Production of TiAl sheet by cold-rolling

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Polysynthetically twinned (PST) crystals of TiAl with a nearly stoichiometric composition have been grown and rolled at room temperature. The maximum total reduction in thickness which is attainable without fracture depends on lamellar orientation with respect to the rolling plane and rolling direction. When specimens are oriented such that shear deformation parallel to the lamellar boundaries is operative during rolling and its operation causes lengthening of specimens, such specimens can be rolled up to about 50 % reduction in thickness.The recovery and recrystallization behavior of cold-rolled PST crystals of TiAl was also investigated. The change in microhardness was found to occur in two stages regardless of the amount of reduction ; the first sage associated with the decrease in dislocation density and the second stage related with the annealing-out of deformation twins introduced in TiAl lamellae during rolling. However, the recrystallization mode was found to vary depending on the amount of reduction in thickness. In specimens rolled to less than 20 % reduction in thickness, th full recovery in hardness occurs preserving the lamellar structure. A mechanism is proposed to interpret the unusual recrystallization behavior of PST TiAl cold-rolled to less than 20 %.At the later stage of this research, environmental embrittlement was found to occur in PST TiAl. Then, alloying attempts were made to reduce the environmental loss in ductility. The Cr, Mo, V or Mn modified PST TiAl was found to show a small environmental loss in ductility as compared with the binary PST crystals. In particular, the addition of Cr is effective in reducing environmental embrittlement of TiAl. Recently, we confirmed that the Cr-modified PST TiAl can be cold-rolled to 50 w reduction in thickness much more easily than the binary PST TiAl.