TY - JOUR
T1 - Microstructural evolution of a nanocrystalline Ti-47Al-3Cr alloy during annealing in the α + γ-phase field
AU - Öveçoglu, M. L.
AU - Senkov, O. N.
AU - Srisukhumbowornchai, N.
AU - Froes, F. H.
PY - 1999/3
Y1 - 1999/3
N2 - Prealloyed, gas-atomized (GA) Ti-47Al-3Cr alloy powder, containing about 70 pct of the α 2 (Ti 3Al) 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.
AB - Prealloyed, gas-atomized (GA) Ti-47Al-3Cr alloy powder, containing about 70 pct of the α 2 (Ti 3Al) 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.
UR - http://www.scopus.com/inward/record.url?scp=0033099021&partnerID=8YFLogxK
U2 - 10.1007/s11661-999-1006-8
DO - 10.1007/s11661-999-1006-8
M3 - Article
AN - SCOPUS:0033099021
SN - 1073-5623
VL - 30
SP - 751
EP - 761
JO - Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science
JF - Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science
IS - 13
ER -