Microstructural evolution of a nanocrystalline Ti-47Al-3Cr alloy during annealing in the α + γ-phase field |
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Authors: | M L Öveçoglu O N Senkov F H Froes N Srisukhumbowornchai |
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Affiliation: | (1) the Department of Metallurgical Engineering, Istanbul Technical University, 80626 Maslak, Istanbul, Turkey;(2) the Institute for Materials and Advanced Processes (IAMP), College of Mines and Earth Resources, University of Idaho, 838744-3026 Moscow, ID;(3) Present address: IMAP, University of Idaho, USA;(4) University of Utah, 84112 Salt Lake City, UT |
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Abstract: | Prealloyed, gas-atomized (GA) Ti-47Al-3Cr alloy powder, containing about 70 pct of the α
2 (Ti3Al) phase and 30 pct of the γ (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 α
2 and γ and then to a fully γ 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 α+γ field, at 1200 °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 γ 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 μm. 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 γ grains as a result of annealing and air cooling and exhibited a common twinning plane of (111) with the matrix phase. The
minimum γ grain size in which twinning occurred in the annealed specimens was determined to be 0.25 μm, which suggests that twinning is energetically unfavorable in the nanometer-sized grains. |
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