Study On Precipitation And Growth Behavior Of MX Phase In 15-15ti Austenitic Stainless Steel

15Cr-15Ni titanium-modified(15-15Ti)austenitic stainless steel is used as the structural material in the fourth generation advanced sodium cold fast reactor core assembly.It has both good radiation resistance and high temperature strength.The cladding material is to be selected for the sodium-cooled demonstration fast reactor.However,Research on 15-15Ti alloy for cladding materials in China is at a preliminary stage,and the materials,properties and preparation techniques of the materials are still not mature.In this paper,15-15Ti austenitic stainless steel is studied by setting different alloy compositions and heat treatment system,the precipitation mechanism of MX phase in different Ti/C ratio alloys at high temperature and the aging of carbide growth with different Mo contents at medium temperature are systematically studied.To provide reliable experimental data and theoretical analysis for the independent research and development of fuel component cladding materials in China.The main research results obtained in this paper are as follows:(1)The primary precipitation phase in 15-15Ti as-cast alloy mainly includes granular TiC and massive TiN.When the Ti/C ratio increases from 4 to 6 and 8,the amount of TiC gradually increases.The amount of precipitated MC in the as-cast alloy is significantly reduced after forging.The Mo content in the precipitated phase increased from 4wt.%to 13wt.%.(2)The type of MX phase precipitated by the alloy above 1200? is related to the contact atmosphere during heat treatment.Granular secondary TiC precipitated under argon atmosphere;dendritic Ti(C,N)precipitated under nitrogen atmosphere.When the Ti/C ratio increased from 4 to 6 and 8,the amount of TiC precipitated by the alloy gradually increased under argon;the amount of dendritic Ti(C,N)precipitated under nitrogen did not change significantly;when the temperature rose to 1250?,the secondary TiC and dendritic Ti(C,N)precipitated in the alloy increased in size and decreased in quantity.(3)The precipitation of dendritic Ti(C,N)is formed by the reaction of Ti and C atoms in the matrix with N atoms in nitrogen when the alloy is exposed to a nitrogen atmosphere during a heat treatment above 1200?.As the nitrogen concentration and the holding time increases,the number of dendritic Ti(C,N)becomes more,and the size gradually grows.(4)By adding Mo element to 15-15Ti,(Ti,Mo)C phase with a special "core-shell" structure is precipitated in the alloy structure after forging,in which the core and shell layers are enriched with Mo element.When aged at 800?950?,the“core-shell”structure of the(Ti,Mo)C phase still exists stably,and its unique Mo-rich "core-shell" structure prevents the matrix elements from diffusing into the precipitated phase,further suppresses the growth and coarsening of micron-scale and intra-crystalline(Ti,Mo)C phases at grain boundaries,and the size of nanocrystalline(Ti,Mo)C is only 100?200 nm.