Experimental Measurements And Thermodynamic Assessments Of The Hf/V Alloyed Nb/Mo-Si Based Superalloy Related Systems

The Nb-Si-based binary system has the two-phase microstructure of Bcc_A2+Nb5Si3 with excellent low-temperature toughness,high-temperature strength and melting point(above 1800℃),Therefore,Nb/Mo-Si-based ultra-hightemperature alloy materials can be used as potential candidates for a new generation of aero engines.However,Nb/Mo-Si-based ultra-high-temperature composites also have disadvantages such as easy oxidation in the high-temperature region and poor high-temperature creep performance in the practical application.At present,the common means to improve the actual performance of Nb-Si-based hightemperature structural materials is to add trace elements for alloy strengthening methods,which cause the matrix to produce solid solution strengthening,precipitation strengthening and other strengthening mechanisms,are used to control the microstructure of the material and improving the mechanical properties of the material by changing the structure of the constituent phases,improving the phase equilibrium relationship,and controlling the number of precipitation phases.The common alloying elements for the Nb/Mo-Si-based ultra-hightemperature alloys include Hf,V,Ta,Re,W,B,Ni,and Co.The present work mainly focused on the ternary system constructed by the alloying elements Hf or V and the matrix binary system Nb-Si or Mo-Si,and the related sub-binary systems involved in the study,and carries out the phase equilibrium experimental measurement and solidification process analysis.Based on the literature information of related subbinary systems,the thermodynamic parameters are optimized and evaluated by combining the experimental results of phase equilibrium measurement.The main research results obtained are as follows.First,for the Mo-Nb-V ternary system and Mo-V sub-binary system,experimental methods such as alloy melting method and diffusion coupling method were used with the aid of Scanning Electron Microscope coupled with Energy Dispersive Spectrom(EDS-EDS),Electron Probe X-ray Micro-Analyser(EPMA),Transmission Electron Microscope coupled with Energy Dispersive X-ray spectroscopy,(TEM-EDX)and Differential Scanning Calorimeter(DSC)were used to determine the phase equilibria and phase transition temperatures of the Mo-V binary system.The equilibrium alloy and diffusion couple specimens indicate that the Mo-V binary system exsits an immiscible area in the low temperature region,and the two phases with the same crystal structure but different compositions are in equilibrium coexistence,constituting a two-phase equilibrium region.Two typical phase decomposition mechanisms,the mucleation-growth and the spinodal decompositions,are comfirmed by means of transmission electron microscopy coupled with Selected Area Electron Diffraction(TEM-SAED).Based on the combination of the thermodynamic data reported in the literature and the measured phase equilibrium relationship in the present work,the Mo-V binary system phase diagram was contrusted and the self-consistent and reasonable thermodynamic parameters were obtained.Furthermore,Mo-Nb-V ternary system alloys of different compositions were melted and prepared by the equilibrium alloying method,and the isothermal sections of the Mo-Nb-V ternary system at 600 and 750℃ were measured after a long isothermal treatment.Combining the thermodynamic data of Mo-Nb and Nb-V binary systems reported in the literature,the thermodynamic data of the optimized Mo-V binary system and the isothermal sections of the Mo-Nb-V ternary system experimentally were measured in the present work,the Mo-Nb-V ternary system was thermodynamical optimized,and the experimental isothermal sections compare well with the result of thermaodynamic calculations.In addition,the extrapolated isothermal sections at the temperatures above the experimental temperatures were calculated to illustrate above the the phase equilibria in the temperature-composition space using the thermodynamic parameters obtained from the optimization in the present work.Secondly,the Mo-Si-V and Hf-Si-V two ternary system composed of the alloying elements Hf/V and basic elements Nb/Mo-Si superalloy,by means of the TEM-EDX、SEM-BSE、EPMA、EDS and XRD experimental characterization.After the reasonable of as-cast alloys microstructure solification process analysis,the primary solidification phase,the univariant reaction lines between two primary solidification phases,and the invariant reaction points of the three primary solidification regions were determined,and finally the liquidous projection of the Mo-Si-V and Hf-Si-V systems were Tconstructed.Afterwards,the isothermal equilibrium alloy method was used,the alloys of two ternary system with different composition were annealed at isothermal termperatrue for a long time.Based on the equilibrium alloys quantitative characterized and qualitative analysis,the Mo-Si-V and the Hf-Si-V two ternary equilibrim relationship are determined,the three phase regions,two phase regions and single-phase regions of two ternary systems are given.Finally,the Mo-Si-V ternary system isothermal sections at1200 and 1450℃and the HF-Si-V ternary system at 1100 and 1300℃ are established.In addition,in the Hf-Si-V ternary system,the solid solubility of the third element in the binary compound was determined by EPMA,and the lattice parameters of each compound were calculated using the JADE software pakage based on the XRD data in this paper.Moreover,the crystal structure of the Hf2V4Si5 phase was determinded by the TEM-EDX,SEAD,SEM-EPMA and XRD characterization methods,and the present work proposed that the Hf2V4Si5 was formed due to the line compound(V,Hf)6Si5 decompose into two phases in the miscibility gap.Finaly,the existing thermodynamic data of Mo-V,Mo-Si,V-Si,and Hf-V binary systems were fully summarized and integrated,and the isothermal sections,equilibrium phase compositions,solubility range,liquidous phase surface projection maps,primary solidification phase regions,univariate reaction lines and invariant reaction points measured et al.One this basis.the CALPHAD technique was used,and selected reasonable thermodynamic models according to crystal structure characteristics of each phase in the Mo-Si-V and Hf-Si-V ternary systems.By the use of the obtained thermodynamic parameaters in present work,the calculated liquidous surface projections and isothermal sections at 1200 and 1450℃of the Mo-Si-V ternary system,and liquidous surface projections and isothermal sections at 1100 and 1300℃ of the Hf-Si-V ternary system are in high agreement with the experimental phase diagram.The results of this paper can enrich and improve the basic data of the multicomponent Nb/Mo-Si-based ultra-high-temperature alloy database,and can provide a useful reference for the Nb/Mo-Si-based ultra-high-temperature alloy design,composition phase control and performance development research.