The available literature on the crystal structure of the metastable alumina polymorphs and their associated transitions is critically reviewed and summarized. All the metastable alumina structures have been identified as ordered or partially ordered cation arrays on the interstitial sites of an approximately close-packed oxygen sublattice (either face-centered cubic or hexagonal close packed). The analysis of the symmetry relations between reported alumina polymorphs having an approximately face-centered cubic packing of the oxygen anions allows for an exact interpretation of all the complex domain structures that have been observed experimentally. Possible mechanisms for the phase transitions between the different alumina polymorphs also are discussed.

Metastable alumina polymorphs : Crystal structures and transition sequences

Thermally grown oxide scales that form beneath ZrO 2 top coats play an important role in determining the performance of thermal barrier coatings. Numerous factors, including the composition of both the alloy substrate and the bond coat, affect adhesion of the α -Al 2 O 3 scale. Three areas of focus in the formation of an `ideal', adherent scale encompass: (1) migration of Al and other elements in the metal substrate, (2) segregation of elements to the metal–scale interface and the scale grain boundaries, and (3) generation of stresses in the scale. Examples of the effects of reactive elements, Pt, indigenous S, and reaction temperature on scale adhesion are discussed.

Substrate and bond coat compositions: factors affecting alumina scale adhesion

Oxidation of NiAl and FeAl

The isothermal oxidation behavior of bulk Ti2AlC in air has been investigated in temperature range 1000–1300°C for exposure time up to 20 hr by TGA, XRD, and SEM/EDS The results demonstrated that Ti2AlC had excellent oxidation resistance The oxidation of Ti2AlC obeyed a cubic law with cubic rate constants, kc, increasing from 238×10-12 to 213×10-10 kg3/m6/sec as the temperature increased from 1000 to 1300°C As revealed by X-ray diffraction (XRD) and SEM/EDS results, scales consisting of a continuous inner α-Al2O3 layer and a discontinuous outer TiO2 (rutile) layer formed on the Ti2AlC substrate A possible mechanism for the selective oxidation of Al to form protective alumina is proposed in comparison with the oxidation of Ti–Al alloys In addition, the scales had good adhesion to the Ti2AlC substrate during thermal cycling

High-Temperature Oxidation Behavior of Ti2AlC in Air

Oxidation behavior of arc melted AlCoCrFeNi multi-component high-entropy alloys

Transient oxidation of NiAl

The microstructural development of oxidizedγ-TiAl is presented with a focus on oxidation inair. The investigations were carried out usingconventional, analytical, and, especially,energy-filtered transmission electron microscopy (EFTEM). Threeimportant points were studied in detail: (1) the“nitrogen effect,” (2) the“surface-finish effect,” and (3) thesubsurface zone. Nitrogen leads to the formation of TiN andTi2AlN at the metal-scale interfaceinterrupting alumina and thereby preventing thedevelopment of a continuous alumina layer. TheAl-depletion layer formed during the oxidation process develops from a single-phaselayer, consisting of a cubic phase, to a two-phaselayer, consisting of the cubic phase andα2-Ti3Al. The cubic phase isnot known in the system Ti-Al-O-N. Oxidation in oxygen depends on the surfacepreparation of the sample with rapid oxidation kineticsfor fine polishing and slow kinetics for a 600-gritSiC-paper finish. The rougher surface finish leads to the development of a recrystallization zonenear the surface and supports the formation of acontinuous alumina layer in the early stages ofoxidation. As for the oxidation in air, the cubic phaseis formed first underneath the oxide scale,followed by α2-Ti3Alformation.

Microstructural study of oxidized γ-tial

The oxidation behavior of undoped and Y-dopedhigh-purity FeCrAl at 1200°C was investigated. Thescale-growth processes were studied using two-stage16O/18O experiments andhigh-resolution imaging SIMS. SEM, TEM, and scanning transmission electronmicroscopy-electron-dispersive X-ray spectrometry(STEM-EDS) studies were performed in order to determinethe microstructure of the oxide and the chemistry of the metal-oxide interface. An equiaxedα-Al2O3 scale was found togrow by simultaneous outward Al and inward oxygendiffusion on the undoped FeCrAl. In contrast, a columnarα-Al2O3 grain structure on the Y-doped FeCrAl was produced by inwardoxygen diffusion; outward Al diffusion was suppressed.Y was shown to segregate toα-Al2O3 grain boundaries andto the metal-oxide interface.

The Effect of Yttrium on the Growth Process and Microstructure of α-Al2O3 on FeCrAl

High-resolution SIMS and TEM have been used to evaluate growth processes and interfacial segregation occurring in α-Al2O3 scales grown at 1200°C on β-NiAl containing zirconium or yttrium.18O/SIMS shows that the extent of aluminum diffusion occurring during α-Al2O3 growth is reduced by the presence of these alloying elements, which are seen by SIMS imaging as oxide particles within the scale. STEM/EDS of the same oxide scales show that zirconium and yttrium also segregated to the oxide-alloy interface to the extent, respectively, of ≈0.15 and ≈0.07 of a monolayer and to oxide grain boundaries (≈0.2 monolayer). The complementary information provided by SIMS, TEM, and STEM provides a better understanding of the role of “reactive elements” in modifying scale-growth processes.

High-resolution SIMS and analytical TEM evaluation of alumina scales on β -NiAl containing Zr or Y

Conventional electron microscopy, analytical electron microscopy, high resolution electron microscopy and high resolution SIMS have been used to investigate the effect of the reactive elements, Yand Zr, on the oxide scale formation on NiAl. Polycrystalline NiAl samples, doped with either 0. 1 wt% Yor 0.2 wt% Zr, were oxidized in air at 1200°C. 18O tracer experiments in conjunction with high resolution SIMS suggest that the reactive elements reduce the outward diffusion of cations. Energy dispersive X-ray spectroscopy on a dedicated STEM showed that the reactive elements segregate to the grain boundaries in the oxide scale and to the metal/oxide interface. The amount at the oxide scale grain boundaries was calculated to be 0.2 monolayers for both Zr and Y doped NiAl. The amounts of segregation were calculated to be 0.15 monolayers (Zr-doped) and 0.07 monolayers (Y-doped) at the metal/oxide interface. The presence of sulfur was detected in Y-rich particles in the NiAl close to the interface.



Untersuchungen zur Segregation von Y und Zr bei der Oxidation von NiAl



Konventionelle, analytische und hochauflosende Transmissionselektronenmikroskopie (TEM) sowie hochauflosende Sekundarionenmassenspektrometrie (SIMS) wurden benutzt, um den Effekt von reaktiven Elementen auf die Oxidation von NiAl zu untersuchen. Polykristalline NiAl-Proben, dotiert mit 0,1 Mas.-% Y oder 0,2 Mas.-% Zr, wurden bei einer Temperatur von 1200°C oxidiert. 18O Tracer-Experimente in Verbindung mit hochauflosenden SIMS-Untersuchungen deuten darauf hin, das reaktive Elemente die nach ausen gerichtete Diffusion von Kationen reduzieren. Energiedispersive Rontgenspektroskopie mit einem dedizierten STEM zeigte, das die reaktiven Elemente an den Korngrenzen in der Oxidschicht und an der Metall/Oxid-Grenzflache segregieren. Der Anteil von Y bzw. Zr an den Korngrenzen in der Oxidschicht betragt 0,2 Monolagen. Demgegenuber wurden an der Metall/Oxid-Grenzflache 0,15 Monolagen (Zr-dotiert) bzw. 0,07 Monolagen (Y-dotiert) gefunden. In Y-reichen Teilchen im NiAl nahe der Metall/Oxid-Grenzflache konnte Schwefel nachgewiesen werden.

E. Schumann

Papers

Transient oxidation of NiAl

Microstructural study of oxidized γ-tial

The Effect of Yttrium on the Growth Process and Microstructure of α-Al2O3 on FeCrAl

High-resolution SIMS and analytical TEM evaluation of alumina scales on β -NiAl containing Zr or Y

Segregation studies of oxidized Y and Zr doped NiAl