Study On Phase Relationships And Thermodynamic Description Of Ternary Systems Related To Co-based Superalloys

At present,the application of the Ni-based superalloys is the widest in high temperature materials.A lot of Ni-based superalloys marks,such as GH4169,GH4133,etc,have been produced and applied in China.The performance requirements at high temperature for high temperature components become higher and higher with the development of aerospace.Therefore,it is extremely urgent to find the high-performance superalloys.The novel Co-based superalloys with a γ’strengthening phase of L12 structure have excellent high temperature microstructure stability and creep resistance,so more and more novel Co-based superalloys with γ + γ’ two-phases microstructure were investigated in recent years.However,the addition of many alloying elements may result in the formation of harmful TCP phase,such as μ,σ and Laves phases,while stabilizing the high temperature γ+γ’ two-phases microstructure.Therefore,the phase relationships and phase stability of different binary and ternary systems in Co-based superalloys were investigated,and then a thermodynamic database of the novel Co-based superalloys was established,which is very important for the composition design of the novel Co-based superalloys.In this paper,three ternary systems Co-Hf-Ta,Ni-Hf-Ta,and Co-Nb-Ti in the Co-based superalloys are experimentally investigated and thermodynamically assessed by CALPHAD technique based on the obtained experimental isothermal sections and liquidus surface projection from present work and the experimental data from literature.The calculated results are in good agreement with the experimental results.The calculated thermodynamic parameters can provide the necessary supplements for constructing Co-based superalloy thermodynamic database and provide necessary data support for the production and development of the high temperature materials.The main research results are as follows:(1)The micro structures and the solidification paths of as-cast Co-Hf-Ta alloys were observed and analyzed,and the liquidus surface projection of this system was established.In addition,the as-cast Co-Hf-Ta samples were annealed at 1373 K and 1223 K for long time,and the phase relationships and the maximum solubilities of compounds were determined in different temperatures,and the isothermal sections at 1373 K and 1223 K were constructed.The Co-Hf-Ta system was optimized based on the obtained experimental liquidus surface projection and isothermal sections.A set of self-consistent thermodynamic parameters was obtained.(2)The annealed microstructures and XRD patterns of Ni-Hf-Ta alloy samples at different temperatures were analyzed,and the isothermal sections at 1373 K,1223 K,and 1123 K were constructed.The different phase regions and the solubilities of third components in all binary compounds were determined in these three isothermal sections,and a new phase τ with Hf2Rh structure was found in the Hf-rich side.This system also was assessed based on the experimental information in this work and literatures.A set of thermodynamic parameters were obtained,and the experimental data were reproduced well.(3)The compositions of primary phases and solidification paths of as-cast Co-Nb-Ti alloys were determined and analyzed,and the liquidus surface projection was constructed.The phase relationships at 1473 K,1373 K,and 1273 K were determined by means of equilibrated alloys and diffusion couple methods,and the isothermal sections of the above three temperatures were constructed.Based on the experimental data in this work and the literatures,the Co-Nb-Ti system was assessed by CALPHAD method,and a set of self-consistent thermodynamic parameters were obtained.(4)The thermodynamic database including Co,Ni,Al,W,Ta,Ti,and Cr elements was constructed.The curves of phase fractions with temperatures of different alloy compositions were calculated during the equilibrium cooling process based on the present thermodynamic database,and the phase fractions ofγ’ phases at different aging temperatures,the solution temperatures of γ’ phases,and the stable temperature ranges of single γ phase in different composition alloys were obtained.Combining with phase consitituents,phase fractions and oxidation resistance of Co-base superalloys,five alloys,in which γ+γ’ two phases,no TCP phase,and Al+Cr≥16 at.%,were designed using the present database,and were prepared by arc furnace.The above five alloys were homogenization heat treated at 1573 K for 24 h,and aged at 1273 K for 240 h,480 h,and 720 h.There existedγ+γ’ two phases and no TCP phase in the microstructures of ageing treatment,which were consistent with the predicted results of the database.Finally,the γ’volume fraction was determined at different aging times using the Image-Pro Plus 6.0 software,and the γ’ volume fraction of most alloys is consistent with the calculated results.