| Oxide dispersion strengthened iron-base superalloy is considered as the critical structure materials in the next generation of nuclear fission/fusion reactor, which attract a large number of researchers to carry out extensive research, due to its low-swelling, irradiatio resistance, oxidation resisitance, corrosion resistance and high creep strength. This work was supported by the National Natural Science Foundation of China. The microstructural evolution and mechanical properties of the Fe-(12-14)Cr-3W-(0.3-0.4) Ti-(0.3-0.4) Y2O3-(1Cu)(wt.%) alloy system have been investigated systematically. Firstly, the technology inducing oxygen was systematically studied in this work, as well as the effect of existence and evolution of oxygen on formation of nanoparticles was explained.The relationship between vacancy and Y-Ti-O nanoparticle was described through formation and evolution of vacancy and vacancy formation enthalpy in this alloy system. Secondly, HRTEM, HAADF-STEM, TEM-EDX analysis were used to investigate deeply on Y-Ti-O oxides and copper precipitation with mean particle sizes about5nm. Finally, the microstructures of the alloy system were modified to improve mechanical properties through micro-alloying and powder metallurgy process. Main conclusions as follows:(1) Introducing oxygen by gas atomized process, the oxygen content in powders and that in gas atomized atmosphere was a linear growth relationship. Oxygen content in powder increased from the inside in turn. Oxygen in the powder can be part of solution. The oxygen on powder surface was formed prior particle boudary composed of oxides in the hot powder consolidation process. After the thermo-mechanical and heat treatment, crushing the original grain boundaries and oxygen resolution to the matrix occurred to form nanoscale dispersed oxides in the matrix due to the prior particle boundaries crushing. After annealling at1400℃for1h and followed by air-cooled, the room temperature tensile strength and elongation were940MPa,7%, respectively.(2) Mixing nano-1wt.%Fe2O3particles in the Fe-Cr-W-Ti-Y pre-alloyed powder, the formation of a variety of composite oxide particles can be promoted in the hot extrusion-hot rolling process. In addition to nano Y2Ti207phase, micron-sized Cr1.3Fe0.7O3,(Cr0.88Ti0,12)203and Y3Fe5012phase were also formed. Formation micron-sized particle was related to agglomeration of Fe2O3powder in the hot extrusion process of and reaction between Fe2O3and strong pro-oxygen atoms (Y, Ti, Cr). Addtion of a small amount of Fe2O3powder can significantly improve microhardness and room temperature, intermediate temperature (550℃) tensile strength of the hot extrusion-hot rolling powder metallurgy iron-based alloy. The room temperature tensile strength of the Fe2O3addtion alloy was1257MPa, compared with the Fe2O3-free alloy that was increased by50.7%, but the elongation decreased from13%to6.5%.(3)In the supersaturated ferrite matrix, the incoherence Cr, Ti-rich oxide phases and coherency Y-Ti-O nanoparticle were co-precipitated. Resolution of Cr, Ti elements in the matrix can be occured to by extending the mechanical alloying time, which is conducive to the formation of small uniform dispersion of Y-Ti-O nanoparticles.(4) In14YWT and14FWT two kinds of ferritic steels, two different crystallographic orientations at least between the Y-Ti-O nanoparticles with a Y2Ti207lattice structure and the matrix have been found. The nanostructural oxide dispersion strengthened ferritic steel prepared using Fe2O3as an oxygen carrier yield an excellent mechanical properties. The vacancy formation enthalpy of the alloy was about1.11eV by calculating, which was significantly lower than the pure iron vacancy formation enthalpy of the pure iron. That provides a favorable structural conditions to formation of the Y-Ti-O nanoparticle.(5) Bimodal grain size distribution structure can effectively improve the ductility of oxide dispersion strengthened ferritic steel. Through theoretical calculation, the ductility of bimodal grain size distribution oxide dispersion strengthened ferritic steel was exceed100%than that of only nanoscale grain14Cr ODS ferritic steel, however theoretical and experimental values was good agreement.(6) Adding copper as micro-alloyed element in bimodal grain size alloy, the microhardness reached a peak value after aging10h at500℃. At peak aging condition, copper-rich precipitation has been formed with a mean particle size about9.6nm and volumn fraction1022m-3. The toughness and strength can be further balanced by adding copper as micro-alloyed element. The copper-rich precipitate did not significantly growth up after500℃,300h prolonged hot exposured. Not increase the relative size distribution of double-crystal copper ferrite steel. The strength of copper addtion micro-alloyed alloy has been greatly improved, comparison to that of copper-free alloy. At room temperature, copper-rich precipitation strengthening effect calculated by Orowan bypass mechanism. |
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