先进镍基单晶高温合金蠕变行为的研究进展

Research Progress of Creep Behaviors in Advanced Ni-based Single Crystal Superalloys

先进镍基单晶高温合金具有优良的成分兼容性,在1 000℃以及更高温度下仍能保持较高的组织稳定性、抗蠕变性、抗疲劳性、抗氧化性和抗腐蚀性能,被广泛应用于现代航空发动机和地面燃气轮机的涡轮叶片等关键热端部件。在服役过程中,镍基单晶高温合金主要发生涡轮叶片旋转造成的蠕变及疲劳变形。另外,现代航空发动机对涡轮进口温度的要求不断提升,使得镍基单晶高温合金的承温承载能力面临着更大的挑战。长期以来,材料科研工作者尝试了许多方法来提升镍基单晶高温合金的蠕变性能:在镍基单晶高温合金中添加了大量的难熔元素(W、Cr、Mo、Re等),降低了元素的扩散速率,从而提高了合金的固溶强化水平;添加了γ'相形成元素(Al、Ti、Ta),形成金属间化合物γ'沉淀相,利用γ'沉淀相与γ基体相之间的相干应变、有序化,以及弹性模量和堆垛层错能差异等沉淀强化机制,提高合金的强度;通过调整热处理制度,进一步优化沉淀相的尺寸、形态以及体积分数,最大化沉淀强化效果;通过调整Mo与Re的含量,提高γ'沉淀相与γ基体相的错配度,细化γ/γ'界面位错网间距,强化γ/γ'相界面强度,提高镍基单晶高温合金的蠕变抗力;同时加入适量的Pt族金属元素,抑制了TCP有害相的析出,进一步稳定了合金组织。然而,镍基单晶高温合金中元素的合金化程度已很高,在CMSX-10中难熔元素的含量高达20.5%,这已经接近镍基体的溶解度极限;同时,也带来了其他一系列问题:组织不稳定性(包括凝固缺陷析出倾向的增加、TCP相的析出)以及合金密度和成本的增加。另外,对于第四代及其后续的镍基单晶高温合金的设计,除依赖提高难熔元素含量和加入铂族元素稳定组织外,并无其他公开、有效的措施。现行措施也与现代工业追求低密度、低成本、环境友好的理念背道而驰。因此,深入认识镍基高温合金成分-组织-结构-性能之间的内在联系十分重要,亟待突破现有的合金设计理论。本文试图从最重要的长时力学性能之一的蠕变性能出发,分别对镍基单晶高温合金成分、组织结构、蠕变行为特点等方面进行了阐述,重点探讨了固溶元素、γ'体积、尺寸、形态、γ/γ'界面、堆垛层错能(SFE)、反相畴界能(APB)等因素对蠕变行为、蠕变机制的影响规律,分析了镍基单晶高温合金蠕变行为研究面临的问题,并展望其研究前景,以期能够深入理解单晶高温合金的强韧化机理,为新一代镍基单晶高温合金的设计提供一些思路。

Advanced Ni-based single crystal superalloys have long been the candidate materials for applications in the critical components of modern aeroengines and in-land gas turbine engines due to their superior composition compatibility,microstructural stability,and creep,fatigue,oxidation,corrosion resistances at temperatures up to 1 000℃and beyond.During services,Ni-based single crystal superalloys are mainly subjected to creep and fatigue deformation caused by the centrifugal forces generated from turbine blade rotation.In addition,the increasing need for the higher turbine entry temperature in modern aeroengines also poses a greater challenge to the temperature and load tolerance of advanced Ni-based single crystal superalloys.In the past decades,considerable endeavors have been made aiming at promoting the creep resistance of Ni-based single crystal superalloys.The addition of refractory elements such as W,Cr,Mo,Re,etc.has led to lower diffusion rates and consequently elevated solid solution strengthening levels in superalloys.A significant number ofγ′-forming elements added to Ni-based single crystal superalloys,e.g.Al,Ti and Ta,have been proved able to impart strengthening via the ordered compound,i.e.,γ′-Ni 3(Al,Ti,Ta)phase,which is known as the precipitation hardening.The coherency strains,existence of order inγ′phase,and differences in elastic moduli and stacking fault energy(SFE)betweenγ′phase andγmatrix,etc.contribute to the improvement of creep resistance of advanced Ni-based single crystal superalloys.By applying a series of adjusted heat treatments,including ramp solution heat treatment,fast solution heat treatment and melting solution heat treatment,etc.,the sizes,morphologies and volume fractions ofγ′precipitates have been further optimized,which achieves full potential of the precipitation hardening.Moreover,following from the consideration that the creep resistance of a superalloy will be enhanced due to the increasedγ/γ′interface strength,moderate Mo and Re elements have been added,resulting in appropriately increasedγ/γ′lattice misfit,improvedγ/γ′interfacial dislocation network density and interfacial strength,all of which benefit the creep resistance.Researchers have also successfully reduced the susceptibility to precipitation of topologically close-packed(TCP)phases by adding Pt-group elements,and in consequence,have made further strides in stabilizing the superalloys’microstructure.However,Ni-based single crystal superalloys already have quite high alloying degree.The refractory elements content in CMSX-10 has approached 20.5 wt%,which has almost touched the ceiling solubility in Ni-matrix.Meanwhile,there have been observed a series of negative consequences of alloying,e.g.microstructural instability(the increasing precipitation tendency of TCP phases and solidification defects),and the rise in alloy density and cost.On the other hand,despite of relying on increasing the content of refractory elements and platinum group elements’addition to stabilize microstructures,there lacks other known and effective ways available for designing next-generation Ni-based single crystal superalloys.And current methods seem more bent on running counter to the prevailing ideal of modern industry that advocates low density,low cost and enviro nment-friendliness.This sobering development situation has highlighted the importance of a deep understanding of the intrinsic relationship of composition-microstructure-performance,and the urgency of a breakthrough in the traditional alloy design theory.From the perspective of one of the most important mechanical properties,i.e.creep resistance,this review provides elaborate descriptions about the composition,microstructure and creep behavior,etc.,in advanced Ni-based single crystal superalloys.We also discussed carefully the action principles of solid solution elements,and sizes,morphologies and volume ofγ′precipitates,as well asγ/γ′interface,stacking fault energy(SFE)and antiphase boundary(APB)energy,etc.,on the creep behavior and mechanism.The current existing challenge and future research aspects on the study of creep in new-generation Ni-based single crystal superalloys were stated and prospected.