Grain size control of TiAl alloy with fully lamellar microstructure using discontinuous coarsening reaction

Discontinuous coarsening reaction have been used to obtain the microstructure of fine-grained lamellae in a Ti-46 mol%Al alloy, which is expected to improve ductility at room temperature. The process mas very simple because the structure change was performed only by aging subsequent to quenching. The refinement mechanism was investigated by optical and scanning electron microscopy. The nuclei of discontinuous coarsening reaction were coarsely spaced lamellae which were generated at grain boundaries and inside grains of finely spaced lamellae. The nucleation of coarsely spaced lamellae during aging increased with increasing cooling rates from the liquid phase and the alpha single phase, When the aging temperature was below 1473 K, the discontinuous coarsening reaction occurred preferentially and the microstructure was refined. The refinement by this process was achieved via the following steps: (1) nucleation of coarsely spaced lamellae, (2) changing the growth direction and (3) suppression of the grain growth. Thermal stability of the fine-grained microstructure: obtained by this process was quite high.