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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 energies of composite constituents, respectively. In addition, analytical methods are 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 a single software program 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 - The capability of the cluster atom model has been expanded by adding an extended set of wave functions that treats the valence state s, p, and d orbitals for Al, Si, Cr, Ti, and Nb. A set of calculations of the total electronic energy in certain reference crystals has been performed, as a function of lattice parameter to determine constants needed in the unstable stacking energy calculations. Software programs for calculating atom positions and slip energetics in a cluster of atoms elongated along the slip plane have been completed. Elastic properties and lattice parameters of Nb solid solution alloys, NbCr2, and Nb5Si3 are being collected from the literature, and these data will be used as input to the cluster atom model for the unstable stacking energy calculation. Analytical models for computing the Peierls-Nabarro barrier energy, fracture resistance, oxidation resistance, and optimum microstructures are being developed.

Materials Research and Structural Mechanics Program
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