| Protective thermally grown oxide(TGO)with the merits of compactness,slow growth and thermodynamic stability is a prerequisite of structural alloys and coatings with superior resistance to high-temperature oxidation.Al2O3 is typical protective TGO that has been deliberately designated for many oxidation-resistant materials to form at high temperatures.It is well-known that Al2O3 usually crystallizes in various lattice stru ctures,such as,?,?,? and so on.Among them,the hexagonal a-phase is the most thermodynamically stable polymorph.?-Al2O3 TGO exhibits a thermal growth rate about two orders of magnitude lower than those of the?-or y-Al2O3 counterparts.Unfortunately,for alumina-forming alloys and coatings containing a high content of Al(e.g.,M,Aly(M= Ni,Co and Fe)),their initial oxidation at temperatures normally not exceeding 1200 ? always form metastable?-or ?-Al2O3,and after further oxidation they will be finally transformed into the stable ?-Al2O3.A slow metastable-to-stable alumina transformation normally leads to a faster degradation of the alumina-forming alloys and coatings and,in the meanwhile,the volume contraction during the phase transformation process induces microcracking in the alumina TGO.Thus,reasons for the phase transformation have been tried to be answered and methodologies to facilitate the alumina phase transformation have been explored in this thesis.The main contents and findings are summarized as follows,(1)The effect of surface Cr2O3 particles on the oxidation behavior of Ni2Al3.We seed a layer of Cr2O3 nanoparticles,by means of a simple technique of electrophoretic deposition(EPD),onto the surface of a Ni2Al3 coating and investigate the evolution of the transient alumina on the naked-and Cr2O3-treated Ni2A13 by using the photostimulated luminescence spectroscopy(PSLS).For oxidation of naked-Ni2Al3 at 1000 ?,the initial themal growth alumina is?-Al2O3,then ?-Al2O3 experiences a 0-to-a Al2O3 transformation and enventually transforms into ?-Al2O3.However,for Cr2O3-treated Ni2Al3,?-Al2O3 directly grow without 0-a phase transformation.It highly decreases the oxidation rate of the aluminide and prevents the phase-transformation-induced negative effects of the formation of microcracks in the alumina scale and cavities at the scale/metal interface.This because oxides(e.g.,Cr2O3)with the same crystallographic structure as ?-Al2O3 can work as the nucleation sites for a-Al2O3 growth.(2)The effect of surface TiO2 particles on the oxidation behavior of Ni2Al3.We report an experimental observation of TiO2 nanoparticles-assisted a-Al2O3 direct growth on the substrate of Ni2Al3 by skipping the undesirable metastable-to-stable alumina transformation during the oxidation process.Further,we investigat the effect of TiO2 on a-Al2O3 formation by transmission electron microscopy(TEM)and build related thermodyamical model by first-principles calculations.This direct a-Al2O3 growth with TiO2 nanoparticles can be attributed to the fact that both a-Al2O3 and rutile-type TiO2 nanoparticles share the well-matching sublattice of oxygen among their energetically favorable interfaces of(100)TiO2//(0001)Al2O3,(010)TiO2//(1210)Al2O3 and(001)TiO2//(1010)Al2O3.Moreover,we elucidate the template effect of TiO2 within the framework of the classical theory of a heterogeneous nucleation and find this well-matched the hexagonal oxygen sublattice significantly decreases the nucleation barrier for?-Al2O3.(3)On the mechanism for 0-to a-Al2O3 phase transformationWe report a detailed theoretical investigation of the ?-to ?-Al2O3 phase transformation based on first-principles calculations.It suggests that ?-to ?-Al2O3 phase transformation obeyed the "synchro-shear"model including the re-stacking of oxygen sublattice from h.c.p to f.c.c through a series shear of the closed-stacking plane(201)along the direction of[102]and the synchronous displacement of metallic ions that Al cations coordinately migrate to the near octahedral interstices to form a "honey comb" lattice. |
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