Studies On Phase Equilibria And Thermodynamic Description Of Ternary Systems Related To Fe-based Alloys

Though a vast scientific and engineering effort to develop the capabilities of non ferrous alloys and non-metallic materials has persisted for several decades,the Fe-based alloys are one of the most important structure materials,and also used widely on account of the economic and societal immense benefits.Therefore,the new generation Fe-based alloys are required for the high strength and toughness with major economic and environmental impacts.The phase diagram is a geometric presentation to the thermodynamic relation in the materials system.And phase diagrams are essential for designing component and microstructure of materials.Experimental phase diagrams are a starting point to guide compositional studies and to build thermodynamic models.Calculated phase diagrams can fill the experimental gap.Consequently,CALPHAD method is one of the most important and powerful tools for materials design.The quality of CALPHAD method depends on the quality of thermodynamic databases used for the calculations.This work deals with the phase equilibrium relationships and thermodynamic description of ternary systems related to Fe-based alloys.The phase equilibrium relationships of the Cu-Nb-Ni,Cu-Nb-Ti,Cr-Mo-Nb and Fe-Mo-Zr systems were experimentally measured by SEM/EDS,DTA and XRD etc.The Cu-Nb-Ni,Cu-Nb-Ti,Ni-Ti-W and Fe-Mo-Zr systems were thermodynamically optimized by means of the CALPHAD method and SGTE standard,and a useful complement to the thermodynamic database of the Fe-based alloys was made.The main research content is as follows:(1)The experimental isothermal sections at 1373,1223 and 1123 K in the Cu-Nb-Ni system were investigated.The solubilities of Cu in intermetallics NbNi3 and Nb7Ni6 at 1373,1223 and 1123 K were determined,respectively.And no ternary compounds were found.Based on the available experimental data in this work and literature,the Cu-Nb-Ni system was thermodynamically assessed.A set of self-consistent reliable thermodynamic parameters describing the Gibbs energy of each individual phase as a function of composition and temperature was derived,which can reproduce the experimental data.(2)The isothermal sections of the Cu-Nb-Ti system at 1173 and 1023 K were investigated.The solubilities of Nb in Cu4Ti,Cu4Ti3,CuTi,CuTi2 and CuTi3 at 1173 and 1023 K were determined,respectively.And no ternary compounds were found.Based on the available experimental data in this work and literature,the Cu-Nb-Ni system was optimized using CALPHAD method and SGTE standard.A set of self-consistent reliable thermodynamic parameters describing the Gibbs energy of each individual phase was derived,which can reproduce the experimental data.(3)The isothermal sections of the Cr-Mo-Nb system at 1173,1073 and 973 K were investigated.The solubilities of Mo in NbCr2 at 1173,1073 and 973 K were determined,respectively.And no ternary compounds were found.These experimental data will be used to thermodynamically optimize the Cr-Mo-Nb system in future.(4)The isothermal section at 1273 K and liquidus surface projection in the Fe-Mo-Zr system are investigated.In the isothermal section of the Fe-Mo-Zr system at 1273 K,there exists nine three-phase regions:bcc(Zr)+C15-Mo2Zr+liq.,C14+C15-Fe2Zr+liq.,C14+C15-Mo2Zr+liq.,fcc(Fe)+bcc(Fe)+Fe23Zr6,C14+C15-Fe2Zr+Fe23Zr6,bcc(Fe)+C14+Fe23Zr6,bcc(Fe)+C14+μ,bcc(Mo)+C14+μ,and bcc(Mo)+C14+C15-Mo2Zr.The solubilities of Mo in C15-Fe2Zr,Mo in Fe23Zr6,Fe in C15-Mo2Zr and Zr in μ at 1273 K are determined as about 4.8,0.6,17.7 and 4.6at.%,respectively.The ternary compound τ is not found in the present experimental results.In the liquidus surface projection,the primary solidification phase regions of bcc(Fe),C15-Fe2Zr,C14,μ,R,σ,bcc(Zr),C15-Mo2Zr and bcc(Mo)are experimentally determined.The Fe-Zr phase diagram is revised by XRD,EPMA and DTA methods.According to the experimental results,the Fe23Zr6 phase is confirmed to be stable below 1615 K in the Fe-Zr system.Based on the available experimental data,the Fe-Zr and Fe-Mo-Zr systems were therdynamically optimized.A set of self-consistent reliable thermodynamic parameters describing the Gibbs energy of each individual phase was derived.(5)Based on the available experimental data,the Ni-Ti-W system was optimized using CALPHAD method and SGTE standard.A set of self-consistent thermodynamic parameters describing the Gibbs energy of each individual phase as a function of composition and temperature was derived.The liquidus surface projection and two isothermal sections at 1173 and 1373 K of the Ni-Ti-W system were calculated.In the liquidus surface projection,six primary solidification regions,fcc(Ni),bcc(W),Ni3Ti,NiTi,NiTi2 and bcc(Ti),were experimentally determined.5 three-phase regions and 6 two-phase regions at 1173 K,and 5 three-phase regions and 5 two-phase regions at 1373 K were confirmed,respectively,which can reproduce the experimental data.With the thermodynamic description available,one can now make various calculations of practical interest.