Serrated grain boundary formation potential of Ni-based superalloys and its implications
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I.
INTRODUCTION
~
THE role
of cellular precipitation in the development of very finely serrated grain boundaries is well established in the literature.l'2'3 In austenitic stainless steels the precipitation of a certain morphology of the grain boundary M23C6 carbides is said to create a serrated grain boundary structure (Figure 1). 4 Serrations can also be generated in Ni-based superalloys, where controlled cooling treatments are employed to control the amplitude and frequency of the serrations 5,6,7 The size and shape of the grain boundary serrations in Ni-based superalloys are, however, considerably different from those observed in cellular precipitation controlled alloy systems (Figure 2). Serrations in Ni-based superalloys contain reasonably well-rounded peaks and valleys (Figure 2(a)), whereas serrations arising from cellular precipitation tend to be fine and angular in nature (Figure 2(b)). The mechanism of the formation of serrated grain boundaries in Ni-based superalloys is associated with the heterogeneous nucleation of y ' at the grain boundaries. Earlier workers 5'6'7 proposed that the serrated grain boundaries were formed due to the migration of grain boundary segments in between primary 3" particles formed upon cooling and that these migrating segments were eventually pinned by the homogeneously precipitated y ' particles at lower temperatures (Figure 3). However, it has been argued that once a material has reached a stable grain size at a given solution treatment temperature, cooling from a temperature above the y' solvus to a temperature below the 3" solvus is not likely to cause sufficient migration of the boundary segments in between the primary 3" particles, s More recently, a mechanism was proposed for the formation of serrations in Ni-based superalloys which was based on grain boundary primary y' movement causing displacement of the local grain boundary segment (Figure 4).9 In accordance with this model, the strain energy differential between the matrix side and the boundary side of the y ' particle-matrix interA. K. KOUL is Research Officer with Structures and Materials Laboratory, National Aeronautical Establishment, National Research Council of Canada, Bldg. M13, Montreal Road, Ottawa, ON K1A 0R6, Canada. R. THAMBURAJ is Research Associate, Department of Mechanical and Aeronautical Engineering, Carleton University, Ottawa, ON K1S 5B6, Canada. Manuscript submitted April 13, 1984.
METALLURGICALTRANSACTIONS A
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/ . . . I C A R B II)F
b
/GRAIN
BOUNDARY
Fig. 1 - - A model for serrated grain boundary formation based on M23C6 precipitation. Straight grain boundaries (a-a) formed on solution treatment and serrated grain boundaries (b-b) formed after M23C~ precipitation (Ref. 4).
face provides a driving force for the movement of the primary y' particles in the direction of the boundary until this force is balanced by the line tension of the boundary. Perhaps, a more important conclusion reached in these studies was that a 3,' solvus temperature that is higher than the solvus temperature o
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