| Ni-base powder metallurgy（PM）superalloys have been widely used in hot end components of aircraft engines and gas turbines due to their excellent comprehensive properties at high temperature.As an important component of the next generation PM superalloy,Nb affects microstructure and mechanical performance of superalloy significantly.However,the influence of Nb on the oxidation behavior of PM superalloy has not been studied and analyzed systematically.Therefore,in this study,the influence of Nb addition（0,0.5,1,2 wt.%）on the isothermal oxidation,cyclic oxidation and stress oxidation behavior of PM superalloys was systematically studied.The oxide scales and internal microstructures after oxidation were observed and analyzed by SEM,TEM,XRD,XPS and other characterization methods.The following conclusions can be drawn:（1）The addition of Nb reduced the isothermal oxidation weight gain at lower temperatures（800-850 oC）,but increased the isothermal oxidation weight gain at higher temperatures（900-1000 oC）.It has been proved by thermodynamic calculation that the addition of Nb inhibited the formation of Cr2O3 while promoted the formation of Ti O2.At lower oxidation temperature,Cr2O3 dominated the oxide scales.Hence,the addition of Nb reduced the oxidation weight gain.However,with the increase of oxidation temperature,the proportion of Ti O2 in the oxidation product gradually increased.The addition of Nb promoted the formation of Ti O2,resulting in higher oxidation weight gain.（2）The addition of Nb accelerated the formation of continuous and dense Ti O2 above the Cr2O3 layer at 1000 oC,reducing the oxygen partial pressure at the Ti O2/Cr2O3 interface,resulting in the reduction of the volatilization of Cr2O3.The addition of Nb promoted the formation of continuous Cr Ta O4 layer under Cr2O3 layer.Cr Ta O4 layer played an important role in improving the oxidation resistance of the alloy by inhibiting the diffusion of oxygen,nitrogen and metal elements.（3）The internal oxidation depth of the alloy after isothermal oxidation increased with the increase of Nb content,while the width of theγ′-depletion zone decreased with the increase of Nb content at 800-1000 oC.The thermodynamic calculation results showed that Nb promoted the partition of Al intoγ′phase,giving rise to more Al atoms“locked”inγ′phase,resulting in lower diffusion rate of Al in alloy matrix.On the other hand,Nb promoted the nucleation of Al2O3,resulting in Al2O3 loosely dispersing in alloy matrix.Hence,the impediment of Al2O3 to O diffusion was decreased by the addition of Nb.Due to the slower diffusion of Al and relatively faster diffusion of O in the alloy matrix by addition of Nb,the internal oxidation in higher Nb content alloys was deeper.（4）The oxide scale spallation area was decreased with the increase of Nb content during the cyclic oxidation.The cracks of oxide scales were initiated at grain boundaries at the earlier period of the oxidation.Then cracks propagated to the oxide scales within grains,resulting in oxide scale spallation.After the oxide scales cracks or spallation,the alloy matrix might be oxidized again,resulting in regeneration of the failed oxide scales.In the alloy with low Nb content,the regeneration of the oxide scales mainly depended on the formation of Al2O3.In the alloy with high Nb content,Nb decreased the diffusion coefficient of Al and promoted the doping of Ti in the Cr2O3 layer,resulting in a faster Cr3+diffusion in oxide scales.Hence,the regeneration of the oxide scales on the high Nb alloy mainly depended on the formation of Cr2O3.Since the oxide scale was mainly composed of Cr2O3,the regeneration of Cr2O3 on the high Nb content alloy induced smaller oxide scales spallation area.（5）The oxidation behavior of the low Nb content alloy was not affected by external tension at 900 oC.But the thickness of oxide scales and the depth of internal oxidation of the high Nb content alloy both decreased with the increase of external tension.This could be ascribed to the addition of Nb accelerating the growth of oxide scales at 900 oC,resulting in high internal stress in oxide scales.Therefore,the external tension had a greater impact on the oxidation behavior of the high Nb content alloy.The internal stress in the oxide scales on the high Nb content alloy decreased significantly with the external tension increasing,resulting in the reduction of buckling and cracking in the oxide scales.Reduction of defects such as cracks in the oxide layer resulted in slower diffusion of elements,thereby reducing the rate of oxidation. |
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