Effect Of Surface Treatment Condition On Oxidation Behavior Of DD6 Ni-based Single Crystal Superalloy

Nickel-based single crystal superalloys have been widely used in aero-engines,gas turbines and nuclear power fields because of their good high-temperature mechanical properties,oxidation resistance and organizational stability.In the high temperature service environment,the oxidation behavior of the alloy tends to cause the decrease of surface strength and easily become the sites of fatigue crack initiation.Therefore,it is of great significance to study the high temperature oxidation behavior and understand their oxidation mechanism for improving the overall performance of the superalloys.In this paper,the second-generation nickel-based single crystal superalloy DD6 which independently developed by our country was selected.The effects of surface treatment condition and temperature on the high-temperature oxidation behavior of the alloy were investigated.Scanning electron microscopy,X-ray diffractometry and electron probing analysis were used to analyze results of experiment,and the mechanism of oxidation was discussed.The results showed that surface states had significant effects on the oxidation behavior of the superalloys.With the increase of surface roughness,the rate of mass gain gradually decreased,and the oxidation resistance gradually increased.The samples ground to 1000 grit and polished both showed the turning phenomenons,and the turning points were 10h.However,the samples ground to 180 grit and 400 grit didn't show the turning phenomenon.The oxide films of samples ground to 180 grit and 400 grit were single Al₂O₃ layer,but the oxide films of the samples ground to 1000 grit and polished both presented a three-layer structure,which contained the outer NiO layer,the inner Al₂O₃ layer and intermediate spinel phases layers that included CrTaO₄ and CoCr₂O₄.The rough surfaces promoted the transformation of Al₂O₃ from internal oxidation to external oxidation:the external oxides on the samples ground to 180 grit and 400 grit were mainly Al₂O₃,and the outer oxides of the samples ground to 1000 grit and polished were NiO.With the increase of the roughness,the cracking of the alloy oxide films was more serious;Recrystallization occurred in the sample ground to 180 grit,and did not appear in other surface states.The samples ground to 1000 grit exhibited good oxidation resistance at 850,900°C and950°C,and no oxide film peeled.As the temperature increased,the oxidation rate of the alloy gradually increased.At all temperatures,the alloy mass gain curves showed turning phenomenons.As the temperature increased,the turning points time advanced and which were 100h,45h and 10h respectively.At three temperatures,the outer layers of the oxide films were all continuous NiO,and the oxide films all presented the multi-layer structure which contained the outer NiO layer,the inner Al₂O₃ layer and intermediate spinel phases layers.