镍基单晶高温合金小角度晶界的形成机制、影响因素与控制措施

Formation Mechanism,Influencing Factors and Control Measures of Low Angle Boundaries in Ni-based Single Crystal Superalloys

镍基单晶高温合金被广泛用于制备先进航空发动机及工业燃气轮机的关键热端部件,随着铸件结构的复杂化和大型化以及合金中难熔元素的增多,凝固缺陷的形成倾向增大。其中,小角度晶界是定向凝固制备单晶高温合金铸件过程中经常出现的一类缺陷,它会破坏单晶的完整性,一旦超过容限就会对铸件的力学性能造成恶劣影响,随着单晶高温合金服役温度的不断提高,小角度晶界对性能的损害会更为严重。因此,小角度晶界日益成为镍基单晶高温合金发展和应用中需要解决的重要课题,受到国内外研究者的广泛关注。单晶中的小角度晶界与传统意义上的小角度晶界有所不同,是指相邻枝晶间的取向偏离。研究者们在不同晶界偏离角及不同温度条件下就小角度晶界对合金持久性能、蠕变性能及疲劳性能的影响进行了研究,结果发现:当偏离角较小时,小角度晶界对合金性能的影响并不明显,但是随着偏离角的增大及温度的升高,合金的性能均会降低。为了探寻有效的预防和控制措施,研究者们就小角度晶界的形成机制及影响因素进行了研究。关于小角度晶界的形成机制,被大家普遍认同的观点是:枝晶在分枝生长过程中发生了塑性变形,从而导致了枝晶的取向偏离,当枝晶再次汇聚时就会产生小角度晶界。但是,关于枝晶变形的原因则没有一致看法。另外,关于小角度晶界影响因素的研究还不是很系统,主要集中在合金成分及晶界强化元素、凝固参数及取向、铸件尺寸等方面。镍基单晶高温合金中的小角度晶界归根结底是由枝晶的取向偏离导致的,而取向偏离的影响因素复杂,因此小角度晶界的出现很难完全避免。目前,主要通过取向控制以减少小角度晶界的产生,并通过晶界强化以提高合金对小角度晶界的容限。本文阐明了单晶高温合金中的小角度晶界的概念,总结了小角度晶界对合金力学性能的影响,综述了小角度晶界形成机制的研究进展,分析了合金元素、微量元素、凝固条件和铸件结构等因素对小角度晶界形成的影响,在此基础上,提出了减少小角度晶界的措施和强化晶界的途径,最后就未来的研究方向进行了展望。

Ni-based single crystal superalloys are widely used to prepare the key hot end components of advanced aero-engines and industrial gas turbines.With the more complicated structure and larger size of the castings,and the increase of refractory elements,the tendency of solidification defects will increase.The low angle boundaries(LABs)are common defects in the casting process of single crystal superalloys by directional solidification.They can destroy the integrity of the single crystal and decrease the mechanical properties of castings once the deviation angle exceeds the tolerance.With the improvement of the service temperature of the single crystal superalloy,the damage of the LABs to the mechanical properties will be more serious.Therefore,the LABs have become an important issue to be solved in the development and application of Ni-based single crystal superalloys,and have been widely concerned by researchers at home and abroad.The LABs in the single crystal are orientation deviations between the adjacent dendrites,different from that of the traditional.The effect of the LABs on rupture lives,creep properties and fatigue properties under different grain boundary misorientations and temperatures have been studied.The results showed that,the effects of LABs on the mechanical properties were not obvious when misorientations were small.However,with the increase of misorientations and the temperature,all the properties deteriorated rapidly.In order to explore the effective prevention and control measures,the formation mechanism and the influencing factors of LABs have been studied.The widely accepted formation mechanism is as follows:the plastic deformations of dendrites occur during the branching growth,resulting in their orientation deviations,when the dendrites converge again,the LABs form.However,there is no consensus on the causes of dendrite deformation.In addition,the research on the influence factors of LABs is not very systematic.It mainly focused on alloy composition and grain boundary strengthening elements,solidification parameters and orientations,and cas-tings size.The LABs in Ni-based single crystal superalloys are caused by the orientation deviation of dendrites,whereas their influencing factors are complex,thus the LABs are almost inevitable.At present,it is mainly through the orientation control to decrease the formation of LABs,and by the grain boundary strengthening to increase the tolerance of the alloys to the LABs.In this paper,the concept of LABs in single crystal superalloys are clarified,the effects of LABs on the mechanical properties are summarized,the formation mechanisms of LABs are reviewed and the influences of different factors are analyzed,such as alloy element,microelement,solidification conditions and casting structure,etc.On this basis,some possible measures to reduce the LABs and the way to strengthen the grain boundaries are proposed,and the future research aspects are stated.