Study Of Microstructure And Propety Of Ultra-Strong Nanocrystalline Austenite Steel With Exceptional Thermal Stability And Radiation Tolerance

Austenitic stainless steel has been widely used in automobile,medical equipment,and nuclear industry areas because of its excellent corrosion resistance,good plasticity and processing performance,and high creep resistance.However,austenitic stainless steel has low strength and poor void swelling tolerance,which seriously limit its application in extreme environments.Nanocrystalline materials play an important role in improving the strength and irradiation resistance of stainless steel due to their high proportion of grain boundary area,but they often suffer from poor thermal stability.In this thesis,304L austenitic stainless steel was used to prepare nanocrystalline materials with ultra high strength,good thermal stability and excellent irradiation resistance.Using thermodynamics and kinetics stabilization mechanisms,bulk nanocrystalline austenitic stainless steel containing 1 at%La(NC 304L-La)was prepared by mechanical alloying and high-temperature/high-pressure powder metallurgy techniques.The microstructure,mechanical properties,thermal stability and radiation resistance of NC 304L-La were studied systematically.Through the microstructure analysis and mechanical property test of 304L alloy containing different alloy elements(Y,La,Hf,W,B,C,Zr,Mo,Nb,Ta),bulk 304L alloy containing La element was screened to have the optimized comprehensive properties.The preparation process of powder metallurgy was optimized,and NC 304L-La samples with an average grain size of 45 nm were obtained under the optimal synthesis conditions(4 GPa and 1000 ?).The yield strength is 2.7 GPa under compression and 2.5 GPa under tension.The compressive yield strength of 304L bulk sample without doped La(NC 304L)under same synthetic conditions is only 1.7 GPa.A series of hardness and grain size data was obtained by isothermal annealing of NC 304L-La.According to Hall-Pacth relation,the increase in the strength of NC 304L-La was mainly attributed to grain boundary strengthening.The samples of NC 304L and NC 304L-La after isothemal annealing were stutied by X-ray diffraction(XRD).The lattice constant,lattice strain and dislocation density of the samples at different annealing temperatures were obtained by using the Nelson-Riley method and the improved Williamson-Hill method respectively.The formation,size,volume fraction and composition of nanoprecipitates(NPs)in NC 304L-La were analyzed by high-resolution transmission electron microsopy(HRTEM)and high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM),and atom probe tomography(APT)techniques.The thermal stability of NC304L and NC304L-La was investigated by TEM and hardness test.The grain size of NC 304L-La remained within the nanometer range(62 nm)after annealing at 1000 ? for 1 h,while the grain size of NC 304L under the same conditions increased to 564 nm.The good thermal stability of NC 304L-La is mainly attributed to the decrease of specific grain boundary(GB)energy due to the segregation of elemental La at GBs,which in turn reduces the thermodynamic driving force for grain growth(thermodynamics effect)and the pinning of GBs by dispersed NPs(kinetics effect).The irradiation resistance of NC 304L-La and coarse-grained 304L(CG 304L)under Me V heavy ion irradiation at room temperature and high temperature was studied.The microscale defects of NC 304L-La after He injection were characterized by positron annihilation spectroscopy(PAS).The phase composition,microstructure and mechanical properties of the irradiated samples were investigated by using grazing incidence XRD and TEM.The element segregation of irradiated samples,volume fraction and number density of NPs were analyzed by APT.Neither dislocation rings and voids nor other defects were detected in NC 304L-La after 6 Me V Au irradiation up to 1.5×10¹⁶ cm²(108 dpa)at room temperature or high temperature(600 ?).Under the same irradiation condition,a large number of dislocation rings and voids were produced in CG 304L after irradiation.The result of 3 Me V Fe ion irradiation is similar to that of Au ion irradiation.The excellent irradiation resistance of NC 304L-La is due to the large amount of GBs and phase boundaries that can provide high sink strength,which suppresses the formation of voids and dislocations.