VARIOUS TRANSFORMATION MODES OBSERVED IN 2-PHASE GAMMA PLUS ALPHA(2) TIAL-BASED ALLOYS

This paper deals with various transformation modes taking place in two-phase gamma+alpha(2) TiAl-based alloys. These transformation modes are: (1) decomposition of the a phase leading to the precipitation of the gamma lamellae (alpha-->alpha+gamma), (2) ordering reaction of the alpha phase (alpha --> alpha(2)), (3) massive transformation (alpha --> gamma), (4) formation of monolithic gamma grains and (5) discontinuous coarsening. By analyzing their kinetics, three types of competitions were identified between these transformation modes: (1) vs. (2), (2) vs. (3) and (4) vs. (5). The occurrence of (2) or (3) and (4) or (5) in binary alloys is strongly dependent on the alloy composition. Therefore, by examining these two competitions in transformation modes, it is possible to evaluate the influence of small additions of ternary and quatemary elements on the response to heat treatments without a precise knowledge of the phase diagrammes of complex alloy systems. Another important finding of the present study is related to the formation of the gamma phase from the alpha phase; this can occur through either lamellar precipitation (alpha --> alpha(2) --> alpha(2)+gamma, or alpha --> alpha+gamma --> alpha(2)+gamma) or massive reaction (alpha --> gamma). The mechanisms involved during these two transformations are investigated and some of the key issues are discussed in the present paper.