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

Abstract

In Al-rich TiAl crystals, several long-period superstructures may appear depending on Al composition, annealing temperature and annealing time. Amongst these, Al₅Ti₃ and h-Al₂Ti contain pure Al (002) layers, as in the L1₀ 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 Al₅Ti₃ long-period superstructure embedded in the L1₀ matrix develops with increasing Al concentration to finally transform fully into h-Al₂Ti for Ti-62.5at.%Al. On the other hand, Al₅Ti₃ precipitates grow with annealing time at 500°C in Ti-58.0at.%Al.

The effects of the Al₅Ti₃ and h-Al₂Ti 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 Al5Ti3 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 Al5Ti3. The CRSS for slip in the <110] direction further increases with the formation of h-Al₂Ti particles within the L1₀ matrix in Ti-62.5at.%Al. By contrast, Ti-62.5at.%Al fully transformed into Al₅Ti₃ exhibits a CRSS significantly lower than that of the two-phase alloy.