Study On The Thermal Stability Mechanism And Mechanical Properties Of Ultra High Strength Nanocrystalline T91 Steel

As a candidate material for the first wall and cladding structure of nuclear fusion reactors,T91 steel has excellent mechanical properties,radiation resistance,corrosion resistance,etc.,and has been widely studied and applied worldwide.At present,many studies have been performed to refine the microstructure and structure of T91 steel through various preparation methods and treatment processes,to improve its mechanical properties and radiation resistance.However,the achieved grain size of these T91 steels is mostly in the ultra-fine grain size range.In contrast,this research aims to prepare a nanocrystalline（NC）T91 steel with ultra-high strength and high thermal stability.Using mechanical alloying and high-temperature and high-pressure sintering techniques,where the latter technique is optimizes as a two-step sintering process（under a pressure of 4 GPa,heated to 750°C and then cooled to 650°C and holding for 30 minutes）,the NC-T91-1at.%Hf and NC-T91 alloys with ultra-high strength and a certain degree of plasticity were prepared.The grain sizes were 37±13 nm and 74±26 nm,and the hardness was 927 HV and 867 HV.The strength is 3478 MPa and 2889 MPa respectively.After NC-T91-1at.%Hf alloy is hot-rolled at 800°C,the grain size is 83±24 nm,there are a large number of fine and dispersed Hf-Si-O nano-oxide precipitates,and the grain size of the precipitates is 4.5±1.2 nm,the number density is 3.4×10²³ m^-3,the volume fraction is1.5%,and it has a semi-coherent relationship with the matrix.The tensile yield strength is3437 MPa,and the elongation at break is 1.2%.The main strengthening mechanism of NC-T91-1at.%Hf alloy as prepared is grain boundary strengthening and dislocation strengthening,and the main strengthening mechanism after 800°C hot rolling is grain boundary strengthening and precipitation strengthening.After NC-T91-1at.%Hf alloy was annealed at 800°C for 1 h,the nanocrystalline size remained at 75±30 nm,and the fine dispersed Hf-Si-O nano-oxide precipitates particle size was 4.7±1.3 nm.After annealing at 1000°C for 1 h,some of the grains grew abnormally and appeared bimodal distribution.The large-sized crystal grain size is about 664 nm,the nano-crystal grain size is 76±15 nm,and the fine dispersed Hf-Si-O nano-oxide precipitates particle size is 5.7±2 nm.The high thermal stability of NC-T91-1at.%Hf alloy is provided by two thermal stability mechanisms.A large number of fine dispersed Hf-Si-O nano-oxide precipitates pin the grain boundaries;Mo and Cr segregate to the grain boundaries to reduce the grain boundary energy.That is,the dynamic Zener pinning stability mechanism and the thermodynamic stability mechanism work together.