Precipitation Of ?? Phase Along Dual Phase Area And Eutectic Area Interface And Cyclic Oxidation Behavior Of Ni₃Al-based Superalloy

Due to the harsh service environment of hot end components of aero-engines,the selected structural materials must have excellent microstructure stability and oxidation resistance.Ni₃Al-based superalloys are widely used in aerospace engine heat-resistant structural components due to their excellent comprehensive properties.As the increase of thrust-to-weight ratio of engines,the requirements for aerospace structural materials also increase,especially formicrostructure stability and oxidation resistance under high temperature conditions.Typical polycrystalline Ni₃Al-base superalloy consists of dual phase area(?+?')and eutectic area(?-?'),and interface area((?+?')/(?-?'))represents its typical structure.Researches on polycrystalline Ni₃Al-based superalloys mainly focused on its deformation and creep properties and evolution of?'phase morphology in dual phase area(?+?').Since the interface area is the unstable region of the alloy during its deformation,and few studies concentrate on the precipitation behavior of strengthening phase under service conditions in interface area and its high temperature oxidation resistance,which limits its application.Basedon these,in this work westudyed the precipitation and growth behavior of the strengthening phase in the interface area of polycrystalline Ni₃Al-based superalloys and its oxidation behavior.The results showed that:For the as-cast alloy,larger mushroom-like?'phase precipitated uniformly in the interface area after short-term cyclic heat treatment.This was mainly owing to that after short-term cyclic heat treatment,a large number of vacancies and dislocations generated in the interface area of the as-cast alloy,which provided nucleation site for the precipitation of?'phase.At the same time,the higher concentration gradient of Al element between the eutectic area(?-?')and dual phase area(?+?')promoted growth of the mushroom-like?'phase.For the solution treated alloy,after a long-term cyclic heat treatment,a small amount of blockynanoscale?'phase precipitated along the interface areas.This was mainly owing to thatrapid cooling process after solid solution treatment generated dislocations in the interface area,and the presence of dislocation promoted nucleation of?'phase.During the long-term cyclic heat treatment,fewer Al atoms in eutectic area(?-?')diffused into the interface area,leading to the formation of nanoscale?'phase.Decrease of Al element concentration gradient in eutectic area(?-?')inhibited the directional growth of nanometer?'phase.During the short-term cyclic oxidation process,obvious Ni O generatedin interface area of the as-cast alloy,while for the solution treated alloy,small amount of Al₂O₃ and Ni Omixture formed in interface area.This was mainly owing to thatduring the short-term oxidation process,Ni atoms with strong oxidizability preferentially diffused into the surface of the alloys along the interface and was oxidized to Ni O.While,the increase of Al content in the interface area after solid solution treatment led to the formation of Al₂O₃during the oxidation process.During the long-term cyclic oxidation process,thicker Al₂O₃ oxide layer existed on the solution treated alloy and only a small amount of Ni O outer oxide layer was observed.During the long-term cyclic oxidation process,O atoms continuously diffused along the oxide pores into the matrix and Al element israpidly captured to generate Al₂O₃,which increased the thickness of Al₂O₃ oxide layergradually.In addition,thickening of Al₂O₃oxide layer also hindered the outward diffusion of Ni atoms.