Phase-field Study On The Rafting Morphology And Creep Property Of Co-Al-W Superalloy

Novel Co-base superalloys have been attractive for the high melting point of Co,good resistance to oxidation and excellent creep property.Since the discovery of L1₂-Co₃(Al,W)in Co-Al-W alloy,the novel Co-base superalloy has been considered to be the new generation of superalloy for aircraft engines.Directional coarsening of??precipitates under external stress and high temperature forms the rafting morphology,which plays a crucial role in the creep property.A straight-forward insight into the element diffusion during rafting provides the guidance for the rafting mechanism and composition design of Co-base superalloy.Also,illustration of the effects of rafting morphologies on creep property will be in favor of property prediction.In this paper,the phase-field model of creep in Co-Al-W superalloy is established on the basis of CALPHAD and crystal plasticity.The element diffusion during rafting and effects of W contents on the creep property are studied.In addition,the evolution of rafting morphology under different stresses and effects of rafting morphologies on creep property are interpreted to realize the control of performance.Rafting and creep roperty are studied from the point of element diffusion.P type rafting parallel to the tensile stress or N type rafting perpendicular to the compressive stress will form during creep,and the diffusion of W is the dominant factor in the process of rafting,which is induced by the potential difference under stress.The effect of W on the creep property is studied.The addition of W content is beneficial to the creep property of Co-Al-W superalloy.The higher volume fraction of??precipitates and higher active energy for diffusion in alloy with high W content are the two reasons for the better creep property.The rate of rafting decreases with the external stress decreasing form 297 MPa to 197 MPa.The lower creep strain and strain rate under 197 MPa can be accounted by low driving force for diffusion of Al and W,more W atoms reserved in the?matrix parallel to the rafting direction,the lag of composition change and lower changing rate of composition.Kachanov's creep law is coupled into the phase-field simulation to study the creep damage in Co-Al-W superalloy,Micro-plastic deformation evolves from the?matrix to the??precipitates in the tertiary stage of creep,and creep damage happens in superalloys.There is a depletion of W in the?matrix after the completion of rafting morphology.The tensile creep property of Co-10Al-9W(at.%)can be enhanced by N-type pre-rafted morphology,and the evolution of the N-type pre-rafted morphology in the consequent creep process can be accounted for by the heterogeneous distribution of diffusion potential induced by stress.