High Temperature Oxidation Resistance Characteristics And Micro-Mechanisms Of Heat Resistant Steels

Heat-resistant steels are widely used in solid-state fuel cells,nuclear power,coal power,metallurgy,construction materials etc.The severe operating conditions are placing increasing demands on their high-temperature oxidation resistance,and the micromechanical understanding of the high-temperature oxidation mechanism is limiting their performance.The high-temperature oxidation resistance of heat-resistant steels depends on the physical properties of two components:the oxide on the surface and the substrate underneath it.The complex composition of heat-resistant steels leads to a variety of oxides generated on their surfaces,which have different and indistinguishable roles,causing problems in the study of the mechanism of high-temperature oxidation resistance of the materials.In addition,the matrix in contact with the oxides is most significantly affected by the element oxygen,and the mechanism of the effect of oxygen on the lattice structure and physical properties under high temperature and magnetic field conditions has not been fully revealed.In the present work,a low-cost heat-resistant alloy with high antioxidant performance was designed and manufactured based on the idea of using silicon and aluminum instead of nickel.The oxides generated on the surface of heat-resistant steel and matrix have been investigated,and the morphology,elemental distribution and crystal structure have been characterized by atomic-level high-resolution transmission electron microscopy and other instruments.On the basis of the first-principles calculations,the evolution of physical properties of oxides and matrix with temperature changing were studied a combination of the complementary methods of crystal orbital Hamilton population analysis,quasi-Debye model,transition state calculations,elasticity theory,electronic structure analysis,and quasi-harmonic approximation.The main research and conclusions of this thesis are as follows.（1）The inner oxide layer with an olivine structure has a filter effect on Fe/Cr diffusion,the main antioxidant element Cr can diffuse through and enrich to form a continuous oxide layer,while the weaker antioxidant element Feis intercepted on the side close to the matrix,remarkably improving the high temperature antioxidant performance of heat-resistant steel.（2）The olivine oxide Cr₂Si O₄ with a lamellar lattice structure has strong metal bonds,enabling it to withstand greater stress without cracking,thereby reducing the generation of cracks in the oxide layer and improving the high-temperature oxidation resistance of heat resistant steel.（3）By changing the lattice constant to simulate the structural distortion of the outer oxide FeCr₂O₄ at different temperatures,a nonrhythmic breathing model of lattice structural distortion is proposed,i.e.,as the lattice constant increases,the octahedral sublattice in spinel produces distortion to angle and bond length both,while the tetrahedral sublattice only has bond length changed,and there is nonperiodic expansion and contraction of both sublattice.（4）The structural distortion of FeCr₂O₄ spinel leads to the transition to non-magnetism,ferromagnetism,anti-ferromagnetism,and sub-ferromagnetism,and the phenomenon of giant magnetic moments were found at some volumes.In addition the alteration of lattice constants leads to the material exhibiting abrupt changes in metallicity,semi-conductivity,and semi-metallicity.This provides theoretical predictions and guidance for the stability studies of oxides at different temperatures and under magnetic field.（5）The interaction between the matrix and O was investigated,and it was found that the O atoms located at the tetrahedral interstices enhance the stability of the surrounding Featoms,and the s orbitals have an important effect on the bonding and stability between Fe-O atoms.In addition,a topological analysis of the electron spin density map revealed that the atomic interactions in the O-containing matrix were mainly provided by the spin-down electrons and that the contribution of the eg band to the interatomic interactions was enhanced with the spin flip of the O atoms.（6）The migration paths of O through the octahedral/tetrahedral interstice were discussed separately,and it was found that 256 K is the critical temperature for diffusion,above which O diffuses faster through the octahedral interstice,otherwise it diffuses faster through the tetrahedral interstice.（7）The location of randomly distributed interstitial solid solution O atoms in the matrix was determined by a combination analysis of atomic-level high-resolution transmission electron microscopy,contrast transfer function simulations,and first principal calculations:oxygen prefers to presents in the octahedral interstitial position.Bing consistent with the results of O diffusion analysis.（8）At high temperatures,the bulk and Young’s moduli of O-containing matrix increase with the loss of electrons,and the anisotropy of Young’s modulus is also increasing with the loss of electrons.The electron localization function calculations illustrated that the electron localization is consistent with the anisotropy of Young’s modulus.（9）The Gibbs free energy composition of O-containing matrix were resolved and it is found that the lattice vibration free energy has the most significant effect on stability,followed by the electron free energy,the endogenous magnetic free energy,and the external field-induced free energy is the smallest.The effect of electron loss is mainly reflected in the lattice vibration and electron free energy,while the effect on the intrinsic magnetic free energy is smaller.