Preparation And Mechanical Properties Of Ultrafine Grained T91 Steel With High Strength And Thermal Stability

As an alternative structural material for the new generation of nuclear reactors,T91 steel has broad application prospects due to its good resistance to radiation creep and hardening.This thesis aims to prepare a new ultra-fine grained T91 alloy with high strength and high thermal stability.In this thesis,added alloying element X(Hf,La,Zr,Y)inhibits grain growth due to both kinetic(Zenner pinning)and thermodynamic(segregation of X solute atoms to grain boundaries reducing grain boundary energy)stabilization mechanisms.A series of ultra-fine grained T91-1at.%X alloys were successfully prepared by mechanical alloying and high-temperature/high-pressure(4 GPa/1100 ?/30 min)sintering techniques.The T91-1at.%Hf alloy with the best overall performance was obtained by studying their thermal stability and mechanical properties.As a basic research,this thesis mainly studies the change of thermal stability and mechanical properties of T91-1at.%Hf alloy with annealing temperature and time,and explores the thermal stability mechanism of Hf element and its high temperature compression properties.The T91-1at.%Hf alloy prepred by consolidating under a high pressure has a grain size of 160 nm,which is 800 nm smaller than that of the traditional T91 alloy,a hardness value of 722 HV,and a compressive yield strength of 2449 MPa.The grain begins to grow slightly after annealing at 600 ?.After annealing at 1000 ?,the grain size is 174 nm whereas the hardness and strength are still high: being 692 HV and 2161 MPa.After annealing at 800 ? for 50 h and 180 h,the grain size of T91-1at.%Hf alloy is 171 nm and 190 nm,respectively.The hardness and strength after annealing at 800 ? for 100 h are 656 HV and 2089 MPa,respectively.The alloy maintains a high thermal stability and excellent mechanical properties during the annealing and maintains a high compressive strength of 1355 MPa at600 ?,showing good high-temperature performance.The Hf element participates in the formation of the Hf-Si-O second phase particle pinning grain boundary,and partial Hf atoms are segregated at the grain boundary to lower the grain boundary energy,thereby achieving excellent thermal stability.Through the research of this subject,the thermal stability and mechanical properties of T91 steel have been further improved,and its important and fundamental problems have been solved for its application in the field of nuclear structural materials.