The Regulation Of Oxide Second Phase Particles In Oxide Dispersion Strengthened Tungsten Alloy And Its Strengthening And Toughening

Compared with the corresponding tungsten alloys,oxide dispersion strengthened（ODS）tungsten-based alloys exhibit significantly enhanced strength,microstructural stability and creep resistance.However,the commonly used oxide doping techniques generally lead to the aggregation and growth of oxide particles at W grain boundaries,greatly weakening the improvement effect of oxide addition.Based on this situation,starting from the preparation of composite powder precursor of ODS tungsten alloy,we prepared oxide-doped W-Y₂O₃ composite nanopowder and Y₂O₃@W core-shell structural composite nanopowder.Using these nanopowders as precursors,the oxide second-phase particles at grain boundaries and within grain interior can be regulated respectively.Then,the effect mechanisms of oxide distribution state in the above composite powders on the grain growth and densification mechanisms of W matrix are investigated deeply,the interfacial structure of oxide second phase particles with W matrix and the strengthening mechanisms of oxide particles are studied.On this basis,we developed the ODS tungsten alloy with excellent match of strength and ductility.The main contents and conclusions are as following:In order to refine and disperse the large-sized oxide second-phase particles at grain boundary of W-Y₂O₃ alloy,various oxide-doped W-Y₂O₃ composite nanopowders are prepared by in-situ chemical powder preparation technique.After low-temperature sintering,it is found that La₂O₃ and HfO₂ doping have the most significant refinement effect on the intergranular Y₂O₃ particles,which is closely associated with the interfacial segregation of La₂O₃ and HfO₂.Oxide chemical potential gradient,elastic strain energy and interfacial energy are three driving forces for the segregation of doped oxides.Besides,La₂O₃ doping refines the grain microstructure,while HfO₂ doping coarsenes the grain microstructure.This is because the W-O chemical bonding at the W/Y₂O₃ interface can be significantly changed by oxide segregation,which will affect the sintering activity of W matrix.La₂O₃ doping can simultaneously refine the intergranular oxide second phase particles and matrix grain microstructure,significantly improving the strength and toughness of W-Y₂O₃alloy.In order to promote more oxide second-phase particles to disperse within W grain interior uniformly,we independently develop the in-situ chemical powder preparation technique of hydrothermal+freeze-drying method,preparing Y₂O₃@W core-shell structural composite nanopowder.Using this powder as precursor,the intergranular oxide particles disappear completely in prepared W-Y₂O₃ alloy,and the oxide second-phase particles are distributed uniformly within W grain interior and keep coherent interface with W matrix.As a result,the fabricated ODS-W alloy possesses both high strength（1390 MPa）and excellent ductility（2.6%）at room temperature,breaking the brittle feature of traditional W-based alloys at room temperature.Its strength at 600℃ reaches 721 MPa with a total elongation of 12%.Its high strength mainly originates from the refined sub-grain microstructure and dislocation shearing effect of intragranular nanoparticles,and its ductility is closely associated with the lamellar microstructure with abundant low-angle grain boundaries and the dislocation pinning and accumulation effects of intragranular oxide nanoparticles.The different oxide distribution states have different effects on the sintering kinetic of W matrix.Based on this,we further studied the influence mechanism of oxide distribution state in the above composite powders on the grain growth and densification of W matrix.It can be found that no matter the oxide is distributed at grain boundary or within grain interior,the mass transport mechanism of W matrix during sintering process does not change and remains as grain boundary diffusion.But the intergranular oxides significantly enhance the activation energy for W grain growth,inhibiting W grain growth.Besides,the intergranular oxides also reduce the grain boundary diffusivity of W,inhibiting the densification of W matrix.In contrast,if the oxides are dispersed within W grain interior,they will not affect the activation energy for W grain growth and the grain boundary diffusivity of W matrix.Thus,the intragranular oxide do not affect the grain grwoth and the densification of W matrix.In the last part of this dissertation,it is found that HfO₂ can be used as both strengthening phase and sintering accelerator for W matrix.During the low-temperature sintering process,HfO₂ can significantly increase the grain boundary diffusivity and the sintering stress of W matrix,so the kinetic and thermodynamic conditions promoting the sintering of W matrix are simultaneously satisfied.In addition,it is found that the sintering kinetics mechanism of W-HfO₂ system was consistent with the kinetic mechanism of W diffusion through HfO₂,indicating that the intergranular HfO₂ particles could provide a fast channel for the diffusion of W atoms along grain boundaries,thus promoting the rapid densification of W matrix during low-temperature sintering.After pressureless sintering at 1480℃ for 2 h,the grain size of W-HfO₂ alloy is only 200 nm,but the relative density reaches 97%.The corresponding compressive strength reaches nearly 3 GPa.Second-phase strengthening and fine-grain strengthening are the main strengthening mechanisms of W-HfO₂ alloy.