Microstructural Stability Of Low-cost Ni-base Superalloys With A High Volume Fraction Of Cuboidal γ’ Nanoprecipitates

The prominent mechanical properties（such as high strength and creep rupture lifetime）at high temperatures（HTs）of Ni-base superalloys benefit primarily from their special coherent microstructures with spherical or cuboidal L1₂-γ/γ（ordered Ni₃Al phase）nanoprecipitates in the face-centered-cubic（FCC）-gsolid solution matrix.In particular,the solvus temperature of cuboidalγ/γphase exceeds 1200°C in cast and single crystal superalloys,which is close to the initial melting temperature of the alloy,and shows excellent high temperature microstructural stability,so they are extensively applied in modern aero-engine and industrial gas turbine.In order to exploit the high-temperature performance potential of Ni-based superalloys,we need to achieve precise control of the alloy composition and process,so that the alloy will have higher L1₂-γ/γsolvus temperature,moderate lattice misfit ofg/γ/γ,excellent high-temperature microstructural stability and mechanical strength,etc.However,the multi-component alloying of more than a dozen elements in Ni-base superalloys induce a serious compositional complex,which needs an appropriate composition design method to match these elements well for achieving a moderateg/γ/γlattice misfit.Therefore,the present work will apply the cluster formula approach to design compositions of Ni-base superalloys in low-cost Ni/Co-Al/Ti/Nb/Ta-Cr/Mo/W system.The composition of[Al-(Ni₁₂)](Al₁（Ti,Nb,Ta）_0.5（Cr,Mo,W）_1.5)is first determined,in which the amounts of（Ti,Nb,Ta）and（Cr,Mo,W）are taken as an equi-molar ratio.Then,we added some Co and change the ratio of Cr,Mo,and W to investigate the effect of elements on the formation of cuboidalγ/γnanoprecipitates after long-term aging at 900°C,further analyzed the relationship between the lattice misfit ofg/γ/γandγ/γparticle morphology,and finally tested the mechanical properties of the series alloys.The main research conclusions are as follows:（1）It is found that the basic Co-free alloy with the addition of（Ti,Nb,Ta）_0.5 and（Cr,Mo,W）_1.5 in an equi-molar mixing has a special coherent microstructure of cuboidalγ/γnanoparticles precipitated into thegmatrix,showing a higher microstructural stability with a slow coarsening rate ofγ/γprecipitates after 500 h-aging.It is mainly ascribed to a moderate lattice misfit（e=-0.53～-0.47%）ofg/γ/γ.Moreover,the volume fraction of cuboidalγ/γparticles keeps almost constant during the aging process,with a high value of f～70%.（2）The addition of Co or the increase of Cr content in the basic alloy can affect the thermal stability ofg/γ/γcoherent microstructure,which is closely related to the lattice misfit ofg/γ/γand a moderate lattice misfit（e=0.44～0.53%）would contribute to the high stability of cuboidalg/γ/γmicrostructure.Also,the alloy with(Cr_1.0Mo_0.25W_0.25)and（Ni₈Co₄）shows a comparable microstructural stability to the basic alloy.（3）The addition of B,Zr,and C elements to the base alloy reduced the grain size,but the precipitation of large-sized massive and rod-likemphases and M₆C carbides at the grain boundaries affected the further improvement of mechanical properties.（4）These alloys exhibit prominent mechanical properties,as evidenced by the fact that the yield strength of the basic alloy is 892 MPa at room temperature and 835 MPa at 850°C,respectively.Moreover,the microhardness keeps almost constant with the aging time,indicating the better microstructural stability.