Behaviors And Effect Factors Of A Re-Free Single Crystal Ni-Based Superalloy During Creep

In this dissertation,the stacking fault energies of nickel-base superalloys are calculated by the thermodynamic and TEM methods,the influence of elements,stacking fault energy on the internal friction stress and creep features of single crystal nickel base superalloys during creep are investigated by means of the measurement of creep properties and SEM,TEM observation,to explore the microstructure evolution regularlity during tensile creep and the influence of the microstructure on the creep lifetimes of superalloys, and in the further,to discuss the creep mechanism of the designed alloy during creep.Some of the conclusions are given as following:The eutectic microstructure of the single crystal nickel-base superalloy formed during solidification consists of the thicker strip-likeγ/γ′phases and finer network-likeγ/γ′phases, thereinto,the thicker strip-likeγ/γ′phases result from the peritectic reaction,and the finer network-likeγ/γ′phases originate form the eutectic reaction.The obvious composition segregation and size difference ofγ′phase in as-cast single crystal nickel-base superalloy apprear in the dendrite and interdendrite regions,which results in a bigger lattice misfit betweenγandγ′phases in the alloy.The heat treatment regimes of the alloy are constituted by means of the DTA curve analysis and a trial and error method.After fully heat treatment,the composition segregation in the alloy was obviously improved,and the cubicalγ′phase was coherently embedded in theγmatrix phase,which decreases the lattice misfit betweenγandγ′phases in the alloy.The stacking fault energy(SFE) of Ni-Al-M alloys may be decreased by adding the element Al,the decreased extent of SFE increases with Al element,and the SFE of the Ni-base superalloys decreases with the elevated temperature.The element Al may decrease the accumulated Gibbs free energy of Ni-base alloy,and promotes the formation of theγ′-Ni₃Al ordered phase,this is a main reason of decreasing the SFE of the Ni-Al-M alloys. The alloys with higher stacking fault energy possess a lower internal fracture stress and creep resistance.The internal fracture stresses and creep resistance of the nickel-base single crystal supralloys are improved with the drop of stacking fault energy,this enhances the creep lifetimes of the superalloys.Compared with the other alloys,the designed Re-free nickel base single crystal superalloy possesses a better creep resistance,and the creep lifetimes of the alloy at 1040℃,137MPa conditions is enhanced to 1280 h.During intermediate temperature and high stress creep,the deformation mechanism of the alloy is theγ′phase sheared by the <110> super dislocations which may move both on {111} octahedral crystal planes and on {100} cubical crystal planes,the super-dislocations resulted from the reaction may be cross-slipped to {100} the cubical crystal planes from the {111} ones.If(1/2)<110> dislocation shears into theγ′phase from theγ′/γinterface to occur the reaction,this may promote the formation of(1/3)<112> super partial dislocation+stacking fault configuration.During the initial stage of high temperature and low stress creep,the deformation mechanism of the superalloy is the slipping of(1/2)<110> dislocations activated on the octahedral slipping planes of theγmatrix channel in the form of cross-slip.After theγ′phase transformed into the rafted structure,the deformation mechanism of the superalloy during creep is the dislocations over the raftedγ′phase by climbing.And in the later stage of creep,the deformation mechanism of the superalloy is the screw or edge <110> super-dislocations in character shearing into the raftedγ′phase.In the range of experimental temperatures and stresses,compared to the P-type structure alloy,the fully heat treated superalloy displays a lower strain rate and longer stress rupture lifetime,the creep activation energies and apparent stress exponents of the fully heat treated and P-type structure alloys are calculated to be Q_a =462 kJ/mol and Q_a=412.5 kJ/mol,respectively,and the apparent stress exponents being n_a=3.5 and n_a=5.2,respectively.During tensile creep,a complicated microstructure evolution occurs during creep of P-type structure alloy in which theγ′phase with the P-type structure is transformed into the N-type rafted structure with a shorter size,so that the moving dislocations are easily over the rafts by slipping.This is a main reason of P-type structure alloy possessing a higher strain rate and shorter creep lifetime.During tensile creep,the superalloys with different compositions display a different rate ofγ′phase directional coarsening.The diffusion rates of the solutes elements andγ′phase directional coarsening are reduced with the increase of the Ta+Mo content and Ta/W ratio in the superalloys.During tensile creep,a shearing stress is applied on the cubical-likeγ′phase interface vertical to the stress axis,which results in the lattice constriction of theγ′phase to repel out the Al,Ta atoms with bigger radius. At the same time,a tension stress is applied on theγ′phase interfaces along the direction parallel to the stress axis,which results in the lattice expansion ofγ′phase to trap the Al,Ta atoms with the bigger radius.This brings out the accumulation of the solute atoms(Al,Ta) to form the N-type rafted structure.Al,Ta atoms with bigger radius diffuse to the {100} plane to form the linked bond of the heterogeneous atoms and the stable stacking mode,this is a main reason of promoting the transformation ofγ′phase into the N-type rafted structure.And the change of the strain energy density in different interfaces of the cubical-likeγ′phase is thought to be the driving force of the elements diffusion and theγ′phase directional growth during creep.