Microstructure evolution in polycrystalline IN738LC in the range 1120 to 1250 degrees C

The evolution of different microstructural features in the superalloy IN738 LC, held in the temperature range 1120 to 1250 degrees C and water quenched , is investigated in this study. The gamma' precipitate formed in IN738LC g rows continuously in cuboidal shape at temperatures up to 1130 degrees C, b ut a distinct duplex-size (very fine + coarse) precipitate morphology devel ops when the alloy is water quenched from the range 1140 to 1150 degrees C. Quenching from 1160 to 1225 degrees C produces a single fine-size precipit ate microstructure, while analogous quenching from 1235 degrees C and 1250 degrees C produces no precipitate at all. Very fine precipitates (similar t o 40 nm size) are formed after a short time dwell in the duplex-size range and they grow to the size of the normal fine precipitates (similar to 70 nm ) obtained upon quenching from the range 1160 to 1225 degrees C. The fine o nes do not grow beyond about 70 nm size in the duplex-size microstructure o r in the single fine-size precipitate microstructure, even after very long time holding at the respective temperature range. Two different heat treatm ents were used to study the mechanism of precipitation leading to the duple x-size and the single fine-size precipitates. In Treatment I, fine gamma' p recipitates (similar to 70 nm in size) obtained after 1200 degrees C/4 h/WQ (water quenching to room temperature) solution treatment were subsequently allowed to coarsen in the temperature range 1140 to 1160 degrees C. In the second treatment, coarsest gamma' precipitates (average size of similar to 700 nm) were first obtained by aging the above solution-treated alloy samp les at 1120 degrees C/24 h/FC (furnace cooling to room temperature) and the ir dissolution and subsequent fine precipitate formation were later studied in the range 1140 to 1250 degrees C. In the first treatment, duplex-size p recipitate distribution after quenching from the range 1140 to 1150 degrees C was first observed with the SEM after about 30 min of soaking time, afte r the initial coarsening of the fine precipitates and partial dissolution o f some of them. Thereafter, at 1140 degrees C, the coarse precipitates of the duplex micros tructure were found to have kept on coarsening in the cuboidal mode up to a bout 4 hours of aging, while the very fine ones coarsened independently up to the fine size level. On the contrary, the coarse precipitates of the dup lex-size microstructure were found to dissolve more easily at 1150 degrees C after reaching a certain maximum size. Dissolution starting from the corn er was observed and the cuboidal morphology changed to near spheroidal thro ugh such dissolution. The latter morphology also was realized through parti cle agglomeration in this temperature range. Coarse precipitates obtained f irst at 1120 degrees C started to dissolve faster and fine precipitates cou ld be obtained within five minutes of dwell at 1140 degrees C. The fine pre cipitates are presumably of the "cooling" type and apparently gain shape du ring the quenching process. It is proposed, however, that their embryos, ri ch in solute content, should have been formed at the solution treating temp erature in the range 1140 to 1225 degrees C, through possibly a spinodal-ty pe transformation, since the fine precipitates cannot grow to the size of s imilar to 70 nm during vary rapid cooling under which they have been obtain ed. Activation energy for the coarsening of precipitates (in Treatment I) a nd for size reduction of the coarse precipitates by dissolution (in Treatme nt II) were calculated using the average sizes of coarse precipitates after agings for various time periods in this temperature range and quenching to room temperature. The values obtained indicate very high activation energy (685 +/- 15 kJ/mol) requirement for the coarsening of precipitates in the duplex-size precipitate formation zone (1140 to 1150 degrees C), but a very low activation energy (85 +/- 20 kJ/mol) is found to be sufficient for the coarse precipitate dissolution in this range of temperature.