Effect Of Zr,Hf Addition On The Microstructure And Tensile Properties Of FeCrAl-ODS Steels

FeCrAl oxide dispersion strengthened?ODS?steels have been considered as one of the candidate structural materials in the future fission and fusion reactors due to their excellent high-temperature profermance and corrosion resistance.However,the presence of Al in matrix tends to coursing the size of oxides which would deteriorate the mechanical properties of ODS steels.In order to improve the profermance and maintain the corrosion resistance of ODS steels simultaneously,Zr and Hf elements were added to FeCrAl ODS steels.Therefore,the effect of Zr and Hf addition on the microstructure and mechanical properties of FeCr Al ODS steels was investigated.ThreecompositionsofODSsteelsFe16Cr3Al1.5W-0.35Y₂O₃,Fe16Cr3Al1.5W0.5Zr-0.35Y₂O₃ and Fe16Cr3Al1.5W0.5Hf-0.35Y₂O₃?wt%?were designed and prepared in this experiment.The metallographic structures,the matrix grain sizes and distribution,the oxide particles size and crystal structures of the three ODS steels were observed and compared by OM,EBSD and TEM.In terms of mechanical performance,tensile tests were carried out at several temperatures?23?,400?,600?,800??,and the fracture mechanism was further analyzed.In addition,the results of tensile tests were compared with two commercial alloys MA956 and PM2000 which have similar components with test samples.In order to study the mechanism of the effect of Zr and Hf addition on the microstructure and properties,Zener-Smith mechanism was used to explain the relationship between grain refinement and the change of oxide particles.The improvement of yield strength caused by grain refinement and the decrease of oxide particle size and spacing were explained by Orowan model and Hall-Petch model.The results showed that the addition of Zr and Hf can effectively refine the size of matrix grains and oxide particles in FeCrAl ODS steels.The inter-particle distance decreased,while the number density of oxide particles increased.The refinement of oxides and the increase of oxides density which can hinder the migration of grain boundaries more effectively is the main reason for the decrease of grain size.And the change in particle distribution is related to the transformation in the crystal structures of the oxide particles.It can be concluded that the oxide particles in ODS steel without Zr and Hf are all coarse Y-Al-O with different crystal structures.The presence of Zr and Hf promoted the formation of finer and more stable Y-Zr-O and Y-Hf-O particles which can partly replace Y-Al-O particles.The addition of Zr and Hf improved the tensile strength of FeCrAl ODS steels which can be explained by dispersion strengthening caused by fine oxide particles pinning dislocations and refined crystalline strengthening caused by grain boundaries.In contrast,the reinforcement of Hf on high-temperature strength is more apparent.The tensile failure mechanism of ODS steels changes with the increasing of temperatures from quasi-cleavage brittle and dimple ductile mixed fracture mode to typical ductile fracture mode and then intergranular fracture.To strengthen the grain boundaries is the effective way to improve the high-temperature strength of ODS steels.To sum up,adding Zr and Hf elements can refine the size of oxide particles,increase the oxides distribution density,and refine the matrix grain of ODS steels,and then achieve the goal about improving tensile properties.