高温氧化对粉末高温合金微观结构和疲劳性能影响研究

Effects of high-temperature oxidation on powder-metallurgy superalloys microstructures and fatigue performance

关键词:  涡轮盘  镍基粉末高温合金  低周疲劳寿命  高温氧化  强化相

Key words:  Turbine disk  Nickel-based PM superalloy  Low-cycle fatigue life  High-temperature oxidation  Precipitate phase

摘要:

为了探究涡轮盘用粉末高温合金表面氧化对其低周疲劳性能的影响，分别针对第三代镍基粉末高温合金的粗晶（Coarse grain，CG）和细晶（Fine grain，FG）材料开展氧化时间对其疲劳性能影响机理的研究。通过在700 ℃空气环境下开展不同时长的高温预氧化实验和低周疲劳（Low-cycle fatigue，LCF）实验，使用SEM和EDS表征LCF断口、表面氧化层结构成分及其强化相形貌变化，揭示LCF裂纹萌生机理。实验结果表明，氧化层厚度随氧化时间而增加，氧元素以氧化侵入的形式进入基体；相同氧化时间下，CG抗氧化性能优于FG；疲劳裂纹萌生于氧化侵入和亚表面夹杂物等应力集中部位，在实验温度下LCF寿命受氧化作用和夹杂物共同影响；高温氧化作用下氧化层呈现分层结构，外层为NiO，中间层为含有Cr₂O₃，TiO₂等复杂氧化物及尖晶石相（NiCr₂O₄）的混合层，内层为Al₂O₃；CG二次γ'相氧化后平均尺寸增加，FG二次γ'相平均尺寸没有明显的变化，但两种组织氧化后在晶界附近均观察到长条状的二次γ'相，表明高温氧化作用下晶界优先遭到破坏。

Abstract:

In order to explore the effects of surface oxidation on the low cycle fatigue （LCF） performance for powder-metallurgy （PM） superalloys of turbine discs， the effects of oxidation time on the fatigue properties were studied for both coarse grain （CG） and fine grain （FG） variants of a third-generation nickel-based PM superalloy. The high-temperature pre-oxidation and LCF experiments were carried out at 700 ℃ in air environment. By using SEM and EDS methods， the LCF sample fractures， the structural composition of surface oxide layer and the precipitates phase morphology were characterized to reveal the mechanism of LCF crack initiation. The experimental results show that the thickness of the oxide layer increases with oxidation time， and oxidative intrusion is found in the matrix. The oxidation resistance of CG is better than that of FG in the same oxidation time. The fatigue cracks mainly initiate from the stress concentration area， such as oxidation intrusion and subsurface inclusions， and the LCF lives are affected by the combination of oxidation and inclusions at experimental temperature. The oxide layers show a hierarchical structure after the high-temperature oxidation， as the external layer contains NiO， the middle layer is a mixture of Cr₂O₃， TiO₂ and spinel phase NiCr₂O₄， and the internal oxide layer comprises of Al₂O₃. The average size of secondary γ' phase in CG increased after oxidation， while no significant change of secondary γ' was found in FG， but long stripes of secondary γ' were observed near the grain boundaries in both CG and FG after oxidation. It shows that grain boundaries were preferentially destroyed under high-temperature oxidation.