Development of microstructures of high grain aspect ratio during zone annealing of oxide dispersion strengthened superalloys

Abstract

Two extruded bars of the nickel base mechanically alloyed materials MA 6000 and MA 760 have been zone recrystallised in a calibrated gradient furnace. Selected area channelling patterns in the scanning electron microscope have been employed to study the crystallographic texture of the grains of large aspect ratio produced by zone annealing, and microbeam electron diffraction has enabled the orientations of the submicrometre sized equiaxed grains in the material behind the (secondary) recrystallisation front to be studied. In both alloys a curved secondary recrystallisation interface is observed, with the surface recrystallising at a lower temperature than the interior. This is considered to result indirectly from the strain gradients occurring during extrusion. A <110> texture is present, and reasons for this are discussed. In MA 6000 progressive grain rotation towards <110> has been measured behind the recrystallisation interface, although this is not observed in MA 760 as it transforms at a lower temperature. Quenching experiments have shown that nucleation of secondary recrystallisation occurs at temperatures higher than that at which the recrystallisation interface grows at the zoning speed employed. It is suggested that the microstructure develops via the thermally activated unpinning of interfaces which have mobility advantages.MST/1948