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 - 12/31/00

Background - Many in-situ composites show potential for high-temperature structural applications, but they need simultaneous improvements in oxidation and fracture resistance.

Approach - 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. Several computational models have been developed for predicting the oxidation and fracture resistance of Nb-based in-situ composites as functions of microstructure and properties of constituent phases. These models have been 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 was 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 Peierls-Nabarro (P-N) barrier energy. Furthermore, a fracture model was 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. A composite design model was developed based on the P-N analytical model, the composite fracture model, and an oxidation model. This computational model is currently being extended in an AFOSR-funded program to optimize the creep, oxidation and fracture resistance of Nb-based solid solution alloys and in-situ composites.

Design of tough Nb-Ti-Cr solid solution alloys using the computational
 tools developed in this program

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