GRAIN-SIZE CONTROL OF TIAL ALLOY WITH FUL LY LAMELLAR MICROSTRUCTURE USING DISCONTINUOUS COARSENING REACTION

Discontinuous coarsening reaction have been used to obtain the microst ructure of fine-grained lamellae in a Ti-46 mol%Al alloy, which is exp ected to improve ductility at room temperature. The process mas very s imple because the structure change was performed only by aging subsequ ent to quenching. The refinement mechanism was investigated by optical and scanning electron microscopy. The nuclei of discontinuous coarsen ing reaction were coarsely spaced lamellae which were generated at gra in boundaries and inside grains of finely spaced lamellae. The nucleat ion 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 r eaction occurred preferentially and the microstructure was refined. Th e refinement by this process was achieved via the following steps: (1) nucleation of coarsely spaced lamellae, (2) changing the growth direc tion and (3) suppression of the grain growth. Thermal stability of the fine-grained microstructure: obtained by this process was quite high.