Evolution of precipitate microstructure in the superalloy IN738LC during compression creep

Abstract

Superalloys are high temperature materials which are used widespread in many industries, especially in several engine components in gas turbines utilized for electricity production, marine applications, and aircrafts. This is due to the fact that they have excellent high temperature strength and corrosion resistance. One of the many nickel-base superalloys is IN738LC which possesses a good high temperature strength and hot corrosion resistance. It is mainly employed 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 evolutions in precipitate morphology and size in IN738LC specimens aged under compressive stress. In order to observe the influence of magnitude of stress, a constant compressive load is applied to conical specimens which are aged in an inert atmosphere for various times. Stress varies along the conical specimens under a constant applied load. Thus, the study of microstructural changes due to various stress levels becomes possible with a single specimen in the same experimental condition. In this study, agings under compressive stress (compression creep agings) at 950°C for 24, 192, 480 and 960 hours and at 1050°C for 12, 24, 192, and 480 hours have been carried out. In addition, two 24-hour agings with no applied stress have been performed at 950°C and 1050°C. Except the 192-hour and 480-hour experiments at 1050°C, single-size distribution of precipitates has been observed. Results indicate that the average precipitate size is inversely proportional to the applied stress but directly proportional to the temperature. The morphology of precipitates is found to be spheroidal and merged spheroidal for all agings at 950°C. For 1050°C agings, the initially cuboidal precipitates transform to a deteriorated cuboidal first, and then they merge to acquire a complex shape. Eventually, they become elongated and faceted with increasing stress and aging time. In addition, rafting is found to be promoted with increasing stress up to some aging time.