Phase Diagram Determination And Thermodynamic Study On The Fe-Ta-Ti/Ni/Nb Alloy Systems

In order to meet the needs of national economic construction and defense construction,the production of alloy steels is developing towards the direction of high performance and multi-variety.High-performance steel materials are often composed of many elements.The proportion,grain size and organization form of each phase in steel have an extremely important influence on the properties of materials.CALPHAD technique can be used to calculate the phase composition and fraction of multi-component material systems,simulate the process of tissue evolution and the phase transformation,provide accurate and valid information for the production and development of alloy steels,so as to rationally design the composition ratio between elements in steels,avoid the formation of harmful phases,optimize productive technology and control the proportion of organization,and thus make the material research and development more directional and prospective.The establishment of a rich and complete thermodynamic database of Fe-based alloy is an indispensable component for the design and process optimization of steel materials which use CALPHAD technique.Therefore,this research mainly focuses on the experimental determination and thermodynamic assessment of the three ternary systems Fe-Ta-Ti,Fe-Ta-Ni and Fe-Ta-Nb,which lack thermodynamic data in the thermodynamic database of Fe-based alloys of our team,and provide necessary data support and theoretical basis for the production and development of relevant materials.The main research results are as follows:(1)The phase equilibria information and the solubilities of the third element in binary compounds in Fe-Ta-Ti ternary system were experimentally determined,and the isothennal sections at 1473,1373 and 1273 K in the Fe-Ta-Ti system were constructed.According to the transition of phase relationsh:ip in the isothermal sections at different temperature,the temperature of a U-type invariant reaction liquid+Fe7Ta6?FeTi +bcc(Ta,Ti)was determined as 1431 K.On the based of the experimental data measured in the present work,the Fe-Ta-Ti system was thermodynamically optimized using CALPHAD technique.A set of thermodynamic data was obtained,which can well reproduce the phase equilibria information of this system,meanwhile,the liquidus surface projection and reaction scheme of this system were deduced by the obtained thermodynamic data.(2)The primary crystallization field of the Fe-Ta-Ni ternary system was established by analyzing the microstructures,constituent phases and solidification paths of the as-cast Fe-Ta-Ni ternary alloys.The phase equilibria information and the solubilities of the third element in binary compounds in Fe-Ta-Ni ternary system were measured by equilibrated alloy method.Based on the thermodynamic parameters of Fe-Ta and Fe-Ni binary systems reported in the literatures and the thermodynamic parameters of Ni-Ta binary system revised in this work,and combined with the above experimental results,the Fe-Ta-Ni ternary system was thermodynamically optimized,and the reaction scheme of this system was deduced according to the obtained thermodynamic data.The calculated liquidus surface projection and isothermal sections were in good agreement with the experimental data determined in the present work.(3)Through the analyses of Fe-Ta-Nb ternary alloys in as-cast and equilibriated states,the primary crystallization field and the isothermal sections at 1473,1373 and 1273 K in the Fe-Ta-Nb ternary system were established,respectively.The Fe-Ta-Nb system was thermodynamically optimized using CALPHAD technique,and a set of self-consistent thermodynamic data for this system was obtained.(4)In this work,three ternary systems Fe-Ta-Ti,Fe-Ta-Ni and Fe-Ta-Nb were optimized for the first time.In consideration of the atomic occupancy and compatibility with the thennodynamic database of Fe-based alloys of our team,the unified two-lattice model(Fe,Nb,Ni,Ta,Ti)2(Fe,Nb,Ni,Ta,Ti)1 and four-lattice model(Fe,Nb,Ni,Ta,Ti)1(Nb,Ta)4(Fe,Nb,Ni,Ta,Ti)2(Fe,Nb,Ni,Ta,Ti)6 were used to described Fe2(Ta,Ti),Fe2Ta and Fe2(Nb,Ta)phases with MgZn2 crystal structure and Fe7Ta6,(Fe,Ni)7Ta6 and Fe7(Nb,Ta)6 phases with Fe7W6 crystal structure in the three ternary systems,respectively.Three set of thermodynamic parameters with high consistency were obtained.Besides the above work,the Hf-Pt binary system was thermodynamically optimized according to the experimental data repor ted in relevant literatures,the calculated results are in good agreement with the corresponding experimental and thermodynamic data.