Preparation,Microstructure And Performance Of ODS FeCrAl Alloys

With the development of the Generation ? nuclear energy system and the concept of accident-tolerant fuel(ATF)after the Fukushima nuclear accident,oxide dispersion-strengthened FeCrAl alloy(ODS FeCrAl)are considered to be one of the ideal candidate materials for advanced fission reactor structures and accident-tolerant fuel cladding.Due to the high-temperature mechanical properties,radiation resistance and good high temperature corrosion resistance.ODS FeCrAl alloy has been a research hotspot in the field of nuclear materials.Although ODS FeCrAl alloy has many excellent properties,there are still some unsolved problems in its research at present,which limit its development and application.For example,uneven distribution of oxide particles(distributed along grain boundaries)can be caused by the current mechanical alloying method,as well as coarse oxide particles caused by Al elements,the processing of ODS alloy tube is difficult,how to avoid the ripening of nano-oxide particles during the processing,mechanical alloying method and Hot Isostatic Pressing(HIP)process are low-efficient and costly and so on.Different solutions for these problems,and a systematic study on them were conducted in this thesis.The main research contents and conclusions are as follows:1.Two kinds of 14Cr ODS FeCrAl alloys were prepared by mechanical alloying method,and the influence of the addition of Zr element on the dispersed oxides and mechanical properties of the alloys was studied.The results show that,with the addition of Zr,the particle size of oxide dispersoids in the alloy is refined,the number density is increased.The nano-oxide phase changes from YAP to Y2(Zr0.6Ti0.4)2O7,and keep a good coherence relationship with the matrix.The denser dispersed oxides and high-density dislocations make the mechanical properties of the alloy significantly improved.2.The powder forging method was used to fabricate ODS FeCrAl alloy,and the effect of powder forging on the microstructure and mechanical properties of the alloy was studied.The results reveal that the ODS FeCrAl alloy prepared by the powder forging method has very high relative density,the YAP oxide particles are uniformly dispersed in the alloy.The alloy has good impact toughness and a lower ductile-brittle transition temperature(DBTT).The grain size and high dislocation density bring it a good ambient and moderate temperature mechanical properties,but the grain boundary slip and relatively high residual porosity deteriorate the mechanical properties of the alloy at 1000?.3.The internal oxidation method was used to fabricate ODS FeCrAl alloy,and the structure evolution and mechanical properties during the preparation process are analyzed.The results show that the preferential combination of Y and O in the alloy powder can be achieved through vacuum treatment and subsequent oxidation treatment,which provid the necessary prerequisite for the generation of Y2O3 and Y-Al-O nanocomposite oxide particles in the alloy during subsequent processing.After the densification by HIP,a large number of oxide particles are generated in the alloy.Alloy has excellent mechanical properties,which is comparable to commercial PM2000 alloy.4.The annealing behavior of ODS FeCrAl alloy tubes at different temperatures were studied,and the effect of annealing temperature on the texture and hoop as well as axial tensile properties of the tube were explored.The results reveal that,with the increase of annealing temperature,the recovery of dislocation and coalescence of subgrain were occurred in the ODS FeCrAl alloy tubes.The bimodal fiber grains show different texture evolution during the annealing process,and excellent hoop and axial tensile properties are attributed to the strengthening mechanisms such as dispersion strengthening,back stress strengthening,and dislocation strengthening.The bimodal grain distribution helps to reduce the anisotropy of fracture model.5.In view of the ripening phenomenon of dispersed oxides in ODS FeCrAl alloys during processing,the ripening behavior of nano-oxide particles during thermal deformation and high-temperature annealing is studied,and a low-temperature relief annealing method was proposed to reduce the degree of ripening.The results show that the high-temperature annealing treatment after cold rolling has a greater impact on the ripening behavior of nano-oxides in ODS FeCrAl alloy than hot deformation,which is mainly attributed to the synergistic effect of grain boundary migration and solute atom diffusion on the grain boundary.Special arrangements of dispersed oxides are caused by grain boundary migration and dislocation movement.And the ripening process is accompanied by the structural transformation of nano-oxides.The coherent relationship between the oxide particles and the alloy matrix will gradually lose.And to reduce the elastic strain energy between the particles and the matrix,the shape of the oxide particles will change from spherical.to ellipsoidal6.The corrosion performance of ODS FeCrAl alloy tubes in a simulated primary loop of Pressurized Water Reactor(aqueous solution,static water environment at 360? and 18.6 MPa)and accident condition(saturate steamsteam environment at 1200?)were studied,and their corrosion behavior in different corrosive environments were also analyzed.The results show that ODS FeCrAl tubes exhibit excellent corrosion resistance in aqueous solution,static water.Due to the low O content in the water environment,the corrosion product is mainly Fe3O4 oxide film.It is estimated that the thickness of the oxide film will reach 1.4%of the tube wall's thickness corroded in dynamic water for 1 year.Densely ?-Al2O3 films are formed on the surface of the alloy during steam corrosion at 1200? and 0.1 MPa,which can effectively protect the tube matrix.In addition,the Zr element is beneficial to inhibit the diffusion of Ti from the matrix,and induce the occurrence of internal oxidation during steam corrosion.