Thermodynamic Investigation On The Subsystems In B-C-Si-Zr-O For Ultra-high Temperature Ceramics

Ultra-high temperature ceramics(UHTCs) mainly refer to the early transition metal(e.g.,Zr,Hf) diborides or carbides with excellent mechanical,thermodynamic properties and thermal shock resistance, oxidation resistance under extremely high temperature conditions.The UHTCs are one class of key material in the study of new spacecraft and carrier and strategic missile,playing an important role in the development of space technology and modernization of national defense.At present, research with respect to UHTCs is an emerging focus in the field of materials.As part of the project "investigation of phase diagram,material preparation and microstructure control of UHTCs" supported financially by National Science Foundation of China(No.50632070),the partial phase equilibrium relationship of the B-C-Si-Zr-O quinary system was investigated through experimental measurements and CALPHAD approach in the present work.The thesis is comprised of seven parts:(1) The binary B-Zr and Si-Zr systems have been reassessed using the CALPHAD method on the basis of the available experimental data. The self-consistent thermodynamic parameters formulating Gibbs energies of various phases in these two binary systems have been obtained.Thermodynamic properties of liquid phases and intermetallics and phase diagrams have been calculated,which are in reasonable agreement with the reported experimental data.(2) Based on the critically assessed B-Si phase diagram,one alloy of Sirich side was selected and the temperature associated with the eutectic reaction was measured to be 1636 K by DSC.And the phase relations of alloy 7at.%B-93at.%Si at 1253K have been investigated by XRD method.The experimental result shows that this alloy consists of two phases,B₃Si and silicon solution.The B-Si system has been reassessed using a regular substitutional model for the liquid.This improved description can be used with confidence to obtain descriptions for higher order alloys.Thermodynamic description for the B-C-Si system has been developed based on critically reviewed experimental data.The calculated phase diagrams are in good agreement with available experimental data.(3) The reaction between Zr and SiC at 1473K in vacuum has been studied.A layered structure was observed after high temperature reactions between Zr and SiC complex.ZrC_x was formed next to the SiC.Thermodynamic description of the C-Si-Zr system was developed based on its constituent binaries and critically reviewed experimental data.The high-temperature Zr₅Si₃ phase and the ternary compound Zr₅Si₃C_x were described as one phase,using the model (Zr)₅(Si)₃(C,Va)₁.Reasonable agreement between the calculated and experimental data was achieved.(4) The phase diagram of B-C-Zr ternary system was thermodynamically assessed.Adopting the enthalpy of formation data at 0 K calculated by first-principles method for assumed compounds ZrC₂ and ZrB (^0KH_Zr:C^ZrB₂ and ^0KH_Zr:B^ZrC_x),the calculated isothermal and isoplethic sections agree well with those of experiments,so do the most of the calculated invariant reactions.In order to simulate the solidification path of the ternary alloy,Scheil-Gulliver model has been employed. The solidification products are obtained by calculation coupling with experimental works.The calculation results show that the experiments can be simulated satisfactorily.(5) By means of annealing method,metallography,scanning electron microscopy(SEM) and X-ray diffraction analysis(XRD),the isothermal section of Zr-rich region at 1253 K and phase constitution of the electric-arc melted cast-state sample were determined.On the basis of experimental information of the phase equilibrium relationship and thermodynamic properties from experimental measurement and literature report,the B-Si-Zr temary system was assessed and a set of thermodynamic parameters was obtained to reasonably describe the Gibbs energy of all the phases.(6) Powder samples 33.33 at.%B₂O₃-66.67 at.%ZrO₂ and 60 at.% B₂O₃-40 at.%ZrO₂ were sintered and prepared with solid state reaction,followed by annealing at 1073 K and 623 K,respectively, for seven days.As is shown from the X-ray diffraction analysis,there were only two phases m_ZrO₂ and B₂O₃ and no intermetallic compound existed in the binary system.Then,three constitutional pseudo-binary systems in the B₂O₃-SiO₂-ZrO₂ system were optimized by evaluating the experimental information of phase diagram and thermodynamic properties in literatures with which the calculated results agreed well.Incorporating the optimized thermodynamic parameters of the constitutional binary systems,the B₂O₃-SiO₂-ZrO₂ pseudo-ternary system was extrapolated.(7) Using the latest lattice stability parameter of ZrO₂,two-sublattice ionic solution model was adopted to describe the Gibbs energy of liquid phases,and then Si-Zr-O,C-Si-O and C-Zr-O ternary systems were assessed successfully.The calculated isothermal sections of Si-Zr-O and C-Si-O agreed well with those of experiments and the phase diagram of C-Zr-O ternary system was predicted through calculations.