Effect Of Ta On The Microstructure And Long-term Creep Behavior Of A Hot Corrosion Resistant Single Crystal Superalloy

In this dissertation,the solidification behavior,as-cast microstructure,heat-treated microstructure,intermediate temperature long-term creep property and the microstructure evolution of three low-Re hot corrosion resistant Ni-base single crystal superalloys(2Ta,5Ta and 8Ta)are systematically investigated by scanning electron microscopy(SEM),electron probe microanalysis(EPMA),transmission electron microscopy(TEM)and energy dispersive spectroscopy(TEM-EDS).The effect of Ta addition on the microstructure are analyzed,and creep mechanisms of the alloys at intermediate-temperature/intermediate-stress(760 ?/5 50 MPa)are discussed.The main results are listed as follows:Ta addition decreases the liquidus and solidus temperatures,leading to a large solidification temperature range as well as alleviated segregation of refractory elements(Re and W).The integration of those two factors leads to an increase in the primary dendrite arm spacing(PDAS)and change of ?' morphology and size.With the increase of Ta addition,the size of ?' precipitates in dendrite core increases substantially,whereas the size of the ?' precipitates in interdendrite region decreases slightly.During the creep at 760 ?/550 MPa,Ta addition prolongs the rupture life from 525 h(2Ta)to more than 6000 h(8Ta),and the steady-stage creep rate is reduced.The size of y' precipitations in dendrite core of all the three alloys increases slightly during creep at 760 °C/550 MPa.At the tertiary creep regime,the directionally coarsening or rafting of y' phases in the dendrite core of all the three alloys occurs.Coarsening parallel to the tensile direction(the tendency of P-type rafting)was observed in 2Ta alloy.For both 5Ta and 8Ta alloy,?' phases in dendrite core exhibit N-type rafting,whereas the ?'phases in interdendritic region exhibit the tendency of P-type rafting.At the primary creep stage,deformation of the three alloys is mainly controlled by a/2(110)matrix dislocation slipping or cross slipping.For 2Ta alloy,due to the high driving force for dislocations to active and low Orowan resistance,dislocations generate quickly and the dislocation density was highest.As creep gone,stacking faults and antiphase boundary(APB)occur subsequently in ?' phases.With increasing Ta content,the formation of stacking faults and APB is delayed.Compared with short-term creep at same temperature and stress level,the dislocation slip during long-term creep is obviously impeded and the dislocation density is also reduced.With the prolongation of creep in high Ta-containing alloys,the long-term creep deformation is mainly dominated by(112){111} slip system.