Directional solidification of TiAl alloys with high Nb concentrations for aligning lamellar microstructures

The TiAl/Ti3Al lamellar structure of TiAl alloys with high Nb concentrations (up to 8 at.% Nb additions) has been successfully aligned parallel to the growth direction through directional solidification by using an appropriately oriented seed from the TiAl-Si system (Ti-43Al-3Si) on the basis of the recently proposed method to predict the appropriate alloy compositions with the concept of 'Al-equivalent'. The value of Al-equivalent for Nb is found to be constant up to 3 at.%Nb but is variable depending on the amount of addition above 3 at.%Nb because of a curvature of the alpha/beta primary phase boundary in the Ti-Al-Nb system. These DS ingots with the lamellar structure all aligned parallel to the loading axis exhibit a very nice balance of mechanical properties such as yield strength, tensile ductility and creep properties, leading to a possibility of alloy design of high property level. (AU)