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

Abstract

When an extruded strain-free RENE 88 Ni-base superalloy about 1 to 2 mum in grain diameter is heat-treated at 1150 degreesC, abnormal grain growth (AGG) begins after 50 hours. When heat-treated at 1200 degreesC, AGG occurs at 15 minutes. Some of the grain 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 degreesC, 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 degreesC. Small deformations drastically alter the AGG behavior. When deformed to 4 pct, AGG begins after heat-treating for 10 minutes at 1150 degreesC, 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 degreesC, the largest abnormal grains are found in the specimens deformed to 2 pct. When the initial grain size is increased to about 14 mum by heat-treating the extruded alloy at 1150 degreesC for 30 minutes, similar low deformation effects on AGG are observed, When these specimens are deformed to 10, 13, and 15.2 pct, primary recrystallization occurs during the heat-treatment at 1150 degreesC, and large abnormal grains are again produced because of the small recrystallized grain size. Therefore, there are two peaks in the grain size vs deformation curve after heat-treating at 1150 degreesC for 1 hour. A pre-heat-treatment of this alloy at 1050 degreesC below the solvus temperature of the gamma' phase greatly reduces the size of the abnormal grains obtained in the specimen deformed to 4 pct after the heat-treatment at 1150 degreesC, probably because some recovery of the dislocations takes place at grain boundaries during the pre-heat-treatment. 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.