al 2 O 3 ), is essential for sustained high temperature and harsh environment exposure without premature failure 1,2 . Of the two family of high-temperature materials, i.e., alumina-formers and chromia-formers, alloys forming protective Al 2 O 3 scales such as iron (Fe)-based (FeCrAls) 1 , nickel (Ni)-based (NiCrAls) 3 , cobalt (Co)based (CoCrAls) 4 and nickel aluminates (NiAl)-type materials 5 are considered for applications with operating temperatures >900°C. This is because α-Al 2 O 3 scale possess several characteristics that are highly desired for the high-temperature oxidation performance of alloys including: (i) they grow very slowly, (ii) thermodynamic stability, and (iii) chemically inert 1,2 .…”

High-temperature oxidation behaviour of AlxFeCrCoNi and AlTiVCr compositionally complex alloys

“…The oxidation morphology of the CCAs was studied using SEM-STEM-EDXS (Figs. [3][4][5]. At both temperatures, the surface of AlTiVCr was fully covered by Ti-and V-rich oxides (Figs.…”

“…As such, the ability of high-temperature alloys to form an adherent, slow-growing and protective surface oxide (scale), nominally consisting of chromia (Cr2O3) and alumina (Al2O3), is essential for sustained high temperature and harsh environment exposure without premature failure [1,2]. Of the two family of high-temperature materials, i.e., alumina-and chromia-formers, alloys forming protective Al2O3 scales such as iron (Fe)-based (FeCrAls) [1], nickel (Ni)-based (NiCrAls) [3], cobalt (Co)-based (CoCrAls) [4] and nickel aluminates (NiAl)-type materials [5] are considered for applications with operating temperatures >900ºC. This is because α-Al2O3 scale possess several characteristics that are highly desired for the high-temperature oxidation performance of alloys including: (i) they grow very slowly, (ii) thermodynamic stability and (iii) chemically inert [1,2].…”

Effect of austenitic stainless steel cladding on the high-temperature oxidation resistance of Ferritic 2.25Cr-1Mo (Grade 22) steel using SMAW process

“…In this sample, some of this free phase was formed due to storage at 950 °C for a long time (60 h), but it had little effect on reducing oxidation resistance due to a low temperature in this experiment. Earlier studies show that the oxidation of ferritic steels can result in the formation of a multi-layered oxide scale on the surface consisting of Fe 2 O 3 , Fe 3 O 4 , and FeO [37,38].…”