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Oxide phase development upon high temperature oxidation of γ‐NiCrAl alloys
Author(s) -
Nijdam T. J.,
van der Pers N. M.,
Sloof W. G.
Publication year - 2006
Publication title -
materials and corrosion
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.487
H-Index - 55
eISSN - 1521-4176
pISSN - 0947-5117
DOI - 10.1002/maco.200503934
Subject(s) - oxide , non blocking i/o , scanning electron microscope , alloy , materials science , phase (matter) , analytical chemistry (journal) , oxygen , kinetics , layer (electronics) , partial pressure , metallurgy , chemical engineering , chemistry , composite material , catalysis , biochemistry , physics , organic chemistry , chromatography , quantum mechanics , engineering
The amount of each oxide phase developed upon thermal oxidation of a γ‐Ni‐27Cr‐9Al (at.%) alloy at 1353 K and 1443 K and a partial oxygen pressure of 20 kPa is determined with in‐situ high temperature X‐ray Diffractometry (XRD). The XRD results are compared with microstructural observations from Scanning Electron Microscope (SEM) backscattered electron images, and model calculations using a coupled thermodynamic‐kinetic oxidation model. It is shown that for short oxidation times, the oxide scale consists of an outer layer of NiO on top of an intermediate layer of Cr 2 O 3 and an inner zone of isolated α‐Al 2 O 3 precipitates in the alloy. The amounts of Cr 2 O 3 and NiO in the oxide scale attain their maximum values when successively continuous Cr 2 O 3 and α‐Al 2 O 3 layers are formed. Then a transition from very fast to slow parabolic growth kinetics occurs. During the slow parabolic growth, the total amount of non‐protective oxide phases (i.e. all oxide phases excluding α‐Al 2 O 3 ) in the oxide scale maintain at an approximately constant value. The formation of NiCr 2 O 4 and subsequently NiAl 2 O 4 happens as a result of solid‐state reactions between the oxide phases within the oxide scale.