| Oxide dispersion strengthened(ODS)steels have been considered to be the most promising candidate structural materials for Generation IV nuclear energy systems and fusion reactors by virtue of their superior high-temperature mechanical properties and excellent irradiation resistance.The excellent properties mentioned above are obtained by extraordinary microstructure that contains a high density of nano-sized oxide particles,which not only effectively reduce both the dislocation mobility and the grain boundary migration,but also act as stable sinks for trapping irradiation induced defects.Therefore,the study of nanoscale oxide particles formation during ODS processing,the mechanism of coarsening,and the thermal stability in high temperature condition is critically important.RFexM12-x compounds with ThMn12-type crystal structure have been extensively studied due to the characteristic of high saturation magnetization,tunable Curie temperature and magnetocrystalline anisotropy.In this paper,the relationship between the structure and magnetism of ErFexMn12-x alloys is investigated for the development of cost-effective magnetic functional materials.The neutron scattering technique has unique advantages in studying crystal structures and nondestructive detection of nanoscale structures.The main parts of this work is as follows:(1)The evolution of the nano-sized oxide particles in mechanical alloying 9Cr-ODS steel powder with annealing temperature was investigated by in situ hightemperature small-angle neutron scattering(SANS)technique.The volume fraction of nanoparticles decreases,while the corresponding radius increases,with increasing the annealing temperature.In comparison to holding time,the temperature is more critical to the precipitation kinetics of nanoparticles in ODS steels during the consolidation process.For 9Cr-ODS steel annealed after hot isostatic pressing,the SANS data show that the nanoparticles are coarsened by Ostwald ripening,and the coarsening data is consistent with a pipe diffusion mechanism with very high diffusion activation energy.(2)The stability of the oxide nanoparticles in 9Cr-ODS steel during thermal aging at 873 K for up to 5000 h was investigated by SANS and transmission electron microscopy analysis.In addition,the changes in grain and mechanical properties during aging were described by electron backscatter diffraction and Vickers microhardness.It was shown that the 9Cr-ODS steel exhibited good thermal stability during long-term thermal aging at 873 K.These nano-oxide particles played a key role in preventing grain growth and pinned dislocations,so the 9Cr-ODS steel also showed good thermal stability in terms of grain structure and mechanical properties.(3)Single-phase ErFexMn12-x(7.0≤x≤9.0)series alloy samples with ThMn12-type structure were successfully prepared by arc melting and heat treatment,and the relative contents of Fe and Mn were obtained using neutron powder diffraction technique,in which Mn preferentially occupies the 8i position.Magnetic measurement systems obtained the magnetic phase diagrams of the ErFexMn12-x(7.0≤x≤9.0)series samples.The fine crystal structure and magnetic phase diagram information were used to thereby finely modulate the exchange bias effect of the material.(4)Based on the interatomic potentials related to the rare earth and transition metals,which are obtained from the Chen-Mobius inversion method.The lattice constants of the ErFexMn12-x system are calculated by the inverted potentials and they are all close to the experimental reports.The differences in lattice constants between the calculated and the experimental values are less than 1.92%.The site preference of the ternary elements Fe in the structure of the rare-earth intermetallic compound ErFexMn12-x was determined based on the lattice inversion potential.The calculated results show that Fe will preferentially subrogate the 8f crystal site of Mn,which is consistent with the experimental results of neutron diffraction in this paper.Based on the site preference,the phonon density of states and the density of partial phonon states corresponding to different atoms in the ErFexMn12-x system are studied for the first time using the Chen lattice inversion potential,and the contributions of different atoms to the lattice vibrations are also analyzed at the atomic level.The thermodynamic properties such as specific heat,vibrational entropy and Debye temperature of ErFexMn12-x were predicted to lay the foundation for the structural and magnetic studies of rare earth intermetallic compounds. |
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