Precipitate Size in the Superalloy IN738LC During Compression Creep

Abstract

Superalloys are high-temperature materials that are used widely in many industries especially in several engine components in gas turbines. One of the many nickel-base superalloys is IN738LC, which possesses a good high-temperature strength and hot corrosion resistance. It is employed mainly in nozzle guide vanes and blades of land-based turbines. The strengthening is provided mainly by coherent γ′ precipitates. The current research is undertaken to determine precipitate size in IN738LC specimens aged under compressive stress. To observe the influence of magnitude of stress, a constant compressive load is applied to conical specimens that are aged in an inert atmosphere for various times [24, 192, 480, and 960 hours at 1223 K (950 °C) and 12, 24, 192, and 480 hours at 1323 K (1050 °C)]. Thus, the study of microstructural changes due to various stress levels becomes possible with a single specimen in the same experimental condition. A single-size distribution of precipitates has been observed except for the 192-hour and 480-hour experiments at 1323 K (1050 °C). The results indicate that the average precipitate size is directly proportional to the temperature but inversely proportional to the applied stress, although in some specimens, maximum size is observed at medium-stress levels. Strain becomes effective at most severely crept specimen by causing a merging of precipitates. Raft degeneration is also observed in this specimen. The growth exponent is found to vary with stress and temperature between 2.97 and 3.78.