Composition Design Of Co-Al-Mo-base Superalloys Based On The Cluster-plus-glue-atom Model

New ?' phase strengthened Co-base superalloys have been a research hot spot in superalloy field because of similar microstructure and properties to Ni-base superalloys,but the poor high temperature stability of ?' and large alloy density limit the further application,so exploring the chemical composition rule can help to get Co-base alloys with excellent microstructure and properties.In this paper,according to the cluster plus glue atom model of face-centered cubic solution,by analyzing composition of Co-Al-W-base alloys,we established the cluster general formula-[Al-CO12]?Co,Al,W?3,a CN12 cluster includes an Al atom as center atom and twelve Co atoms as shell atoms pluses three glue atoms.For multicomponent complex alloys,according to heat of mixing and participation behavior of alloying elements,after dividing alloying elements into ????Co?Cr?Fe?Re?Ni?Ir?Ru??Al?????W?Mo?and ????Ta?Ti?Nb?V?,then analyzed alloy compositions and the ideal cluster formula of Co-Al-W-base superalloys was gained-[Al-???₁₂]???_1.0Al_0.5????,????_1.5(???? _81.250Al_9.375????,????_9.375 at.%).Besides,the ideal cluster formulas of y and ?'phase were also attained:[Al-Co₁₂]???_1.5Al_0.5????,????_1.0(????_84.375Al_9.375????,????_6.250)and[Al-???₁₂]???_0.5Al_0.5????,????_2.0(????_78.125Al_9.375????,????_12.500).As a result of large atomic mass and high melting temperature of W,Co-Al-W-base superalloys had high density and difficulty in melting.So we replaced W by Mo and changed the atom kinds and numbers of Ta and Co,designed two series of Co-Al-Mo-base superalloys:Co-Al-Mo-Ta/Nb?-Cr?and Co-Al-Mo-Ti-Ni/Cr series alloys.For Co-Al-Mo-Ta/Nb?-Cr?series alloys,because of much refractory elements,lamellar ?phase precipitated between dendrites in cast microstructure.However,solution heat treatment could not only eliminate ? phase but also made it bigger There were ?' phase precipitated in all alloys after aging treatment,and Ta could greatly increase volume fraction and lattice misfit of ?' phase,at the same time,Ta also could decrease ?' phase size.Besides ?' phase,?phase inherited from solution treatment still existed in grain boundary and in the grains;x phase precipitated from ? phase being in grain boundary.Therefore,the Co-Al-Mo-Ta/Nb?-Cr?series alloys all stayed in y+?'+x three phases equilibrium.As for as alloy density,because of nonexistence of W,Co-Al-Mo-Ta/Nb?-Cr?series alloys had smaller density than Co-Al-W-base superalloys.For Co-Al-Mo-Ti-Ni/Cr series alloys,using Ti completely to replace Ta and Nb made no chemical composition segregation at both of cast and solution state.After aging treatment,?'phase precipitated in all alloys,but only the 6Ni alloy had a microstructure of ?+?'two phases.Compared with Co-Al-Mo-Ta/Nb?-Cr?series alloys,cubic degree and volume fraction of ?'phase decreased in all alloys.The existence of ? and ? phase meant that not only refractory elements were still excess but also had much Ti and Al.It was notable that the number and size of second phase was obviously decreased,so,these second phases had little influence on alloy hardness,the trendy of alloys at different states were identical to microstructure,specially,6Ni alloy had the largest hardness.Besides,the density of Co-Al-Mo-Ti-Ni/Cr series alloys were smaller than Co-Al-Mo-Ta/Nb?-Cr?series alloys,which could be compared with Ni-base superalloys.