Changes of γ′ precipitate characteristics in damaged superalloy IN738LC during different stages of rejuvenation heat treatment cycles

The present study investigates the evolution of gamma' precipitate microstructure in a creep damaged IN738LC superalloy during different stages of rejuvenation heat treatment cycles. Dissolution of coarsened gamma' precipitates in the creep damaged alloy occurred during different solution treatments. The precipitates that did not dissolve completely during solution treatment and those formed during subsequent cooling were considered to be as remnant and cooling precipitates respectively. It was found that higher solution treatment temperatures result in lower volume fraction of remnant precipitates and further formation of cooling precipitates. Slower cooling rates after solution treatment gave the chance for the existing gamma' particles to be coarser. Moreover, aging treatment increased the volume fraction of gamma' precipitates. Double solution treatment at 1190 degrees C/4 h/furnace cooling+1120 degrees C/4 h/air cooling followed by aging at 845 degrees C/24 h/air cooling dissolved nearly all the coarsened precipitates and produced a new bimodal gamma' microstructure analogous with that of the original alloy.