Electron Microscopy Investigation Of Microstructure And Creep Mechanism Of Nickel-based Single Crystal Superalloys

In this dissertation,the microstructure evolution of two nickel-based single crystal superalloys has been systematically investigated by using transmission electron microscopy(TEM),scanning transmission electron microscopy(STEM),scanning electron microscopy(SEM)and energy dispersive spectroscopy(EDS)techniques.The two superalloys are one without Re(ORe)during creep at 760? under 850 MPa?600 MPa and 550 MPa,and the other one with 4wt.%Re(4Re)during creep at 1120?/140 MPa,compared to the aging treatment at 1120 ?,During the creep at low temperature(760?)for the alloy without Re,the formation of stacking faults depends on both the reaction of two different a/2<110>dislocations and the dissociation of single a/2<011>dislocation,but mainly depends on the former(such as 600 MPa).The proportion of the latter one increases with the increasing stress(such as 850 MPa).The deformation mechanism at the initial creep stage for ORe alloy at 550 MPa mainly relies on the movement of dislocations in y matrix without the deformation of ?'precipitates.With the increasing stress,dislocations at ?/?' interface start to shear into the ?'precipitates and form stacking faults there.Under higher stress,dislocations in multi-slip systems form and shear into ?' precipitates,thus introduce multi-directional stacking faults.The different stacking faults interact with each other and form various locks.These locks can impede the movement of dislocations or the shearing of stacking faults in ?' precipitates and thus increase the creep resistance of the superalloy.During the creep deformation for 4Re alloy at 1120?/140 MPa,more complex grooves and dense interfacial dislocation networks are found at the initial stage.The formation of grooves is related to the movement of dislocations and the diffusion of solute elements,and both the dislocation lines and grooves at the ?/?' interface are enriched with Re,Co and Cr.In the process of creep,the two interfacial dislocations shear into y'precipitates together and form a<101>and a<010>superdislocations.The a<101>superdislocation slips from {111} plane to {010} plane and forms K-W locks.All the grooves and ledges are the result of local dissolution events which are triggered by dislocations at ?/?' interface.Through their stress fields,dislocations can affect the local chemical potential at ?/?' interfaces and can drive diffusional fluxes which result in groove formation.In the process of creep deformation,the secondary y' partials are observed in ?matrix,and we belive that the formation of secondary ?' partial is related to the local dissolution of ?' precipitates.Under the extermally applied stress,the increase of interfacial dislocations and grooves causes the increase of dissolution of ?' precipitates,thus results in the increase of secondary ?' partials.During the aging treatment for 4Re alloy at 1120?,there are abundant dislocation at ?/?' interface.During the process of long-term aging treatment,many dislocations form and slip in the ? matrix channel.When the different dislocations slip into ?' phase,dislocations in multi-slip systems intersect with each other and form the complex networks in ?/?' interface.However,no dislocations exist inside ?'precipitates.The coarsening of ?' precipitates is along the[100],[010]and[001]direction in the process of long-term aging treatment.