Precipitation Kinetics and Morphology of Grain Boundary Carbides in Ni-Base Superalloy Haynes 282

Abstract

Precipitation of grain boundary carbides in a mill-annealed Haynes 282 in the temperature range 650 °C to 1120 °C was investigated. The kinetics of M23C6 was significantly faster than that of M6C. With increasing aging temperature, the morphology changes from continuous film to an interconnected brick wall structure and finally to discrete particles. No morphological changes were observed with aging time. Serrated grain boundaries formed during aging around 750 °C. The solvus temperature for both M23C6 and M6C was approximately 1100 °C.

Microstructural Evolution of Large Cast Haynes 282 at Elevated Temperature

“…No Mo-rich borides which were previously found in wrought Haynes 282 [19] were found in the as-cast condition. This result is also supported by [16]. The authors stated that the absence of Mo-rich borides could be explained by its very small size and volume fraction.…”

“…It was a combination of the equilibrium prediction (Figure 2a) and the solidification prediction (Figure 2b). M6C was not found on the grain boundary in the as-cast condition because of the slow precipitation kinetics of M6C [16]. During solidification, Cr atoms at the grain boundary combined with C atoms to form M23C6 phase; due to the sluggish M6C precipitation, the grain boundary was dominated by M23C6 precipitates, and Mo atoms segregated to the grain boundaries to form μ phase due to insufficient C content.…”

“…Therefore, after the standard heat treatment, grain boundaries in AC Haynes 282 were 100% covered by discrete M6C and M23C6 precipitates. In general, three morphologies of grain boundary precipitates were found in Haynes 282: filmlike, brick wall, and discrete precipitates [16,20]. Vattappara et al and Joseph et al [16,21] studied the effect of aging temperature on grain boundary morphology.…”

“…In general, three morphologies of grain boundary precipitates were found in Haynes 282: filmlike, brick wall, and discrete precipitates [16,20]. Vattappara et al and Joseph et al [16,21] studied the effect of aging temperature on grain boundary morphology. They found that, with increasing aging temperatures-from below 650 °C to 1050 °C-grain boundary morphology changed from a continuous film (650-750 °C) to a brick wall structure (750-950 °C) and finally to discrete precipitates (above 1000 °C).…”

“…A general microstructure of wrought Haynes 282 includes MC, intergranular and intragranular Cr-rich M23C6, and a 20 nm uniformly distributed γ' [2,5,[12][13][14]. Mo-rich M6C has been found in additive manufactured [11] and cast Haynes 282 [15], as well as in wrought Haynes 282-with alternative heat treatments due to its sluggish kinetics [16]. A lamellar constituent consisting of Mo and Cr-rich secondary carbide phases together with γ phase was found in a vacuum cast thin wall component (4.6 kg) [7].…”