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2003
Journal Article
Titel
In situ - Investigation of kinetics and stresses in alumina scales on NiAl during the phase transformation theta to alpha
Abstract
NiAl pure and NiAl doped with Hf were oxidised in air for 100 h at 950°C. The residual stresses and the oxide scale growth were measured in situ by high temperature X-ray diffraction and evaluated as a function of time. A mathematical model was applied to describe the alumina growth during the transformation and oxidation process. After the isothermal period the samples were cooled to room temperature, reheated to 950°C and cooled again. The residual stresses were measured every 200°C and plotted as a function of temperature. In the first 45 h for NiAl and 35 h for NiAl+Hf -Al 2O3 is observed, which transforms into -Al 2O3. In this time interval compressive stresses of -200 MPa were measured, and after the disappearance of the -Al 2O3 nearly constant tensile stresses of +500 MPa were detected. On cooling compressive stresses reaching -1500 MPa at room temperature were formed due to the different thermal expansion coefficients of the metal and the alumina. On reheating t he stresses decreased again to +500 MPa as before cooling. On the second cooling again a compressive stress of -1500 MPa was found.
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