Correlations between microstructure evolution and mechanical behavior of a nickel-based single crystal superalloy with long-term aging effects

Highlights

• Quantitative description of materials microstructural evolution depending on exposure time and temperature.

• Quantitative model for the materials property degradation as a function of exposure time and temperature.

• Micromechanical mechanisms for microstructural changes in nickel-based single crystal alloys.

Abstract

The microstructure evolution and mechanical property degradation are investigated for a nickel-based single crystal superalloy during 1000~1100°C thermal exposure for up to 1000 h. Microstructure analyses are aimed at the coarsening of γ′ precipitates and the formation of rafted structure. The morphology of γ/γ′ phase is described by the coarsening parameter λ′ and the degree of rafting ξ depending on aging time and temperature. The non-monotonic change in the volume fraction of γ′ phase, caused by aging, is also considered. Multi-directional observations reveal that the aged superalloy displays cubic anisotropy with layered rafting structure blocks. The compression experiment results show that elastic modulus is independent of the aging process, but the yield strength degraded exponentially and drops close to 30% after 500 h aging. Relationships among microstructure evolution, aging time/temperature, and macroscopic mechanical property are discussed and proposed. It is confirmed that the material degradation is not only related to the morphology of γ/γ′ phase, but also depends on the precipitation of harmful phases, such as the TCP phase from thermal exposure. The material strength of the aged single crystal superalloy is expressed as a function of microstructure characteristics.

Introduction

Nickel-based single crystal superalloys are widely used as the high-pressure turbine blade material. The alloys consist of high volume fractions of regularly arranged cubical strengthening γ′ precipitates embedded in the γ matrix [1,2]. During its high-temperature service, the γ/γ′ microstructure of the nickel-based superalloys gradually degrades with the operating time [3,4], which has a great influence on the mechanical properties. Therefore, the life of blades is not only affected by the high-temperature performance of the original superalloy as well as the applied loads but also strongly related to the microstructure stability during the service.

The microstructure stability at elevated temperatures has received extensive attention. It is known that, during load-free thermal exposure, the γ′ particles coarsen with time. Tang et al. [5,6] studied the coarsening behavior and its growth kinetics of the γ′ precipitates without applied stress in detail. Another phenomenon closely linked to the coarsening is topological phase inversion, which has been analyzed via experiments and phase-field simulations [7]. In addition to the resistance against rafting, the microstructure stability concerns more general microstructure variations, including the precipitation of topologically close-packed (TCP) phase and the evolution of carbides, etc. [[8], [9], [10]].

To investigate the effects of long-term thermal exposure on mechanical properties, various types of experiments on aged materials were performed. An important aspect of the works is on the tensile and compressive mechanical property [11,12]. It was found that both tension and compression yield stresses reduce significantly with the increasing aging time, which arises from the coarsening of γ′ and a decrease in the γ′ volume fraction [13]. Furthermore, the creep resistance is another crucial property for the nickel-based superalloy. A large number of results show that the stress rupture lives deteriorate obviously with the rise of exposure time and temperature [14,15], which is attributed to coarsening of the γ′ precipitates, the formation of the TCP phase and the deposition of dislocation during thermal exposure [9,16]. The very high cycle fatigue (VHCF) property, as well as the intermediate temperature brittleness behavior, also change due to the microstructure degradation caused by the long-term aging [17,18]. All the investigations confirmed unstable material behavior in the long-term aging, however, a quantitative description of the degradation of mechanical properties is still an open issue.

Different constitutive models were proposed to account for the γ′ rafting in single crystal superalloys. Under these models, the influence of rafting on creep deformation is incorporated in different plasticity models through the Orowan stress as a function of the γ channel width [[19], [20], [21]]. Specifically, these models focused on the rafting effects during creep loading, which has external stress except the high temperature. Fedelich [22] extended the model to represent rafting during high-temperature exposure without external load. The influence of the high-temperature aging was not directly considered in such models. There are only a few models that can be used to describe the constitutive behavior of aged single crystal superalloys.

In summary, the microstructure evolution and mechanical property degradation caused by long-term thermal exposure play an important role in the reliable use of the nickel-based superalloys. However, published investigations are limited to illustrate the degradation phenomena of microstructure or reduction of the mechanical property after aging. The relation between microstructure and material behavior is of importance but has not to be established. The present work studies the microstructure evolution and mechanical properties of a nickel-based single crystal superalloy under different thermal exposure conditions and is trying to provide a quantitative correlation between material property and microstructure.