PHASE-TRANSFORMATION OF AL2O3 SCALES FORM ED ON AL-DEPOSITED FE-CR-ALALLOY FOIL

The phase transformation of Al2O3 formed on an Al-deposited Fe-Cr-Al a lloy foil and an Fe-20Cr-5Al alloy foil of the similar composition, an d their oxidation kinetics have been studied in air at 1073-1373K usin g XRD, SEM and SIMS. it was found that the deposition of Al promotes t he growth of metastable aluminas. The transformation of metastable Al2 O3 to alpha-Al2O3 formed on the Al-deposited foil is slower than that on the alloy foil. The values of lip for the growth of alpha-Al2O3 on the Al-deposited foil is smaller than that on the alloy foil in the te mperature range 1273 to 1373K. These are attributable to the purity of the Al2O3 initially formed on Al-deposited foil. The phase transforma tion of Al2O3 is reflected by the scale morphology. The nucleation of theta-Al2O3 occurs on the surface of granular gamma-Al2O3. theta-Al2O3 having a shape of whisker grows on the oxide surface. The morphology of whiskers is more round, after theta-Al2O3 transforms to alpha-Al2O3 . We obtained the TTT diagrams for the phase transformation of Al2O3 s cales formed on both foils in air in the temperature range 1073 to 137 3K.