Effects of trace elements on mechanical properties of superalloys

A review is given of the range of properties of superalloys that can be affected by trace elements and their dependence on various solutes showing the different response of a given alloy to different solutes, the different sensitivities of different alloys to the same solute, and the importance of both microstructural form and mechanical-test conditions. The main emphasis is on understanding and modelling the mechanisms by which trace elements can influence mechanical properties significantly. Trace elements at ppm concentrations are only likely to be effective with respect to mechanical behaviour if there is a mechanism leading to local enrichment. The various means of achieving this are considered including different types of segregation and the formation of precipitate particles. The factors leading to segregation of solute to, and the embrittlement of, grain boundaries are considered. Guidelines evolved to identify embrittling species in ferrous alloys are modified to predict those most likely to segregate to and embrittle grain boundaries in superalloys. The roles of segregants in modifying mechanical properties are outlined. Three types of change are envisaged, namely (a) energetic (lowering of surface energy), (b) kinetic (changing diffusivities), and (c) mechanical (weakening the interface). The available experimental evidence is examined in the light of these effects to identify the dominant factors. Solutes which react with constituents of the alloy to form precipitate particles can affect the mechanical behaviour in less direct ways. Thus the pick-up of nitrogen during processing of superalloys leads to the formation of TiC particles which appear to nucleate carbide precipitation and affect the incidence of porosity. Gaseous trace elements can also be introduced by diffusional processes through the solid superalloy. Evidence of reductions in ductility due to oxygen ingress during heat treatment, high-temperature testing, or service are presented. Current explanations in terms of the formation of CO or CO₂ bubbles at the grain boundaries due to oxygen reaction with carbides are outlined.