High Temperature Oxidation Behavior Of Ni-10Cr-5Al Superalloy Containing Rare Earth Elements

Ni-Cr-Al based alloy is one of the most important nickel-based superalloys,with an excellent mechanical properties and good oxidation resistance at high temperatures,and thus widely used for aero-engines.It is commonly accepted that the addition of rare earth elements can improve the oxidation resistance of the superalloy.However,the over-doping of rare earth?RE?elements directly results in severe internal oxidation and accelerated oxide scale growth,and insuficient doping weakens their the anti-oxidation effect.Therefore in the present work the oxidation behavior and mechanism of alloys Ni-10Cr-5Al?wt.%?containing different RE elements under high temperatures were studied.Seven groups of Ni-10Cr-5Al?wt%?alloys with different compositions were prepared by adding 0.1wt% one or two types of RE elements?Y,La and Ce?.The isothermal oxidation kinetics?at 900 ?,1000 ?,1100 ??and the cyclic oxidation kinetics?at 1100 ?,1200 ??of the alloys were studied by thermogravimetric method.The oxidation resistance of the alloys was evaluated by the growth rate and adhesion of the oxide films.The influence of RE elements on the oxidation behavior of the alloys was investigated by analyzing the phase,microstructure,and elements distribution of the oxides on the alloy surface.The high temperature oxidation mechanism of Ni-10Cr-5Al alloy and the role of rare earth played were proposed.The results show that the oxidized law of the alloy is changed from the law of straight line?transient oxidation stage?to parabolic?steady-state oxidation stage?during the high temperature oxidation of 900 ?,1000 ? and 1100 ?.The addition of RE elements does not only shorten the transient oxidation stage and reduce the oxidation rate in the steady-state oxidation stage,but also reduce the exfoliation of the surface oxide layer,thus enhancing the high temperature oxidation resistance of the alloys effectively.Considering the cyclic oxidation process of the alloys,the addition of RE elements increases the adhesion of the oxide layers due to the formation of thinner oxide layers and wedge-shaped internal oxides at 1100 °C.But the exfoliation of the oxide layers aggravates due to severe internal oxidation along the grain boundary at 1200 °C.In addition,the surface scale of alloy without RE grows by "advances gradually" at 1000 ?,but the surface oxides of alloy with RE "preferentially" form at the grain boundary,and then extend from the grain boundary to the whole surface of the alloy gradually.The addition of RE elements,on the one hand,promotes the selective oxidation of Al,which promotes the nucleation of ?-Al₂O₃ at the initial stage of oxidation and the formation of continuous inner Al₂O₃ layer at the later stage of oxidation.This does not only slow down the oxidation rate of the alloy,but also reduce the thickness of the surface oxide layer.On the other hand,an appropriate amount of internal oxide will have a "pinning effect" on the surface oxide film,which enhances the adhesion of the oxide layer.Moreover,Y is favorable for the transition of ?-Al₂O₃ to ?-Al₂O₃ and the formation of spinel oxides.La also has a positive effect on the growth of NiO,while CeO2 forms on surface scale of alloy after Ce addition,which promotes the nucleation and growth of the surface oxides and makes the surface scale thicker and retain more unprotective ?-Al₂O₃.