- Title
- Engineering the Alpha Two Phase Morphology in Gamma TiAl Based Alloys
- Author
- Meisenkothen, Frederick
- Degree
- Doctor of Philosophy, Ohio State University,
Materials Science and Engineering, 2003.
- Advisor
- Hamish L Fraser
- Pages
- 181p.
- Abstract
- Conventionally processed, fully lamellar, TiAl Based alloys ordinarily possess alpha2 laths having a thickness on the order of hundreds of nanometers. Polycrystalline samples, having compositions between Ti-42Al and Ti-46Al, can be solution heat treated in the alpha phase field, and then rapidly quenched, to produce a microstructure that consists entirely of supersaturated alpha2 phase. Upon aging the supersaturated solid solution, in the alpha2 + gamma phase field, a fully lamellar structure results. In contrast to the conventionally processed fully lamellar TiAl based alloys, the laths resulting from the aging treatment have a thickness on the order of tens of nanometers. Additionally, this nanoscale microstructure consists of rigidly alternating laths of gamma and alpha2. A transformation mechanism from the supersaturated alpha2 to the ultra-fine lamellar structure, via nucleation and glide of dislocation loops is proposed. Consideration is also given to the question of whether or not these ultra-fine lamellar structures can aid slip transmission between the two phases. The gettering of interstitial contaminants, by the alpha2 laths in fully lamellar TiAl based alloys, has been shown to be responsible for the enhanced slip activity observed in the gamma laths of these alloys. However, absorption of the impurities from the gamma, limits the number of active slip systems that can operate in the alpha2 phase. The result is that the alpha2, an already brittle phase, becomes even less likely to undergo plastic deformation. The brittle alpha2 laths constrain the gamma phase, and the overall ductility of the fully lamellar material is reduced. By suitably engineering the sequence of solid state phase transformations leading to microstructural development in TiAl based alloys, it is possible to employ the gettering benefits provided by the alpha2 phase, while eliminating the constraints imposed by it on the gamma phase. The addition of small amounts of the ternary alloying element, Ta, makes it possible to subject the material to a gamma solution heat treatment. Upon cooling from the single phase gamma region, multiple variants of the alpha2 phase precipitate directly from the gamma phase allowing for the formation of discrete alpha2 precipitates in the matrix.
- Keywords
- gamma; TiAl; alpha2; ultra-fine lamellar structure; Ti3Al; slip transmission; phase transformation; precipitation of alpha2 from gamma
Document number: osu1040673861.
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