Novel Fabrication Technology And Microstructure Study Of Oxide Dispersion Strengthened Ferrite Alloy

Oxide dispersion strengthened(ODS)ferrite alloys are considered to be one of the most promising materials for reactor fuel cladding due to its excellent high-temperature mechanical properties and resistance to neutron irradiation.The traditional mechanical alloying process for ODS alloy has the disadvantages of high cost,low efficiency and difficulty in mass production,which seriously restrict the application of ODS alloy in nuclear power materials.Therefore,it is necessary to study the new preparation technology of ODS alloy.The conventional fabrication process of ODS alloy includes powder preparation,mechanical alloying,thermal consolidation and plastic working.Mechanical alloying and thermal consolidation are two key steps,which are time-consuming and costly.In order to reduce the preparation cost and simplify the process,the idea of using low temperature oxidation instead of mechanical alloying process to introduce oxygen element into the alloy powder is firstly determined by reviewing a large amount of literature and summarizing the previous work.On the basis of extensive experiments,a reasonable oxidation regime is established to realize the oxidation of powder and control the oxygen content and oxidation products.The oxidized powder is then consolidated by a hot isostatic pressing process and the FeCrAl ODS alloy is successfully prepared.However,there is a problem of uneven distribution of nanoparticles in the ODS alloy prepared by the above method,so the explosive compaction method is used to replace the traditional hot isostatic pressing and hot extrusion process to control the microstructure of the alloy,and the FeCrAl ODS alloy with fine and uniform nanoparticles is obtained.Finally,in order to further simplify the process and reduce the cost,FeCr ODS alloy is successfully prepared by combining low temperature oxidation technology with powder forging technology.In this thesis,we mainly use scanning electron microscopy(SEM),X-ray photoelectron spectroscopy(XPS),electron microprobe(EPMA),transmission electron microscopy(TEM)and other analytical means to study the change of microstructure characteristics of the powders,the types and structural information of nanoparticles in the alloy during oxidation treatment.In addition,the formation mechanism and various factors affecting the formation of nanoparticles in ODS alloy prepared by different processes are also explored.The main conclusions are as follows:(1)Low temperature oxidation technology is used to introduce O elements into the powder.By precisely controlling the oxidation temperature and oxygen partial pressure,a metastable oxide film composed of iron oxides can generate on the surface of FeCrAl alloy powder.This oxide film can "decompose"during the subsequent heating process,and the oxygen generated by the decomposition of the oxide film can diffuse into the powder and combine with Y and Al elements to generate YAlO3 nano-oxide particles.(2)Explosive compaction technology is used to shape and densify the alloy powder.When the oxidized powders are formed by explosive compaction,the powder will be deformed and broken.Then,the oxide film on the powder surface will also be broken and redistributed,which is conducive to the sintering of the powder and the uniform distribution of oxygen elements.A small amount of oxide nanoparticles are produced in the compacts obtained after explosive compaction,but further sintering is still needed to promote the formation of a large number of nanoparticles.A large number of nanoparticles can be produced in the alloy sintered at 800? after explosive compaction.When the sintering temperature is 1000?,the number of nanoparticles further increases and the prior particle boundaries completely disappears.The number density of nanoparticles in the alloy is 5.6× 1022 m-3,which is equivalent to that of ODS alloy fabricated by mechanical alloying process.However,when the sintering temperature is 1200?,the nanoparticles in the alloy will coarsen through Ostwald ripening mechanism,resulting in an increase in the size and a decrease in the number density of the oxide nanoparticles.(3)The powder forging process is used to shape and densify the alloy powder.A low-cost and simple process for the preparation of ODS alloys was designed by combining the low-temperature oxidation technology with the powder forging process.A large number of uniformly distributed Y2TiO5 nanoparticles with a number density of 1.55×1022 m-3 are generated in the FeCr ODS alloy prepared by this technique,and a small amount of large Y2O3 particles exist at the grain boundaries.(4)Study on the formation of nanoparticles in alloys and their influencing factors.When FeCr ODS alloy is prepared by oxidation method,the distribution of nanoparticles in the alloy is the same as that of Y element.The diffusion coefficients of Y,Ti and O elements in this alloy system are calculated by using Thermal-Calc software,and we find that the diffusion rate of Y element is the slowest.The oxide nanoparticles in the alloy are mainly formed by the diffusion of these three elements,so the distribution of nanoparticles in the alloy ultimately depends on the distribution of element Y.In addition,the types of oxide nanoparticles in the alloy may also vary when the relative contents of Y and Ti are different.The lower Ti/Y value is favorable for the formation of Y2TiO5 phase,and the higher Ti/Y value is favorable for the formation of Y2Ti2O7 phase.(5)Research on new ODS alloy forming process and fabrication technology.Explosive compaction and powder forging processes are both special forming processes in powder metallurgy with the characteristics of thermomechanical forming.Their characteristic is that they can cause severe plastic deformation of the powder,resulting in a large number of dislocations,vacancies and other defects in the powder.These defects can promote the diffusion of elements,and are favorable for the nucleation of nanoparticles.The new forming processes result in a more uniform distribution of the nanoparticles in the alloy and an increase in the number density,which is expected to provide the alloy with desirable mechanical properties.Therefore,densification of oxidized powders by thermo-mechanical forming is a new ODS alloy preparation technique with great potential.