Project No: FE0003798
Performer: 


Contacts

Duration
Award Date:  06/17/2010
Project Date:  12/30/2013

Cost
DOE Share: $280,000.00
Performer Share: $0.00
Total Award Value: $280,000.00

Performer website:  - http://www.tnstate.edu

Crosscutting Research - University Training and Research

Computational Studies of Physical Properties of Nb-Si Alloy

Project Description

Niobium-silicon based alloys are highly promising materials for use in next generation turbines operating at the elevated temperatures required for advanced power generation systems. Niobium has an array of attractive properties for high-temperature applications, but it suffers from low oxidation resistance and modest high-temperature mechanical properties. Alloying Nb with Si introduces intermetallic phases that significantly improve its high-temperature mechanical properties. The intermetallic phases have a high melting point, but are brittle at room temperature. However, by incorporating a ductile Nb solid solution phase, the resulting composite can be strengthened. Some roadblocks to their eventual industry acceptance remain. Alloying with a third element has been shown to improve oxidation resistance. The evolution of Nb-Si alloy development is very complex with the alloy often being composed of more than seven elements.

In an effort to manage this complexity and further the development of a suitable Nb-Si based alloy, the project team will provide physical properties data supplementing experiments for thermodynamic modeling, phase field simulation for microstructure, and continuum simulations for mechanical properties of these alloys. The project team will further develop their Gibbs free energy package to support free energy calculations of structure types. The project team will then develop an interface between the Gibbs free energy package G(P,T) and the popular Calculation of Phases Diagram (CALPHAD) software Thermo-Calc®. The integrated tool will be utilized to study thermodynamic properties of various phases found in Nb-Si-based alloy with a focus on the quaternary system Nb-Si-chromium (Cr)-X—where X is some other metal, such as titanium (Ti), aluminum (Al), hafnium (Hf), or molybdenum (Mo).

The project will also study interfaces found in the Nb-Si based alloys. Structural models for the interfaces among the main bulk phases, the Nb solid solution, and Nb silicides will be developed. Formation energies of the undoped and doped Nb-Si-Cr will be calculated and compared. Interfacial mechanical properties will be evaluated using similar approaches developed for ceramic interfaces.


Program Background and Project Benefits

The intent of the Historically Black Colleges and Universities and Other Minority Institutions (HBCU/OMI) Research and Development (R&D) Program within the U.S. Department of Energy (DOE) Office of Fossil Energy (FE) is to establish a mechanism for cooperative FE R&D projects between DOE and the HBCU/OMI community; foster private sector participation, collaboration, and interaction with HBCU/OMI in FE R&D; provide for the exchange of technical information to raise the overall level of HBCU/OMI competitiveness with other institutions in the field of FE R&D; enhance educational and research training opportunities for tomorrow’s scientists by developing and supporting a broad-based research infrastructure; and position HBCU/OMI graduates to assume mainstream leadership and technical roles in America’s FE commerce.

The Crosscutting Research (CCR) Materials Program addresses materials requirements for all fossil energy systems, including materials for advanced power generation technologies such as coal gasification, coal fuel technologies, heat engines such as turbines, combustion systems, fuel cells, and carbon capture technologies. The program is led by the DOE National Energy Technology Laboratory (NETL) and is implemented through R&D agreements with other national laboratories, industry, and academia. The program strategy is to provide a materials technology base to ensure the success of advanced power generation systems being pursued by DOE.

In alignment with these programs, NETL is partnering with Tennessee State University (TSU) to develop computer-aided material design to accelerate the development of niobium (Nb)-silicon (Si) based refractory alloys for advanced energy applications.

This project will contribute to the development of materials to be used in power plant steam turbines operating at high temperature and pressure. Development of thermodynamic modeling and other simulations to guide the optimal design of Nb-Si-based alloy for use in advanced power generation systems will lead to higher operating efficiency and a corresponding reduction in carbon dioxide emission.

Goals/Objectives

The overall goal is to provide insight into the mechanisms and processes that could lead to next generation hot section material operating at temperatures beyond 1350 °C, which could play an important role in achieving energy production with reduced harmful environmental effects. Specific objectives include (1) developing a supercell approach to evaluate physical properties of alloys which maintains various order and disorder bulk phases and interfaces, and (2) applying the supercell approach to Nb-Si based alloy to compute physical properties data that can be used for thermodynamic modeling and other simulations to guide the optimal design of Nb-Si-based alloy.


Accomplishments

TSU has demonstrated the Gibbs free energy module for calculating temperature pressure dependent elastic constants on several systems including the Nb metal, Nb-Si alloys, and silicon carbide (SiC). For insulator SiC, the calculated temperature coefficients of elastic constants for both cubic and hexagonal SiC are in excellent agreement with the experimental data. For Nb, the calculated elastic constants at low temperature showed notable error. The error has been traced to insufficient k-space sampling and supercell size used in the phonon calculation. More accurate calculations are planned. TSU researchers have completed the calculations of Gibbs free energy, specific heats, thermal expansion coefficients, entropy, and elastic constants for all known crystal phases of the binary Nb-Si alloys and are currently working on thermodynamic databases for Nb-Si-Cr ternary systems for selected crystal phases. TSU is continuing to improve the post-processing codes for Gibbs free energy calculation, temperature and pressure dependent elastic constants calculation, and other thermodynamic properties. The draft temperature pressure dependent elastic constants of SiC have been completed and will be submitted for review soon. TSU is currently developing a flexible cluster variation method that will enable computation of the free energy and stress for disordered systems and refining the calculation of elastic constants based on free energies in proximity to the temperature-hydrostatic pressure surface of states.