The Deformation Mechanism And η Precipitation Behavior Of K4750 Ni-based Superalloy

In order to achieve greater thrust and higher thrust-to-weight ratio,aero-engines continue to call for materials of higher temperature bearing capacity.The inclined support frame is an important large-scale complex thin-walled integral casting in aeroengines,whose service temperature in the new generation of high thrust-to-weight ratio aero-engines has reached more than 700℃,so the active alloy K4169 with a maximum temperature capacity of 650℃ needs to be replaced.In order to meet the requirements of components for manufacturing and serving at higher temperatures,the Institute of Metal Research of Chinese Academy of Sciences adopted the idea of low Al+Ti content and high Ti/Al ratio,to develop a new K4750 nickel-based superalloy with a temperature capacity of 750℃.The successful application of K4750 alloy solves the problem that there is no material available for the component whose service temperature is raised with the whole machine.Although the new K4750 alloy has been previously verified possessing excellent room temperature and high temperature strength,the relationship between strength and deformation mechanism is still lacking in-depth understanding.In addition,due to the high Ti/Al ratio,the K4750 alloy has the problem of mechanical properties degradation caused by the transformation of γ’→η,but the precipitation mechanism of η phase has not been clarified.Therefore,this paper systematically studied the deformation mechanism of the γ/γ’ two-phase structure,the main source of strength of K4750 alloy,and clarified the relationship between the deformation mechanism and the important room temperature and high temperature tensile properties and high temperature fatigue properties which were of vital for the alloy application.At the same time,the precipitation mechanism of η phase during overtemperature aging of the alloy was systematically studied,and the transformation mechanism of γ’→η was revealed.Below are key research findings.The effect of γ’ phase size on the room temperature tensile strength and deformation mechanism of K4750 alloy was investigated.After aging at 800℃ for 0～1000 h,γ’ phases with different sizes were prefabricated.The coarsening of γ’ phases follows the Lifshitz-Slyozov-Wagner ripening theory,and the coarsening rate is about 353.2 nm3/h.With the increase of γ’ size,the tensile strength at room temperature first increases and then decreases.The aging time of the peak strength is 20 h,and the corresponding average γ’ phase size is about 42.3 nm.With the increase of γ’ size,the evolution of deformation mechanism is as follows:weakly-coupled dislocation(WCD)shearing→strongly-coupled dislocation(SCD)shearing→SCD+Orowan looping.The critical γ’ phase size of the WCD→-SCD transformation corresponds to the highest room temperature tensile strength.Taking the anti-phase boundary energy as 0.15～0.175 Jm2,the size range is 40.1-46.8 nm,according to which the heat treatment procedure is proposed:1120℃/4 h,air cooling+800℃/20 h,air cooling.Based on the theoretical calculation of the dislocation pair shearing mechanism,the precipitation strengthening contribution of the y’ phase(487.3 MPa)accounts for more than 62%of the total yield strength of the alloy(784.6 MPa),and the solid solution strengthening contribution(266.4 MPa)accounts for about 34%.The contribution of the grain boundary strengthening(30.9 MPa)accounts for less than 4%.The effect of temperature on the tensile properties and deformation mechanism of K4750 alloy was investigated.The results show that with the increase of temperature,the tensile strength of the alloy first decreases(room temperature～650 ℃),then increases(650～750℃),and finally decreases(750～850℃).The dominant deformation mechanism at the initial plastic deformation(about 1%)is anti-phase boundary(APB)shearing→Orowan looping→cross-slip+climbing.The theoretical calculation results of the critical shear stress(r)show that both τAPB and τOrowan decrease with the decrease of the shear modulus of experimental alloy with increasing temperature.However,τOrowan decreases more rapidly and becomes lower than τAPB at about 650℃,leading to the transition of APB shearing→Orowan bypassing at 650 ℃.The deformation mechanism of the alloy in the critical service temperature range of 650～750℃ is planar slip,and the dislocation loops left after dislocations bypassing γ’ phase significantly hinder the movement of subsequent dislocations,resulting in a high level of dislocation entanglement and a high dislocation density in the heterogeneous slip bands.This additional strengthening effect caused by the planar slip and Orowan looping leads to the yield strength anomaly at 650～750℃.However,at 750～850℃,the cross-slip and climbing mechanisms are activated,the deformation mode changes to non-planar slip,the γ’ phase could not effectively hinder the dislocation movement,and the strength decreases continuously.The deformation mechanism and fracture behavior of low cycle fatigue(LCF)and high cycle fatigue(HCF)of K4750 alloy at 600℃ were explored.The strain(Δεt)-life(Nf)relationship in LCF is obtained as Δεt/2=0.0066(2Nf)-0.0421+0.0162(2Nf)-0.3449,and the conditional fatigue strength of in HCF is 615 MPa.It is found that most of the cracks in LCF originate from surface and propagate perpendicular to the loading direction in a striations-assisted Stage Ⅱ manner.Whereas,the cracks in HCF mostly initiate at large-size inclusions and propagate following a crystallographic Stage Ⅰ mode.The crack propagation in LCF can be accelerated by the MC carbides which cause stress concentration and induce numerous second cracks in front of the primary crack tip.The dangerous grains in HCF are those which contain large-sized inclusions,are of large size or large Schmid factor,which are easy to cause Stage Ⅰ cracking.It is found that the fatigue fracture mode is related to the deformation mechanism.The LCF deformation structure contained slip bands of small spacing and uniform distribution,which coordinate the crack propagation direction perpendicular to the normal stress.Whereas,the slip bands in HCF are less in number and distributed heterogeneously,and the propagation of cracks along the highly-isolated slip bands results in Stage Ⅰ cracking.The precipitation mechanism of η phase in K4750 alloy was clarified.The orientation relationship between η/γ is<1120>η//<110>γ,{0001}η//{111｝γ,and twelveη phase variants with an misorientation of 70.5° of each other can be observed.The{0001}η//{111}γ interface marked as A-type is coherent,the {1120}η//{110}γ marked B-type is incoherent,and the preferential growth of the high-energy B interface will induce a lamellar or thin-plate morphology of η phase.The growth of η phase after its nucleation at grain boundaries(GBs)can through intragranular diffusion(850～900℃)and GB short-range diffusion(1000℃).In the latter case,the GBs migrate from the ηnucleating side into the adjacent grains along with the η/γ A-type interface and form a cellular structure.It is found that the metastable extra-large γ’(EL-γ’)plays a role in all the solid-state phase transformation reactions involved with the precipitation of intragranular η phase,such as produced by MC decomposition in reaction:(ⅰ)MC+γ→M23C6+EL-γ’,(ⅱ)EL-γ’→η,induced by M23C6 precipitation in reaction:(ⅰ)γ→M23C6+EL-γ’,(ⅱ)EL-γ’→η,and absorbing γ’ phase in reaction(ⅰ)γ’→EL-γ’,(ⅱ)EL-γ’→η.The composition of EL-γ’ and η phases was similar(69.8Ni-14.2Ti-8.6Al1.7Nb and 71.6Ni-15.2Ti-4.6Al-2.8Nb,respectively,in at%),but the stability of EL-γ’is lower than η phase(the calculated formation energies of L12-Ni3Ti and D024-Ni3Ti are-0.4684 and-0.4863 eV/atom,respectively),which is the thermodynamic driving force for the EL-γ’→η transition.The structure difference of the two phases can be attributed to the arrangement order of the close-packed planes and could be smoothed out through stacking faults,which is the kinetic mechanism that the intermediate step EL-γ’→η is prone to occur.