Precipitation of Carbides in a Ni-based Superalloy

Sammanfattning: Alloy B is relatively new precipitation hardening superalloy. It´s applications are in the hot sections of the aero engines, rocket nozzles, gas turbines and in the chemical and petro-leum applications. The alloy is characterized by keeping high strength at elevated tempera-tures and high creep resistance. It´s excellent mechanical properties and corrosion resis-tance are due to the balanced amount of the coherent γ' matrix, combined with other alloy-ing elements and carbides. There are three types of carbides which can be found in nickel-based superalloys: MC, M 23C6 and M6C. Primary MC carbides act as source of carbon for the secondary carbides, which precipitate at the grain boundaries. They can have strengthening effect by hindering the movement of dislocations. In this work both simulation and experimental analysis are conducted in order to investi-gate the behaviour of the secondary carbides. JMatPro simulation is used to predict the behaviour of the material. Heat treatments are conducted at soak temperatures ranging from 920 °C to 1130 °C, with steps of 30 °C, and dwell times of 0.5, 1, 2 and 24 hours. Experimental methods included analysis at LOM, SEM, EDS, manual point counting and hardness tests. Main results show chromium rich M 23C6 carbides are stable at lower temperature compared to molybdenum rich M6C. Both appear as fine and discrete particles at the grain boundaries at 1070 °C. This morphology is believed to be beneficial for the mechanical properties of the alloy. The volume fraction varies between 0.6 and 1.3%. Hardness values are relevant in the range of 920-1010 °C. Above this range there is sudden drop of the hardness