The coarsening of lamellae in Ti48Al2Mn2Nb

The thermal stability of as-cast Ti48Al2Mn2Nb alloys has been investigated as a function of time at temperatures of 1200, 1350 and 1420°C. The resultant structures have been characterised using optical microscopy, electron probe microanalysis and transmission electron microscopy. The as-cast structure consists mainly of lamellae of α₂ and γ. Annealing at 1350 and at 1420°C resulted in the continuous and discontinuous coarsening of the primary lamellae. Discontinuous coarsening, which often originates at the sample surface, results in branched irregular secondary lamellae. The average interlamellar spacing of the secondary lamellae is up to a thousand times larger than the primary interlamellar spacing. The extremum principles of maximum velocity and maximum rate of entropy production, used to predict the secondary interlamellar spacing, are inconsistent with the experimental observations. The theoretical predictions of the velocity of the discontinuous coarsening by Livingston and Cahn are used to estimate the value of grain boundary diffusivity at 1350°C. The temperature dependence of discontinuous coarsening is weak, in contrast to the strong effect of lamellar orientation on the rate of discontinuous coarsening.