Preparation,Structure And Properties Of Ultrastrong And Thermally Stable Nanocrystalline 14YWT ODS Steel

Oxide dispersion strengthened(ODS)steel is recognized as one of the most promising alternative materials for the first wall/cladding structure of fusion reactors,However,the ODS steel with a moderate density of precipitates exhibits such problems as radiation hardening and increased ductile-brittle transition temperature after low temperature(300 ~oC)radiation.The large number of grain boundaries in nanocrystalline(NC)materials can promote the recombination of vacancies and interstitial atoms,reduce the aggregation of point defects and improve the radiation tolerance of the materials.At the same time,the excellent mechanical properties of nanocrystalline materials can also improve the design stress of the materials.Note that grains often rapidly grow in NC materials at a temperature of above 0.3 Tm.Therefor,the research goal of this dissertation is to prepare a NC ODS steel with a high strength and a high thermal stability.In this project,14YWT alloy was selected as the original sample.High-energy ball milling was used to dope 0.5at.%Zr element in 14YWT powder.A high temperature(900°C)and high pressure(4 GPa)synthesis technique was then used to consolidate the ball-milled powders to form bulk specimens.NC 14YWT-0.5at.%Zr alloy with a grain size of 50±15 nm was achieved.Atom probe tomography results indicates that Zr element is segregated on the grain boundary whereas Y,Zr,Ti,and O elements are riched in the precipitates.The precipitaes is with a size of 3.18±2.5 nm,a volume fraction of 2.27%,and perfectly coherent with the matrix.Under the same preparation conditions,the grain size of the 14YWT alloy is 153±30 nm whereas the size and volume fraction of precipitates are 6.3±4.1 nm and 0.84%,respectively.It was found that the improve a thermal stability of NC 14YWT-0.5at.%Zr alloy resulted from 1)the segregation of Zr elements at the grain boundaries,which thermodynamically reduced the driving force for grain growth,and 2)the Y-Zr-Ti-O precipitates,which kinetically increased the pinning resistance to suppress the grain growth.The tensile yield strength of NC 14YWT-0.5at.%Zr alloy after hot rolling at 800°C is 2730 MPa,which is nearly 65%higher than the tensile yield strength of 14YWT alloy,whereas the elongation at fracture of NC 14YWT-0.5at.%Zr alloy is 4.7%,which is close to that of 14YWT alloy.Calculation and analysis suggest that the improved yield strength is mainly caused by the dispersion strengthening and grain boundary strengthening.The yield strength and grain size of NC 14YWT-0.5at.%Zr alloy after annealing at 1000°C for 1 hour are 2237 MPa and 75±23 nm,respectively.The yield strength and grain size of NC 14YWT-0.5at.%Zr alloy after annealing at 800°C for 180 hours are 2618 MPa and 58±23 nm,respectively.The yield strength of NC 14YWT-0.5at.%Zr alloy under high temperature compression at 650°C reaches 670 MPa.Thus,the NC 14YWT-0.5at.%Zr alloy shows excellent thermal stability and mechanical properties,which can in turn improve the safety service conditions of the first wall/cladding structure material in nuclear fusion reactor.Through the research of this project,NC 14YWT-0.5at.%Zr alloy with high strength and high thermal stability was successfully prepared.This new alloy has the potential to become the first wall/cladding structure material of the new generation fusion reactor.