Cyclic Oxidation Behavior Of Two Novel Co-Base Superalloys

Co-base superalloys with the same γ’-strengthening method to Ni-base superalloys have received wide attention.The previous studies mainly focuses on the mechanical properties of Co-base superalloys.But the excellent oxidation resistance is also indispensable for the safe service.Therefore,in this paper,the cyclic oxidation behavior of OC and 5C superalloys was studied at 900℃ and 1000℃.And the efects of preoxidation on oxidation behavior of the two superalloys were analyzed at 1000℃.Cyclic oxidation behavior of 0C and 5C superalloys were studied at 900℃ and 1000℃.The results show that the oxidation resistance of 0C and 5C superalloys is complete anti-oxidation grades at 900℃ and antioxidant at 1000℃.Selective oxidation of the superalloys occurs,as Al2O3,Cr2O3 and a small amount of TiO2 formes on the surface of 0C and 5C superalloys in the initial stage of oxidation.As time goes by,the oxidation layer can be divided into two or three layers:(Co,Ni)Cr2O4+oxides riched in Ta and W,Al2O3 at 900℃ and(Co,Ni)O,(Co,Ni)Cr2O4+oxides riched in Ta and W,Al2O3 at 1000℃ from the surface to the inside.Carbides in 5C superalloy are also oxidized quickly.And cracks can be found.Oxygen diffuses to carbide/mtrix interface and leads to the formation of a continuous and dense Al2O3 layer.The effects of pre-oxidation(at 900℃ and 950℃ in air for 72h)on the oxidation behavior of 0C and 5C superalloys at 1000℃ were studied.The results show that preoxidation in air improves the oxidation resistance of the superalloys.During cyclic oxidation,oxidation products on the surface of the superalloys change from Cr2O3 and(Co,Ni)Cr2O4 to(Co,Ni)O.Pre-oxygen scales play an important role in hindering the diffusion of alloying elements and oxygen.The effects of low oxygen partial pressure(1000Pa)pre-oxidation(at 1000℃ for 72h)on the oxidation behavior of the superalloys at 1000℃ were studied.The results show that pre-oxidation with low oxygen partial pressure improves the oxidation resistance of 0C and 5C superalloys.After cyclic oxidation,the thickness of oxide layers are greater than that of pre-oxid samples at 950℃.Therefore,oxides formed after low oxygen partial pressure pre-oxidation are more likely to hinder the diffusion of oxygen.