Phase Equilibria Of The Co-Al-V Ternary System

Co-based alloys exhibit a better resistance to hot corrosion,but because of the lower high-temperature strength compared with Ni-based superalloys,they have not found widespread usage.However,recent studies have determined that cobalt-based superalloys reinforced by γ’ Co3(Al,W)phases have higher high-temperature strength than conventional Ni-based superalloys.Co-based alloys are thus regarded as promising candidates for applications at high temperatures.In adding elements to Co-based alloys,V and Al play an important role in Co-based high-temperature alloys.The main work of this paper is to study on phase relations of Co-Al-V system,to provide theoretical basis for the research and development of new Co-based superalloy.The isothermal section of Co-Al-V ternary system at 800℃ and 1100℃ have been determined by means of X-ray diffraction(XRD)and scanning electron microscopy(SEM)coupled with energy dispersive X-ray spectroscopy(EDS).A new ternary compound named ’T’ phase(Al2CoV)is found in this study which possesses a face-centered cubic(fcc)structure.In 800℃ isothermal section,thirteen three-phase regions have been confirmed experimentally.i.e.,Al9Co2+Al3V+L-Al,Al9Co2+Al3V+Al13Co4,Al13Co4+Al3V+Al3Co,Al3Co+Al3V+Al5Co2,T+Al3V+Al8V5,T+Al3V+Al5Co2,T+AlCo+Al5Co2,T+α-V+Al8V,T+AlCo+α-V,CoV3+α-V+σ-CoV,AlCo+α-V+σ-CoV,Co3V+AlCo+σ-CoV,Co3V+α-Co+AlCo.The T phase can be in equilibrium with Al3 V,Al8V5,α-V,Al5Co2,and Al Co,comprising 47.9-53.7 at.% Al and 21.2-27.6 at.% V.In 1100℃ isothermal section,three-phase regions have been confirmed experimentally.i.e.,L-Al+Al3V+Al3Co,Al3Co+Al3V+Al5Co2,Al3V+Al8V5+Al5Co2,T+Al8V5+Al5Co2,T+AlCo+Al5Co2,T+α-V+Al8V5,T+AlCo+α-V,the T phase can be in equilibrium with Al8V5,α-V,Al5Co2,and Al Co,comprising 50.5-54.3at.% Al and 20.9-25.2 at.% V.In the 800 ℃ and 1100℃ isothermal section,the maximum solubility of V in AlCo phase is 35.0 at.% and 54.0 at.%.