The Study On In-Situ Oxidation/Hydrogenation Reaction Mechanism Between Magnesium And Its Alloy With Gases

Magnesium alloys are the least dense structural materials and have many excellent properties including high specific strength,good castability,good electromagnetic shielding properties,good impact resistance,good shock resistance,good anti-vibration,excellent heat dissipation and damping performance,which make them broad applied in the fields of automobile,transportation,aviation,aerospace,rocket,national defense military,chemical industry,biomedicine,sports equipment and electronic products.As one of the active metals,the role between magnesium(magnesium alloys)and gas is an inevitable issue in the process of smelting,preparation,processing,transportation and application.The study on those issues mentioned above not only shows outstanding scientific significance,but also has important industrial value.However,due to the high activity of magnesium and the technical limitation,the atomic scale experimental observation and theoretical simulation of the reaction process between magnesium and gas are still lacking,which seriously restricts the application of magnesium alloys.To solve this issue,in situ aberration corrected environmental transmission electron microscopy(ac-ETEM)observations and density functional theory(DFT)calculation were combined to reveal a unique inter-layered hydrogenation process and a unique incipient interval-layered oxidation mechanism of single-crystal Mgat the atomic scale.The hydrogenation process and mechanism of Mgover catalyst and the hydrogenation process of a magnesium alloy were studied.The sublimation mechanism of pure magnesium and a magnesium-lithium alloy under electron irradiation in carbon dioxide atmosphere were clarified.The specific conclusions are as follows:Using in situ environmental transmission electron microscopy and density-functional theory,we firstly clarify the oxidation process of single-crystal Mgat the atomic scale by using a new double-hole technique.A unique incipient interval-layered oxidation mechanism of single-crystal Mghas been confirmed,in which O atoms intercalate through the clean(2110)surface into the alternate-layered tetrahedral sites,forming a metastable HCP-type MgO_0.5 structure.Upon the increased incorporation of oxygen at the neighboring interstitial sites,the HCP-type Mg-O tetrahedron structure sharply transforms into the FCC-type MgO oxide.In addition,a typical anisotropic growth mechanism of oxides has been identified,wherein it involves two routes:the epitaxial growth of the MgO layer and the inward migration of the MgO/Mginterface.The whole oxidation rate of single-crystal Mgis mostly determined by the inward migration rate of the MgO/Mginterface,which is about six times higher than that of the epitaxial growth rate of the MgO layer along the same orientation planes.Moreover,the inward migration rate of the(020)_MgO||(011^—0)_Mg interface is about twice as large as that of the(200)_MgO||(0002)_Mginterface.This continuous oxide growth is mainly related to the defects in the MgO layer,which builds effective channels for the diffusion of O and Mgatoms.In situ ETEM observations have been performed to investigate the unique sublimation of Mgunder CO₂ irradiation conditions.Differing from the oxidation of Mg,severe sublimation phenomenon is detected due to the coexistence of electron irradiation and CO₂.The fundamental reason is related to the formation of amorphous MgCO₃.The direct sublimation of MgCO₃ phase accounts for the continuous sublimation of pure Mgunder a mild electron dose.In turn,the decomposition of amorphous MgCO₃ plays a crucial role in accelerating the sublimation of Mgunder a harsh electron dose,resulting in a unique oscillatory phenomenon.The reason for oscillation stems from the first-order reversible phase transformation of MgCO₃.These observations provide a thorough understanding of the interactive role between Mgand CO₂ under e-beam irradiation and point toward new routes in the design of Mgmaterials with enhanced anticorrosion and welding properties under CO₂ conditions.A unique inter-layered hydrogenation process of single crystal magnesium has been firstly confirmed by in situ aberration corrected environmental transmission electron microscopy together with density function theoretical calculations.Firstly,in hydrogen(oxygen is more than 0.01%),under focused e-beam irradiation,a large number of dislocations were generated and the product is mainly MgO and part of MgH₂.Secondly,in vacuum,under focused e-beam irradiation,oxidation reacted from the edge to the center at room temperature.The product is small rectangular island MgO.Third,(oxygen is less than 0.001%),under focused e-beam irradiation,hydrogenation process was detected.Differing from the surface dominated process,the incipient hydrogenation of magnesium is mainly related to an intercalation reaction mechanism,resulting in the incorporation of hydrogen into the bulk.Two metastable phases-such as HCP-MgH_0.5 and HCP-MgH_1.0have well detected by in-situ ETEM observations.The density-functional theory calculations demonstrate that the interval-layer hydrogenation is mainly related to the special adsorption site of H atoms on the surface.The stable bridge site on the surface prohibits the adjacent adsorption,facilitating interval-layered adsorption.In-situ environmental transmission electron microscopy observations combining density-functional theory calculations have been performed to investigate two main chemical reactions of Mg:hydrogenation and oxidation processes.The results reveal the oxidation reaction plays a crucial role in tuning the phase transformation of Mgeven under hydrogen environments.In addition,with the presence of Mg₆Pd catalyst,the MgH₂compound is prone to form in the interface of Mg/Mg₆Pd.However,the MgH₂ compound will spontaneously change to MgO with retarding time.The DFT calculation of the MgO oxide formation energy is consistent with the facile oxidation of Mg.The observations on two processes extend the understanding on the fundamental characteristics of Mg-based materials.In situ transmission electron microscopy was used to observe the hydrogenation process of Mg₁₂Ni Y alloy with LPSO structure.The high chemical active Y element in Mg₁₂Ni Y firstly reacted with hydrogen to form YH₂.Subsequently,YH₂ was further reacted with hydrogen to form YH₃.The Mg₂Ni H₄ and MgH₂ were also detected.The results show that Mg₁₂NiY alloy with LPSO structure will be gradually destroyed during the hydrogenation process,resulting in the occurrence of pulverization,which is consistent with the actual hydrogen storage.