A Game-Changing Innovation in Crystal Viscoplasticity Modeling of Directionally Solidified Nickel-Base Superalloys

Superalloys are high-performance alloys that can operate at exceptionally high temperatures, often exceeding 0.7 of the absolute melting temperature. These alloys exhibit remarkable mechanical properties, creep, oxidation and corrosion resistance, which are also considered important design parameters for high-temperature applications. Superalloys can be based on different metals, mostly cobalt, iron and nickel. Notably, nickel-base superalloys are the best suited for gas turbine and airplane applications.

Directionally solidified (DS) nickel-base superalloys (NBSAs) are widely used in developing gas turbine components owing to their remarkable mechanical properties at extremely high temperatures. DS NBSAs generally consist of columnar grain structures aligned in a parallel direction to the stress axis to enhance their strength at high temperatures. In most cases, the in-service failure of different components, such as gas turbine blades, is induced by the high mechanical and thermal cycling loads. Thus, in order to improve the service life of these materials, it is important to pair the mechanical design process with the material modeling process for a detailed evaluation of the material behaviour for a wide range of loading conditions.

In most studies involving DS NBSAs, the crystal viscoplastic (CVP) model is one of the methods used to simulate the effects of temperature and orientation dependence under different loading conditions. Though initially developed for single crystal material, DS materials have been extended to represent columnar grain structure and model the individual grains. However, CVP models for DS materials have been mainly developed by implicitly or explicitly applying SX CVP model to DS materials. Additionally, these approaches require the application of the SX-CVP model to multiple grains, which makes these approaches computationally expensive.

Herein, Professor Navindra Wijeyeratne, Dr. Firat Irmak and Professor Ali Gordon from the University of Central Florida presented a comparative study of the implicit and explicit approaches for modeling NBSAs with the DS-CVP model. The DS-CVP model for DS materials was developed to circumvent the expensive modeling of individual grains. Their main objective was to use three types of DS-CVP models to investigate the performance of the same materials. The previously developed single-crystal CVP model was the basis of the present study. Comparisons were performed at different temperatures and orientations to establish the advantages and disadvantages as well as the applicability of the models. Their research work is currently published in the research peer-reviewed Journal of Engineering for Gas Turbines and Power.

The authors revealed that CVP theory simulated the material response with higher precision compared with other modes. This was mainly attributed to its ability to integrate the effects of plasticity at the slip system level, allowing more accurate description of the underlying material physics. Moreover, all three approaches accurately fitted the monotonic experimental data for a wide range of temperatures and orientations. The main notable differences were in the computational processing time. Compared with the explicit and implicit approaches, the DV-CVP required less computational time, which was evident when performing cyclic and thermomechanical fatigue simulations. Furthermore, the explicit model requires complex geometry to represent material grains, making it unsuitable for practical applications.

In summary, this study utilized constitutive modeling to simulate the material behaviour under different loading conditions. It is worth noting that the study is the first comparison of these three modeling approaches for the same material. The comparison showed the superiority of DS-CVP model. However, both the DS-CVP model and implicit approaches were suitable for practical applications, while explicit approaches were not due to their disadvantages. In a statement to Advances in Engineering, Professor Navindra Wijeyeratne pointed out that the study findings would provide excellent insights into the usability of these constitutive models and the design of superalloys.