| The effect of alloying elements specifically, Nb and Mn and heat treatment variables such as temperature, time and cooling rate on phase transformations in TiAl-based alloys, was evaluated on small alloy samples by heat treatment and subsequent microstructural characterization by optical, scanning and transmission electron microscopy. The alloys used in this investigation had nominal compositions of Ti-45Al (alloy I), Ti-45Al-2Nb (alloy II), Ti-45Al-2Nb-0.4Mn (alloy III) and Ti-45Al-2Nb-2Mn (alloy IV), all compositions in atomic percent. The effect of alloying elements (Nb and Mn) and cooling rate on phase transformations was studied by cooling small samples from the alpha-phase field at various cooling rates. Small specimens (∼12mm x 6mm x 6mm) were encapsulated in vycor capsules that were evacuated and backfilled with commercial purity argon. The encapsulated specimens were annealed at 1350°C (in the alpha-phase field) for 30 minutes and subsequently cooled to room temperature by furnace cooling (FC), air cooling (AC), oil quenching (OQ) and water quenching (WQ). The cooling rates induced by FC, AC, OQ, and WQ were measured to be 2--3°C/min, 25°C/sec, 155°C/sec and ∼1000°C/sec, respectively. The microstructure of the heat-treated alloys was observed to vary significantly with cooling rate, and the cooling rate dependence of microstructural evolution was observed to be a strong function of alloy composition. FC resulted in the formation of equilibrium lamellar microstructure. The alloys' susceptibility to massive transformation was found to increase with increased cooling rate. The critical cooling rate required for massive transformation was also observed to vary with alloy composition. The susceptibility to massive transformation decreased with an increase in Mn concentration, that is, the critical cooling rate increased with increase in Mn concentration in the alloy. The rate of transformation, on the other hand, was found to increase with Mn concentration in rapidly quenched specimens. The effect of alloying elements on microstructural evolution has been rationalized in terms of their site occupancies in TiAl and their influence on phase boundaries and on grain size. Based on the microstructural characterization by TEM, tentative mechanism of massive transformation in TiAl-based alloys has been proposed. Schematic CCT diagrams of the four alloys used in this investigation have also been proposed. Alloys III and IV, that contain Mn, exhibit greater potential for microstructural modification.; The effect of grain size on the rate of massive transformation was also investigated on alloy III. Small samples were first wrapped in Ta-foil, encapsulated in vycor capsules and heated at 1350°C for various periods of time, specifically, 30 min, 90 min and 150 min so as to obtain different grain sizes and were subsequently quenched. It was observed that at a fixed composition and cooling rate, an increase in grain size resulted in a decrease in the rate of massive transformation. Schematic CCT diagrams have also been proposed that demonstrate the effect of grain size on the mode of transformation.; The grain growth kinetics of alloy II and alloy III at 1350°C was studied by annealing the alloy samples for various time periods ranging from 15 to 240 minutes. Grain growth equations for both the alloys have been established. The rate of grain growth in alloy II was found to be slower as compared to alloy III. The grain growth exponents for alloy II and III were determined to be 0.38 and 0.52, respectively. |
