Microstructure-Sensitive Crystal Plasticity Modeling for Austenitic Steel and Nickel-Based Superalloy Under Isothermal Fatigue Loading

Abstract

Intermittent mechanical loads combined with high temperatures appear during the operation of turbines in jet engines or in power plants, which can lead to high-temperature fatigue or to thermomechanical fatigue. Since the assessment of fatigue properties is a complex and time-consuming process, it is essential to develop validated material models that are capable of predicting fatigue behavior, thus allowing the extrapolation of experimental results into a broader range of thermomechanical conditions. To accomplish this, two representative volume elements (RVEs), mimicking the typical microstructure of single crystal Ni-based superalloys and polycrystalline austenitic steels, respectively, are introduced. With the help of these RVEs, the temperature and deformation-dependent internal stresses in the microstructure can be taken into account. In the next step, phenomenological crystal plasticity models are implemented and parameterized for cyclic deformation of these two materials. The RVE, constitutive model, and the material parameters for the Ni-based superalloy are taken from a former study. For the austenitic steel, however, an inverse procedure has been used to identify its material parameters based on several isothermal fatigue tests in a wide temperature range. With the identified material parameters, a valid description of the isothermal fatigue behavior at different temperatures is possible. The most important conclusion from the comparison of the isothermal fatigue behavior of the two different materials is that the kinematic hardening, which is responsible for the shape of the hysteresis loops, is entirely described by the internal stresses within the typical microstructure of the Ni-based superalloy, which is modeled in a scale-bridging approach. Hence, no additional terms for kinematic hardening need to be introduced to describe the cyclic plasticity in the superalloy. For the austenitic steel, in contrast, the Ohno–Wang model for kinematic hardening needs to be considered additionally to the internal stresses in the polycrystalline microstructure to obtain a correct description of its cyclic plasticity.

Experimental Assessment and Micromechanical Modeling of Additively Manufactured Austenitic Steels under Cyclic Loading

“…The results of the micromechanical model show a generally good agreement with the experimental hysteresis curves with the largest discrepancies occurring in the transition regions between elastic and plastic material response. Here, the [22] 200 [22] 170 [22] 1250…”

“…It is noted here again that the constitutive parameters for the crystal plasticity model were fit to match the cyclic plasticity of sample A in a wide temperature range and that the main focus of the fitting was to match the stress amplitudes for different applied strain cycles. [ 22 ] Furthermore, in the present work, a local crystal plasticity model was applied in which length scales are not considered explicitly such that grain size effects are not captured by the model. Consequently, the numerical results reflect mostly the influence of crystallographic texture, grain morphology, and grain arrangement in the RVE.…”

“…[30]. The plastic and kinematic hardening parameters defined in Equation ( 6) and ( 7) are calibrated in previous work [22] by an inverse analysis using experimental data of sample A for two different strain amplitudes and in a wide temperature range between room temperature and 750 °C. Even if in the present work the constitutive model was applied only for room-temperature simulations for one strain amplitude, the parameters were not changed.…”

“…In a further step, a micromechanical model including a representation of the true microstructure and a crystal plasticity‐based constitutive law are implemented and applied to simulate the material behavior under cyclic loading with material parameters determined in an inverse procedure in previous work. [ 22 ]…”

DRAGen in Application—An Approach for Microstructural Fatigue Predictions of Non-Oriented Electrical Steel Sheets

“…Multiple studies show that numerical simulations in combination with microstructure models have a high potential for predicting fatigue properties of metallic materials. One noteworthy approach for predicting these properties lies in the statistical analysis of strain fields from multiple simulations shown by Mughrabi [ 6 ], Sharaf et al [ 7 ], Cruzado et al [ 8 ] and Shahmardani and Hartmaier [ 9 ]. These studies utilize the statistical nature of the Representative Volume Elements ( s) as a linking element between simulation and experimental results.…”