Preparation,Microstructure And Property Of Oxide Dispersion Strengthened Nickel-based Superalloys

Oxide dispersion strengthened（ODS）nickel-based superalloys are deemed as promising candidate materials to meet the corrosion,erosion,oxidation,mechanical strength,and creep challenges imposed by the harsh operating environments of components such as gas turbines,heat exchanger tubes and nuclear reactors.In order to satisfy more stringent requirements,it is essential to improve the property of ODS nickel-based superalloys.In this paper,ODS nickel-based superalloys are prepared by mechanical alloying（MA）,hot isostatic pressing process（HIP）,hot extrusion（HEX）and simulation method of consolidating powder.The microstructure of samples is studied by scanning electron microscope（SEM）,electron backscattered diffraction（EBSD）and transmission electron microscopy（TEM）.The tensile properties,creep behaviors and thermal stability are tested.The mechanisms of microstructure evolution during the preparation and post-treatment process of the alloys are systematically investigated,which provide theoretical and data support for the improvement of ODS nickel-based superalloys.Accordingly,several results are concluded as following:（1）The microstructures and tensile properties of ODS nickel-based superalloys with additions of YH₂,Y₂O₃,La₂O₃and Ce O₂are characterized.The results show that the mechanical property of alloy prepared by YH₂with smaller nano-oxide is higher than other alloys.The contributions of Hall-Petch strengthening,solution strengthening,dislocations and dispersion strengthening to yield strength at room temperature are estimated.The formation and evolution mechanism of nano-oxides are discussed.Nano-oxides riches in M-Al-O（M=Y,La,Ce）are preferentially formed,due to the higher diffusion rate of Al atom and high M/Al value,then Ti and O atoms diffuse to the surface of nano-oxides.（2）An simulation method of consolidating powder is introduced to optimize the preparation process of ODS nickel-based superalloys.The influences of composition,consolidation temperature,strain rate and ball milling time on the microstructure and hardness of sample are studied.Combined with machine learning,the relationship among process,microstructure and hardness is established.The results indicate that the cause of crack formation in sample is related to prior particle boundary.Long ball milling time,moderately low consolidation temperature and high strain rates are beneficial to the formation of fine-grains and obtaining excellent properties.Basically,in order to obtain fine-grains,excellent properties and less cracking risk,the ball milling time of 24 h,heating temperature of 1130°C and extrusion ratio of 16:1 are suggested.（3）The microstructure and tensile properties of samples are charactered,the influences of nano-oxides introducing methods on the microstructure and properties of alloys are discussed.The microstructure of tensile fractures of samples are analyzed,the influences of temperature and oxidation damage on the tensile ductility of the alloy are studied.The results indicate that the ductility of alloys decrease is affected by stress-assisted grain boundary oxidation,the grain boundary slip and grain boundary migration play key role in improving the ductility of alloys at elevated-temperature.The bimodal-grained alloys owned superior combination of strength and ductility due to the combination of fine grain boundary strengthening and good plasticity of coarse grain.The creep properties of samples are tested,TEM observations indicates that dislocation climbing controlled is the main creep deformation mechanism.In addition,the evolution of microstructure and mechanical properties of the alloy during high temperature exposure is studied,the growth of grain and the decrease of hardness are related to the coarsening of nano-oxides.（4）To conquer the poor processibility of ODS nickel-base superalloys,caused by large deformation resistance,narrow hot processing window and uniform microstructure.Hot compression experiments are carried out at the temperatures ranging in 950-1100°C and strain rates of 0.001-1 s^-1.The constitutive equations and hot processing map of the alloy were established.The results show that the peak stress region of energy consumption mainly occurs at 958-1030°C and 0.008-0.1s^-1,the instability zones of the alloy are mainly in the high strain rate zone at low temperature and the medium-low strain rate zone at high temperature,the instability mechanism is the generation of wedge cracks.The microstructure evolution of the alloy during hot deformation is studied.The increase of temperature and decrease of strain rate are favorable for the dynamic recrystallization nucleation and grain growth.No obvious change of nano-oxide is observed during the hot deformation.