The effect of heat treatment on characteristics of the gamma prime phase and hardness of the nickel-based superalloy Rene®80

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Highlights

  • •

    Effect of heat treatment on characteristics of the γ' phase of Rene80 was evaluated.

  • •

    Increasing the holding time led to an increase in the γ-γ′ lattice mismatch.

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    Increasing the temperature, 871–1130 °C led to decrease the γ-γ′ lattice mismatch.

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    Decreased amount of γ' at high temperatures and extended time, decrease hardness.

Abstract

In this study, the effect of heat treatment on characteristics of the γ'-Ni3(Al, Ti) phase, as the main strengthening precipitates of the nickel-based superalloy Rene®80 was evaluated in terms of morphology and distribution of the γ' phase, γ (matrix)-γ′(strengthening precipitates) lattice mismatch, and hardness of the alloy. For this purpose, at first, the solution heat treatment was performed at 1204 °C on the alloy. Then, heat treatment cycles were carried out at three temperatures, namely 871 °C, 1095 °C, and 1130 °C, for different periods of time. The microstructures were analyzed by optical microscope and scanning electron microscope. Also, X-ray diffraction analysis was conducted to determine variations of the γ-γ′ lattice mismatch, followed by hardness measurement test, which was performed after each stage of heat treatments. The results showed an increase in the amount and size of the γ' phase with holding time at 871 °C, while the values were seen to remain constant and even decrease at 1095 °C and 1130 °C, respectively, due to simultaneous dissolution of the γ' phase during extended periods of time. At all of the three temperatures, the γ-γ′ lattice mismatch increased with time and showed larger negative values, while an increase in temperature from 871 °C to 1130 °C led to lower γ-γ′ lattice mismatch. The largest lattice mismatch (−0.653%) was observed at 871 °C after a heat treatment time of 1000 s. Hardness of the Rene®80 alloy at 871 °C increased with time, while different behaviors were observed at 1095 °C and 1130 °C, where the hardness decreased for extended heat treatment times. The most significant cause of such changes in hardness might be the coherency elastic strain between the γ and γ' phases, which is affected by the characteristics of the γ' phase and its lattice mismatch with the γ phase.

Introduction

Rene®80 (General Electric Company trademark), as a nickel-based superalloy, has been widely used in the manufacturing of the jet engine turbine blades. This superalloy provides excellent mechanical properties at elevated temperatures [1,2]. In most cases, this precipitation hardened superalloy is generally used at temperatures in the range of 760–982 °C. The microstructure of Rene®80 consists of γ matrix, γ' phase precipitates in the γ matrix and the carbides [3]. This alloy also, strongly affected by heat treatment processes, therefore, the selection of a proper heat treatment cycle for this superalloy is of paramount importance. The reasons for the heat treatment of Rene®80 can be summarized as follow; homogenization of casting microstructure, proper distribution of alloying elements, elimination of the adverse eutectic γ-γ′ region, achieving proper distribution of carbide phase (especially along grain boundaries) and γ'-Ni3(Al, Ti) phase to retain the desired mechanical properties at high temperatures [3]. Distribution and size of the γ' strengthening phase with FCC crystalline structure and L12 order impose the largest contribution into strength properties of Rene®80 alloy. Morphology and size of this phase are extremely dependent on heat treatment temperature and time, with its geometry varying from spherical to cubic. Increasing the γ-γ′ lattice mismatch led to major changes in the morphology of the γ' phase in the following order: spherical, globular, blocky and cuboidal [4]. Due to slight differences in lattice constants of both phases, γ and γ', a lattice mismatch exists. It should be mentioned that in some nickel-based superalloys, undesirable phases (e.g., η-Ni3Ti or δ-Ni3Nb) are formed within the alloy microstructure at high γ-γ′ lattice mismatch and temperatures beyond 700 °C, therefore, the lattice mismatch between the γ matrix and the γ' precipitates is one of several factors which can influence the strength and creep resistance of these alloys [4,5]. Regarding the effect of γ' phase on mechanical properties, Kakehi [6] reported that the best high-temperature tensile and creep ductility values of Rene®80 alloy were obtained in the presence of primary and secondary γ' phases with a bimodal size distribution. Moreover, Aghaie and Hajjavady [7] referred to the volume fraction increase in this phase in the microstructure of nickel-based superalloys as a reason backing the preservation of strength properties at high temperatures. Given the importance of γ', which, as a secondary precipitated phase, plays a key role in strengthening the Rene®80 alloy [8], the present work attempted to evaluate the effects of heat treatment conditions on morphology, the amount and size of this phase. Also, the lattice mismatch of the γ' phase with γ matrix and the relationship between the characteristics of the γ' phase and changes in the hardness of the nickel-based superalloy (Rene®80) were further studied.

Section snippets

Experimental procedure

The alloy used in the present study was the Rene®80 nickel-based superalloy. The chemical composition of this alloy is listed in Table 1, as determined by an American quantometer named ARL-3560-OES, indicating its compliance with AMS 5403 [9] aviation material standard. Cylindrical bars, of 12 mm in diameter and 120 mm in length, were produced by the investment casting using a German Laybold furnace with a maximum capacity of 50 kg under 10−4 bar vacuum. Samples were sectioned in 10 mm height,

Microstructure characterization of as-cast Rene®80

Fig. 1 shows the optical microscopy and scanning electron microscopy images of the as-cast samples. Fig. 1-a shows the dendritic microstructure of the as-cast Rene®80 alloy studied in the present work. As shown in this figure, the primary dendrites are spaced by 250 μm, i.e. more than twice as wide as the secondary dendrites. Fig. 1-b shows the γ-γ′ eutectic region formed in the space between the dendrites and is enriched in titanium and aluminum. This region shows high hardness, and its

Conclusion

  • 1.

    Increasing heat treatment time led to an increase in the size of γ' phase at all of the three temperatures studied in this research, but only at 871 °C, the phase amount maintained its increasing trend for extended holding times. Increased time at 1095 °C and 1130 °C led to a relatively constant and significantly decreasing the γ' amount, respectively. The most important reason backing these observations is the dissolution of γ' phase at an extended holding time at high temperatures.

  • 2.

    At all of

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