Investigation on the Precipitation Behavior of a Ni-Based Alloy Containing Tungsten during Thermal Exposure at 850 °C

Precipitate evolution characteristics of a Ni-based alloy containing tungsten during thermal exposure at 850 °C from 100 to 3000 h and its influence on the impact toughness were investigated. Transmission electron microscopy, scanning electron microscopy, and precise chemical analysis were used to analyze precipitate shape, type, composition, and growth mechanism. Experimental results proved that the μ, σ, M23C6, and M6C phases were formed and grew during thermal exposure at 850 °C. The μ phase was mostly formed within the grain or around the grain boundary in a strip-like or irregular shape and a stacking fault interior. M23C6 was formed at twin boundaries in a quadrilateral shape with one pair of parallel sides. However, at the grain boundary, the precipitate was mainly M6C and shaped in an irregular polygon. In addition, during long-term thermal exposure at 850 °C, M6C grew along the grain boundary and formed a chain-like structure. The μ phase can nucleate and grow independently or in clusters. Particularly, M6C and Ti(CN) or TiC can be the nucleation core for the μ phase. The precipitate size and quantity both increased with longer exposure time. Meanwhile, mechanical property testing indicated that the impact toughness worsened at the initial stages of aging, but remained relatively constant during thermal exposure from 500 to 3000 h. Microchemical phase analysis and EDS results showed that impact toughness deterioration for samples exposed to less than 500 h thermal treatment may be caused by the coarsening of grain boundary carbides and chemical composition fluctuation of the precipitate.