Microstructural evolution of a forged TiAl based alloy during heat treatment from 1 180 ℃ to 1 300 ℃ was investigated. The grain sizes of the alpha phases as well as the sizes and the volume fractions of the gamma phases were evaluated as a function of heat treatment temperature and time. When the alloys are isothermally heat treated at subtransus temperatures, the sizes of gamma phases(D γ) increase slightly with heat treatment temperature and time and those of alpha phases(D α) and the volume fractions of gamma phases(φ γ) vary significantly with holding time in the early stages of heat treatments, but after heat treatments for 2 h, φ γ reveal little variations with holding times and D α approach limits, which can be described by D α0 =0.65D γ/φ γ. Besides, it has been found that the alpha phases in the specimens heat treated at 1 260 ℃ and 1 300 ℃ contain lamellar structures, at low temperatures, however, appear featureless.
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