College Park, Maryland June 6 - 10 , 2004
WP15: Study of Oxygen Dissolution in a2-Ti3Al: Combining the Analysis of Pair Distribution Functions from Total Scattering with First Principles Calculations
I. Peral (NIST Center for Neutron Research; Department of Materials Science and Engineering, University of Maryland), E. Copland (Case Western Reserve University and Materials Division, NASA Glenn Research Center, Cleveland, OH 44135 USA), X. Qiu (Department of Physics and Astronomy, Michigan State University, E. Lansing, MI, 48824), S. Short (Materials Science Division, Argonne National Laboratory), C. Y. Jones (NIST Center for Neutron Research)
The most promising of the g-TiAl-based intermetallic alloys for application as high-temperature structural components are g-TiAl+a2-Ti3A mixtures based on the composition Ti-48 atom% Al. Oxygen interstitials stabilize a2-Ti3Al relative to g-TiAl, thus in a2-Ti3Al+g-TiAl mixtures a2-Ti3Al behaves as an oxygen scavenger. However, limited data are available on the dissolution of O in a2-Ti3Al.
Recently, high-resolution constant wavelength neutron powder diffraction data were used to refine the crystal structures of several a2-Ti3Al alloys with varying O content . This study confirmed that O resides in octahedral sites surrounded by six Ti atoms, as proposed by Menand et al . From this work, it was possible to obtain the average structure, that is, the atomic composition in the unit cell and the atomic positions of the atoms. However, there is no complete information about the local environment of the atoms. A more thorough investigation of the effect of oxygen interstitials on the structure and stability of a2-Ti3Al is required to understand the dramatic difference in O affinity and the behavior of the duplex alloys.
We have measured the total scattering from two specimens of a2-Ti3Al alloys with O weight percentages of 0.025% and 7.7%, to determine from the Pair Distribution Function (PDF) the various correlation lengths for O, Ti, and Al. The comparison of the experimental PDF to the calculated PDFs of a number of local structure models shows that the average structure can describe most of the features of the experimental PDFs and that the PDF is sensitive to the different possible local environments of the atoms. The sampling of the different local structure models is a work still in progress as well as the model refinement to the experimental PDFs. We use Density Functional Theory to probe the energy of models that describe different local structures.
From our preliminary modeling and model refinement, it is observed that the agreement experiment-model is better for local structural models where the atoms that present positional disorder in the system (the aluminum atoms that can be distributed over the titanium sites and the oxygen atoms) are distributed randomly, rather than for models where the aluminum atoms cluster around the oxygen atoms.
1. Jones C. Y., Luecke W. E. and Copland E. (submitted to Intermetallics)
2. Menand A., Huguet A., Nerac-Partaix A., Acta Mater. 1996, 44(12), 4729-4737.
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