Structural Utilization of the Intermetallic Compound TiAl - an Attempt to Ductilize TiAl by Controlling its Lamellar Structure -

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Ti-rich TiAl has been recognized as a high-temperature structural material of high strength/density ratio. A major problem has also been recognized, poor ductility at ambient temperatures. The present authors' group has succeeded in controlling the micro-structure evolution during the unidirectional solidification of TiAl compounds. We have made a study of the uniaxial deformation and rolling of TiAl with such a controlled microstructure and obtained the results which can be summarized as follows.(1) Yield stress is high when the lamellae are parallel or perpendicular to the loading axis and it is very low for intermediate orientations.(2) When the lamellae are parallel or perpendicular to the loading axis, shear deformation proceeds across the lamellar boundaries, while shear deformation occurs parallel to the lamellar boundaries for intermediate orientations.(3) A room temperature tensile elongation of about 20%, was obtained for a specimen whose tensile axis is inclined at 511 to the lamellar boundaries.(4) Deformation mechanisms of TiAl with the TAl two phase structure are found to be the true twinning of the {111}-type and slip along <110].(5) When the two-phase TiAl compounds with the controlled microstructure can be rolled to as much as 50% reduction in thickness even at room temperature.(6) When TiAl compounds are rolled at room temperature and subsequently annealed in the range 900, recovery in microhardness occurs in two stages. The first and the second stages correspond respectively to the rearrangement and annihilation of dislocations and the annealing-out of deformation twins.(7) Recrystallization mode of cold-rolled PST crystals depends on rolling reduction. When the rolling reduction is smaller than 20%, recrystallization occurs without disturbing the lamellar structure characteristics. Therefore, it should be possible to attain a rolling reduction higher than 50% thrrough successive roll-and-anneal process.