Session: 18-07: Failure Prediction and Life Asssessment II

Submission Number: 177029

Quantitative Evaluation of Thermo-Mechanical Fatigue Damage in Ni-Based Superalloy Gas Turbine Blades Using EBSD and X-Ray Crystallite Orientation Deviation (XCOD) Method 

As the global energy landscape shifts toward carbon neutrality by 2050, renewable energy sources are increasingly adopted. However, thermal power generation, particularly Gas Turbines (GTs), continues to play a vital role in ensuring stable electricity supply due to their excellent start-up characteristics. GT blades, operating under high-temperature conditions, are subject to creep damage caused by centrifugal forces. Consequently, extensive research has been conducted on developing creep-resistant materials and quantitatively evaluating creep damage.

We have enabled quantitative estimation of creep life by analyzing characteristic misorientation associated with creep damage in Ni-based superalloys through observations using Electron Backscatter Diffraction (EBSD). On the other hand, non-destructive technique is required to evaluate individual blades, and in collaboration with Rigaku Corporation, we have developed a laboratory-based X-ray Crystallite Orientation Deviation (XCOD) Method applying Laue method and successfully evaluated creep damage in Ni-based superalloys.

However, in recent years, the number of GT start-stop cycles has increased, raising concerns about reduced component life due to fatigue damage. Although quantitative evaluation of fatigue damage in GT blades made of nickel-based superalloys is in high demand, assessing the damaged regions remains a significant challenge.

In this study, a Finite Element Method (FEM) analysis was first conducted to simulate the start-up and shut-down operations of a GT, enabling the identification of regions susceptible to fatigue damage, particularly at the blade top tip and the platform on the suction side. These regions were found to be subjected to high temperatures and compressive stresses during rated operation, suggesting the occurrence of Thermo- Mechanical Fatigue (TMF) damage under out-of-phase conditions, where temperature and stress cycles are misaligned. Visual inspection of actual turbine blades revealed microcracks and delamination of the Thermal Barrier Coating (TBC) in these areas, suggesting the presence of TMF damage.

To investigate the damage process quantitatively, TMF tests were conducted under strain-controlled conditions that included high-temperature compressive hold, reflecting the FEM results. Specimens corresponding to 25%, 50%, 75%, and 100% of the failure life were prepared.

EBSD analysis of the internal microstructure of these specimens revealed a trend of increasing crystallographic misorientation with damage progression. In particular, the Kernel Average Misorientation (KAM) values exhibited a linear and monotonic increase with the fraction of fatigue life. This trend is attributed to the accumulation of dislocations resulting from compressive creep and tensile plastic deformation.

In parallel with EBSD, Full Width at Half Maximum (FWHM) measurements were performed using XCOD method applying Laue method on specimens with varying damage levels. The results similarly showed a monotonic increase in FWHM values with TMF life fraction.

In previous studies, it has been reported that evaluation parameters obtained from EBSD and XCOD tend to increase with the fraction of TMF life when macroscopic deformation, such as creep strain or plastic strain, is involved. However, in this study, even in the absence of macroscopic deformation, both KAM values from EBSD and FWHM values from XCOD increased monotonically. This phenomenon is considered to result from the accumulation of inelastic strain within the material, leading to increased dislocation density.

In conclusion, under the present conditions, this study successfully identified fatigue-prone regions through FEM analysis and demonstrated that EBSD and XCOD can quantitatively evaluate fatigue damage under TMF conditions. Future work will explore the applicability of these methods to in-phase TMF and low-cycle fatigue tests.

Presenting Author: Shun Miyaoka CHUBU Electric Power Co., Inc.

Presenting Author Biography: Shun Miyaoka（CHUBU Electric Power Co., inc.）
Shun Miyaoka is a Manager (R&D) at Chubu Electric Power Co., Inc., where he is engaged in material evaluation and life assessment of power generation equipment using Finite Element Method (FEM). With over 10 years of experience in FEM, he specializes in stress analysis, heat conduction analysis, and creep analysis. He is particularly interested in identifying root causes of material and structural failures, as well as solving issues related to malfunctioning components.

Authors:

Shun Miyaoka CHUBU Electric Power Co., Inc.
Shinya Iwasaki CHUBU Electric Power Co., inc.
Daisuke Kobayashi CHUBU Electric Power Co., Inc.
Akihiro Ito CHUBU Electric Power Co., Inc.
Toru Utsunomiya CHUBU Electric Power Co., Inc.