A TEM Study Of The Initial Oxidation Behavior Of Typical Alloys

Oxidation of alloys at high temperatures often involves two consecutive steps: the initial oxidation and the steady-state oxidation. Unveiling the structural evolution during initial oxidation might provide new insights into the understanding of the formation mechanism of the steady-state oxide film. The oxide film formed at the initial stage of oxidation is rather thin, making it difficult to characterize the chemical composition and the microstructure by conventional methods such as Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD). In this study, by using Analytical High Resolution TEM, we investigated the oxidation behavior of CeO2 dispersion modified Ni2Al3 high temperature coating and the 2:17 type SmCo-based high temperature magnets, by systematically characterizing the chemical composition and microstructure of the oxide film formed in initial oxidation. The major findings are summarized as follows:(1) Initial oxidation behavior of CeO2 dispersion modified Ni2Al3 alloy:revealing Reactive Element Effect (REE) of rare earth oxides.Alumina coatings have been widely used as Al2O3 layer thermal growth high temperature protective coating, in which the rare earth oxide is often added to slow down the growth rate of the Al2O3 layer. Its mechanism is often interpreted in terms of a "dynamic segregation model":driven by the oxygen potential, part of the rare earth oxide dissolve in the metals and release the rare earth element atoms, which diffuse toward the boundary between oxide film and the metal, enter the oxide layer and segregate on the grain boundaries. The segregated rare earth element atoms diffuse along the grain boundary towards the sample surface, which suppress the diffusion of Al3+ and O2- ions that are essential to growth of Al2O3 layer. However, there is a unsolved problem in this model:if the rare earth oxide are more stable than normal thermal growth oxides (such as NiO, O2O3 and Al2O3), when and how could they dissolve and release rare earth elements in the metal matrix and diffuse towards the metal/oxide boundaries?Aiming at solving this problem, by chemical and structural characterizations of the initial oxide film formed at 1100℃ of CeO2 dispersion modified Ni2Al3, we found that:(1) the Al2O3 films are composed of a inward-growing α-Al2O3 layer and an outward-growing γ-Al2O3 layer; (2) the Ce ions segregate onto the grain boundaries in the inner α-Al2O3layer; (3) there are new Ce2O3 particles dispersed along the twin boundaries in the outer γ-Al2O3 layer; (4) Ce was found neither on the grain boundaries nor in the grains in the metallic matrix. Based on these experimental observations, we propose an improved dynamic segregation model that involves REE of rare earth oxide:during oxidation, the γ-Al2O3 layer forms and grows first; then underneath this layer, the α-Al2O3 nucleates and grows inward; and in consequence, the CeO2 dispersion in Ni2Al3 was enclosed and embedded in this layer; the CeO2 dissolve and release Ce ions, which segregate on the grain boundaries of α-Al2O3 the Ce ions diffuse outword under the driving force of oxygen potential and along the grain boundaries, and eventually react with oxygen in the outer oxide layer and form Ce2O3 oxide.2. Initial oxidation behavior of 2:17 type SmCo alloy-a systematic description of the mechanisms of the inner oxidation and outer oxidation.Because of the high Curie temperature (～850℃) and excellent magnetic stability, the 2:17 type SmCo-based magnets are one of the most promising candidates for high temperature applications. However, considerable irreversible magnetic loss can happen at a temperature well below the Curie temperature in this material. It is generally believed that the oxidation is the major cause of this phenomenon. However, the mechanism of the oxidation and its relation to magnetic loss still remain unclear.In this study, by characterizing the chemical composition and structure in the oxide formed at initial stage of oxidation of Sm(CobaiFe0.22Cu0.08Zr0.02)7.5, we found that:1) the oxide layer is composed of a thin outer Cu2O and CuO oxide layer, and a thick spinal (CoxFe1-x)3O4 layer, which reveals the evolution of outer oxidation with time.2) In the inner oxidation region, the Sm oxidizes and forms nanoscale Sm2O3 particles, and with increasing oxidation depth, the shape of the Sm2O3 particles changes from spherical to rod shape. It indicates a transformation of inner oxidation from nucleation-dominated process to a growth-dominated process. The oxidation of the magnet material leads to the dissociation of 1:5 and 2:17 magnetic phases, and eventually gives rise to the decreased coercive force.