Thermodynamic Study Of Ni-Al-RE(RE=La,Nd,Pr) System

Nickel-based superalloys are used extensively in the blades of power turbines in modern aero-engines because of their excellent high-temperature properties,such as a high-temperature strength,excellent creep and fatigue resistance,a good oxidation resistance and a hot corrosion resistance.The excellent performance of nickel-based superalloys is attributed to the addition of elements,such as Al,Mo,Cr,W,V and Ti.Among them,the addition of a small amount of rare-earth element（RE）can improve the high-temperature oxidation resistance,corrosion resistance and manufacture performance significantly.However,excessive addition of RE elements can result in incipient melting followed by formation of some detrimental topologically close-packed phases and deteriorates the alloy properties,it is thus of vice importance to control the addition of the RE.Thermodynamic database and CALculation of PHAse Diagram（CALPHAD）method which can reduce the experimental time,shorten the development cycle and provide theoretical guidance for the design of new nickel base superalloy.However,the calculation of phase diagrams of multicomponent nickel-based alloys containing RE has been studied rarely.Therefore,it is of great significance to establish a database of multi-component nickel-based superalloys containing REs and provide theoretical guidance for the development of new nickel-based superalloys.The present work aims at performing thermodynamic assessments for the Ni–Al–La,Ni–Al–Nd and Ni–Al–Pr ternary systems and their corresponding sub-systems by the CALPHAD method.The phase equilibrium information of 800 and 1000℃ in the nickel-rich region of Ni–Al–La system was determined by equilibrium alloy method,and providing a basis for the establishment of thermodynamic database of multi-component Ni-base superalloys which containing REs.The main contents of this paper are as follows:1)The phase equilibrium information of 800 and 1000℃ in the nickel-rich regionof Ni–Al–La system was determined.The solubility of Al in Ni₅La was 23.53 at.% at 800℃ and 19.84 at.%at 1000℃ by means of scanning electron microscope, energy disperse spectroscopy and X-ray Diffraction.2)A new ternary intermetallic compound,terned as Ni₂AlLa,was first confirmed in the experimental determination of Ni–Al–La system at 800℃.Its X-ray Diffraction pattern and space groups were determined by total pattern solution （TOPAS）,and the appropriate lattice parameters were fitted by Pawley method and selected area electron diffraction.Ni₂AlLa crystallizes in the trigonal system with a space group R3（no.146）and a=4.1985?,c=13.6626?.3)Considering the solubility of La in fcc（Al）and fcc（Ni）,the thermodynamic database of Al–La and Ni–La binary system was re-established by CALPHAD method.Based on the literature data and our experimental data,a thermodynamic database of Ni–Al–La system was established by using the CALPHAD approach.4)The Al–Nd binary system was re-evaluated in present work because of the optimized Al–Nd binary system by the literature cannot be reproduced the experimental data well.The thermodynamic database of the system was established using CALPHAD approach.The calculated equilibrium imfoamation and enthalpy of Al–Nd system are in good agreement with the experimental reports.5)Combining the Al–Nd,Ni–Al databases and Ni–Nd database reported by literature,the thermodynamic database of Ni–Al–Nd system was established by thermodynamic extrapolation and thermodynamic optimization based on the literature data.6)Considering the solubility of Pr in fcc（Al）,the thermodynamic database of Al–Pr binary system was established by CALPHAD method.Combining the Al–Pr,Ni– Al databases and Ni–Pr database reported by literature,the thermodynamic database of Ni–Al–Pr system was established by thermodynamic extrapolation and thermodynamic optimization based on the literature data.