Microstructural evolution of a nanocrystalline Ti-47Al-3Cr alloy during annealing in the {alpha} + {gamma}-phase field
- Istanbul Technical Univ. (Turkey). Dept. of Metallurgical Engineering
- Univ. of Idaho, Moscow, ID (United States)
Prealloyed, gas-atomized (GA) Ti-47Al-3Cr alloy powder, containing about 70% of the {alpha}{sub 2} (Ti{sub 3}Al) phase and 30% of the {gamma} (TiAl) phase, was fully amorphized by mechanical alloying. The amorphous phase was stable during heating to 600 C, but decomposed at higher temperatures, with an exothermic reaction peak at 624 C as the material transformed to a mixture of {alpha}{sub 2} and {gamma} and then to a fully {gamma} structure at 722 C. A nanocrystalline compact with a mean grain size of 42 nm was obtained by hot isostatic pressing (HIP`ing) of the amorphous powder at 725 C. Isothermal annealing experiments were conducted in the two-phase {alpha} + {gamma} field, at 1,200 C, using holding times of 5, 10, 25, and 35 hours, followed by air cooling. The X-ray diffractometry and analytical transmission electron microscopy investigations carried out on annealed and air-cooled specimens revealed only the presence of the {gamma} grains, which coarsened on annealing. Initially, the grains grew, followed by a saturation stage after annealing for 25 hours, with a saturation grain size of about 1 {micro}m. This grain growth and saturation behavior can be described with a normal grain growth mechanism in which a permanent pinning force is taken into account. Twins formed in the {gamma} grains as a result of annealing and air cooling and exhibited a common twinning plane of (111) with the matrix phase. The minimum {gamma} grain size in which twinning occurred in the annealed specimens was determined to be 0.25 {micro}m, which suggests that twinning is energetically unfavorable in the nanometer-sized grains.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 335294
- Journal Information:
- Metallurgical and Materials Transactions. A, Physical Metallurgy and Materials Science, Vol. 40, Issue 3; Other Information: PBD: Mar 1999
- Country of Publication:
- United States
- Language:
- English
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