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  • 1
    ISSN: 1573-4889
    Keywords: Ni-Cr-Al alloys ; alloy microstructure ; oxidation ; refractory-metal additions
    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 at 1023 and 1123 K in air has been followed for several Ni-Cr-Al ternary alloys and with small additions of refractory elements Nb, Mo, and Ta. Prior to testing, the alloys were characterized by microscopic and microanalytic methods. Microstructural observations have shown that the formation of oxide layers depends on the original microstructure. Discontinuous thermogravimetric tests were used to relate the behavior of different chromium, aluminum, and refractory-element additions, and eventually to service conditions in similar environments in which general oxidation takes place. The addition of refractory elements generally impaired the oxidation resistance.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Oxidation of metals 51 (1999), S. 159-180 
    ISSN: 1573-4889
    Keywords: MA 6000 ; LONG-TERM OXIDATION ; OXIDATION KINETICS ; SCALE COMPOSITION ; SCALE MORPHOLOGY ; OXIDE UNDER STRESS
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract MA 6000 is a mechanically alloyed, Ni-base ODSalloy. Excellent high-temperature strength makes it astrong candidate for application in high-temperatureindustrial processes. In order to assess its usefulness for high-temperature structural components,in-depth knowledge of its oxidation behavior,particularly long-term exposure, is necessary. Thepresent work deals with studies of the cyclic andisothermal oxidation of MA 6000 in the temperature range900-1050°C, with emphasis at 1050°C. A fewcomplementary studies have been carried out on the oxideintegrity under creep conditions for exposure times of up to 11,000 hr. The results have shown thatoxidation of MA 6000 involves rather complex mechanismsand alterations of the oxidation behavior still occurafter long-term exposure. Excellent oxidation resistance is based on the formation of an internalcontinuous Al2O3 layer.
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  • 3
    ISSN: 1573-4889
    Keywords: Oxidation behavior ; ODS ferritic alloys
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract This investigation was undertaken in an attempt to gain a fundamentalunder-standing of the oxidation behavior of a fine-grain MA 956 in thetemperature range 800 to 1200°C, with emphasis placed on the scalemorphology and oxidation kinetics. Oxidation reaction led to the formationof a thin and dense alumina scale, irrespective of the oxidationtemperature. At intermediate temperatures plateletlike oxides of alumina,probably θ-Al2O3, covered the surface, whereas at othertemperatures small nodules of Fe-, Cr-, Ti or Y-rich oxides were observed inthe outer part of the alumina scale. The temperature dependence of theparabolic rate constant revealed a change in the oxidation mechanism with atransition at about 1000°C. The high-temperature mechanism is controlled bythe formation of α-alumina, whereas the low-temperature oxidationmechanism is controlled by formation of metastable alumina. Comparison withthe reported oxidation kinetics for coarse-grain MA 956 is inconclusive,since microstructure (grain size, texture) of the substrate changed duringoxidation in the high-temperature range.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1573-4811
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1573-4889
    Keywords: INTERMETALLICS ; Ni3AL PARTICLE SIZE ; OXIDATION KINETICS ; POWDER METALLURGY
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The influence of particle size on the oxidationbehavior of Ni3Al prepared by powdermetallurgy (PM) was investigated in the temperaturerange of 535 to 1020°C for exposures up to 200 hr.Four alloys were obtained, each one processed with a differentpowder particle size fraction (〈25, 25-50, 50-100,and 100-200 μm). For temperatures below 730°C,the scale consists of an outer NiO layer, a thindiscontinuous intermediate nickel layer, and an internaloxidation zone. The lowest oxidation rate corresponds tothe material with the smallest particle size. Thisresults from its higher grain-boundary density; the boundaries act as easy-diffusion paths foraluminum leading to the rapid formation of a continuousinner alumina layer. At temperatures above 730°C, athree layered scale is observed consisting of an outer NiO layer, an intermediate layer that,depending on temperature, consisted of a mixture ofnickel and aluminum oxides orNiAl2O4, and an inner layer ofAl2O3, which accounts for thehigher oxidation resistance. The oxidation attack is characterized byintrusions of the scale into the alloy, the intrusionnumber increasing as the particle size decreases. It isassumed that oxide particles and impurities present at the original particle boundaries facilitatealumina growth along these regions. Thus, the lowestoxidation rate for the highest temperature rangecorresponds to the largest particle-sizematerial.
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  • 6
    ISSN: 1573-4889
    Keywords: Cr-IMPLANTATION ; Ni3Al ; INTERMETALLICS ; ISOTHERMAL OXIDATION
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The isothermal-oxidation behavior ofnonimplanted and Cr-implanted Ni3Al obtainedby powder metallurgy (PM) was investigated at 585, 635,and 900°C for exposures up to 200 hr. For thenonimplanted material, the mass gain at 900°C was lower than thatfound at 635°C, whereas for the implanted materialthe oxidation rate increased with temperature, asexpected. Furthermore, Cr implantation significantlyreduced the oxidation rate with respect to thenonimplanted material, particularly at the lowertemperatures. Cross-sectional examination of theimplanted alloy revealed a heterogeneous attackcharacterized by a thin Al-rich oxide layer, occasionallydisrupted by isolated zones with a thicker three-layerscale, the morphology of which was similar to that ofthe nonimplanted alloy. The density of these zonesdecreased with increasing temperature. The beneficialeffect of Cr is associated with the rapid establishmentof a protective Al-rich oxide layer.
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  • 7
    Electronic Resource
    Electronic Resource
    Springer
    Oxidation of metals 48 (1997), S. 143-170 
    ISSN: 1573-4889
    Keywords: Ni3Al ; oxidation kinetics ; spallation ; scale roughening
    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 a Ni3Al powder-metallurgical (PM) alloy doped with boron was investigated by means of discontinuous isothermal tests in the temperature range of 535° to 1020°C for exposures of up to 150 hr. The oxidation kinetics were characterized by a sharp decrease in the oxidation rate at about 730°C which is associated with a change in the oxidation mechanism. Below 730°C, the scale exhibited an outer NiO layer and an internal-oxidation zone consisting of a fine dispersion of alumina in a diluted Ni-Al solid solution. Between these two layers a very thin layer of nickel could be observed. Above 730°C, a three-layered scale was observed consisting of an outer NiO layer, an intermediate layer that depending on temperature consisted of a mixture of nickel and aluminum oxides or NiAl2O4, and an inner layer of Al2O3, which accounts for the higher oxidation resistance. Oxidation at the higher temperatures resulted in extensive void formation at the scale/metal interface which led to poorly adherent scales. It is worth noting that at the early oxidation stage the scale was characterized by planar interfaces. Roughening of the air/scale and, specially, the scale/metal interfaces after long exposures at the low-temperature range or after short times at higher temperatures could be related to the formation of the inner Al2O3 layer at the grain boundaries which favor oxygen penetration through the grain interior.
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  • 8
    ISSN: 0021-9304
    Keywords: magnetic behavior ; ferritic superalloy ; alumina coatings ; thermal oxidation treatment ; surgical implants ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine , Technology
    Notes: MA 956 superalloy is a ferritic stainless material which develops a fine, dense, and well-adhered α-alumina layer upon heat treatment at elevated temperatures. This unique capability makes MA 956 attractive for surgical implants. In this work, the magnetic behavior of the material before and after thermal oxidation treatment required to develop the alumina layer is investigated. The thermal oxidation treatment yields a microstructure of elongated grains and a significant change in the texture. Despite these strong microstructural differences between the as-received and heat-treated materials, the hysteretic behavior is not greatly affected by them. MA 956 is a soft magnetic material irrespective of the material condition. The coercitive force and residual magnetization of the material are somewhat lower under heat-treated conditions than in the as-received condition. © 1997 John Wiley & Sons, Inc. J Biomed Mater Res, 37, 453-456, 1997.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
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