Date of Award
Doctor of Philosophy (PhD)
Metallurgy and Materials Science
The oxidation properties of Fe-Al alloys containing 1.5,3,4,5,7,8,12,16,20 atomic percent aluminum were investigated in dry oxygen, wet oxygen, laboratory air and alloy/(Fe+FeAl₂O₄+Al₂O₃) diffusion couples at 1173K. The reaction kinematics were determined thermogravimetrically and by layer thickness measurements. The reacted specimens were analyzed using light microscopy, electron metallography, x-ray diffraction, Auger electron spectroscopy and Mössbauer spectroscopy. Particular interest was given to the growth of Al₂O₃on these alloys. A duplex external scale consisting of Fe₂O₃at gas/scale interface, (Fe,Al)₃O₄ at scale/subscale interface and a subscale of FeAl₂O₄ platelets with Al₂O₃tips grew on alloys containing up to 5 a/o Al oxidized in dry oxygen. The growth of the FeAl₂O₄-Al₂O₃ platelet precipitates, which is interpreted by a (Fe,Al)₃O₄/Al (alloy) solid state displacement reaction, is controlled by oxygen diffusion torugh the subscale in the very early stage of oxidation. A oxidation continues, the oxygen pressre at the scale/subscale interface decreases due to increase of aluminum concentration of (Fe,Al)₃O₄ by dissolution of FeAl₂O₄ precipitates. The diffusion outward from the bulk alloy plays and important role as the velocity of precipitation front decreased by decreasing oxygen activity which leads to the development of a continuous Al₂O₃film at the subscale front in Fe-5 a/o Al alloy and isolation of the subscale from the alloy substrate. Multilayered scale consisting of Fe₂O, Fe₃O, FeO+Fe₃O, FeO+FeAl₂O₄ and subscale grew on alloys containing up to 5 a/o Al alloy oxidized in air or wet oxygen. The FeO+FeAl₂O₄ layer and subscale grew by inward oxygen migration generated by dissociation of wustite. The high rate of oxygen transfer inward with hydrogen generated by dissociation of water vapour, accelerated the oxidation processes and did not allow aluminum diffusion outward from the bulk of the alloy to affect the oxidation behavior. For Fe-7 and 8 a/o A alloys, a thin duplex scale was formed consisting of Fe₂O₃ layer at oxygen/scale interface and Al₂O₃ layer at scale/alloy interface. An oxidation model was advanced to describe the scale, development involving iron diffusion through leakage paths in the Al₂O₃ formed initially to form Fe₂O₃ layer at the scale/gas interface. Only the Al₂O₃ is formed on Fe-12, 16 and 20 a/o Al alloys. An oxidation mechanism is proposed based on inward oxygen diffusion down the grain boundaries and outward aluminum diffusion to account for the formation of hollow nodules in the scale. Alloys containing 3, 5, 7 and 8 a/o Al oxidized internally when coupled with (Fe+FeAl₂O₄+Al₂O₃) mixture. The Fe-3 a/o Al alloy oxidized linearly but the Fe-5, 7, and 8 a/o Al alloys at internal oxidation zone/alloy interface. However, an Al₂O₃ film was formed at (Fe+FeAl₂O₄+Al₂O₃)/alloy interface with no internal oxidation in Fe-12 a/o Al alloy diffusion couple. Alloy composition at which transition from internal to external scale formation of Al₂O₃ occurs, is calculated using available models and compared with the experimental results.
Ahmed, Hafiz Abdel Azim, "Oxidation Properties of Fe-Al Alloys at 1173K" (1984). Open Access Dissertations and Theses. Paper 3278.