IN738LC is a modern, nickel-based superalloy utilized at high temperatures in aggressive environments. Durability of this superalloy is dependent on the strengthening of ␥' precipitates. This study focuses on the microstructural development of IN738LC during various heat treatments. The 1120 ЊC/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 ЊC/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 ЊC/4 h/AAC conditions. Agings at lower temperatures after 1200 ЊC/4 h/AAC or 1250 ЊC/4 h/AAC or WQ conditions yield analogous microstructures. Agings below ϳ950 ЊC for 24 hours yield nearly spheroidal precipitates, and single aging for 24 hours at 1050 ЊC or 1120 ЊC produces cuboidal precipitates. Two different ␥' 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 ЊC to 1050 ЊC and 1050 ЊC to 1120 ЊC, 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.
I. INTRODUCTIONSUPERALLOYS are widely used in a variety of applications, at temperatures ranging from 650 ЊC to 1050 ЊC, in aggressive atmospheres such as those experienced in rocket and jet engines and in land-based gas turbines, high-temperature catalytic reactors, etc. In order to function satisfactorily in such severe environments, the alloys are required to possess at high temperatures good corrosion resistance, optimal thermal properties, strength coupled with ductility, creep and fatigue resistance, and optimal impact and wear resistance. The unique set of properties required in these alloys can be obtained by having an fcc Ni-based solid solution matrix which is hardened by solutes and precipitates. The precipitates in nickel-based superalloys are primarily the Ni 3 (Al,Ti,Nb)-type intermetallic compounds and carbides, with suitable structure, shape, size, and distribution, which give the desired combination of properties and resistance to microstructural changes at high temperatures. [1] The microstructural aspects affecting the high-temperature properties of the superalloys are very important, and there is no complete investigation on microstructure development in these alloys. In order to have a better understanding of the behavior of the superalloy IN738LC, especially at high temperatures, the investigators are studying the microstructural dependency of its high-temperature ...
