Formation of η and σ phase in three polycrystalline superalloys and their impact on tensile properties
Introduction
In the past decade, development of industrial gas turbine (IGT) calls for blade materials with good high temperature creep strength and corrosion resistance. In order to meet this requirement, new corrosion resistant nickel based superalloys are continuously designed with increased levels of Al, Ti and Cr concentration, which significantly improve the alloys’ oxidation resistance and hot corrosion resistance. The most typical polycrystalline alloys are GTD111 [1], IN738 [2], and IN792 [3]. All of them consist of a solid solution strengthened austenitic nickel matrix γ, and hard γ′ precipitates of stoichiometric Ni3(Al,Ti). For their similar usage purpose, these alloys’ chemical compositions were designed with several characteristics in common, which are a relatively large amount of Cr (>12 wt.%) and high Ti/Al ratio (bigger than one). This is because both Cr and Ti are known to form Cr/Ti-sulfide near the surface of the alloy, thus prevent further progress of sulfidation into the alloy when the alloy is faced on the sulfate-induced hot corrosion environment [4], [5]. In addition, by increasing the Ti/Al ratio when the Cr contents are sufficiently high, hot corrosion resistance can also be improved [5].
However, the above-mentioned modification of chemical composition may bring along some drawbacks to the solidification of nickel based superalloys for IGT blade materials. Increasing of the elements that are prone to interdendritic segregation has critical influence on the hot tearing susceptibility and interdendritic phase constitution. A high Ti content will favor the formation of cracks in the final stage of solidification [6], [7]. Also, in some cases, minor phases like η phase and σ phase which usually form at solid state in superalloys may form at the end of the solidification owing to the influence of compositions. The η phase has a hexagonal close-packed (hcp) structure and has the basic stoichiometry of Ni3Ti, while the better known σ phase has a tetragonal topologically close-packed (TCP) structure. Both η and σ phase are generally considered as deleterious minor phases by alloy designers as they usually pose negative impact on superalloys’ mechanical properties. Many previous studies are focused in analyzing the precipitation mechanism of η and σ phases at solid state in superalloys. η Phase may form either during casting or thermo exposure process in nickel based superalloys. Bouse [8] has reported that η or platelet phases formed in the as-cast microstructure of alloys containing large amounts of Ti, such as IN792 + Hf, IN939, GTD111, and IN6203. Other studies [9], [10], [11] also reported the occurrence of η phase at the periphery of the γ/γ′ eutectics, respectively. Recently, the η phase formed by MC carbide decomposition during heat treatment or thermo exposure is gaining researchers’ interests [10], [12], [13]. However, there are quite a few studies have been done to characterize the formation of η phase during both solidification and subsequent heat treatment process. σ Phase generally precipitates from γ phase at solid state and seriously degrades superalloys mechanical properties due to its plate-like morphology and high refractory alloying containing [14]. Also a recent study [15] reported that it can act as nucleating site for other TCP phases, such as μ phase or P phase. Fully understanding of σ phase's transformation mechanism is of crucial importance for alloy designers. As almost no studies have reported formation of σ phase during casting and its phase transformation during heat treatment as well as possible impact on mechanical properties of superalloys, a detailed characterization of this transformation process will provide knowledge for the better understanding of σ phase.
In this study, η and/or σ phases were found in the as-cast microstructures of three polycrystalline experimental nickel based superalloys. To address the formation mechanism of both η and σ phase in the three alloys and explore their possible impact on phase stability and mechanical properties is meaningful in both alloy design and subsequent casting and treating aspects. Thus, phase evolutions in the three alloys were characterized in detail in this paper. Tensile tests were done on the heat-treated three alloys to evaluate the possible influence of η and σ phases on mechanical properties as well.
Section snippets
Experimental procedure
The chemical compositions of the three alloys were given in Table 1, compositions of alloy GTD111 were listed for reference as well. Master ingots were melted and cast into rods of 13 mm in diameter under vacuum, and a standard heat treatment (1100 °C/2 h/AC + 843 °C/6 h/AC) was used to treat the as-cast rods. Some heat-treated specimens were chosen to further thermally exposed at 850 °C for 100 h to test their phase stability, and some were tensile tested at temperatures from 25 °C to 950 °C to test
Solidification characteristics of three alloys
As-cast microstructures of the three alloys are shown in Fig. 1. In alloy DK3 (Fig. 1(a)), it can be seen that blocky phase existed at the eutectic periphery. EDS analysis showed it had high Ti content and was identified as η phase. In alloy DK4 and DK5 (Fig. 1(b and c)), besides the blocky η phase which was the same as that observed in alloy DK3, nodular shaped particles were also observed at the eutectic periphery. The EDS spectrum (Fig. 1(d)) shows that they had high Cr content, which
Conclusion
The final stages of solidification in the three nickel based polycrystal superalloys were completed with the formation of η phase or (η + σ) phases after eutectic reaction. A high Ti/Al ratio in the interdendritic area favored the formation of η phase, and high (Ti + Al) value of the alloy chemical composition may favor the formation of σ phase during solidification.
During heat treatment, in alloy DK3, η phase nucleated from the coarse γ′ with high Ti content in the eutectic peripheries and grew
References (26)
- et al.
Acta. Mater.
(2002) - et al.
Mater. Sci. Eng. A
(2008) - et al.
Acta. Mater.
(2001) - et al.
Metallography
(1973) - et al.
Mater. Sci. Eng. A
(2005) - et al.
Metallogragphy
(1973) - et al.
J. Eng. Gas Turbines Power
(1998) - G.C. Bieber, R.J. Mihalisin, Second International Conference on the Strength of Metals and Alloys, vol. IV, Asilomer,...
- et al.
Cited by (60)
High temperature microstructure stability of Waspaloy produced by Wire Arc Additive Manufacturing
2023, Journal of Alloys and CompoundsMicrostructure and mechanical properties of medium-entropy alloys with a high-density η-D0<inf>24</inf> phase
2022, Materials CharacterizationMicrostructure evolution and phase transformation in a nickel-based superalloy with varying Ti/Al ratios: Part 1 - Microstructure evolution
2022, Materials Science and Engineering: AMicrostructure evolution and phase transformation in a nickel-based superalloy with varying Ti/Al ratios: Part 2 – Phase transformation
2022, Materials Science and Engineering: A