The effects of deformation and pre-heat-treatment on abnormal grain growth in rene 88 superalloy

When an extruded strain-free RENE 88 Ni-base superalloy about 1 to 2 mu in grain diameter is heat-treated at 1150 deg C, abnormal grain growth (AGG) begins after 50 hours. When heat-treated at 1200 deg C, AGG occurs at 15 minutes. Some of the gain boundaries are faceted with hill-and-valley structures when observed in transmission electron microscopy (TEM), and the occurrence of AGG is consistent with the boundary step and dislocation mechanism for the migration of singular boundaries with faceted shapes, as observed and proposed in other pure metals and alloys. The dissolution of abundant gamma precipitates (with a solvus temperature of 1107 deg C), which are coherent with the matrix and hence strongly pin the grain boundaries, does not cause AGG during early stages of heat-treatment at 1150 deg C. Small deformations drastically alter the AGG behavior. When deformed to 4 pct, AGG begins after heat-treating for 10 minutes at 1150 deg C, compared to the apparent incubation time of 50 hours for an undeformed specimen, and very large abnormal grains are produced. With increasing deformation to 6 and 9.2 pct, the abnormal grain size decreases. These results are qualitatively similar to those observed in Cu. This deformation effect on AGG is attributed to the absorption of lattice dislocations in the grain boundaries, which produces nonequilibrium structures that, in turn, can apparently cause rapid boundary migration. When heat-treated at 1200 deg C, the largest abnormal gains are found in the specimens deformed to 2 pct. When the initial grain size is increased to about 14 mu m by heat-treating the extruded alloy at 1150 deg C for 30 minutes, similar low deformation effects on AGG are observed. The deformation effect on AGG observed in this alloy is qualitatively similar to that previously observed in Cu and appears to be consistent with the boundary step and dislocation mechanism for AGG.