Influence of various heat treatments on the microstructure of polycrystalline IN738LC

IN738LC is a modem, nickel-based superalloy utilized at high temperatures in aggressive environments. Durability of this superalloy is dependent on the strengthening of gamma' precipitates. This study focuses on the microstructural development of IN738LC during various heat treatments. The 1120 degrees C/2 h/accelerated air-cooled (AAC) solution treatment, given in the literature, already produces a bimodal precipitate microstructure, which is, thus, not an adequate solutionizing procedure to yield a single-phase solid solution in the alloy at the outset. However, the 1235 degrees C/4 h/water quenched (WQ) solution treatment does produce the single-phase condition. A microstructure with fine precipitates develops if solutionizing is carried out under 1200 degrees C/4 h/AAC conditions. Agings at lower temperatures after 1200 degrees C4 h/AAC or 1250 degrees C/4 h/AAC or WQ conditions yield analogous microstructures. Agings below similar to 950 degrees C for 24 hours yield nearly spheroidal precipitates, and single aging for 24 hours at 1050 degrees C or 1120 degrees C produces cuboidal precipitates. Two different gamma' precipitate growth processes are observed: merging of smaller precipitates to produce larger ones (in duplex precipitate-size microstructures) and growth through solute absorption from the matrix. Average activation energies for the precipitate growth processes are 191 and 350 kJ/mol in the ranges of 850 degrees C to 1050 degrees C and 1050 degrees C to 1120 degrees C, respectively, calculated using the precipitate sizes from microstructures in the WQ condition, and 150 and 298 kJ/mol in the analogous temperature ranges, calculated from precipitate sizes in the microstructures in the slow furnace-cooled condition.