During the operation of turbines in jet engines or in power plants, high thermal and intermittent mechanical loads appear, which can lead to high-temperature fatigue failure or to thermomechanical fatigue (TMF). In the aerospace industry, nickel-based alloys, titanium alloys, and stainless steels are the key materials of turbine blades able to endure a wide range of stresses and temperatures. Since fatigue is a complex and time-consuming process, it is important to develop realistic numerical models to predict fatigue behavior and to extrapolate the limited experimental results into a wider range of thermomechanical conditions [1].

To accomplish this, two reference volume elements (RVEs), mimicking the typical microstructure of a nickel-based single crystal superalloy and polycrystals of 316L, respectively, are introduced. The cubic RVE of CMSX-4 consists of one central cubic γ′ precipitate surrounded with six half-width channels of γ matrix [2]. The cubic RVE of 316L represents a polycrystals with randomly oriented grains derived from EBSD analysis. Both RVEs represent the simplest self-repeating structural unit of its microstructure. Volume fraction variation of the γ′ precipitate and the γ matrix in CMSX-4 are considered in the numerical modeling.

With the help of these RVEs, the temperature and deformation-dependent internal stresses in the microstructure can be taken into account in a realistic manner, which proves to be essential in our understanding of the isothermal fatigue and TMF behavior of these two materials. The material behavior in the elastic regime for both CMSX-4 and 316L is described by temperature-dependent anisotropic elastic constants. The flow rule for plastic deformation in CMSX-4 is governed by the thermal activation of various slip systems in the γ matrix, the γ′ precipitate and by cube slip along the γ/γ′ microstructure. These different mechanisms are into account in a phenomenological crystal plasticity/creep model [2]. The plastic deformation in 316L is described by temperature dependent crystal plasticity model adopting the Ohno-Wang relation for kinematic hardening.

Both proposed constitutive laws are parameterized based on experimental data from the literature, covering isothermal fatigue at elevated temperatures and TMF tests for both materials. It is shown that with a consistent set of material parameters for each material, our constitutive models are able to predict isothermal fatigue behavior at different temperatures as well as TMF behavior. The most important conclusion from isothermal fatigue behavior of CMSX-4 at various temperatures is that the kinematic hardening, which is responsible for the shape of the hysteresis loops, is completely described by the internal stress within the γ/γ′ microstructure. Hence, no additional terms for kinematic hardening need to be introduced to describe the cyclic plasticity in the superalloy, while in 316L, the Ohno-Wang relation could fully describe kinematic hardening during plastic deformation.

References

[1] B. Fedelich, A. Epishin, T. Link, H. Klingelhöffer1, G. Künecke, P. D. Portella, Superalloys 2012, 2012.

[2] M. Shahmardani, A. Hartmaier International Journal of Fatigue, 2021, 151, 106353.