Microstructure Evolution Of Binary TiAl Peritectic Alloys During Directional Solidification

The regularity of microstructure evolution and the microstructure selection of TiAl peritectic alloy during directional solidification were systemically studied. The possibly existed phases and microstructure morphologies are theoretically predicted. Experimentally, both the evolutions of interfacial morphology, and the typically solidified features, such as competitive growth of the two phases, coupled growth and banding microstructure were observed, which provide the clue for the further control of the microstructure of TiAl alloy.With the interface response function (IRF) of each solidified phase of Ti-(44-50)Al alloy, and the criterion of the highest interface temperature, the phase selection and microstructure evolution between the primaryβand peritecticαwere theoretically studied under directional solidification condition and a microstructure-selection map of Ti-(44-50)Al as the function of composition and the G/V ratio was constructed. On the other hand, based on the criterion of nucleation and constitutional undercooling (NCU), the microstructure selection map of Ti-(44-50)Al was also constructed, which is in good agreement with the one by IRF in low growth velocity. Then, the microstructure selection map of Ti-(50-58)Al was constructed by NCU. Those maps were used to select the compositions and growth parameters.Four different compositions in two peritectic reaction regions of Ti-Al alloys were adopted in directionally solidified experiments with temperature gradient 100K/cm and 80K/cm and growth velocity ranging from 2μm/s to 25μm/s. The influences of composition and growth velocity on the directionally solidified structure were analyzed. There has the competitive growth of the columnar grains of Ti-(44-50)Al during the directional solidification. With the coupled growth ofβcellular/dendrite andαplanar interface, the competitive growth between two phases was investigated and the influnence of peritectic reaction on the solidified microstructure were analyzed, which clarified the competitive growth mechanism between the primary phase and peritectic phase. The evolution of interfacial morphology of Ti-(44-50)Al peritectic alloys are investigated, which changed from planar to cellular/dendrite morphologies. The theoretical selection maps of Ti-(44-50) Al matches well with the experimental results.When the temperature gradient is 100K/cm and growth velocities are 2.8μm/s and 6.7μm/s, a banding ofαphase andγphase were obtained in the directionally solidified microstructures of Ti-52.5Al. When the alternating growth ofαandγphase has finished, it comes into the coupled growth ofβphase,γphase andαphase with cellular/dendrite interface.When the temperature gradient is 80K/cm and growth velocity is 6.7μm/s, a coupled growth in the directionally solidified microstructure of Ti-52.5Al alloy takes place. The basic characteristic for coupled growth was investigated. The coupled growth models for eutectic and peritectic alloys are compared, with an aim to determine the condition for the coupled growth in peritetic alloy, and a steady superheat growth model was presented. During the directionally solidified process of Ti-52.5Al alloy, there is the coexistence of coupled growth ofαandγ, discrete bandingα, islandα, oscillatory microstructure and mixed band microstructure of single phase ofα/coupled growth ofαandγ/single phase ofα.The formation of banding growth and coupled growth is significantly affected by temperature gradient, alloy composition, nucleation velocity, and growth velocity of each phase. The mechanisms of banding growth, coupled growth and unsteady oscillation are discovered by analyzing the competitive growth between the primary phase and peritectic phase on the basis of nucleation, alloy composition, growth velocity.