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|Title:||High Temperature Sulfidation Properties of Fe-Al Alloys|
|Authors:||Patnaik, Chandra Prakash|
|Department:||Metallurgy and Materials Science|
|Keywords:||Materials Science and Engineering;Metallurgy;Materials Science and Engineering|
|Abstract:||<p>The sulfidation properties of the Fe-Al alloys containing 6, 9, 18 and 28 atomic percent Al were investigated in alloy/FeS diffusion couples, in sulfur vapour at the dissociation pressure of the FeS and in H₂S+H atmosphereat 1173K. The reaction kinetics were determined thermogravitmetrically and by layer thickness. measurements. The reaction specimens were analyzed using light microscopy, X-ray diffraction and electron metallographic techniques (SEM, EPMA) along with EDAX spectrometry. Particular interest was given to the mode of precipitation and growth of sulfide phases. Diffusion coupling of Fe-Al alloys with FeS in the form of compacts results in periodic precipitation of sulfide (FeS+FeAl₂S₄) bands in the alloy. The thickness of sulfide bands increases with depth satisfying the Jablczynski's relationship for Liesegang type of precipitation process. The initial stage of sulfide band formation is explained by a model involving precipitation of Al₂S₃which is subsequently converted to FeAl₂S₄. Growth of each sulfide band or layer is supported by raid diffusion of iron and aluminum in the FeS phase. It is suggested that aluminum depletion from the alloy in the front of a sulfide layer ultimately leads to its cessation of growth. A new sulfide band begins to form at a distance ahead of the sulfide layer. where the concentration product for the precipitation of Al₂S₃ is satisfied. The mode of precipitation of sulfide changes from bands parallel to the original alloy surface to the platelets normal to alloy surface when Fe-6, 9 and 18 Al alloys are sulfidized in sulfur vapour supplied by a mixture of Fe and FeS. The acicular internal sulfide precipitates of FeAl₂S₄ and Al₂S₃ in these alloys are elongated in the growth direction. Their growth is interpreted by a model involving enhanced sulfur diffusion along the incoherent interfaces between the internal sulfides and alloy matrix. The alloy composition at which transition from the internal to external scale formation of Al₂S₃occurs, is calculated using available models and compared with the experimental results. Fe-9 Al alloy sulfidizes parabolically in H₂S+H atmospheres giving rise to a scale consisting of an outer Al-doped FeS layer, an inner FeS+FeAl₂S₄ layer and Al₂S₃+alloy internal precipitation zone. the Fe-18 Al alloy is sulfidized by a two-stage kinetics. In the initial stage, growth of the FeS+FeAl₂S₄ nodules is observed accompanied by the internal sulfidation beneath these nodules. The final stage of the reaction curve commences when an inner film of Al₂S₃ forms at the external scale/alloy interface. Models based upon diffusion and the thermochemistry of the sulfidation reactions are advanced to account for the reaction kinetics and sulfide morphologies. The ternary Fe-Al-S isothem is determined experimentally and it is used to interpret the scale microstructures which grew by parabolic kinetics.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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