| In recent years,the demand for structure materials,which are environmentally benign,energy efficient,light weight,and reliable in design is increasing..The next generation of high-performance structural materials must have high specific strength,good fracture toughness,and good joining capabilities.Steel is one of important structure materials.Therefore,the next-generation high-performance steel is required for the high strength and toughness,such as ultrastrong steel.In general,an increase in strength is accompanied by a decrease in toughness,which results in stress-induced cracking and difficulty in forming.The addition of molybdenum,niobium and zirconium to steel can effectively improve its strength and toughness.Molybdenum,niobium and zirconium are strong carbide-forming elements in steel.The carbides produced,such as MC and M2C,can obstruct the motion of dislocations and prevent austenite grain growth during austenization.The fine precipitates of C14 phase can effectively improve the strength and creep resistance of steel.The addition of molybdenum,niobium and zirconium contributes to the formation of the C14 phase.Zirconium and sulfur can form ZrS,,which can prevent hot embrittlement of steel.Excessive Mo content increases the probability of formation of the a phase.This phase promotes the formation of cracks and deteriorates the toughness of the steel during loading.Therefore,the amount of alloying elements added to steel must be strictly controlled.Phase diagrams provide the basic data of designing material compositions and heat treatment processes.Thus,it is important to obtain the phase diagrams of relevant systems for the research and development of ultrastrong steel.The phase diagrams of Fe-Zr,Fe-Mo-Nb,Fe-Mo-Zr,and Fe-Nb-Zr systems were experimentally measured and thermodynamically optimized by means of alloying method,diffusion-couple technique and calculation of phase diagram(CALPHAD)method in the present work.In addition,thethermodynamic parameters of Fe-Nb-Zr system was obtained.The main research content is as follows:(1)The phase diagram of Fe-Zr binary system was revised by means of X-ray diffraction(XRD),electron probe micro-analyzer(EPMA),and differential thermal analysis(DTA).The stability of the Fe23Zr6 phase in Fe-Zr system was confirmed by analyzing the diffusion couple samples.The temperatures of invariant reactions related to Fe23Zr6 and hex.(Fe2Zr)were measured by DTA.The reaction temperature of bcc(Zr)?FeZr3+hcp(Zr)was revised.(2)The primary crystallization fields of Fe-Mo-Nb,Fe-Mo-Zr,and Fe-Nb-Zr ternary systems were determined by analyzing the microstructures of as-cast alloys using XRD,EPMA,DTA,and scanning electron microscope(SEM).According to the constituent phases of different composition alloys at different temperatures,the isothermal sections of Fe-Mo-Nb system at 1373 and 1473 K and of Fe-Mo-Zr and Fe-Nb-Zr systems at 1273 K were constructed.(3)The reasonable thermodynamic models were selected by means of CALPHAD method.Based on the experimental results of present work and literatures,the Fe-Zr,Fe-Mo-Nb,Fe-Mo-Zr,Fe-Nb-Zr,and Mo-Nb-Zr systems were optimized,and self-consistent thermodynamic parameters were obtained.The thermodynamic database of Fe-Mo-Nb-Zr system was constructed.In addition,the liquidus surface projection and the isothermal section at 1273 K of the Ni-Ti-V system were experimentally constructed in this work.Based on the experimental results,this system was optimized,and the obtained calculated results were in good agreement with the experimental data. |
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