Thermodynamic Study On Phase Composition Of Co-8.8Al-9.8W Alloy By Several Alloy Elements

Traditional Co-based superalloys have much better performance than Ni-based superalloys,but because the strengthening mechanism of traditional Co-based superalloys is different from that of Ni-based superalloys,which leads to the lower working temperature of traditional Co-based superalloys.Therefore,the Ni-based superalloys strengthened by two-phase coherent strengthening ofγ/γ′have been widely used in high temperature environment.In 2006,a new Co-Al-W superalloy withγ/γ′two-phase coherent strengthening was first discovered.It not only has excellent properties of traditional Co-based superalloy,but also it’s performance showed in high temperature environment can be comparable to those of Ni-based single crystal superalloy.Since then,the alloy has attracted wide attention from scholars at home and abroad.Based on this,the following work has been carried out in this paper on the basis of previous research on Co-Al-W alloy.（1）The study of Co-8.8Al-9.8W alloys with 2%（at.%）Nb,Mo,Ni,Ta and Ti elements shows that after adding different alloying elements,the intergranular phase with segregation will appear in alloy organization,γ′strengthening phase and different secondary phases occur after heat treatment.XRD analysis shows that the alloys after heat treatment containγmatrix phase,γ′phase and CoCx phase,while the2Nb,2Mo,2Ta and 2Ti alloys also have the secondary phase ofμ-Co₇W₆,in which both 2Mo and 2Ti alloys contain Co₃W phase and metastable ordered phase of Co₃Ta and Co₃Ti,respectively.Through study of the thermodynamic calculation shows that,at 600℃,the precipitation amount of 5 kinds alloy’sγphase in turn is 2Ni>2Mo>2Ta>2Ti>2Nb,the precipitation ofηphase is 2Nb>2Ti>2Ta>2Mo>2Ni,and the precipitation ofμphase is 2Mo>2Ta>2Ni>2Nb>2Ti.The initial precipitation temperature ofγphase is 2Ni>2Mo>2Ti>2Nb>2Ta,the initial precipitation temperature ofηphase is 2Nb>2Ti>2Ta>2Mo>2Ni,and the initial precipitation temperature ofμphase is 2Ta>2Nb>2Mo>2Ti>2Ni.As the temperature decreases,W and Al precipitated from theγphase respectively provide conditions for the formation ofμ-Co₇W₆ andη-Co₃Al,respectively.Nb、Ni and Ta provide conditions for the formation ofμandηphases,while Mo and Ti provide conditions for the formation ofμandηphases,respectively.（2）A study on the thermodynamics for phase precipitation behavior of Co-8.8Al-9.8W alloy of different Ta（0%、0.5%、1%、1.5%and 2%,at.%）elements shows that with the increase of Ta element,the precipitation amount ofγphase decreases and the initial precipitation temperature decreases.The precipitation amount ofηphase andμphase increases,and the initial precipitation temperature increases.With the increase of Ta element,the critical temperature of transformation betweenγphase and Co-hcp phase increases,while the critical temperature of transformation betweenηphase andδphase decreases.As the temperature decreases,W and Al elements precipitated from theγphase provide conditions for the formation ofμ-Co₇W₆ andη-Co₃Al,respectively.After the disappearance of theη-Co₃Al phase,Al elements provide conditions for the formation of theδ-Co₃Al phase,and Ta elements precipitated from theγphase provide conditions for the formation of bothμphase andηphase.（3）The study for Co-8.8Al-9.8W alloys with different contents of Ni（5%、15%、25%and 35%,at.%）was found that the alloys after heat treatment all containγmatrix phase,γ′phase and CoCx phase.Theγ′strengthening phase in the alloys is cubic and evenly distributed inγmatrix,but no CoCx phase was found in the microstructure of the alloys.The Ni element has certain influence on the morphology,size and q uantity of the gamma’phase in the alloy.Through the study of thermodynamic calculation shows that the increase of Ni element can promote the formation ofγ′phase and inhibit the formation ofη-Co₃Al phase,which results in the decrease ofηphase until it disappears.With the increase of Ni element,the content ofγphase decreases and the precipitation temperature decreases.Ni element can not only increase the initial precipitation temperature ofγ′phase,but also promote its precipitation.With the increase of Ni element,W and Al elements precipitated from theγphase provide conditions for the formation ofμ-Co₇W₆ andη-Co₃Al,respectively.However,after the disappearance ofη-Co₃Al phase,Al and Ni elements provide favorable conditions for the formation and growth of theγ′phase.