Solidification Microstructure Evolution And Oxidation Behavior Of Ni₃Al-based Alloys

The solidification structure of Ni₃Al-based superalloys is complex,and their ultimate performance is closely related to the formation and evolution of solidification structure.It is of great theoretical significance and practical value to systematically clarify the microstructure evolution and phase transformation behavior in a wide range of cooling rate for understanding the solidification behavior of Ni₃Al-based alloys and the development of non-equilibrium solidification technology.In this paper,a Ni₃Al-based alloy was used as the experimental material.The conventional solidification process with low solidification rate and the rapid solidification technologies of spray cast and atomization were used to study the solidification microstructure evolution and phase transformation mechanism of the alloy from low solidification to rapid solidification rates.On this basis,through the comparative study of rapidly-solidified alloy and conventionally-solidified alloy,the influence of the original microstructure difference caused by rapid solidification on the phase transformation,microstructure evolution and high temperature oxidation behavior of the alloy in the process of thermal exposure were explored.During rapid solidification,the size ofγ’phase changes from bimodal distribution（conventional solidification）to unimodal distribution,and the secondγ’phase disappears.Besides,theγ’-phase envelope at the interface along interdendritic regions disappears.Martensitic transformation occurs in the interdendriticβphase.And the coherent stress at the interface between theα-Cr precipitates and the martensitic matrix promotes the transformation of dislocations on the{111}atomic planes to stacking faults and microtwins,forming martensites with high density stacking faults and microtwins.As the result,the martensites with high density stacking faults and microtwins are formed.Under the condition of rapid solidification,the volume fraction of interdendritic region increases with the increase of cooling rates,while the secondary dendrite spacing,size ofγ’phase in dendrite and the amount of carbides decrease.Cr and Fe elements mainly segregate in the dendriteγphase of Ni₃Al-based alloys,and rapid solidification improves the solubility of Cr and Fe atoms in theγ’phase.Moreover,Cr and Fe atoms preferentially replace the Al atoms inγ’phase.As a result,the lattice constants ofγandγ’phases in（γ+γ’）dendrite decrease,and the degree of lattice mismatch increased,resulting in the formation of edge dislocations at theγ/γ’interface.During thermal exposure at 600°C,theγ’-phase envelope in the conventionally-solidified Ni₃Al-based alloy is obviously widened,and a large number of bulkyγ’phases and sub-spherical or rod likeα-Cr phases are generated within the interdendriticβphase.The primary carbide Cr₇C₃ in theγ’-phase envelope decomposes to form secondary carbide Cr₂₃C₆ andγ’phases.The interdendritic twin martensite plates in rapidly-solidified Ni₃Al-based alloy directly transform intoγ’phases with high density microtwins.When the temperature increases to 900°C,the number ofγ’phase andα-Cr phase in the interdendriticβphase of conventionally-solidified Ni₃Al-based alloy decreases obviously.For the rapidly-solidified Ni₃Al-based alloy specimen,the martensite with stacking faults and microtwins in the interdendritic region completely transforms to ordered B2-βphase at first,and then precipitatedγ’phase without obvious dislocations and twins.Isothermal oxidatized at 600°C,the oxide film was mainly composed of NiO,Al₂O₃,Cr₂O₃ and NiFe₂O₄.In conventionally-solidified Ni₃Al-based alloy,theγ’-phase envelope preferentially oxidizes to form obvious cellular NiO and NiFe₂O₄ mixed oxides.However,the preferential oxidation at the interface is avoided due to the disappearance ofγ’-phase envelope in the rapidly-solidified Ni₃Al-based alloy.