H. Al-Badairy

A comparison of breakaway oxidation in wedge-shaped and parallel sided coupons of FeCrAl alloys

Abstract At elevated temperatures, FeCrAl based alloys go into non-protective breakaway oxidation when the aluminium content drops below a critical level and α-alumina ceases to form. For 2-mm thick coupons, even at 1,300°C, this may take several thousand hours of isothermal oxidation and it can be quite difficult to obtain samples in the critical period immediately before breakaway commences. An alternative approach has been developed, which involves the oxidation of wedge-shaped samples with a 7° taper. The surface to volume ratio varies along the wedge, hence the aluminium reservoir is depleted first at the thin end of the sample, where breakaway commences, and then moves progressively up to the wedge. There is always a region on each sample which can be studied next to the breakaway area. Although the test can be used to rank alloys by the ease with which breakaway progresses, there is clearly a difference between a parallel sided coupon just prior to breakaway and a wedge where one adjacent region has already gone into breakaway. Electron microscopy and analysis have been used to explore these differences and preliminary results are reported.

FEG-SEM examination of alumina scales formed on FeCrAl alloys

Abstract Field emission gun scanning electron microscopy (FEGSEM) with electron backscattered diffraction (EBSD) has been used to investigate the microstructure and the oxidation behaviour of ultra-high purity Fe–20Cr–5Al model alloys and a commercial Fe–20Cr–5Al alloy. The model alloys contain controlled additions of phosphorus and carbon impurities and increased levels of more beneficial elements including yttrium, hafnium and titanium. The samples studied were oxidised at 800°C and 1200°C in humidified air for up to 3100 h, and 900°C and 1000°C for 1 h in laboratory air. At the higher temperature, well-adhered, compact and highly protective α-alumina scales formed, whereas at the lower temperature the scales formed were a less protective type of metastable alumina. Preliminary examination showed that the texture of the formed alumina scale was unaffected by the texture of the underlying substrate and the substrate compositions. At the higher temperature, the study revealed that the alumina scale comprised two distinct regions; the outer region at the scale/gas interface contained small, equiaxed (0.5–1 micron) grains and the inner region at the scale/metal interface contained, columnar grains that are 2–3 times larger than the equiaxed ones. However, at the lower temperature these two distinct regions were not apparent. Instead, grains of predominantly metastable alumina were observed. The links between texture morphology and oxide growth mechanisms will be discussed in this paper.

Significance of minor alloying additions and impurities on alumina scale growth and adherence in FeCrAl alloys

AbstractUltra-high purity Fe–Cr–Al–Y model alloys with controlled additions of impurities such as phosphorus and carbon, and potentially more beneficial elements such as titanium and zirconium have been prepared by induction melting in water-cooled, silver crucibles. 1 mm thick samples were then prepared by hot and cold rolling and annealing prior to cyclic oxidation in air at temperatures in the range 1100–1300°C. Other impurities were kept to a minimum of <10 ppm. scanning electron microscopy, Auger surface analysis and Scanning Transmission Electron Microscopy were used to characterise the samples both before and after oxidation.Weight gain studies during oxidation showed that the high phosphorus containing alloy went into breakaway very quickly, after only 200 hours at 1300°C, while the Ti and Zr rich samples lasted for 1900 hours and 3300 hours respectively. In some cases, chromium or titanium rich precipitates were found along the alloy grain boundaries, often associated with carbon, while in other ...

Air oxidation of commercial FeCrAlRE alloy foils between 800 and 950°C

Abstract The oxidation behaviour of 30 μm and 50 μm thick Aluchrom YHf and 50 μm thick Kanthal AF foils have been examined during discontinuous exposure (100 h cycles) to laboratory air at 800, 850, 900 and 950°C. Oxidation was continued out to 3000 h or until chemical failure ensued, whichever was the shorter. Resulting observations include two oxidation temperature domains, a two-stage oxidation process leading to a duplex (transitional-α) alumina scale and ultimately, a two-stage chemical failure process of the oxide scale leading to breakaway/non-protective attack. To unravel the underlying mechanisms involved, the scales were examined by a range of microscopy and analytical techniques, including optical and scanning electron microscopy (SEM), electron microprobe analysis (EPMA), energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD).

Microstructural development of α alumina scales developed on FeCrAl alloys

Abstract A scanning electron microscope (SEM) study was conducted of α alumina scales developed on the surface of FeCrAl alloys during high temperature oxidation, using backscattered electron imaging, cathodoluminescence (CL) imaging, electron backscatter diffraction (EBSD) and X-ray microanalysis. CL variations relate to trace element distributions and to crystallographic orientations. The compositional variations suggest that grain boundary diffusion was dominant in the scale. The α alumina grains are equiaxed at the outer scale surface and columnar between here and the metal oxide interface; both grain types have a very strong crystallographic preferred orientation (CPO): a fibre texture of r axes perpendicular to the sheet. The CPO cannot be explained by substrate control or by plastic deformation. There is a correlation of the orientation of the anisotropic elastic properties of the scale and the likely thermal stress systems: one possible explanation is that the CPO may relate to oriented oxide nucleation during heating.

Characterisation studies of chromia formation on commercial FeCrAlRE alloy foils following chemically induced failure at 900°C

Abstract The chemical failure of commercial FeCrAlRE (where RE is a reactive element) alloys, such as Aluchrom YHf and Kanthal AF foils (30–50 μm thick), is induced by oxidation, in air, at 800–950°C. This is also the life limiting process of this family of alloys at higher temperatures (≥1100°C) and occurs when the Al activity in the alloy becomes depleted below a critical level, as a result of oxidation. At this junction, the integrity of the protective alumina scale is not sustainable but continued ‘pseudo-protection’ can be provided by chromia formation beneath the alumina layer. Oxidation continues at a slow linear rate controlled by transport through the alumina scale. The duration of the pseudo-protection region can extend to several hundred hours and has considerable technological implications, if it could also be used in defining component lifetimes. Greater understanding is, therefore, required concerning the formation and growth of the chromia subscale, as this has received only limited attention to date. Of particular possible significance is the mode of formation of the subscale. At temperatures above 1000°C, this forms a continuous, essentially uniform layer. In contrast, at ∼900°C scalloped shaped pits of chromia develop beneath the outer alumina scale. Hence a detailed characterisation study has been undertaken of the formation and development of this chromia attack; the results of which form the basis of this paper. The study has involved the deployment of a range of microscopy and analytical techniques, including optical and scanning electron microscopy, electron microprobe analysis (EPMA) and energy dispersive X-ray analysis (EDX).