Effects Of Element Re On Microstructure Stability And Creep Properties Of Nickel-base Superalloys

In this paper,by means of Md and Nv method,five kinds of nickel-based superalloy compositions have been designed to predict the tendency of TCP phase precipitated.The evolvement regularity of the TCP phase in the alloy has been investigated by means of the aged treated for longer time and microstructure observation.The effects of elements Re on the lattice parameter and misfit of the alloy are analyzed by means of the X-ray diffraction analysis.The creep behavior of the single crystal nickel-based superalloys was investigated by the measurement of tensile creep curves.Results show that the elements W,Re may intensively promote the precipitation of TCP phase.During aged for longer time of the richer W and Re alloy at 1100℃,the TCP phase is precipitated in the form of the coherent lattice along the {111} planes and <110> orientation and may be arranged in the upright each other or 60°angle each other on the different crystal planes,and which is determined asμphase by means of the electric diffraction analysis.In the initial stages of aging,theμphase is precipitated along the planes parallel to the {111} planes ofγ' phase in the form of the sheep-like structure due to the role of the coherent lattice strain energy.The thickness of the strip-likeμphase is increased as the aged time is prolonged,which decreases the lattice strain energy ofμ/γ' phases interfaces due to the loss of the coherent interfaces.Thereafter,the groove appears in the local regions of the strip-likeμphase according to the principle of the interfaces energy minimum,which enhances the chemical potential energy of the elements W and Re. In the further,the difference of the chemical potential energy in the different regions of the strip-likeμphase promotes the diffusion of the elements W,Re,which results in the decomposition of theμphase up to the occurrence of the spheroidized.The change of the lattice strain and interfaces energirs is thought to be the driving force of the groove dissolved and spheroidized ofμphase.The lattice parameters ofγ' andγphases in alloy are increased with the element Re,but the absolute value of the lattice misfit decreases.And the absolute values of the lattice parameter and misfit in the alloys increase with temperatures.The solution strengthening extent of the richer W,Re single crystal nickel-based superalloy decreases due to the precipitation of TCP phase to consume the more refractory elements,and which interrupts the raftedγ' phase so that results in the bigger strain rate and shorter creep lifetimes.The activation energy of the alloy during the steady state stage of creep is calculated to be 413.3 kJ/mol.Compared to the single crystal nickel-base superalloy with the richer elements W and Re,the one with the poorer W and Re displays a lower strain rate and longer creep lifetimes.Thereinto,the cubicγ' phase in the alloy during creep is transformed into theγ' rafted structure along the direction vertical to the applied stress axis,which can effectively inhibit the movement of dislocation.This is a main reason of resulting in the alloy possessing the better creep resistance and longer creep lifetimes.