Effects of Long-Term Thermal Exposure on the Microstructure and Properties of a Cast Ni-Base Superalloy

Abstract

Microstructural stability and microstructure-property relationship during long-term thermal exposure in K452 alloy (a new Ni-base cast superalloy with~21 pct Cr, 11 pct Co, 3.5 pct W, 2.5 pct Al, 3.5 pct Ti, and others) are investigated. It is found that exposure temperature and time have significant effects on the microstructure and properties of the alloy. During exposure, the microstructure is degraded by c¢ coarsening, MC carbide (M mainly represents Ti, W, and Nb) degeneration, precipitation and evolution of grain interior (GI) M 23 C 6 carbide, evolution of grain boundary (GB) microstructure, and precipitation of g phase. Among them, the c¢ coarsening is the leading reason for the decrease of strength of the alloy. The GI M 23 C 6 and the g phase have negligible influence on the properties due to their relatively small populations. Blocky, closely spaced GB M 23 C 6 particles engulfed in c¢ increase the stress-rupture life, whereas the formation of a continuous GB M 23 C 6 chain has an opposite effect. A life peak occurs when the M 23 C 6 /c¢ structure at the GBs is in an optimal form. The degenerated MC is the preferred initiation site of microcracks. Its presence at the GBs promotes the onset of intergranular fracture, and leads to the decrease in mechanical properties.

Microstructure of an experimental Ni base superalloy under various casting conditions

“…High purity alumina crucibles were used for all samples. Due to the severe segregation that occurs in these alloys [17], cylindrical samples with a minimum size of approximately 4 mm diameter × 4 mm length were used. Data was acquired only on heating to better characterize the effect of the thermal processing and to avoid the effects of super-cooling.…”

“…Other studies also reported the occurrence of phase at the periphery of the ␥/␥ interdendritic zones [13][14][15]. Recently, the phase formed by MC carbide decomposition during heat treatment or thermo exposure is gaining researchers' interests [16,17]. However, quite a few studies have been made to characterize the formation of phase during both solidification and subsequent heat treatment processes.…”

Effects of cold rolling on the precipitation kinetics and the morphology evolution of intermediate phases in Inconel 718 alloy

“…In recent decades, wide spread interest has been developed in nickel-base superalloy due to their excellent M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 2 properties at high temperatures [1]. These materials have a satisfactory performance-price ratio, and are extensively used in gas turbines of jet engines and related aerospace applications, power generation plants, petrochemical industry because of their superior mechanical properties and structural stability at elevated temperature [2].…”

Fatigue Life of the Repair TIG Welded Hastelloy X Superalloy

“…Furthermore, it is well published that M 6 C carbides is an unstable carbide as it can decompose to the new precipitate during aging. 12,13) Bai et al 14) published that M6C carbide can decomposed into M 32 C 6 carbide with a prolonged exposure. After exposure in elevated temperature for a long time, the TCP phases were precipitated in the region near the prior carbides.…”