Phase-Field Study On The Precipitation Of D0₁₉ Phase In Co-Al-W Single Crystal Superalloy

The L1₂-γ’-Co₃（Al,W）phase in Cobalt-based superalloy can greatly improve the creep performance and oxidation resistance.However,γ/γ’two-phase region is narrow and the cubicγ’phase is unstable under high aging temperature.Theγ’phase will be transformed into stripe-shaped D0₁₉-χ-Co₃W phase after long-term aging.Therefore,the investigation of the precipitation characteristics ofχphase and the mechanism of the decomposition ofγ’phase have important reference value for predicting the structure evolution of novel Cobalt-based superalloys and designing composition.In this paper,theγ/χtwo-phase free energy is established based on the phase field method,coupled with the sublattice model and the Microelasticity theory.Element diffusion behavior during precipitation ofχphase,the influence of temperature and the influence of composition on the precipitation ofχphase are studied.Furthermore,much work is done to study the precipitation kinetics ofγ’+χphase and the mechanism of the transformation fromγ’phase toχphase.The morphology evolution and element diffusion behavior ofχphase precipitation are revealed through compositional evolution.W atoms diffuse from the verticalγmatrix channel in the[111]_γdirection to the horizontalγmatrix channel in the[11（?）]_γdirection which promotes theχphase particles grow up and coarsen along the[111]_γdirection,forming strip particles.The elastic energy dominates the connecting process.The influence of concentration and temperature on the precipitation ofχphase are clarified.The ordering ofχphase is faster than the composition clustering,there is a transition structure fromγmatrix toχphase.The high temperature accelerates theχphase structure transformation.Although the high initial W concentration makes theχphase precipitate earlier and more,it has little effect on the structure transition speed.The transformation mechanism of theγ’phase toχphase in the Co-Al-W alloy is studied by establishing theγ/γ’+χthree-phase precipitation model.The precipitation ofγ’phase in Co-Al-W alloy precedes that ofχphase.As the aging progresses,theγ’phase becomes unstable and transforms into theχphase,which causes stress concentration in the decomposed particles.The high diffusion potential difference promotes the continuous decomposition of the surroundingγ’phase to form strip-shapedχphase.The orientation of the strip-shapedχphase and theγ’phase has an angle of 60°.