Silicon distribution and silicide precipitation during annealing in an advanced multi-phase γ-TiAl based alloy

Intermetallic TiAl alloys solidifying via the disordered beta phase, such as the TNM alloy, are promising gamma-TiAl based materials as balanced mechanical properties are attained by conventional manufacturing routes and subsequent heat-treatments. However, a further enhancement of creep resistance is required to exceed present service temperature limitations. In this regard, alloying with light elements, such as C and Si, is of particular interest. Underlying strengthening mechanisms are well established in case of C, but matter of debate in case of Si. In this study the microstructural evolution and preferential distribution of the alloying elements in the constituent phases of a derivative of the TNM alloy containing Si is investigated after a two-step heat treatment by atom probe tomography, scanning and transmission electron microscopy as well as X-ray diffraction. During gamma lamellae formation upon annealing, Si is repelled from the gamma phase and accumulates in the alpha(2) phase. In both phases Si preferentially resides on Al lattice sites as evidenced by ab initio calculations. Neither a Si excess nor precipitate formation at lamellar alpha(2)/gamma interfaces is observed. Precipitates are, however, perceived to form in the regions between the colonies, more precisely in the beta(0) phase. These particles are identified as zeta-silicides, which are enriched in Nb and Al. Accompanying ab initio calculations, used to study the phase preference, provide evidence that in beta(0) phase containing alloys, silicides most likely precipitate in this phase, whereas silicide precipitation within the other phases, gamma, alpha(2), or omega(0) is improbable as these phases are energetically stabilized by the incorporation of Si atoms. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.