Carbon distribution in multi-phase γ-TiAl based alloys and its influence on mechanical properties and phase formation

Advanced intermetallic gamma-TiAl based alloys are attractive light-weight materials for high-temperature application. In order to extend their service temperature limits, alloying with low-density elements, such as C, is of particular interest and has been shown to effectively increase high-temperature strength as well as creep resistance. In the present study the local chemical composition of the constituent phases of the so-called TNM alloy and a C-containing derivative thereof is characterized by atom probe tomography. In both alloys Mo is found to preferentially locate in the beta(o) phase, in contrast to Nb, which is dispersed in similar levels in all phases. In the C-containing alloy, C is enriched in the alpha(2) phase, dissolved in the gamma phase, but depleted in the beta(o) phase. Furthermore, the investigation of interfaces through site-specific sample preparation reveals segregation of C at phase interfaces and their close vicinity. Finally, a correlation of the mechanical properties with the C distribution is established by nanoindentation technique. Both the gamma and the alpha(2) phase significantly harden through the addition of C, which is in good agreement with the C concentration present within these phases as observed by atom probe tomography. However, the beta(o) phase softens through the addition of C, which is not a direct consequence of the C distribution, but follows from the absence of finely dispersed omega(o) particles in the beta(o) phase of the C-containing alloy. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.