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Stéphane Baleix
Abstract
Oxidation and spallation of cast refractory alloys for superplastic forming tools
Stéphane Baleix - 16 december 1999.
Superplastic forming process (SPF) of titanium sheets is performed at temperatures around 900°C. Forming tools, manufactured with cast refractory alloys, are thermally and mechanically damaged during forming campaigns. Oxidaiton and spallation kinetics of four alloys, either austenitic or ferritic, are investigated in isothermal (750°C - 1050°C) and cyclic conditions, as well as the influence of thermal cycling and the temperature drop on spallation. The nature of oxides is determined using SEM, SIMS and XRD techniques. Nature, size and proportion of spalled areas are investigated using the image analysis technique. Thermo-elastic models are used in order to determine the stress levels in the oxide layers and the critical parameters for spallation. Energetic models, which take into account the metal/oxide interfacial interactions, are also investigated. These models, describing two distincts spallation routes, allow the interfacial fracture energy determination using experimental parameters such as the critical temperature drop available by acoustic emission during cooling and the size of spalled areas available by image analysis. The evolution of the fracture energy with oxidation exposure is discussed through microstructural considerations relative to diffusion process during oxidation.
Keywords :
cast refractory alloys, superplastic forming, tooling,
oxidation, spallation, thermal stresses, fracture energy,
acoustic emission, image analysis
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