Oxygen diffusion kinetics of an advanced three step heat treatment for zirconium alloy ZrNb7

ZrNb7 was exposed to a very specific three step heat treatment. The alloy is first oxidized in ambient air. The second step is a solid state reduction in vacuum, during which the oxide scale is dissolved and the oxygen diffuses into the substrate. Within a third step, the alloy is oxidized again to...

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Bibliographic Details
Published in:Surface & coatings technology 2018-04, Vol.339, p.139-146
Main Authors: Mosbacher, Mike, Scherm, Florian, Glatzel, Uwe
Format: Article
Language:English
Subjects:
Ceramic coatings
Deoxidation
Deoxidizing
Diffusion
Diffusion coating
Heat treating
Heat treatment
Kinetics
Oxidation
Oxidized zirconium
Oxygen
Oxygen solubility
Process parameters
Reaction kinetics
Reduction (metal working)
Scale (corrosion)
Substrates
Zirconium alloys
Zirconium base alloys
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Summary:ZrNb7 was exposed to a very specific three step heat treatment. The alloy is first oxidized in ambient air. The second step is a solid state reduction in vacuum, during which the oxide scale is dissolved and the oxygen diffuses into the substrate. Within a third step, the alloy is oxidized again to generate the desired ceramic coating on the outside followed by an oxygen hardened diffusion zone, that gradually reduces the hardness of the material from the outer surface to the inside. The high oxygen solubility of up to 29 at.% is distinctive in zirconium. Only titanium and hafnium show similar properties. Therefore, this rare phenomenon is further investigated with the main focus on the oxygen diffusion mechanisms through the oxide scale and the substrate. The reaction kinetics are evaluated and activation energies, parabolic oxidation constants and diffusion coefficients are determined for the two diffusion controlled processes: the oxidation and deoxidation of zirconium. This article reveals why the diffusion hardened zone does not appear in the desired thickness within a simple oxidation step and ideal process parameters are given concerning oxide thickness, cracks in the oxide scale and oxygen diffusion depth. •Very specific three step heat treatment was applied to specimen.•Diffusion kinetics were determined for all three heat treatment steps.•Results show why common oxidation cannot form a thick diffusion hardened zone.•Ideal heat treatment parameters were determined.
ISSN:0257-8972
1879-3347
DOI:10.1016/j.surfcoat.2018.02.015