Instability mechanisms in lamellar microstructures

The shape instabilities associated with a lamellar structure when exposed to elevated temperature have been analysed. These instabilities were compared with experimental data from Ti-47Al, Ti-47Al-140 a.p.p.m. B and Ti-47Al-0.5 at.% W-140 a.p.p.m. B alloys. Instability maps, which were constructed for an isolated plate, show that the dominant mechanism controlling the coarsening of lamellae changes with time and also show that coarsening of lamellae with an initially large aspect ratio is controlled by termination migration, until the aspect ratio falls to some critical value below which cylinderization becomes the dominant mechanism. Instability maps were also constructed using the criterion of maximum mass transport rate per unit volume. Such volume instability maps show that the coarsening rate associated with a unit volume of the lamellar structure depends on the density of faults and that termination migration and boundary splitting occurs at a considerable rate as compared with other continuous coarsening mechanisms. The effect of alloying additions of B and W to the Ti-47Al alloy was to reduce the coarsening rate of the initial lamellar structure.