Correlation between microstructure and creep behavior of the high-temperature Ti alloy IMI 834

The creep behavior of the near alpha Ti alloy IMI 834 was in vestigated in the temperature regime around 600 degrees C, comparing a fully lamellar and two bi-modal microstructures consisting of 15 and 40 vol.% equiaxed primary alpha phase in a lamellar matrix of transformed beta phase. The width and length of the alpha lamellae of all three microstructures was varied by changing the rate of cooling from the recrystallization temperature. The width of the alpha lamellae decreased continuously with increasing cooling rate. The creep resistance exhibited the generally observed dependency to increase slightly with increasing cooling rate, to pass over a maximum and then to decrease drastically. This cooling rate dependent creep resistance is tentatively thought to be controlled by two competing mechanisms, an increasing amount of deformation strengthening as a result of the continuously decreasing interface distance and an increasing rate of recovery of dislocations at interfaces as a result of the drastically increasing density of interfaces which depends in a first approximation on the reciprocal interface distance. The observed lower creep resistance of the bi-modal as compared to the fully lamellar structure is thought to result from the alloy element partitioning effect reducing the strength of the lamellar matrix of the bi-modal in comparison to the fully lamellar microstructure.