Effects of long-period superstructures on plastic properties in Al-rich TiAl single crystals

摘要

In Al-rich TiAl crystals, several long-period superstructures may appear depending on Al composition, annealing temperature and annealing time. Amongst these, Al_5Ti_3 and h-Al_2Ti contain pure Al (002) layers, as in the L1_0 structure of the matrix, alternating with Ti (002) layers that exhibit an ordered arrangement of the Al atoms in excess. In single crystals with compositions ranging from Ti-54.7at.%Al to Ti-62.5at.%Al annealed at 1200°C, the Al_5Ti_3 long-period superstructure embedded in the L1_0 matrix develops with increasing Al concentration to finally transform fully into h-Al_2Ti for Ti-62.5at.%Al. On the other hand, Al_5Ti_3 precipitates grow with annealing time at 500°C in Ti-58.0at.%Al. The effects of the Al_5Ti_3 and h-Al_2Ti superstructures on slip properties of 1/2 < 110] ordinary dislocations are examined both at a macroscopic and a microscopic level. The CRSS for 1/2 < 110] ordinary slip increases with Al_5Ti_3 ordering depending on Al composition, or of annealing time in the case of Ti-58.0at.%Al. Dislocations with 1/2 < 110] Burgers vector group into fourfold configurations to avoid the trailing of extended APBs in Al_5Ti_3. The CRSS for slip in the < 110] direction further increases with the formation of h-Al_2Ti particles within the L1_0 matrix in Ti-62.5at.%Al. By contrast, Ti-62.5at.%Al fully transformed into Al_5Ti_3 exhibits a CRSS significantly lower than that of the two-phase alloy.