Materials Property Modeling for Tougher Turbines
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Computational materials science enables researchers to predict the behavior of materials, and reduce the difficulty, time and expense of experimentation. Using advanced computing power and software, researchers are able to design, study, test, and optimize materials in a virtual setting. For NETL researchers, computational materials modeling is a powerful tool.

One research area where NETL focuses its materials property modeling is the technology needed for increased turbine temperatures. Ultra-efficient power production requires high-temperature, high mechanical stress operating conditions. Components of turbines operating at these demanding conditions are subject to damages such as “creep”—the tendency for materials to sag or deform at high-temperatures, and “fatigue”—the propagation of cracks over time. Advanced materials are needed to increase turbine power and efficiency while maintaining low capital costs and protecting components to enable longer service lifetimes. The advanced turbines and turbine-based systems enabled by these materials will operate cleanly and at low cost.

As part of this effort, NETL manages projects under DOE’s Office of Fossil Energy Advanced Turbines Program, which supports universities and small businesses to conduct materials R&D. One of the goals of the program is to design and develop new alloys that can be used effectively as large industrial gas turbine (IGT) blade components, withstand harsh operating conditions, and provide superior performance compared to state-of-the-art turbine blades.

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Developing Stronger, More Durable Alloys through Integrated Computational Materials Engineering

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Researchers create computer models of the cracked structure to trace microstructure evolution and plasticity during cracking.

In the world of alloy development, nickel is the material of choice for high-temperature applications because of its outstanding high-temperature strength and corrosion resistance. Alloying nickel with other select materials produces super tough alloys known as “superalloys.” As the need for higher efficiencies grow, superalloys must be made even tougher.

Under an NETL-managed project, QuesTek Innovations LLC is designing and developing an innovative superalloy for industrial gas turbine (IGT) blade components that is more cost-effective and easier to manufacture compared to state-of-the-art superalloys. QuesTek used its proprietary integrated computational materials engineering (ICME) tool and design platform to address the task. ICME tools enable researchers to link materials models at multiple scales to better understand how fabrication methods produce structures on the micro-scale, how those structures affect material properties, and what materials will be most suited for particular applications.

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QuesTek designed a new single-crystal nickel-based superalloy called QTSXTM that exhibits superior high-temperature properties—especially creep resistance—and can be cast effectively as large, defect-free components. Major improvements in alloy performance have been associated with optimizing alloying elements, especially reducing rhenium while maintaining creep strength since rhenium significantly drives up costs. Because of its low availability relative to demand, rhenium is among the most expensive of metals. QuesTek’s QTSX superalloy composition achieves the same superior creep performance of a rhenium-containing alloy, but without the added expense. Another benefit is the QTSX’s castability. Alloys must be easily cast (manufactured) without defects for industry adoption. Traditionally, “freckling” has been an obstacle during casting—a major defect that poses a weakness in the alloy as a crack-initiation site and degrades the mechanical properties. The QTSX single crystal superalloy, developed by use of a novel computational modeling tool, has demonstrated freckle-free casting of full-scale IGT blades. Moreover, preliminary assessments show that QTSX is comparable to alloys containing more than twice the amount of rhenium (1 weight percent versus 3 weight percent) regarding microstructural stability, oxidation resistance, and creep performance. Casting trials at full-scale blade have shown that QTSX is defect free, which would drastically improve manufacturing yields of IGT blades.

Using Computational Modeling to Improve Component Life-Prediction Methodologies

Creep is a permanent deformation that occurs under stress at elevated temperatures. Fatigue is localized, progressive structural damage that occurs under cyclic loading. For designers of high-temperature components creep and fatigue are serious problems. Materials in high-temperature systems operate at the conditions that will result in both types of damage. Most gas turbine users want increased operating lifetimes and the flexibility to enable various operating cycles or “cyclic loading.” The starts and stops and changes in power generating output imposed on gas turbines during these transient operations can lead to significant materials damage due to thermal and mechanical stress.

Evaluating the creep-fatigue interactions in IGT materials through computational materials results in increased efficiency and reduced costs by enabling more precise materials design.

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Purdue University, as part of an NETL-managed project, is developing tools to predict creep-fatigue interactions in nickel-based superalloys to predict creep-fatigue crack growth, interaction, and life prediction methods for Inconel 718—a nickel-based superalloy that is highly oxidation- and corrosion-resistant. The alloy is also well suited for service in extreme pressure and heat environments.

A team led by Dr. Thomas Siegmund is developing a model to predict creep-fatigue under various loads that will be tested and validated by experiments. Once developed, the Purdue team’s model could be embedded into standard finite element software* as an add-on analysis tool for gas turbine designers. The work will improve the cost-effectiveness of turbine design while ensuring a high level of safety for turbine operation. In addition, the new model can reduce turbine manufacturing costs and maintenance costs through increased blade life, leading to lower plant costs and lower electricity costs.

In another project managed by NETL, Dr. Rick Neu at Georgia Tech is developing a microstructure model for single-crystal nickel-based superalloys to assess long-term creep fatigue interactions and life prediction for turbine components. Advanced computational techniques enable researchers to predict the structure properties of materials at very small scales. A material’s microstructure—the small-scale structure of a material—can strongly influence physical properties including strength, corrosion resistance, and temperature resistance. The Georgia Tech team will investigate microstructural properties of single-crystal nickel-based superalloys to understand how long-term exposure to the stresses of IGT service environments degrades the microstructure and what this evolution means for performance of IGT components. The model developed under this project will allow improved component life, which will enable decreased maintenance costs and reduced costs for electricity. The model will be validated by long-term experimental studies that systematically ages the alloy under different stress conditions with specific emphasis on the role of microstructure.

Computational materials modeling is enabling the rapid design and simulation of new and novel alloys that are essential for advanced power generation systems to achieve performance, efficiency, and cost goals.

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