Morphology And Kinetic Evolution Of Bimodal ?? Phase Using Phase-field Method

Nickel based alloys has excellent persistent,resistance to fatigue,oxidation when served at high temperature,these performances benefit from the morphology and spatial configuration of ?? phase which coherently embedded in the ? phase.While the bimodal ??phase can bring increased yield strength and creep resistance without loss the ductility.In this paper,the bimodal ?? phase in nickel based superalloys was obtained by preaging and continuous cooling by phase-field simulation,the unidirectional tensile stress along x axis applied to alloy during secondary isothermal aging.The effects of cooling rate,preaging time and tensile stress on the morphology and kinetic evolution of bimodal ?? phase are studied in this paper and provide a hybrider way to regulate the morphology of nickel based alloy.The formation mechanism of bimodal ?? phase and characters of primary and secondary ??phase have been clarified.The cubic primary ?? phase formed at preaging with large size,its particle size distribution(PSD)close to the prediction of LSW theory.The fine and nearly spherical secondary ?? phase formed at continuous cooling or interrupted ageing which distribute in the matrix between primary ?? phase with high number density,the content of Al in secondary ?? phase is far away from the equilibrium composition at cooling terminal temperature,and its PSD close to the prediction of BW theory.The influence of cooling rate,preaging time and tensile stress on the morphology and kinetic evolution of bimodal ?? phase are clarified.The high cooling rate lead to the small volume fraction and average radius of primary and secondary ?? phase,the high number density of secondary ?? phase,and large size difference of primary and secondary ?? phase.A long time isothermal preaging promotes the growth of primary ?? phase,whlie reduces the size,content of Al and coarsening rate of secondary ?? phase.The ?? phase is stretched along the y axis when the unidirectional tensile stress along the x axis is applied to the Ni–Al alloy with bimodal ?? phase.The tensile stress promotes the connection and combination of bimodal ??phase along the y axis and the ??-raft are formed.The content of Al in longitudinal ?? phase rised with tensile stress increased.At the same time,the tensile stress promotes the dissolution of the smaller ?? phase.