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

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 castings 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.