The thermal stability of the microstructure of γ-based titanium aluminides

The thermal stability of the microstructure of γ-based titanium aluminides after aging at elevated temperatures was studied by transmission and analytical electron microscopy (TEM and AEM, respectively) as well as by optical microscopy. Predominantly lamellar microstructures of Ti-47Al (at.%), Ti-47Al + B (140appm) and Ti-47Al + B + W (0.5at.%) alloys were produced by heat-treatment at 1400°C for 1h followed by furnace cooling. The average lamellar spacing of the as-heat treated was measurably finer in both alloys with solute additions (400nm in Ti-47Al, 170-190nm in alloys modified with B or W + B). The α₂ lamellae were especially finely distributed and continuous in the T1-47Al + W + B alloy. These alloys were then aged at 800, 1000 or 1200°C for 168h. The alloys aged at 800°C did not show much change, whereas aging at 1000°C produced significant lamellar break-up and coarsening in the Ti-47Al and B-modified alloys, but not much change in the alloy modified with W + B. In addition, coarsening by grain boundary migration led to the formation of new coarser, irregular and branched lamellae. Aging for 168h at 1200°C significantly degraded the originally fine lamellar structure. In addition to the lamellar degradation processes observed at 800-1000°C, spheroidized precipitates of α₂ and γ were also observed at 1200°C. At all three aging temperatures the cut surfaces of the samples showed additional effects of aging not observed in the bulk. These included the loss of Al from the sample during vacuum annealing, and the recrystallization and growth of new grains into the lamellar structure. After aging at 800 and 1000°C, these sub-surface microstructural changes were found to be greatest for the case of the binary alloy and least for the Ti-47Al + W + B alloy. The observed changes of the lamellar structure during aging were found by TEM to be related to the following defects present in the lamellar structure prior to aging: low angle grain boundaries, misoriented lamellae, lamellar interfaces and lamellar colony boundaries. Some of the observed modes of microstructural instability were fault and edge migration, the dissolution of lamellae at lamellar interfaces, and the formation of new y grains within the lamellar structure. At 1200°C, even the coarsened lamellar structure ultimately spheroidizes, particularly in the binary alloy. Other kinds of microstructural instability observed included the migration of colony boundaries, the penetration of one colony into an adjacent colony, and the growth of y grains found at colony boundaries into the lamellar structure of that same colony. The effect of alloying additions of B and W to the binary alloy is to refine the initial lamellar structure produced by heat-treatment at 1400°C, and then to stabilize that original lamellar structure (particularly B + W) during subsequent aging at 800-1000°C.