Microstructure Evolution Of Eutectic Area And Hot Deformation Performance Of Ni₃Al-based Superalloy By

Due to its excellently comprehensive performances,Ni₃Al-based superalloy has become an ideal alternative material of traditional nickel-based superalloy for hot-end components of aero engines.However,high volume fraction ofγ′phase and complex chemical composition exert a significant effect on their microstructure,thermal stability and mechanical properties.In this work,we focused on a new polycrystalline Ni₃Al-based superalloy,aiming to obtain the optimal solution treatment parameters through systematical solution treatment experiment design.Precipitation,growth and evolution mechanism ofγ′phase in eutectic area during solution treatment were studied.Furthermore,the influence of thermal exposure temperature on the microstructure of eutectic area was explored.Based on these,the effect of temperature on hot deformation behavior and microstructure of eutectic area,interface area was analyzed and the main findings are as follows:After solution treated at 1270℃ for 9 hours,eutectic area boundaries became smoother and carbide existed in the forms of Hf C which possess a stable state.Meanwhile,interfaces ofγchannels were smooth and the main strengthing phase ofγ′presented as cubic with uniform size in dual phase area.Moreover,micro-hardness difference between eutectic areas and dual phase areas was the smallest,and microstructure of alloy was uniform without obvious grain boundaries.During the solution treatment process,plate twins ofγ′phase precipitated in eutectic areas with the sizes and density increase with eutectic area dimensions increasing.Besides this,γ′phase in eutectic areas went thorugh the process of“sphere→precursor plate→plate”.Vacancies,lattice distortion and thermal stress in eutectic area were the main reasons for the nucleation and precipitation of sphericalγ′.Micro-segregation of Al element,preferential growth direction ofγ′phase and the decrease of interfacial energy were the main reasons for its morphological evolution.When the thermal exposure temperature was 850°C,γ′phase existed as plate shape in eutectic areas without twins and dislocation tangles appeared along the interfaces ofγ′phase,andγ′phase in dual phase areas gradually rafted with thermal exposure time prolonged and L-shaped raftingγ′phase occurred.When the exposure temperature was1150°C,twinsγ′phase appearred with blocky-shape in eutectic areas and plateγ′phase disappeared.In addition,the dimension of cubicγ′phase in dual phase areas increased unevenly and the phenomenon of splitting occurred inγ′phases.When the hot deformation temperature was 850°C,the flow stresses decreased linearly after reaching the maximum values.At this time,although the microstructure in eutectic areas was similar to that at the thermal exposure temperature of 850°C,the density and size of plateγ′phase were higher and smaller respectively.In addition,a large number of cracks propagated along interface areas（eutectic area/dual phase area）in the alloy.Enrichment of Cr and Hf elements was the main factor causing the propagation of cracks in this area.When the deformation temperature was 1150°C,the flow stresses reached the maximum value and then entered into the steady-state stage.The microstructure of eutectic areas was still dominated by blocky twinsγ′phase,the alloy underwent significant dynamic recrystallization.When work hardening and the softening effect of recrystallization reached equilibrium,deformation steady-state stage appeared.