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

摘要

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 degrees C for 1 h, the Al5Ti3 superstructure develops in the L1(0) (gamma) 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 L1(0) 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 [118.6] 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 di. culty to move ordinary dislocations. Under the [118.6] 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 L1(0) phase into a higher energy complex extrinsic stacking fault.