Development of fundamental processes of directional solidification of TiAl-base Alloy

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Mechanical properties of the lamellar microstructure of TiAl-base alloys are extremely anisotropic with respect to the lamellar orientation. However, if the lamellar microstructure can be aligned parallel to the growth direction, the resulting material should possess a good combination of mechanical properties. The seeding and directional solidification (DS) techniques were developed for the lamellar boundaries to be aligned parallel to the growth direction. The lamellar structure of a wide variety of TiAl-base alloys can be controlled using the techniques. DS ingots of these alloys exhibit an excellent combination of room temperature tensile properties and high temperature strength, in particular, creep strength. Attempts were made to increase the size and growth rate of DS ingots for larger-scale production of TiAl castings through seeding and directional solidification. In order to obtain information on the mechanical behavior of DS ingots under the state of stress with multi-components, compression tests of DS ingots with various lamellar orientations were made under the constraint imposed along a direction perpendicular to the loading axis. Such constraint was found to significantly increase the yield stress of DS ingots with intermediate lamellar orientations. Slip systems which operate under the constraint were examined and the results were found to be in good agreement with those of the deformation analysis based on the description of anisotropic elastic continuum.