Effects of Al concentration and resulting long-period superstructures on the plastic properties at room temperature of Al-rich TiAl single crystals

Abstract

The role of Al5Ti3 and h-Al2Ti long-period superstructures on the plastic properties of TiAl at room temperature is investigated on five single crystals with aluminium content comprised between 54.7 at.%, and 62.5 at.%. After annealing at 1200°C for 1 h, the Al5Ti3 superstructure develops in the L10 (γ) matrix upon increasing Al concentration except for Ti–62.5 at.%Al where h-Al2Ti substitutes for Al5Ti3. The CRSS for <110]{111} first increases abruptly with the development of the Al5Ti3-type ordering. Then, the CRSS reaches a plateau at which dislocations assemble in groups of four to prevent extra anti-phase boundary (APB) from being engendered during glide throughout the Al5Ti3 phase. In Ti–62.5 at.%Al, the CRSS for ordinary slip further increases upon the precipitation of h-Al2Ti in the L10 phase, whereas it decreases when the crystal is fully transformed into single-phased Al5Ti3. <101] superlattice dislocations are primarily activated under both the [210] and [1 1 8.6] load orientations irrespective of the Al concentration, but the dislocation microstructure strongly depends on orientation as well as on the degree of Al5Ti3 ordering. In the [210] orientation, the frequency of the decomposition of <101] dislocations into 1/2<110] and 1/2<112] dislocations decreases abruptly with the development of Al5Ti3. This is interpreted in terms of the increased difficulty to move ordinary dislocations. Under the [1 1 8.6] orientation, the density of faulted dipoles diminishes remarkably with the development of Al5Ti3. This is consistent with the transformation of the low energy extrinsic stacking fault of the L10 phase into a higher energy complex extrinsic stacking fault.