Development of a Computational Material Science-Based Methodology for Alloy/Composite Design, 18-9135

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Principal Investigators
Kwai S. Chan
Robert E. Beissner
David L. Davidson

Inclusive Dates: 04/01/99 - Current

Background - Many in situ composites show potentials for high-temperature structural applications, but they need simultaneous improvements in oxidation and fracture resistance. The objective of this program is to develop a computational material-science-based methodology to aid the design of alloy composition and microstructure of in situ composites with sufficient oxidation and fracture resistance for high-temperature service. The proposed methodology is generic, but is applied in this program to Nb-based in situ composites containing Cr-, Ti-, Al-, and Si-alloying elements and Nb5Si3 or NbCr2 intermetallics. This material system has been selected because of its potential applications in advanced gas turbines.

Approach - Several computational methods are being used to develop the composite design methodology. For designing fracture-resistant materials, a quantum mechanics-based cluster atom model and an analytical model are being developed to predict the unstable stacking energy and Peierls-Nabarro (P-N) energies of composite constituents, respectively. In addition, analytical methods are also being developed to predict the oxidation and fracture resistance of the in situ composite as functions of microstructure and properties of constituent phases. These models will be incorporated into one computer software so that the composition and microstructure of candidate in situ composites can be optimized to attain the desired levels of fracture and oxidation resistance.

Accomplishments - A quantum mechanics-based cluster atom model with expended s, p, and d orbital wave functions has been developed for computing the unstable stacking energy of Nb-Ti-Si-Al solid solution alloys. Analytical methods were also developed to compute the unstable stacking energy and P-N barrier energy. Furthermore, a fracture model has been developed to treat brittle-phase embrittlement and ductile-phase toughening in in situ composites. Computational results indicated that the unstable stacking energy is insensitive, but the P-N barrier energy is very sensitive to alloy composition. As a result, the design of ductile alloy and composite compositions is best based on the P-N barrier energy, which is a measure of dislocation mobility. A composite design model is currently being developed, which includes the P-N analytical model, the composite fracture model, and an oxidation model that will allow optimization of oxidation and fracture resistance of Nb-based solid solution alloys and in situ composites.

Materials Research and Structural Mechanics Program
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