Microstructure, Creep Behaviors and High Temperature Oxidation Features of Single Crystal Ni-based Superalloys
|School||Shenyang University of Technology|
|Keywords||Single crystal Ni-based superalloy heat treatment lattice misfits creep features TCP phase high temperature oxidation|
By means of the analysis of the differential thermal curve （DTA）, and microstructure observation, heat treatment conditions of the superalloy have been determined. The regularity of the microstructure evolution during the alloy heat treated at different conditions is investigated by the analysis of finite element method. The crystal lattice parameters and misfits of the y7y phases in the alloy have been calculated by means of X-ray diffraction analysis. The creep curves and internal friction stresses of dislocation motion for the superalloy have been measured at elevated temperature, and the creep activation energy and the apparent stress exponent during the steady stage creep have been calculated, and in further setting up the synthetic creep equation for describing creep regulation of the superalloy. The effects of the elements W and Co on the microstructure and properties of the alloy are studied by means of creep testing, and observation of SEM and TEM.Results show that the difference of the γ’ phase sizes between dendrite and indendritic is decreased by means of the solution treatment, the fine γ’ phase is dispersedly precipitated in the γ matrix. In the role of the interface and misfit strain energies, the γ’ phase has grown into the morphology with cubic after during first aging. During second aging, the cubic γ’ phase has slightly grown up in the form of the regular arrangement. According to "steps" mechanism, the γ’ phase is coherently grown into the cuboidal morphology along the <100> orientation. The lattice misfit of the alloy at room temperature is measured to be about —0.3121%. The average internal frictional stresses （σ0） of dislocations movement during steady state creep are slightly improved with the enhancement of the applied stress, but obviously reduced with the increase of temperatures, the internal frictional stress （σ0） of dislocations movement is more sensitive to temperature in the range of the applied high stresses. The dislocation networks on the y// phases interfaces are formed by the dislocations reaction with different Burgers vectors during primary creep. When the elements Al and Ta of γ’-former exceed the solubility in the matrixduring creep in the role of the applied stresses and temperatures, these supersaturated elements are congregated to precipitate some of the finer / particles from y matrix. The alloy containing 4wt% W> Co elements displays a better property at elevated temperature. With the increment of the elements W and Co concentration, the needle-like u phase is precipitated during the elevated temperature aging, which reduces severely the creep properties of the alloy due to exhausting the elements W and Mo in the matrix of alloy.The surface oxidation of the elements occurs during high temperature exposure. The oxides of the elements Al and Cr are formed on the surface layer of alloy. The internal oxide of AI2O3 was formed in the sub-surface layer in the form of the rod and block. The thickness of internal oxidation layer, with the increase of time, displays the regularity of the parabolic curve. The / phase in the internal oxidation layer is dissolved as a result of the exhausting of the element Al, which is one of the main reasons for resulting in the creep lifetimes decreased of the alloy.