Thermodynamic Study On Some Of The Binary And Ternary Systems Of Nb/Mo-Si Based Superalloy

As a new generation of the aeroengine materials,the application of the Nb-Si based superalloys is of the great value to replace the Ni based single crystal material.The Nb-Si based superalloy is a duplex alloy composed of the high toughness phase bcc solution and the high strength phase NbsSi3 compound,with many advantages of mechanical properties such as low temperature toughness and high temperature strength.However,the week high-temperature anti-oxidation and the inferior high-temperature anti-creep abilities are the main limitations to its practical application.Currently,adding alloying elements is the important approach to conquer the weakness of the Nb-Si based superalloys.Through the ways of solution strengthening,precipitation strengthening,eliminating high temperature metastable phase and improving microstructure,the property requirements for the Nb-Si based superalloysare meted.The alloying elements for the Nb-Si based superalloys includeMo,Ti,V,Hf,Sn,Cr,Mn,Al,Ge,C,B and so on.The present work mainly focused on the binary and ternary systems composed of the alloying elements Mo,Hf and V as well as the basic elements Nb and Si.The phase-equilibrium measurements and the thermodynamic optimizations were performed,and the solidification processes were analyzed thermodynamically.The following results were obtained:Firstly,in the Nb-V binary system,a solidus miscibility gap was observed at low temperatures.In the present work,by the use of high temperature melting method,diffusion couple method,differential thermal analysis,scanning and transmission electron microscopes,the isothermal equilibrium compositions,the interfacial equilibrium relations,the phase transition temperatures,the microstructures and the phase decomposition mechanisms,including the Spinodal decomposition and the nucleation-growth decomposition,were determined and analyzed comprehensively.The obtained experimental results can confirm with each other.On this basis,CALPHAD method was used to assess the thermodynamic data of the Nb-V binary system by establishing the reasonable models for all of the phases of the system,and then the critical temperature and the temperature-composition space of the miscibility gap and the Spinodal decomposition in the BCC phase region at low temperature were determined.Secondly,the experimental and optimized phase diagram of the Hf-Si binary system has been revised recently.It is necessary to re-optimize the thermodynamic parameters of this system.In the present work,the reasonable thermodynamic models were set up according to the characteristics of the crystalline structures of all the phases.Under the consideration of the literature reported phase equilibrium data(including the solubility ranges of solution phases,the liquidus data,as well as the temperatures and the compositions of phase transformations)and the thermochemical properties(including the activities of components,the enthalpies of compounds and the mixing enthalpy of liquid),especially the new experimental reports,the Hf-Si binary system was critically re-assessed.A set of self-consistent thermodynamic parameters was obtained.The thermodynamic calculations can reproduce the experimental results very well.Thirdly,the Nb-Si-V and the Mo-Si-Hf ternary systems,composed of the alloying elements V/Hf and the basic elements Nb/Mo-Si,were studied in order to obtain enough phase equilibrium data for the thermodynamic assessments of these systems.The liquid-solid phase transformations and the isothermal phase equilibrium experiments were designed and performed for the alloys of different compositions.After the reasonable analyses for the solidification processes of alloys,the liquidus surface projections of the Nb-Si-Hf and the Mo-Si-Hf ternary systems were determined,including the composition ranges of primary solidification phases,the univariant reaction lines between two primary solidification regions and the invariant intersecting points of three primary solidification regions.Afterwards,the Nb-Si-V and the Mo-Si-Hf ternary alloys with different compositions were treated isothermally at 1300? for 20-40 days.The equilibrium microstructures,the constituent phases and the phase compositions of the quenched samples were measured and analyzed,and the isothermal sections at 1300? were constructed for both of the systems.The isothermally equilibrated phase relations were made clearly.Especially,the equilibrium phase compositions of the three-phase equilibria were determined by electron probe microanalysis.These experimental data are necessary and useful for the later thermodynamic optimization.Fourthly,based on the presently obtained liquidus surface projection and the isothermal section at 1300?,the Nb-Si-V ternary system was assessed thermodynamically.By the use of the newly optimized thermodynamic parameters,the phase equilibrium calculation not only can reproduce the experimental liquidus surface projection and the isothermal section,but also can simulate the solidification processes of the ternary alloys with different compositions.And then the isothermal sections at different temperatures were predicted,and the temperature-composition space of the Nb-Si-V ternary system was sketched and the invariant reaction scheme of the system was presented.Finally,under the inspiration of the study on the solidus miscibility gap of the Nb-V binary system,the experimental methods for measuring the liquidus miscibility gaps were tried in the present work.The Fe-Cu binary system with a meta-stable liquidus miscibility gap and the Fe-Sn binary system with a stable liquidus miscibility gap were selected.On the one hand,the powder metallurgy method was adopted to prepare the Fe-Cu samples with relatively uniform microstructure morphology and chemical composition.By the use of differential thermal analysis,the phase transformation temperatures under different cooling rates were measured and those under the proposed 0?/min were extrapolated.Consequently,the meta-stable liquidus miscibility gap of the Fe-Cu binary system was determined.One the other hand,the high temperature melting method was adopted to prepare the Fe-Sn samples for isothermal treatment to reach the equilibrated compositions and to form the layered morphology after rapid cooling.So that the stable liquidus miscibility gap of the Fe-Sn binary system was determined.On the basis of the experimental results,both the Fe-Cu and the Fe-Sn binary systems are re-assessed thermodynamically.The present results are available for enriching the thermodynamic database of the Nb-Si based multi-component system and are of important reference to the composition design and the process control of the Nb-Si based high temperature superalloys.