Discontinuous coarsening of the primary $\alpha_2/\gamma$ lamellar sturcture in Ti-44at.%Al-0.5at.%Nb alloy was studied in a temperature range between 900 and 1150℃, using optical microscope and transmission electron microscope. On holding the specimen at isothermal aging temperatures lower than the eutectoid temperature, the interlamellar spacing of the primary lamellar was nearly constant with aging time. But, the primary lamellar spacing tends to continuously increase with the aging time at temperatures higher than the eutectoid temperature.
The coarse secondary lamellar structure nucleated at the α grain boundaries and grew into adjacent grains accompanying the grain boundary migraion. The growth rate of secondary lamellar structure increases quite linearly with the aging time at temperature below eutectoid temperature. The growth rate of the moving boundary tends to continuously increase with aging temperature up to the eutectoid temperatures. Secondary lamellar growth slightly slower in Ti-44at.%Al-0.5at.%Nb as compared to the case of corresponding binary intermetallic compound of Ti-44at.%Al. The lamellar spacing of both the secondary and primary lamellar structures increase with the increase in the aging temperature. The primary lamellar spacing in the intermetallic compound was larger as compared to the case of binary intermetallic compound at the otherwise same aging condition.
These kinetic data on the secondary and primary lamellar structures have been analyzed using the kinetic model of Fournelle. The free energy change has been calculated using Chang's model for the ordered phase. The volume fraction of $\gamma$ phase has been determined using TEM study.
The result yields that the activation energy for the growth rate of secondary lamellar structure is 342.7kJ/mole. The diffusivity calculated to be $1.087*10^{11}$ at 900℃. This diffusivity is similiar to grain boundary diffusivity in $\alpha_2$. However the above activation energy appears to be too large for a grain boundary diffusion.
