Multimodal Precipitation in the Superalloy IN738LC
- PDF / 375,578 Bytes
- 8 Pages / 593.972 x 792 pts Page_size
- 98 Downloads / 233 Views
INTRODUCTION
IN738LC is a nickel-based, polycrystalline cast superalloy, which is used especially in land-based gas turbines. This superalloy has been the sole blade material for electricity producing gas turbines since 1971, and it is still in use with other recently developed alloys.[1] IN738LC was developed in 1969[2] with an intention to combine the mechanical strength expected of high-temperature alloys and especially a good hot corrosion characteristic required in combating the aggressive combustion products. Superior properties of this material are obtained through strengthening the c Ni-base matrix solid solution by the c¢ Ni3(Al,Ti) precipitates. MC and M23C6 type carbides also aid in preventing the grain boundary sliding. About 3 vol pct c-c¢ eutectic phase also exists in the structure along with other low melting point boronand zirconium-based compound phases in a lesser amount.[3,4] Since performance of this superalloy is strongly influenced by the size and the morphology of the precipitates, microstructural control is extremely important. To this extent, correct knowledge of the phases present in the IN738LC and their transformation behavior is crucial. Differential elemental segregation during solidification into the dendritic and interdendritic regions forms precipitates of different compositions in these regions. In dendritic cores, the precipitates are of aluminum-rich type, whereas those in the interdendritic regions contain more titanium. Dissolution (solvus) temperatures of these ERCAN BALIKCI, Assistant Professor, and DINC ERDENIZ, MSc Student, are with the Department of Mechanical Engineering, Bogazici University, Istanbul, Turkey. Contact e-mail: ercan.balikci@ boun.edu.tr Manuscript submitted September 17, 2009. Article published online April 16, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
two types are naturally different, 1393 K (1120 C) for the Al-rich precipitates and 1453 K (1180 C) for the Ti-rich ones.[5,6] A numerical simulation study reports stoichiometric c¢ solvus as 1409 K (1136 C).[7] These suggest that the standard, 2 hours at 1393 K (1120 C), solution treatment recommended in the literature[8] for this alloy is insufficient for full solutionizing. Precipitate particles grow obeying the Ostwald ripening, during which precipitate volume fraction increases as solute is rejected by the matrix to the precipitate particles. The precipitates then may grow further at a constant volume fraction, which is called coarsening. Coarsening phenomenon has been formulated by Lifshitz and Slyozov[9] and by Wagner[10] separately, and these two works have been combined into LSW theory. Several other coarsening theories have been suggested based on LSW theory.[11–16] Subsequent to reaching a critical size, precipitates start to dissolve back into the matrix.[17] A full circle of all these stages may be defined as the life cycle of a precipitate. The precipitate size distribution may be unimodal or multimodal (bimodal, trimodal, etc). While a unimodal size distribution is represented by a single Ga
Data Loading...
