Formation Mechanisms Of The Dispersed Oxides Strengthening Phase In High Chromium Ferritic Heat-resistant Steels

Oxide dispersion strengthened （ODS） ferritic steels are believed to be the mostpromising candidates of structural materials for advanced nuclear systems for theirsuperior radiation resistance, excellent high temperature creep and tensile properties.A key strategy for designing high-performance materials is based on the introductionof dispersed nanoscaled oxide particles which are achieved by the new technology ofmechanical milling （MM） and subsequent hot pressing （HP）.In this thesis, fundamental issues such as elaboration, mechanism and phasetransformation of ODS steel were investigated. The main content of the thesis:formation mechanism of the nanoscaled oxide particles, microstructural evolution ofODS steels during elaboration processes, the elaboration of MgAl₂O₄ODS steel andformation of nanoscaled MgAl₂O₄particles at lower sintering temperature, kinetics ofaustenitisation process in steels and the elaboration of Fe-Cr-W-Ti-Al-Y ODS alloyby the addtion of Fe₃Al as aluminum carrier on the basis of Fe-Cr-W alloy powders..The research provides a base for exploration and application of ODS steel based onexperimental and theoretical investigations.Evolution of Y₂O₃during mechanical milling and subsequent annealing wasinvestigated. The particle size and the grain size of the mixed powder decrease in themechanical milling process, they almost do not decrease along with the formation ofdynamic equilibrium between cold-welding and repeated fracture after longer millingtime.The Y₂O₃is gradually fractured and nanocrystals are formed after MM. Growthof Y₂O₃nanocrystals takes place during subsequent annealing due to Ostwaldripening. The formation processes of Y₂O₃nanocrystalline may follow the sequence:ordered phase disordered phase （loss of long-range order） fine-grained（nanocrystalline） phaseThe formation mechanism of nanoscaled Ti-Y bioxide particles was investigated.Three types of nanoscaled oxide particles were observed in HPpedFe-12Cr-0.2Ti-0.3Y₂O₃ODS steel: fully crystallized particles, partially crystallizedparticles and particles with a structure characterized by crystalline domain whereamorphous domains are distributed inside. Nanoscaled Y₂O₃fragments andamorphous phase of[YMO]amorphousare formed after MM. The formation process of mixing of nanoscale fragments Y₂O₃and amorphous phase can be illustrated as:.The formation of Y₂Ti₂O₇crystallineduring subsequent HP process involves reactions between Y₂O₃fragments andtitanium （Ti） and crystallization of[YMO]_amorphous and the formation process can beillustrated as:Microstructural evolution of ODS steels during elaboration processes wasinvestigated. The HPped steels exibit a full ferritic microstructure and the grains areequiaxed with a micrometric size. The nanoscaled oxide particles are dispersed in thegrains of the HPped steels. Some unreinforced domains without the nanoscale oxideparticles indicate that there still exist inhomogeneous areas through the size of thoseoxide particles reaches nanoscaled. Calculated threshold stress varies at differentdomains randomly selected due to various dispersion states of the nanoscale oxideparticles such as the size and relatively homogeneousness. That may be the reasonwhy the threshold stress cannot be clearly achieved by the results of creep tests. Thecalculated results of threshold stress （360MPa） indicate that the size and dispersion ofthe nanoscaled oxide particles are beneficial to excellent properties of materials.MgAl₂O₄ODS steel was successfully elaborated by MM and HP. The MgO andAl₂O₃do not dissolve in Fe-Cr binary system during MM. The combination betweenmagnesium （and aluminum） atoms and oxygen atoms isn’t fractured by severe plasticdeformation.The MgAl₂O₄nanoparticles are formed at relatively lower temperature.The increase in interfacial energy due to the size decrease of the oxides and thechange of volume fraction of grain boundary of Fe crystals are considered to be thedriving force for the amorphization of the oxides during MM. The strengtheningphase of MgAl₂O₄formed due to crystallization of the amorphous phase during HPprocess.Phase transformation kinetics from ferrite to austenite in Fe-Cr ODS ferrite steelswas investigated. The kinetics analysis indicates that the nanoscaled oxide particles inreinforced alloys exhibit great influence on the growth process of austenite throughtheir pinning effect on the movement of interface. This pinning effect against themotion of/interface leads to the decrease of the values of pre-exponential factorV0with increase of additive. Moreover, the values of the pinning force exerted by those nanoscaled oxide particles were quantitatively evaluated, which agrees with theretardation phenomenon of austenitisation process according to the modified JMAKmodel. The values of the pinning forcep pinare0.78MJ/m³and2.99MJ/m³forFe-9Cr-0.3Y₂O₃alloy and Fe-9Cr-0.2Ti-0.3Y₂O₃alloy, respectively. Compared withunreinforced alloy Fe-9Cr, the value ofp pinincreases with the addition of yttria andtitanium in Fe-9Cr-0.3Y₂O₃alloy and Fe-9Cr-0.2Ti-0.3Y₂O₃alloy. Finer dispersionof nanocaled oxide particles （Y₂Ti₂O₇） in Fe-9Cr-0.2Ti-0.3Y₂O₃than that （Y₂O₃） ofFe-9Cr-0.3Y₂O₃is beneficial to provide more efficient blocks to the motion ofinterface, which is reasonably consistent with the fitted results of the values ofpre-exponential factorV0for the movement of interface/phase by phasetransformation model.Fe-Cr-W-Ti-Al-Y ODS alloy was elaborated by the addtion of Fe₃Al as aluminumcarrier on the basis of Fe-Cr-W alloy powders. The formation of Y-Al-O bioxideparticles are composed of two stages:（a） the aluminum component precipitated fromthe matrix and （b） the reaction between the aluminum component and yttria. Thealuminum component substituted the titanium component during the formationprocess of Y-Al-O bioxide particles due to its higher oxygen affinity. Thetransformation of the phase of the dispersion after apperence of the aluminumcomponent probably comes from a competition between the aluminum componentand titanium component. The calculated yield strengths results are in good agree withthe experimental ones when0.5and1, the better results of yield strengthscome from grain refinement strengthening and dispersion strengthening.