Study On Phase Equilibria Of The Ni-Cr-Al-Pd Quaternary System And Related Ternary Systems At 1150℃

The application of aluminide coatings on the surface of nickel-based superalloys can improve the performance of nickel-based superalloys in hot end components.Aluminide coating has good oxidation resistance,but it has the disadvantages of poor hot corrosion resistance,poor brittleness,and rapid degradation.Adding modifying elements can improve the oxidation resistance and hot corrosion resistance of aluminide coatings.The addition of chromium to modify the properties significantly improves the hot corrosion resistance of the coating,and the addition of palladium to improve the high temperature oxidation resistance of the coating.The phase relationship of the Ni-Cr-Al-Pd alloy system can provide theoretical guidance for the design of chromium and palladium modified aluminide coatings with better compositions.In this paper,the alloy samples were prepared by the equilibrium alloy method,and the microstructure observation,composition analysis and phase analysis of the alloy samples were carried out by means of scanning electron microscopy（SEM）,energy dispersive spectroscopy（EDS）and X-ray diffractometer（XRD）.In this work,the isothermal section of Ni-Al-Pd ternary system at 1150℃,the isothermal section of Al-Cr-Pd ternary system at1150℃,the isothermal section of Ni-Cr-Pd ternary system at 1150℃,and the isothermal section of Ni-Cr-Al-Pd quaternary system with 53at.%Ni at 1150℃were studied;（Ni,Pd）Al phase was studied by X-ray diffraction.Isothermal sections of Al-Cr-Pd and Ni-Cr-Pd ternary systems at 1150℃was calculated by Pandat software.The main conclusions were obtained as follows:Four three-phase regions exist in the isothermal section of the Ni-Al-Pd ternary system at 1150°C.The three three-phase regions were experimentally determined,which are（Ni,Pd）Al+Al Pd₂+Al₃Pd₅,Ni₃Al+（Ni,Pd）+（Ni,Pd）Al,（Ni,Pd）Al+（Ni,Pd）+Al Pd₂.A three-phase region（Ni,Pd）+Al Pd₂+L was speculated.And the maximum solubility of Ni in Al Pd₂ and Al₃Pd₅ phase is 14.0at.%and 5.2at.%respectively,the maximum solubility of Pd in Ni₃Al is 16.4at.%.Ni Al can form a continuous solid solution with Al Pd.Six three-phase regions in the isothermal section of the Al-Cr-Pd ternary system at 1150°C,and 4 three-phase regions were experimentally determined:Al₈Cr₅-H+β-Al Pd+（Cr）,（Cr）+Al Pd₂+（Pd）,（Cr）+Al Pd₂+Al₃Pd₅ and L+Al₈Cr₅-H+β-Al Pd,two three-phase regions are inferred,namelyβ-Al Pd+Al₃Pd₅+（Cr）and Al Pd₂+（Pd）+L;The maximum solubility inβ-Al Pd,Al₃Pd₅ and Al Pd₂ is 18.5at.%,8.0at.%and 7.1at.%,respectively.There is no three-phase region in the isothermal cross section of Ni-Cr-Pd ternary system at 1150℃,and the boundary between（Cr）and（Ni,Pd）is determined by experiments.Three three-phase regions of the isothermal section of Ni-Cr-Al-Pd quaternary system 53at.%Ni at 1150℃were measured respectively:（Ni,Pd）+Al Pd₂+（Ni,Pd）Al,（Cr）+（Ni,Pd）Al+（Ni,Pd）and（Ni,Pd）+（Ni,Pd）Al+Ni₃Al.It is speculated that there are two quaternary regions,（Ni,Pd）+Al Pd₂+（Ni,Pd）Al+（Cr）,（Ni,Pd）+（Ni,Pd）Al+（Cr）+Ni₃Al;The 15 two-phase region alloy samples obtained through experiments approach the boundary of the two-phase region.In this work,no new ternary and quaternary compounds were found.According to the experimental results of the isothermal cross section of the Al-Cr-Pd and Ni-Cr-Pd ternary systems at 1150°C,combined with the Calphad technology,the thermodynamic optimization of the two ternary systems was carried out,and a set of reasonable and self-consistent thermodynamic parameters was obtained.