| In order to increase the operation temperature of nickel base superalloys to meet the need for increased efficiency of airplanes engines, some refractory elements like Mo, W are added to increase the solution strengthening. The alloys with high contents of these elements, however, often exhibit poor process-ability so that they have to be produced by powder metallurgy, resulting in unacceptable cost and density. Besides, excessive use of such refractory alloying elements aggravates segregation which promotes the precipitation of some TCP phases and degrades the mechanical properties of superalloys. These drawbacks limit the application of superalloys. Therefore, the superalloys strengthened by suitable Al, Ti and Co contents have been developed. These alloys have not only improved mechanical properties at high temperature, but also broad working window. In this paper, new Ni-Co base superalloys, which are strengthened by Co3Ti have been studied to investigate the effects of Ti, Al and Co elements on the mechanical properties and work-abilities by thermodynamic calculation and experiments.Firstly, the effects of the Ti, Al and Co elements on the equilibrium precipitations, alloys'work-ability andγ/γ′misfit have been studied by thermodynamic calculation. Thermodynamic properties have been investigated using JMatPro and the latest relevant database for Ni-base superalloys. It is found that theγ/γ′mismatch increases with the increase of Ti/Al (atomic ratio) and the volume fraction ofγ′is proportional to the Ti+Al contents (atomic percent). Therefore, by increasing the Ti/Al ratio and Ti+Al contents, the yield strength of alloys can be improved. On the other hand, theγ′solvus temperature decreases by Co additions. As a result, the range of processing temperature is extended. Theγ/γ′mismatch also increases with Co additions, which adds additional strengthening to the alloy. Moreover, high Ti/Al ratio or low Co content promotes the precipitation of theηp?hase. Thus, Ni-Co base superalloys with high Ti+Al contents, Ti/Al ratio bear an improved strength, but too high Ti/Al ratio will damage the microstructure stability.Besides, four kinds of Ni-Co base superalloys with different Ti, Al and Co contents have been prepared to evaluate their work-ablities and microstructures. The experiments reveal that Ni-Co base superalloys have good work-ablities as they can be rolled at suitable processing temperature. After rolling, the alloys have improved homogeneities of microstructures.γ′phases, which include some large primaryγ′precipitations and other relatively small secondary and tertiaryγ′precipitations have been precipitated after solution and aging treatment.Moreover, some lathηphases are found in an alloy whose Ti/Al ratio is 1.8, while such phases do not precipitate in other alloys with low Ti/Al ratios. The tensile testing of alloys under different temperatures from room temperature to 850 degrees C shows that the alloys with the highest Ti/Al ratio and Ti+Al content have relatively higher yield strength. However, the yield strength of alloy whose Ti/Al ratio is 1.8 decreases seriously above 750 degrees C. On the other hand, the alloy with relatively high Ti+Al contents still maintain high yield strength. The calculation and experiment results indicate that the Ti+Al content should be increased to improve the yield strength of Ni-Co base superalloys at high temperature, but the Ti/Al ratio should be controlled to avoid TCP phases precipitation.
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