Morphological and size evolution of γ' phases during aging in nickel-based single crystal superalloy

Abstract

A multistage solution treatment process was applied for nickel-based single crystal superalloys, complemented by various aging durations and cooling rates. The microstructure was characterized by scanning electron microscopy (SEM) to observe the γ' phase. Additionally, phase field simulations were conducted to model the growth of γ' precipitates during aging and analyze their morphological evolution. The experimental results demonstrated that the multistage solution treatment effectively eliminated eutectic phases and carbides. Moreover, samples aged for 10 min exhibited larger and more rectangular γ' precipitates compared with those aged for 5 min. Notably, secondary γ' precipitates were observed in samples subjected to water cooling. Two indices for quantifying rectangularization were proposed and successfully applied. Based on the simulation results, lattice mismatch induced coherency stresses and elevated stress triaxiality along the 〈111〉 direction contributed to the rectangularization of the γ' phase.

1. Introduction

Nickel-based single crystal (SC) superalloys have earned a pivotal role in aerospace engineering, particularly in producing high-performance turbine blades used in aeronautical engines [1−3]. However, the as-cast state of SC superalloys may exhibit mechanical weakness due to the presence of eutectic phases and carbides, which creates challenges for their practical use. Consequently, heat treatment plays a crucial role in addressing these deficiencies and improving the alloy’s mechanical performance [4−6]. The conventional heat treatment process involves a sequential solution treatment followed by a two-stage aging process, widely adopted in nickel-based single crystal superalloys [7−9]. The primary objectives of the solution treatment are to dissolve unwanted eutectic phases and carbides as thoroughly as possible and to achieve an optimal supersaturated solid solution. The subsequent two-stage aging process facilitates the formation of neatly arranged γ' precipitates, significantly enhancing the service performance of alloy [10−12]. Understanding the influence of temperature and holding time during these treatment stages is crucial, as they significantly impact the microstructural evolution and, consequently, the mechanical properties of nickel-based single crystal superalloys [13−16].

Recent investigations have highlighted the advantages of using elevated solution treatment temperatures during solid solution processing to mitigate compositional segregation [17−19]. It is generally believed that, while avoiding initial melting, high solution treatment temperatures lead to improved service performance. The “solutioning window”, between the γ' solvus temperature and the initial melting temperature, is critical in providing sufficient opportunities for dissolving undesirable phases. To expand this “solutioning window”, researchers have introduced multistage solution heat treatments that raise the initial melting temperature, effectively broadening the range for the dissolution of unfavorable phases. HEGDE et al [20] used stepwise homogenization solution heat treatments and found that steps below the γ' solvus temperature stabilized the eutectic phase, while steps above the γ' solvus temperature improved the homogenization and reduced the eutectic phase fraction. LV [21] applied several stepwise solution heat treatments to DD5 superalloy, which are (1280 °C, 2 h) + (1315 °C, 2 h) + (1320 °C, 2 h) + (1325 °C, 6 h). AC solution route produced a favorable microstructure. In addition to the aforementioned stepwise solution heat treatments, innovative techniques suc...