Effect of B addition on thermal stability of lamellar structure in Ti-47Al-2Cr-2Nb alloys

Thermal stability of fully lamellar microstructures in Ti-47Al-2Cr-2Nb (at.%) alloys with and without B additions has been evaluated in the temperature range of 800 to 1200 degrees C. The alloy with 0.15B exhibits an alpha(2)+gamma fully lamellar microstructure containing ribbon-like structures (TiB2+beta) inside the lamellar colonies. During aging at 800 and 1600 degrees C, beta particles surrounded by gamma grains form adjacent to the ribbon-like structures. The formation of these grains is attributed to the precipitation of thin plate-like 0 phase along {1100}(TiB2) within the TiB2 ribbon particles, resulting in the loss of alpha(2) plates around the ribbon-like structure by scavenging Ti. This allows coarsening gamma lamellar plates to form new gamma grains, and then the ribbon-like structure completely dissolves to leave only particles. At 1200 degrees C, discontinuous coarsening (DC) of the lamellae becomes the dominant mode of microstructural change for all the alloys. However, the ribbon-like structures in the alloy with B additions play a role in pinning the DC cells, which leads to higher thermal stability of the lamellar microstructure in that alloy.