Influences Of Re On TCP Phase Morphology And Creep Behavior Of Single Crystal Nickel-Based Superalloys

In this dissertation, the precipitation trend of TCP phase are predicted and verified by means of the alloys compositions design of using the Md and Nv methods, and the morphology features and evolution regularity of TCP phase precipitated during high temperature aging of the Re containing/free single crystal nickel-based superalloys are detected, and in the further the influences of the element Re on the TCP phase configuration and stress rupture properties of the alloys are investigated. The lattice parameters and misfits of theγ′,γphases in the different superalloys are calculated by means of measuring X-ray spectrums, and investigating the effecting regularity of the element Re and temperatures on the misfits ofγ′,γphases. And the creep behaviors and effect factors of the different superalloys are investigated by measuring creep properties and microstructure observation.Results show that the precipitated trend of TCP phase in the nickel-based superalloys can be predicted by using Md and Nv method of the alloys design, and the critical values of TCP phase precipitated in the Re containing single crystal nickel-based superalloys are defined as Md > 0.98 and Nv > 2.1. The trend of TCP phase precipitated in the single crystal nickel-based superalloys increases with the amount of the elements W,Re,Mo addition, and TCP phase precipitated in the Re containing/free superalloy is identified as theμp?hase. During long term aging at 1040℃and 1100℃, theμphase precipitated in the 6%W alloy and 4.5%Re alloy is grown along the <110> orientation on {111} planes in the form of the strip-like structure. Thereinto, theμphase on {100} planes displays the strip-like configuration arranged at the parallel or upright each other, and theμphase on {111} planes displays the strip-like configuration arranged at the angle of 60°each other. The element Re has an obvious influence on the morphology ofμphase in alloys.During long term aging at 1040℃, theμphase precipitated in 6%W alloy is shorter in size, no spheroidized feature of the TCP phase is detected. But theμphase precipitated in 4.5%Re alloy during aging at 1100℃is longer in size, and theμphase is regularly coarsened, as the aging time prolongs, for displaying the accidented feature up to transformed into the sphere-like morphology. The difference of the chemical potential at the different regions of the slice-likeμphase promotes the solute elements diffusing to the adjacentγ′phase, which is thought to be a main reason of resulting in theμphase dissolved and transformed into the sphere-like morphology. The refractory elements in the alloys are consumed due to the precipitation ofμphase, therefore the stress rupture lifetimes of the alloys are obviously decreased. The stress concentration is easily generated in the regions near the strip-likeμphase to accelerate the initiation and propagation of the cracks during creep, which is thought to be the main reason of the creep lifetime deterioration, to a great extent, of 6%W alloy. But the stress concentration is not easily generated in the regions near the sphere-likeμphase, which is thought to be a main reason of depressing creep lifetime of 4.5% Re alloy to a smaller extent.The different size and morphology ofγ′phase are displayed in the interdendritic and dendritic areas, which are related to the segregation of the alloying elements in the areas. Thereinto, the elements W, Cr, Mo and Re are richer in the dendritic area, and the elements Co, Ta, Al are richer in the interdendritic area. And the segregation extents of the elements in the areas decrease with the enhancing temperature of the solution treatment, which may obviously improve the stress rupture lifetimes of the alloys.Theγ′,γphases in as-cast single crystal nickel-based superalloy have a bigger lattice parameters and misfit. After high temperature solution and fully heat treated, the lattice parameters and misfits ofγ′,γphases in the alloys is slightly diminished due to the cubicγ′phase embedded coherently in theγmatrix. After long time age under the applied stress/unstress, theγ′phase in alloy is coarsened and the dislocation networks are appeared on the interfaces ofγ,γ′phases, which increases the lattice parameters and misfit ofγ,γ′phases. The lattice parameters increase with the Re content, but the absolute value of the misfit decreases, which decreases the rafting rate ofγ′phase and enhances the creep resistance of the alloys at high temperature. But the lattice parameters and absolute value of misfit increase with temperature.Comparing to 2%Re alloy, 4.2% Re alloy displays the better creep resistance and longer creep lifetime, which indicates that the creep resistance and stress rupture lifetimes of the alloys may be obviously improved by addition the element Re. In the ranges of the applied temperatures and stresses, the activation energies of the alloys during steady state creep are calculated to be Q1 = 461.5kJ/mol and Q2 = 497.9kJ/mol, respectively. During the initial stage of high temperature and low stress creep, the deformation mechanism of the single crystal superalloys is that the (1/2)<110> dislocations activated in the octahedral slip system of the matrix channel in the form of cross-slipping. And the deformation mechanism of the alloy during steady state creep is the dislocations climbing over the raftγ′phase, while the deformation mechanism of the alloy during the later stage of creep is the <110> super-dislocations shearing into the raftedγ′phase, thereinto, the dislocation which shears into theγ′phase may be decomposed to form the configuration of the partial dislocation and stacking fault, which can hinder the cross-slipping of dislocations to improve the creep resistance of the alloy.