Microstructure And Al-Re-Nb Phase Equilibrium Of The Nickel-based Superalloy Containing Re

Nickel-based superalloy is widely used in hot end parts of engine.The addition of refractory elements such as Re and W will lead to the precipitation of harmful topologically close-packed phases（TCP）in the alloy,which accelerates the formation and propagation of cracks and reduces the durability of the alloy.Nb increases the strength,hardness and ductility of nickel-based superalloy.But it’s harmful to the alloy when Nb is excessive,which slows down the increase of the strength and decreases the ductility sharply.By studying the interaction of Re and W elements and the effect of solution-aging treatment on the microstructure of nickel-based superalloy,the formation law of TCP phase was revealed.The research results can provide important guidance for the design of new alloys to inhibit the precipitation of TCP phase and the prediction of the safety performance of aircraft in the process of cyclic flight.The results of phase relationship research can enrich the thermodynamic database of nickel-based superalloys and provide important basic data for the study of phase equilibrium of high element systems.The main research results are as follows:（1）The thermodynamic software TC was used to calculate and predict the phase transition of the alloy with different W and Re contents,and the alloy samples with different W and Re contents were selected for experimental study.The results of TC calculation and prediction showed that there was onlyμphase precipitated during solidification when the content of Re was less than 4.9 wt.%.μand P coexist in a certain temperature range when Re content was more than 5.6 wt.%.The effect of Re content on the microstructure of nickel-based superalloys in as-cast and thermal exposure conditions was investigated by SEM-EDS.The results showed that the size ofγ’phase in the as-cast alloy decreased with the increase of Re content.And the content of TCP phase gradually increased with the increase of thermal exposure time,The phase transition temperature of the alloy was determined by means of a differential thermal analyzer.It has a reasonable agreement between the calculated results and the experimental results.（2）The thermodynamic software TC was used to calculate and predict the phase transition rule of the alloy with different W and Re contents,and the alloy samples with different W and Re contents were selected for experimental study.The effects of the interaction of W and Re on the microstructure of the nickel-based superalloy under slow solidification and solution-aging conditions were investigated by SEM-EDS and TEM-EDS.TC thermodynamic calculation and prediction showed that onlyμphase was precipitated during solidification when Re content was 2～4 wt.%and W content was 7～9 wt.%.When it came to 5～7 wt.%Re content and 4～6 wt.%W content,P phase precipitated at high temperature,which coexisted withμphase in a certain range with temperature decreasing.P phase disappeared with temperature decreasing further.The results of the slow solidification experiment showed that theμphase with the shape of plate distributed parallelly in different orientations,whileμ/Ρphase with the shape of short rods coexistented,which has a coherent relation,leading to stress concentration and micropores easily.After solution and aging treatment,the amount of TCP phase in the alloy decreases and is uniformly distributed in the matrix,and the micropores in the alloy disappear.（3）The equilibrium-alloy approach was adopted to determine the phase relations of the Al-Nb-Re ternary system at 1150℃.The results showed that two new ternary compoundsΤ₁ andΤ₂ were found and eight three-phase equilibrium regions were determined in the isothermal section at 1150℃,which were L-Al+Al₃Nb+Al₄Re,Al₃Nb+Al Nb₂+Τ₁,Al₃Nb+Al₁₁Re₄+Τ₁,Al₁₁Re₄+Τ₁+Τ₂,Al Nb₂+Al Nb₃+Τ₁,Al Nb₃+Τ₁+Τ₂,Al Nb₃+Τ₁+β-Nb,Al Re₂+α-Re+Τ₂.The solubility of the third component in the binary compound was determined.