Formation of Z-Ti{sub 50}Al{sub 30}O{sub 20} in the sub-oxide zones of {gamma}-TiAl-based alloys during oxidation at 1000 C
- Univ. of New South Wales, Sydney , New South Wales (Australia). School of Materials Science and Engineering
- Univ. of New South Wales, Sydney, New South Wales (Australia). School of Materials Science and Engineering
The oxidation behavior of {gamma}-TiAl and {alpha}{sub 2}-Ti{sub 3}Al + {gamma}-TiAl alloys was studied at 1,000 C in pure oxygen up to 250 h. All alloy coupons were polished to a 1 {micro}m finish prior to oxidation. A series of {gamma}-Ti{sub 48}Al{sub 52} coupons polished to a 600-grit finish were also prepared. The type of scale which formed during oxidation and the consequent subsurface changes in the alloy were found to depend on the presence and amount of {alpha}{sub 2} in the starting alloy microstructure, surface finish, and oxidation time. The {gamma}-alloy with a 600-grit finish formed a protective Al{sub 2}O{sub 3}-rich scale during the early stages of oxidation, whereas the same alloy with a 1 {micro}m finish formed a less protective Al{sub 2}O{sub 3} + TiO{sub 2} scale. In the former case, a continuous subsurface layer of Z-Ti{sub 50}Al{sub 30}O{sub 20} formed from {gamma} by essentially equal amounts of oxygen enrichment and aluminum depletion; essentially no titanium diffusion was required. With continued oxidation, the continuous Z-layer destabilized at the Z/{gamma} interface into a mixture of {alpha}{sub 2}(O) + Z. The presence of {alpha}{sub 2}(O) + Z in the subsurface zone always corresponded to the growth of a non-protective Al{sub 2}O{sub 3} + TiO{sub 2} scale, suggesting that {alpha}{sub 2}(O) precipitation initiates the breakdown of the initially formed Al{sub 2}O{sub 3}-rich scale. The subsurface zone in the {gamma} alloy with a 1 {micro}m finish varied, containing either internal Al{sub 2}O{sub 3} and no Z phase or {alpha}{sub 2}(O) + Z. The {alpha}{sub 2} + {gamma} alloys formed a non-protective Al{sub 2}O{sub 3} + TiO{sub 2} scale and subsurface zone of {alpha}{sub 2}(O) + Z at all the times studied. The discontinuous distribution of {gamma} in the two-phase alloys prevented formation of a continuous Z layer. The Z phase was determined to be metastable on the basis of equilibration experiments with Ti-Al-O alloys.
- OSTI ID:
- 684394
- Journal Information:
- Acta Materialia, Vol. 47, Issue 10; Other Information: PBD: 10 Aug 1999
- Country of Publication:
- United States
- Language:
- English
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