Improved Ductility And Hightemperature Mechanical And Oxidative Properties Of Ultra-strong Nanocrystalline Austenitic Stainless Steels

Conventional austenitic stainless steel has relatively low room-temperature and high-temperature strength.Nanocrystalline austenitic stainless steel with a high grain boundary volume fraction can greatly improve its room-temperature strength.However,grain size of conventional nanocrystalline austenitic stainless steel is easy to grow up at high temperature and shows a very poor thermal stability.Therefore,this dissertation takes 304L austenitic stainless steel as the research object and uses the dual stabilization mechanism of thermodynamics and kinetics to prepare nanocrystalline 304L austenitic stainless steel with high room-temperature strength,high high-temperature creep resistance,and high hightemperature oxidation resistance.Nanocrystalline 304L austenitic stainless steel（NC304L-La）containing 1 at%La was successfully prepared by mechanical alloying and high-temperature and high-pressure（HT/HP）technique.Then,its room-temperature mechanical properties and microstructure,high-temperature properties（high-temperature strength,high-temperature creep,and high-temperature oxidation）and microstructure,and plastic improvement were systematically investigated.The mechanisms of NC 304L-La with high room-temperature strength,high-temperature high strength,high-temperature resistance creep,high-temperature resistance oxidation and plasticity improvement were described in detail.Through room temperature compression experiments,it was determined that NC 304L-La prepared at 4 GPa-1000 ℃ had the best room temperature mechanical properties,and it was analyzed that the high strength of NC 304L-La is mainly due to fine-grain strengthening.The excellent thermal stability of NC 304L-La is due to the dual effect of pinning of grain boundaries by nanoprecipitates（NPs）（kinetic mechanism）and grain boundary energy reduction（thermodynamic mechanism）caused by the segregation of La elements.Comparative experiments on the construction of nanocrystalline 304L without La element doping（NC 304L）and NC 304L-La.On the one hand,it is found that the addition of La element could significantly improve the phase stability of NC 304L.On the other hand,it was found that NC 304L-La had a higher compressive yield strength than NC 304L in the medium and high temperature ranges.It is believed that the deformation mechanism of NC304L during the hot pressing process at 600℃ is grain boundary mediated,while the deformation mechanism of NC 304L-La during the hot pressing process at 600℃ is dislocation mediated.The microstructure of NC 304L and NC 304L-La were analyzed by TEM and APT after hot pressing at 600 ℃.Elemental segregation and NPs were present in NC 304L and NC 304L-La.Compared to NC 304L,elemental segregation in NC 304L-La had a greater ability to reduce grain boundary energy whereas NPs had greater resistance to grain boundary slip and rotation at high temperatures,resulting in higher strength for NC304L-La at high temperature.Compare the high-temperature creep of NC 304L and NC 304L-La at different temperatures and stresses.The results showed that the creep rate of NC 304L-La was lower than that of NC 304L,indicating that the addition of La could improve the high-temperature creep resistance of NC 304L.It is believed that creep mechanism of NC 304L-La at 600 ℃ and 700 ℃ was glide-controlled creep,while the creep mechanism of NC 304L-La at 800℃ was dislocation climb-controlled creep.The mechanism of high temperature creep resistance of NC 304L-La is attributed to two aspects:1)The interaction between NPs and dislocations in NC 304L-La can enhance the storage of dislocations within the grain,thereby suppressing dislocation annihilation at grain boundaries.2)High density NPs can exert enhanced pinning force on grain boundaries,thereby limiting grain boundary activity such as grain rotation and grain boundary sliding.Comparative high-temperature oxidation experiments were conducted on commercial coarse-grained 304L stainless steel（CG 304L）and NC 304L-La at 900 ℃ and 1000 ℃.The microstructure and oxidation mechanism of CG 304L and NC 304L-La after high temperature oxidation were studied.It was found that the oxidation layer of NC 304L-La had a three-layer structure.The outermost layer was the Mn-rich oxide layer,the middle layer was the Cr/Mn-rich oxide layer,and the inner layer was the continuous SiO₂ oxide layer.It is assumed that the formation of continuous SiO₂ healing layer inhibited the diffusion of Cr from the nanocrystalline substrate to the upper Cr-rich oxide layer,which made NC 304L-La exhibit excellent oxidation resistance at high temperature.A process for producing high strength and high plasticity 304L austenitic stainless steel using micro-nano composite method（heterogeneous 304L）.Deeply explored the origin of high strength and high plasticity of heterogeneous 304L.It is believed that the high strength was mainly caused by the ultrafine grain（UFG）phase.The existence of UFG phase was mainly due to the utilization of high sintering pressure,which reduced the diffusion rate and thus the grain growth rate.The high ductility was mainly caused by the microcrystalline（MC）phase,which caused the formation of a large number of dislocations and the generation of nano-twins.