TEM Observation of Phase Transformations of Alumina Scales Formed on Al-Deposited Fe-Cr-Al Foils

Andoh, Atsushi; Taniguchi, Shigeji; Shibata, Toshiharu

doi:10.4028/www.scientific.net/msf.369-372.303

Cited by 30 publications

(18 citation statements)

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“…This behavior agrees well with current understanding of the oxide development on this alloy. At 1000°C, α-Al 2 O 3 is the major phase on FeCrAl after 16 minutes [25]. Relaxation of the compressive stress must be achieved by the wrinkling of the scale and the FeCrAl alloy, which is known to occur during oxidation, and the phenomenon is believed to be driven by the biaxial compressive stress in the oxide film [26,27].…”

Section: Fe-40almentioning

confidence: 99%

Growth Strains and Stress Relaxation in Alumina Scales during High Temperature Oxidation

Hou

Paulikas

Veal

2004

MSF

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Abstract.A novel X-ray technique was used, exploiting synchrotron radiation at the Advanced Photon Source at Argonne National Laboratory, to investigate the growth stresses in α-Al 2 O 3 . In-situ measurements of Debye-Scherrer diffraction patterns from the scale were recorded during oxidation and cooling, and the elliptical distortion of the diffraction rings was analyzed to yield the in-plane strain. Fe28Al, and Ni-50Al (at. %) were studied. Data were acquired in air at temperatures between 950-1100 o C and during cool down. In all cases, the steady stage growth strain was relatively low (<0.1%) and was either tensile or compressive depending on the alloy. A higher tensile strain often existed during the initial oxidation period when transition alumina was present. Thermal stresses imposed on NiAl by reducing the sample temperature to 950 o C for a period of time showed noticeable stress relaxation by creep. Different degrees of relaxation were also found during cooling depending on alloy composition and scale microstructure. On all Fe-based alloys, the first formed α-Al 2 O 3 was highly textured with the degree of texture decreasing with further oxidation. The relationships between stress development, scale wrinkling, oxide phase changes, and the effect of reactive element addition on growth stresses are discussed. Results are compared with other reports of growth stresses in Al 2 O 3 scales.

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Section: Fe-40almentioning

confidence: 99%

Growth Strains and Stress Relaxation in Alumina Scales during High Temperature Oxidation

Hou

Paulikas

Veal

2004

MSF

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“…[11] and for a FeCrAl alloy with an aluminium coating produced by vapour deposition in ref. [12,13]. As is known for yttrium containing FeCrAl alloys [2,14,15] the a-alumina scale grows mainly by inward diffusion of oxygen along the grain boundaries of the scale.…”

Section: Discussionmentioning

confidence: 99%

A new austenitic alumina forming alloy: an aluminium‐coated FeNi32Cr20

Hattendorf¹,

Hojda²,

Naumenko³

et al. 2008

Materials & Corrosion

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The FeCrAl alloys owe their low oxidation rate to the formation of a slow growing α‐aluminium oxide scale. Therefore they are used, for example, as a substrate material in metal‐supported automotive catalytic converters. Increasing exhaust gas temperatures mean that, in addition to the oxidation properties, high temperature mechanical properties should also be improved. Compared to the ferritic FeCrAl alloys, austenitic alloys possess the required high mechanical strength at higher temperatures. However for most commercially available materials the oxidation resistance is not sufficient due to a low aluminium content. High aluminium contents are avoided in austenitic alloys, since they cause severe workability problems, even at aluminium contents, which are below the necessary amount to get a pure alumina scale. The newly developed material Nicrofer 3220 PAl (coated FeNiCrAl) consists of an austenitic FeNi32Cr20 alloy coated with aluminium on both sides. It combines the outstanding oxidation resistance of an alumina forming FeCrAl alloy with the advantage of the high temperature strength of an austenitic alloy. Additionally the oxidation is even lower than the oxidation of the commercial grade Aluchrom YHf (FeCr20Al6)—conventional homogenous FeCrAl. Aluminium coated FeNiCrAl can easily be formed into its final shape. Prior to service, an in situ heat treatment is recommended in order to optimize the properties.

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“…When an Al 2 O 3 forming alloy is oxidized, a transition alumina film generally develops first, and it later transforms to the thermally most stable α-Al 2 O 3 . These transition aluminas have been identified to be either the θ [13][14][15][16] or the γ phase [17][18][19]. They grow 1-2 orders of magnitude faster than α-Al 2 O 3 [14,20], and the growth is dominated by aluminum outward transport [21].…”

Section: Resultsmentioning

confidence: 99%

“…Corundum type (Fe,Cr) 2 O 3 phases were indeed detected by the x-rays on these alloys. Furthermore, TEM studies have shown that the fine-grained scale that formed during the early stage on a FeCrAl alloy with similar composition as the one tested here [18] and on the Cr-containing Fe 3 Al [27] also contain fine α-Al 2 O 3 crystallites. Fig.…”

Section: Resultsmentioning

confidence: 99%

Strains in Thermally Growing Alumina Films Measured In-Situ Using Synchrotron X-Rays

Hou

Paulikas

Veal

2006

MSF

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Abstract. Strains in thermally grown oxides have been measured in-situ, as the oxides develop and evolve. Extensive data have been acquired from oxides grown in air at elevated temperatures on different model alloys that form Al 2 O 3 . Using synchrotron x-rays at the Advanced Photon Source (Beamline 12BM, Argonne National Laboratory), Debye-Scherrer diffraction patterns from the oxidizing specimen were recorded every 5 minutes during oxidation and subsequent cooling. The diffraction patterns were analyzed to determine strains in the oxides, as well as phase changes and the degree of texture. To study a specimen's response to stress perturbation, the oxidizing temperature was quickly cooled from 1100 to 950 o C to impose a compressive thermal stress in the scale. This paper describes this new experimental approach and gives examples from oxidized β-NiAl, Fe-20Cr-10Al, Fe-28Al-5Cr and H 2 -annealed Fe-28Al-5Cr (all at. %) alloys to illustrate some current understanding of the development and relaxation of growth stresses in Al 2 O 3 .Introduction. The existence of a growth stress associated with oxidation has long been envisioned [1], since most oxides have higher volumes than their corresponding metals. Later, it was proposed that growth stress can arise from oxide formation within the scale as the diffusing cations and anions meet and react, particularly along oxide grain boundaries [2]. Several other mechanisms have also been proposed to explain the origin of growth stresses [3]; however, actual in-situ measurements of these stresses are scarce. Values are usually deduced from the difference between measured residual stresses at room temperature and the calculated thermal stress based on thermal expansion coefficients (CTE) of the oxide and the alloy. However, this method depends on CTE values, which are often unreliable, cannot properly account for stress relaxations and fails to follow the development of growth stresses with time.

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TEM Observation of Phase Transformations of Alumina Scales Formed on Al-Deposited Fe-Cr-Al Foils

Cited by 30 publications

References 0 publications

Growth Strains and Stress Relaxation in Alumina Scales during High Temperature Oxidation

Growth Strains and Stress Relaxation in Alumina Scales during High Temperature Oxidation

A new austenitic alumina forming alloy: an aluminium‐coated FeNi32Cr20

Strains in Thermally Growing Alumina Films Measured In-Situ Using Synchrotron X-Rays

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