Abstract: The γ/γ′ microstructure of a Re-containing Ni-based single crystal super alloy after a two-step aging was studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The crystals were grown by the floating zone (FZ) method. Both cuboidal and spherical γ′ precipitates were formed after a two-step aging. The size of the cuboidal γ′ phases first increased and then decreased with the extension of the second-step aging time.Re, Co and Cr strongly concentrated in the γ phase whereas Ni and Al enriched in the γ′ phase. Thermodynamic calculation by JMatPro was performed to explain the experimental observations.
1 Introduction
Ni-based single-crystal superalloys, which consistof a high volume fraction of ordered L12 γ′ precipitates coherent with the disordered FCC γ matrix, are widely used for turbine engine blades employed for jet propulsion where superior high-temperature strength and creep resistance are required [1−4]. The morphology, size and distribution of the γ′ precipitates in the γ matrix of Ni-based single-crystal superalloys have been shown to drastically influence the high-temperature creep properties [5−7]. Heat treatment is important to obtain a desired γ/γ′ microstructure [8]. Therefore, it is essentialto study the γ/γ′ microstructure evolution during heat treatment. Moreover, a certain amount of refractory elements such as Re, W, Mo and Co for bothsolid-solution strengthening in the γ matrix and precipitation strengthening via the γ′ formation, are added to enhance the high-temperature creep properties [9−11]. Furthermore, the precipitate morphology, lattice misfit and composition of the γ/γ′ phases are interrelated[12]. Thus investigation of the elemental partitioning behavior of the γ/γ′ phases could clarify the relationship between the nominal alloy composition and the local chemistry in γ/γ′ phases.This may shed light on the maximum benefits of the alloying elements. In the present investigation, the morphology and chemical composition of the γ/γ′ phases in a Re-containing Ni-based single-crystal superalloy after two-step aging were studied.
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