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  • TiAl  (2)
  • oxide-scale failure  (2)
  • 1
    Electronic Resource
    Electronic Resource
    Mechanism of isothermal oxidation of the intel-metallic TiAl and of TiAl alloys (1992)
    Springer
    Oxidation of metals 38 (1992), S. 425-464 
    Details
    ISSN: 1573-4889
    Keywords: oxidation mechanism ; TiAl ; TiAl alloys ; air ; oxygen ; nitridation ; Ti-Al-O phase diagram
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The oxidation behavior of Ti36Al, Ti35Al-0.1C, Ti35Al-1.4V-0.1C, and Ti35 Al-5Nb-0.1C (mass-%) in air and oxygen has been studied between 700 and 1000°C with the major emphasis at 900°C. Generally an oxide scale consisting of two layers, an outward- and an inward-growing layer, formed. The outward-growing part of the scale consisted mainly of TiO2 (rutile), while the inward-growing part is composed of a mixture of TiO2 and α-Al2O3. A barrier layer of Al2O3 on TiAl between the inner and the outer part of the scale was visible for up to 300 hr. Under certain conditions, the Al2O3 barrier dissolved and re-precipitated in the outer TiO2 layer. This “shift” leads to an effect similar to breakaway oxidation. Only the alloy containing Nb formed a longlasting, protective Al2O3 layer, which was established at the metal/scale interface after an incubation period of 80–100 hr. During this time, Nb was enriched in the subsurface zone up to approximately 20 w/o. The growth of the oxide scale on TiAl-V obeyed a parabolic law, because no Al2O3 barrier layer formed; large Al2O3 particles were part of the outward-growing layer. A brittle α2-Ti3Al-layer rich in O formed beneath the oxide scale as a result of preferential Al oxidation particularly when oxidized in oxygen. Oxidation in air can lead also to formation of nitrides beneath the oxide scale. The nitridation can vary between the formation of isolated nitride particles and of a metal/Ti2AlN/ TiN/oxide, scale-layer system. Under certain conditions, nitride-layer formation seemed to favor protective Al2O23 formation at the metal/scale interface, however, in general nitridation was detrimental with the consequence that oxidation was generally more rapid in air than in oxygen.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00665663
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  • 2
    Electronic Resource
    Electronic Resource
    Behavior of oxide scales on alloy 800 H and HK 40 during thermal cycling (1993)
    Springer
    Oxidation of metals 40 (1993), S. 37-63 
    Details
    ISSN: 1573-4889
    Keywords: cyclic oxidation ; alloy 800 H ; HK 40 ; acoustic emission ; oxide-scale failure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Cyclic oxidation causes degradation of alloys used in the chemical and petrochemical industries. In this paper, the behavior of the protective oxide scales formed on Alloy 800 H and HK 40 was investigated under thermalcycling conditions with upper-hold temperatures of 900°C (Alloy 800 H) and 950°C (HK 40). The atmospheres in the tests were air, air+0.5% SO 2 and Ar-5%H 2 -50%H 2 O. Tests were accompanied by acoustic-emission measurements in order to detect scale failure in situ during the experiments. During cooling the scales were under compression which led to spalling when critical stress values were reached in the scales. The outer-spinel partial layers are more prone to spallation, and the presence of SO 2 increases the amount of acoustic-emission activity (scale damage). In the case of HK 40 the oxide scales on the as-cast surfaces showed better spallation resistance than those on the ground surfaces. Quantitative model considerations were able to describe the spallation behavior of the protective scales investigated, and critical-temperature-drop diagrams for scale failure are given. The model approach was supported by results from the acoustic-emission measurements.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00665258
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  • 3
    Electronic Resource
    Electronic Resource
    Cyclic-oxidation behavior of TiAl and of TiAl alloys (1993)
    Springer
    Oxidation of metals 39 (1993), S. 93-106 
    Details
    ISSN: 1573-4889
    Keywords: cyclic oxidation ; TiAl ; TiAl alloys ; scale spallation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The cyclic-oxidation behavior of (in w/o) Ti-36Al, Ti-35Al-0.1C, Ti-35Al-1.4V-0.1C and Ti-35Al-5Nb-0.1C was studied between 800 and 1000° C in air. A few experiments were also performed in oxygen. Scale spallation after oxidation in air occurs during cooling on TiAl, TiAl-C, and TiAl-V at or close to the metal/scale interface when a critical scale thickness has been achieved. This process repeats and can lead to a stratified scale. These three materials form scales composed of an inward-growing fine-grain mixture of TiO2-Al2O3 and an outward-growing coarse-grain TiO2 layer or TiO2+Al2O3 mixture. The TiAl-Nb alloy had a significantly different behavior. The scale on this material grew very slowly because a protective Al2O3 layer formed at the metal/scale interface. This behavior resulted in much better resistance to spallation because the critical scale thickness was reached only after a much longer time, and is different from the behavior of the other three alloys. Oxidation in air leads to slight nitridation of the subsurface zone beneath the scale. In comparison to oxidation in air, oxidation in oxygen improves the cyclicoxidation behavior. Whereas the scale formed in air was uniformly thick over the entire surface, the scale grown in oxygen varied locally in structure and thickness. A large fraction of the surface was covered with a thin Al2O3 layer, while the remaining part formed a two-layer scale similar to that formed in air. The results are discussed briefly in the light of a recently published model for scale spallation under compressive stress, however, quantitative estimations are not possible due to a lack of relevant data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00666612
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  • 4
    Electronic Resource
    Electronic Resource
    Behavior of oxide scales on 12Cr-1Mo steel during thermal cycling (1993)
    Springer
    Oxidation of metals 39 (1993), S. 389-410 
    Details
    ISSN: 1573-4889
    Keywords: cyclic oxidation ; 12Cr-1Mo steel ; acoustic emission ; oxide-scale failure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Soot-blower operation leads to thermal-cyclic-oxidation conditions of heat-resistant steels in conventional power stations. The consequence may be failure of the protective oxide scales and increased corrosive attack. The behavior of protective oxide scales on 12Cr-1Mo steel was investigated under isothermal conditions at 650°C and under thermal cycling conditions between 650 and 300°C (200°C). The tests were performed in air, air + 0.5%SO2, simulating the fire side, and Ar-5% H2-50% H2O, simulating the steam side. Complete heat-exchanger tubes were used as specimens. The main instrument for the detection of scale failure was acoustic-emission analysis. In air and air + 0.5% SO2 the M2O3 scales with M = Fe, Cr were very thin and did not show significant failure either during isothermal or during cyclic oxidation. The thicker scales formed in Ar-5% H2-50% H2O, consisting of several partial layers, failed even during isothermal oxidation due to geometrically-induced growth stresses in the scale. Thus, in the thermal-cycling cooling periods there was only very little additional scale cracking. The scale behavior can be explained consistently by applying the existing quantitative models.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00664663
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